SKULL MORPHOLOGY AND PHYLOGENETIC RELATIONSHIPS OF A NEW DIMINUTIVE BALAENID FROM THE LOWER PLIOCENE...

SKULL MORPHOLOGY AND PHYLOGENETIC

RELATIONSHIPS OF A NEW DIMINUTIVE

BALAENID FROM THE LOWER PLIOCENE

OF BELGIUM

by MICHELANGELO BISCONTIDipartimento di Scienze della Terra, Universita di Pisa, via Santa Maria 53, 56126 Pisa, Italy; e-mail: [email protected]

Typescript received 26 November 2003; accepted in revised form 10 May 2004

Abstract: A new small balaenid is described and compared

to all fossil and living balaenid taxa. The specimen represents

a new genus and species and is named Balaenella brachyrhy-

nus. It was discovered in the Lower Pliocene of Kallo (north-

west Antwerp, Belgium) and adds new information on the

diversity and evolution of Balaenidae. Based on both com-

parative morphology and phylogenetic analysis, Balaenella

brachyrhynus is morphologically closer to the genus Balaena,

including the living Greenland Bowhead whale (B. mystice-

tus), and two Pliocene species (B. montalionis and B. ricei)

from central Italy and the eastern USA. Balaenella brachyrhy-

nus has very short nasals, a short rostrum relative to the total

skull length and a horizontal supraoccipital. A cladistic treat-

ment of 81 morphological character states scored for 10

balaenids and nine non-balaenid cetaceans revealed that the

other small balaenids generally included in the genus Balaen-

ula (including Balaenula astensis, B. balaenopsis and a Plio-

cene Balaenula sp. from Japan) are closer to the living genus

Eubalaena (the Right whale). As the new skull is so different

from the nominal Balaenula species, and as it is more closely

related to Balaena than to Eubalaena, it is concluded that a

small body size was a common condition in different Bala-

enidae clades.

Key words: Balaenula, Balaenidae, Belgium, Cetacea, Mysti-

ceti, Phylogeny, Pliocene.

In zoological textbooks, balaenids are usually regarded as

gigantic, slow-swimming Mysticeti (Mammalia, Cetacea)

with an arched rostrum and long baleen (Ridgway and

Harrison 1985; Berta and Sumich 1999), but it is often

forgotten that small and more streamlined forms have

flourished during their evolutionary history. Small balae-

nids are represented by a conspicuous and widespread

fossil record. They occur in the Upper Miocene–Lower

Pliocene of California (eastern Pacific; Barnes 1977), and

in Pliocene sediments of Belgium (north-east Atlantic;

Van Beneden 1880, 1878), Italy (central Mediterranean;

Trevisan 1941; Pilleri 1987; Bisconti 2000), Japan (western

Pacific; Excavation Research Group for the Fukagawa Fos-

sil Whale 1982), and eastern USA (western Atlantic;

Whitmore 1994). Despite their worldwide occurrence,

only a few species have been described and figured in

detail; these are: the Japanese Balaenula sp. (Excavation

Research Group for the Fukagawa Fossil Whale 1982), the

Italian Balaenula astensis (Trevisan 1941; Pilleri 1987; Bis-

conti 2000) and, to some extent, the Belgian Balaenula

balaenopsis (Van Beneden 1878, 1880).

The small balaenids are usually grouped within

the genus Balaenula Van Beneden, 1880. Balaenula is

based on a fragmentary skull and associated skeleton

but unfortunately an unambiguous character-based diag-

nosis of the genus has been lacking until recently (Bis-

conti 2003a). Abel (1941) re-assessed some Belgian

material pertaining to Balaenula and provided a recon-

struction of the type skull (Van Beneden did not desig-

nate any holotype material). However, some

morphological details (such as the dorsal surface of the

supraoccipital, the relationships of maxilla and frontal,

the morphology of the distal portion of the rostrum,

and part of the architecture of the temporal fossa) are

definitively lost and their reconstruction is based on

inference only.

In his original descriptions of the Balaenula type skull

(namely Balaenula balaenopsis), Van Beneden (1880,

1878) also discussed the morphology of periotic and

tympanic bones. Unfortunately, it is not certain that

these bones belong to the type skull because they were

found disassociated from it (Van Beneden 1880, 1878),

and the features of the petrosal are so different from

what is observed in other balaenids that it is likely that

it does not belong to a balaenid at all (but see Bisconti

2003a).

[Palaeontology, Vol. 48, Part 4, 2005, pp. 793–816]

ª The Palaeontological Association 793

New discoveries made during the twentieth century

provided materials for the detailed descriptions of two

well-preserved skulls and associated petrotympanic com-

plexes of small balaenids from Italy and Japan (Trevisan

1941; Excavation Research Group for the Fukagawa Fossil

Whale 1982). The new specimens were assigned to the

genus Balaenula based on the small size of the crania. Bis-

conti (2000) assessed the phylogenetic relationships of the

Italian specimen (Balaenula astensis) on cladistic grounds

and found that it was close to the Belgian Balaenula

balaenopsis; however, the Japanese Balaenula sp. was not

included in the ingroup and its phylogenetic position is

still unclear. Bisconti (2000) found differences between

the Italian and the Belgian species in the architecture of

the temporal fossa and the brain size, and found a

weakly supported relationship between Balaenula astensis,

B. balaenopsis and the living Right whales (Eubalaena) to

the exclusion of the extant and fossil species of Balaena

that were known at that time. However, the low resolu-

tion of his cladograms demands a new and comprehen-

sive study in order to fix systematic relationships among

the Balaenidae.

In a review of the evolutionary history of balaenids,

Bisconti (2003a) provided redescriptions of several fossil

taxa from Europe and emended some morphological

diagnoses including that of Balaenula. He also furnished

a redescription of some type materials belonging to the

Belgian Balaenula balaenopsis. Furthermore, he updated

the systematics of fossil Balaenidae and stated that, cur-

rently, three species belong to Balaenula: the Italian

B. astensis, the Belgian B. balaenopsis and an unnamed

Balaenula sp. from Japan. The diagnosis of Balaenula is

now unambiguous and mainly based on the orientation

of the squamosal (in Balaenula it is directed anteriorly),

the position of the cranio-mandibular joint (which, in

Balaenula, is located under the orbit), and the height

of the exoccipital relative to the orbit (which, in

Balaenula, is very low).

A new small balaenid is described in this paper based on

a fairly well-preserved skull. The specimen is described

and compared to the other giant and small, living and fos-

sil balaenids. The phylogenetic position of the new skull is

explored by means of a cladistic study of the best-pre-

served fossil balaenids, together with representatives of the

living taxa. Based on the phylogenetic results and the com-

parative analysis, it is concluded that the skull shows mor-

phological features that distinguish it from the Italian,

Belgian and Japanese small balaenids at generic level.

Indeed, it represents a lineage unrelated to Balaenula,

being closer to the giant Balaena (the Greenland Bowhead

whale). The skull documents an unsuspected diversity

among the small balaenids and, based on its phylogenetic

position, suggests that in the past few million years a small

size was a common condition in different balaenid clades.

The new balaenid was found by E. T. H. Van Tuiyll

in 1974 at Kallo, north-west Antwerp (Belgium; Text-

fig. 1), during the construction of the First Channel dock

(Eerste Kanaaldok), 18 m below sea level. The First Chan-

nel dock is one of a series of channels connecting Ant-

werp with the Schelde River (and indirectly with the

North Sea). The specimen was displayed in Van Tuiyll’s

palaeontological collection until March 2001, when the

whole collection was donated to the Natuurmuseum

Brabant in the town of Tilburg, Holland.

MATERIAL AND METHODS

Institutional abbreviations. MSNT, Museo di Storia Naturale

e del Territorio, Universita di Pisa, Pisa, Italy; MGP, Museo

Geopaleontologico G. Capellini, Universita di Bologna, Bologna,

Italy; IRSN, Institute Royal des Sciences Naturelles, Bruxelles,

Belgium; NMB, Natuurmuseum Brabant, Tilburg, The Nether-

lands; SMNS, Staatliches Museum fur Naturkunde, Stuttgart,

Germany; USNM, United States National Museum of Natural

History, Smithsonian Institution, Washington, DC, USA; ZMA,

Instituut voor Systematiek en Populatiebiologie ⁄ZoologischMuseum, Amsterdam, The Netherlands; ZML, Zoologisch

Museum, Leiden, The Netherlands.

Anatomical terms and abbreviations. The anatomical termin-

ology describing the general features of the cetacean skull fol-

lows Kellogg (1965, 1968a). Petrosal terminology is in accord

with Geisler and Luo (1996, 1998) and Luo and Gingerich

(1999); tympanic terms are from Oishi and Hasegawa (1994),

Luo (1998) and Luo and Gingerich (1999). Abbreviations as

follows: an, antorbital notch; ap, anterior process of petrosal;

apat, anterior pedicle for the articulation with the tympanic;

exocc, exoccipital; fm, foramen magnum; ip, infraorbital pro-

cess of the maxilla; irfr, interorbital region of the frontal; lp, lat-

eral protrusion of the lambdoidal crest; lpap, lateral projection

of the anterior process of the petrosal; lsc, lateral squamosal

TEXT -F IG . 1 . Location of Antwerp and some of the principal

towns of Belgium (west of the River Mose), southern Holland

(north of the River Mose) and north-west Germany (south and

east of the River Rhine); the black rectangle indicates the

discovery area.

794 PALAEONTOLOGY, VOLUME 48

crest; mx, maxilla; mxf, maxillary foramen; n, nasal; nf, narial

fossa; oc, occipital condyle; p, parietal; pbsf, posterior border

of the stylomastoid fossa; pc, pars cochlearis; pgl, postglenoid

process; plc, posterolateral corner of the pars cochlearis; pmx,

premaxilla; ppp, posterior process of the petrosal; ow, oval

window; si, sutura interfrontalis; soc, supraoccipital; sop, supra-

orbital process of frontal; ttg, tensor tympani groove; zp,

zygomatic process of squamosal; VII-g, groove for the facial

nerve.

Sources of comparative data

Eubalaena glacialis. MSNT 264: skeleton and left petrotym-

panic; ZML without inventory number (abbreviated w.i.n.):

skeleton and both petrotympanics; USNM 267612, 3339990,

23077, 301637: skulls and petrotympanics. Based on molecular

analyses, Rosenbaum et al. (2000) and Malik et al. (2000)

proposed that Eubalaena should comprise at least three differ-

ent living species. However, morphological features useful to

distinguish these three supposed species are not currently

available (Bisconti 2002), and for that reason here I

accept the traditional, morphological definition of the genus

Eubalaena (Cummings 1985; Mead and Brownell 1993), and

consider that at present the genus includes two geographically

distinct species, Eubalaena australis and E. glacialis. General

descriptions and illustrations can be found in Cuvier (1823),

Van Beneden and Gervais (1880), True (1904) and Cummings

(1985).

Eubalaena belgica. Type material described by Abel (1941) and

Plisnier-Ladame and Quinet (1969) as Balaena belgica. The

taxon is represented by the IRSN holotype skull, Ct.M. 879a–f

(including portions of squamosals, maxilla and frontal).

McLeod et al. (1993) suggested that this form is so similar to

Balaenula balaenopsis in the morphology of the zygomatic

process of the squamosal that it should be referred to Balaen-

ula as the new combination Balaenula belgica. I examined the

type skull and confirmed this observation, but it should be

noted that the type skull of this taxon is very large and

heavy, differing from the small, slender forms belonging to

Balaenula. Moreover, in this large Belgian species, the lateral

borders of the supraoccipital are parallel whereas they are

anteriorly convergent in Balaenula astensis and the Japanese

Balaenula sp.; therefore, the morphology observed in the Bel-

gian species resembles Eubalaena glacialis and not Balaenula.

A slender zygomatic process of the squamosal was described

by Nishiwaki and Hasegawa (1969) in a Pleistocene Eubalaena

glacialis from Japan; therefore this feature does not appear to

be useful for generic diagnosis. Bisconti (2000, 2003a) sugges-

ted that Balaena belgica should belong to Eubalaena based on

the morphology of the supraoccipital, a strong temporal

ascending crest on the supraorbital process of the frontal, and

the angle between rostrum and frontal.

Balaena mysticetus. IRSN 1532: skeleton; ZML 1680, 3997,

2563, 2001, two ZML w.i.n. (both petrotympanics labelled ‘Bal-

aena japonica’), and ZML w.i.n. (right petrotympanics); USNM

257513: skull. Descriptions and illustrations can be found in

Cuvier (1823), Van Beneden and Gervais (1880), Reeves and

Leatherwood (1985) and Burns et al. (1993).

Balaena montalionis. MSNM MC CF31 holotype skull (see

Capellini 1904; Pilleri 1987; Bisconti 2000, 2003a for descriptions

of the specimen).

Balaena ricei. Westgate and Whitmore (2002).

Balaena primigenius. Type material described by Van Beneden

(1880, 1878), held by IRSN and represented by Ct.M. 887

(left petrosal), Ct.M. 886, Ct.M. 884 (both right tympanic

bullae). Bisconti (2003a) re-assessed the type materials, conclu-

ding that the taxon is too poorly known to receive a scientific

name; therefore, Balaena primigenius is a Balaenidae gen.

indet.

Balaenula balaenopsis. Type material described by Van Beneden

(1880, 1878), held by IRSN and represented by Ct.M. 858a–b

(right petrosal), Ct.M. 853d (posterior process of petrosal),

Ct.M. 859–863 (tympanic bullae), Ct.M. 865a–b, 867a–c, 868,

869a–b (cervical vertebrae). Additional material held in MGP is

represented by a skeleton from Poggio alle Talpe including both

petrotympanics (central Italy; 1CMC29 ⁄ 8929; Capellini 1877;

Portis 1883).

Balaenula astensis. MSNM MC CF35 holotype skull and associ-

ated petrotympanics (see Trevisan 1941; Pilleri 1987; Bisconti

2000, 2003a for descriptions of the specimen).

Japanese Balaenula sp. Excavation Research Group for the

Fukagawa Fossil Whale (1982).

Balaenotus insignis. Type material described by Van Beneden

(1880, 1878), held by IRSN and represented by Ct.M. 832,

833a–b (both specimens are right petrosals), Ct.M. 834, 835

(both specimens are tympanic bullae), Ct.M. 836a–c (a is a

distal fragment of right supraorbital process of frontal, b and

c are rostral elements: a fragment of premaxilla and a prox-

imal portion of maxilla), Ct.M. 837 (fragment of left squamo-

sal), Ct.M. 838 (fragmentary supraorbital process of the

frontal), Ct.M. 840, 841a–b, 842a–b, 844a–b (cervical verteb-

rae), Ct.M. 850 (left scapula) and Ct.M. 856 (left fragment of

mandibular condyle). All these materials have been redescribed

by Bisconti (2003a). Additional material is held in MGP

(Capellini 1877; Portis 1883): the Poggiarone skeleton

(1CMC23 ⁄ 8923, 26 ⁄ 8926, 22 ⁄ 8922), the cast of a right petro-

sal from Pieve di Santa Luce (near Pisa, central Italy,

1CMC87 ⁄ 8987), the cast of a right petrosal from Monte Aper-

to (near Siena, central Italy, 1CMC ⁄ 8987). Balaenotus is a

problematic genus. It was based on a very incomplete skull,

petrotympanics and some postcranial bones by Van Beneden

(1880, 1878); however, this poor material is diagnostic at the

genus level (see Bisconti 2003a); therefore, in this work the

name Balaenotus insignis is retained.

Morenocetus parvus. Cabrera (1926).

B ISCONTI : DIMINUTIVE PLIOCENE WHALE 795

SYSTEMATIC PALEONTOLOGY

Class MAMMALIA Linnaeus, 1758

Order CETACEA Brisson, 1762

Suborder MYSTICETI Cope, 1891

Family BALAENIDAE Gray, 1825

Genus BALAENELLA gen. nov.

Text-figures 2–7

Derivation of name. Balaena, Latin name for the Greenland

Bowhead Whale; -ella, Latin, diminutive suffix indicating a form

smaller than Balaena.

Type species. Balaenella brachyrhynus sp. nov.

Diagnosis. Balaenella brachyrhynus belongs to the family

Balaenidae owing to the following features: maxilla trans-

versely compressed in its anterior three-quarters, rostrum

highly arched, supraoccipital extending anteriorly to the

orbit, supraorbital process of the frontal gently descending

from the interorbital region, parietal not exposed on the

cranial vertex, squamosal mainly developed along the

dorsoventral axis, lateral squamosal crest forming a strong

anterior convexity, triangular-shaped lateral projection of

the anterior process of the petrosal, posterodorsal corner of

the stylomastoid fossa rounded in dorsal view and far from

AB

CD

E

TEXT -F IG . 2 . Balaenella brachyrhynus holotype skull (NMB 42001). A, right lateral view. B, right premaxilla. C, left premaxilla.

D, left lateral view. E, anterior view. Scale bar represents 200 mm.


the posterior wall of the pars cochlearis, long and shallow

stylomastoid fossa, tympanic bulla dorsoventrally com-

pressed, tympanic cavity shallow. It shares the following

features with Balaena: supraorbital process of the frontal

and lateral process of the maxilla posteriorly orientated so

that a continuous arch is developed by rostrum and frontal,

squamosal posteriorly orientated, posterior wall of tem-

poral fossa observed when the skull is in lateral view, infra-

orbital plate absent laterally (feature figured by Cuvier

1823, pl. 25). The following features are exclusively diag-

nostic of Balaenella brachyrhynus: skull length slightly more

than 1000 mm, length of rostrum approximately 50 per

cent of the total skull length, nasal bones about 60 per cent

shorter than nasals of giant balaenids (Balaena and

Eubalaena) and about 75 per cent shorter than nasals of

Balaenula astensis, nasal bones lacking the anterior notch;

nasal bones forming a triangular complex displaying an

anterior apex, anterior two-thirds of supraoccipital hori-

zontal, rostrum triangular in dorsal view, lateral process of

the maxilla strong.

Balaenella brachyrhynus sp. nov.

Derivation of name. Greek, brachy, little; rhynus, nose, denoting

a small Balaena with a little nose, and the marked reduction of

the nasal bones of the holotype skull.

Holotype. Specimen 42001, Natuurmuseum Brabant, Tilburg,

Holland. The specimen will be referred to as NMB 42001 or the

Tilburg skull in the text.

Type locality and horizon. The specimen was found during the

construction of the First Channel dock (Eerste Kanaaldok) at

Kallo (north-west Antwerp, Belgium; Text-Fig. 1), 18 m below

sea level in the Zanden van Kattendijk (Kattendijk Sand) For-

mation. Janssen (1974) and Nuyts (1990) provided data on

the fossil content, biostratigraphy and lithostratigraphy of the

deposits near Kallo. The specimen was found in what Nuyts

(1990) described as clay, bioturbated at the top, infilled with

greyish green sands. The clay belongs to the Kattendijk For-

mation and is Early Pliocene in age based on its foraminiferal

content. The Kattendijk Formation contains a foraminiferal

association that is typical of the BFN 4 (Florilus boueanus-

Monspeliensina pseudotepida Assemblage Zone; Nuyts 1990).

Janssen (1974) stated that the Kattendijk Formation is Scaldi-

sian or ‘Kattendijkien’ in age; Hoedemakers and Marquet

(1992) correlated the ‘Kattendijkien’ to the Mediterranean

Tabianien. The latter is now completely incorporated in the

Zanclean (Monegatti and Raffi 1996), i.e. Early Pliocene

(2Æ5–5Æ3 Ma; Sprovieri 1992; Rio et al. 1994), and this is the

age proposed for NMB 42001.

Diagnosis. As for genus.

Description

The skull is substantially complete; it lacks dentaries, the right

petrotympanic complex, the anterior end of maxillae, right orbit,

jugals, lacrimals and some portions of the squamosals. The pre-

maxillae are largely preserved but are detached from their nat-

ural position by the collector. Taken as a whole, this skull is one

of the best-preserved balaenid specimens in the world. Measure-

ments of skull and petrotympanic complex are given in Tables 1,

2, respectively. The skull as preserved is illustrated in Text-

figures 2, 3; a reconstruction of the skull is presented in Text-

figure 4.

Rostrum: maxilla, premaxilla, nasal. The left maxilla is almost

complete, lacking only the anterior apex. Only the posterior half

of the right maxilla is preserved. The maxillae are transversely

compressed for the main part of their length but become wider

approaching the frontal. Anteriorly, the lateral borders of the

maxillae converge toward the long axis of the skull (Text-

Fig. 4B); in fact, the transverse diameter of maxillae is 140 mm

at the anterior end of the rostrum (as inferred by duplicating

the distance between the lateral border of the left maxilla and

the longitudinal axis) and an increment of 10 mm is observed

200 mm posteriorly to the apex. The width of the rostrum

increases to 220 mm at 400 mm behind the apex and reaches

670 mm between the lateral apices of the infraorbital processes

of maxillae at c. 560 mm behind the anterior end of the left

maxilla. In dorsal view, the rostrum appears triangular (Text-

Fig. 4B).

In lateral view, the maxilla forms a complete arc anterior

and under the supraorbital process of the frontal (Text-figs 2,

3A). Differing from living balaenids (Balaena mysticetus and

Eubalaena), the maxillae are depressed in front of the frontal,

and the posterior three-quarters of their dorsal border are

ventrally directed. The lateroventral margins of both maxillae

form a dorsoventrally and laterally concave border running

onwards, and slightly converging towards the long axis of the

TEXT -F IG . 3 . A, right side of the holotype skull (NMB 42001)

as preserved. B, left side of the holotype skull as preserved. The

scale bar represents 200 mm. See text for explanation of

labelling.


skull. The anteriormost quarter of the rostrum abruptly pro-

jects downward over the last 57 mm.

Both maxillae bear four maxillary foramina. The infraorbital

processes are very robust and triangular in dorsal view. There

is a slight depression 110 mm in length running above the

ventrolateral border of the infraorbital process that corres-

ponds to the antorbital notch. Rare vascular sulci related to

the blood supply for the baleen are present on the ventral

surface of the maxillae.

The maxilla articulates with the frontal through a wide, flat

process that is formed by its posteromedial corner. This pro-

cess superimposes on the anteromedial portion of the frontal

lateral to the nasals, and it is round in posterior outline. The

infraorbital process of the maxilla and the supraorbital process

of the frontal are divided by a space that widens distally. The

infraorbital plate is developed medially under the supraorbital

process of the frontal, but it is absent more laterally so that

it cannot be observed when the skull is examined in lateral

view.

TABLE 1 . Skull measurements of Balaenella brachyrhynus.

NMB 42001, holotype; measurements in mm.

Condylobasal length 1080

Maximum width of skull (between postorbital

processes of supraorbital processes of frontal)

800

Anteroposterior diameter of supraorbital process

of frontal

130

Laterolateral diameter of supraorbital process

of frontal

550

Nasal length along medial border 43

Maximum width of both nasals at anterior end 25

Maximum width of both nasals at posterior end 58.5

Maximum width of narial opening anterior

to nasal end

140

Length of supraoccipital 310

Linear length of left maxilla 560

Length of left maxilla along external curvature 560

Posterior width of left maxilla (posterior border

of infraorbital process)

375

Linear length of right maxilla 750

Length of right maxilla along external curvature 770

Posterior width of right maxilla (posterior border

of infraorbital process)

345

Maximum width of supraoccipital between

exoccipitals

460

Length of left premaxillary fragment 650

Anterior width of left premaxillary fragment 23

Posterior width of left premaxillary fragment 40

Length of anterior fragment of right premaxilla 330

Length of posterior fragment of right premaxilla 410

Width of supraoccipital at midlength 330

Anteroposterior diameter of temporal fossa 90

Laterolateral diameter of temporal fossa c. 250

Laterolateral diameter of left occipital condyle 75

Anteroposterior (dorsoventral) diameter of

left occipital condyle

100

Estimated laterolateral diameter of foramen

magnum

64

Estimated anteroposterior (dorsoventral)

diameter of foramen magnum

80

Distance between lateral border of left

occipital condyle and lateral border of exoccipital

150

Distance between antorbital and postorbital

processes of supraorbital process of frontal

105

TABLE 2 . Periotic and tympanic measurements of Balaenella

brachyrhynus, NMB 42001, holotype; measurements in mm.

Length of posterior process of petrosal 96

Width of posterior process of petrosal at midlength 25

Length of anterior process of petrosal 42

Width of anterior process of petrosal excluding

lateral projection

28

Width of anterior process of petrosal including

lateral projection

46

Anteroposterior diameter of pars cochlearis 26

Lateromedial diameter of pars cochlearis

(maximum protrusion of promontorium)

18.5

Length of tympanic bulla 84

Anterior width of tympanic bulla 60

Height of tympanic bulla (from convex face to

dorsal rim of tympanic medial prominence)

38

Height of tympanic bulla (from lateroventral

keel to dorsal rim of tympanic medial prominence)

62

Maximum height of tympanic cavity 30

Maximum length of tympanic cavity 63

Maximum width of tympanic cavity 20

TEXT -F IG . 4 . Reconstruction of the Tilburg skull (NMB

42001). A, left lateral view. B, dorsal view. The scale bar

represents 200 mm. See text for explanation of labelling.


The premaxillae display a strong transverse compression and a

round dorsal profile. The ventral border is broken along the

entire length of the bones. The nasal cavity is located between

the posterior portions of the premaxillae, and its anteroventral

edge is represented by an anterodorsally ascending relief of the

medial wall of the premaxillae.

The nasal bones display a very characteristic morphology

observed in no other living and fossil balaenid (Text-fig. 4).

They are very short and their lateral borders converge anteriorly,

forming a triangular complex slightly raised relative to the max-

illae. The anterior edges of the nasals are transversely and dors-

oventrally round, lacking the typical notch of the other

balaenids. Posteriorly, the nasals are divided by a triangular, for-

ward-growing portion of the frontal.

The rostral length (560 mm between anterior and posterior

end of maxilla) approaches 50 per cent of the condylobasal

length (1080 mm). This situation is observed in newborn living

balaenid species, but the holotype of Balaenella brachyrhynus is

not a juvenile individual because all of the sutures of the skull

are fused. Balaenella brachyrhynus shares two features with juve-

niles and newborns of living balaenids, namely the horizontal

development of the supraoccipital (see above) and the ratio of

rostral length to total skull length.

Frontal. The frontal includes two supraorbital processes and a

small interorbital region (Text-figs 2–3, 5). The right supraor-

bital process is broken at 410 mm from the sagittal axis of the

skull but the left one is complete. The supraorbital processes are

developed along an oblique plane descending from the higher

interorbital portion. They strongly project rearward and out-

ward; the ventral angle between them is largely obtuse,

approaching 170 degrees. The dorsal surface of the supraorbital

process is distally flat, but medially displays a slight temporal

ascending crest that makes the dorsomedial portion of the bone

prismatic and more robust.

The antorbital process is more robust and higher than the

postorbital process. The sulcus for the optic nerve is shallow and

narrow, and it develops on the posteroventral surface of the

supraorbital process. The frontal is exposed dorsally and forms a

short interorbital region, which is 32 mm in length. The anterior

process of the supraoccipital is superimposed on the dorsal sur-

face of the posterior portion of the interorbital region of the

frontal. The frontal is anteriorly divided by an unfused sutura

interfrontalis, and posteriorly bounded by the coronal suture

between frontals and the parietals.

Temporal fossa: parietal, squamosal. The coronal suture is evi-

dent in lateral view but not in dorsal view owing to the super-

imposition of the anterior process of the supraoccipital on the

parietal and the frontal. In lateral view, the squamosal suture (of

parietal and squamosal) begins from the lambdoidal suture (of

squamosal, parietal and supraoccipital) and develops ventrally.

The lambdoidal suture is marked by a robust lateral tubercle

protruding from the posterior temporal crest.

Anteriorly, the parietal is superimposed upon the lateral and

medial portions of the frontal covering the posterodorsal region

of emergence of the supraorbital processes. Presumably, the

supraoccipital superimposes on the parietal. The dorsal border

of the parietal forms the temporal crest overhanging the tem-

poral fossa (i.e. the lateral walls of the skull behind the supraor-

bital processes of the frontal are not seen in dorsal view); the

temporal crests follow the dorsal borders of the supraoccipital

and converge anteriorly towards the sagittal line.

The squamosal is developed along the dorsoventral axis. It

projects downward and posteriorly, but it is less posteriorly ori-

entated than the supraorbital process of the frontal. The squ-

amosal forms the posterior wall of the temporal fossa, which is

strongly concave in this skull. This wall is laterally bound by an

undulating lateral squamosal crest (lsc). The lsc originates dor-

sally from the lambdoidal crest and projects forward; then it

continues ventrally generating a convex curve (anterior convex-

ity); finally, the crest projects downward and posteriorly. The

lateral surface of the squamosal is flat. The posterodorsal profile

of the bone is rounded. Both the zygomatic and the postglenoid

processes are missing; thus, the morphology of the cranioman-

dibular joint is unknown.

The tubercle located on the lambdoidal suture and the con-

vex curve on the lsc limit the space occupied by the lambdoi-

dal crest. That crest is very round, differing from the anterior

A

B

TEXT -F IG . 5 . A, nasal bones of the holotype skull (NMB

42001) as preserved. B, vertex structure of Balaenella

brachyrhynus. Scale bars represents 85 mm. See text for

explanation of labelling.


temporal crest and the ventrolateral lsc; in fact, it is not a true

crest but only a round convexity of the dorsal borders of the

squamosal. The lambdoidal crest represents the connection

between the temporal crest and the lsc, and its posterior apex

is behind the level of the lsc but anterior to the occipital

condyles.

Occipital region: supraoccipital and exoccipital. The supraoccipi-

tal is large and wide (Text-fig. 4B); it does not display a dome

(like Eubalaena), and its anterior end is slightly raised. The lat-

eral borders are posteriorly convex, but lateral to the anterior

process they are concave (i.e. the anterior process of the supra-

occipital is laterally compressed). The anterior process is small

and narrow, and its rostral border is linear. The anterior process

is more advanced than the supraorbital processes of the frontal

(i.e. in dorsal view, the anterior process ends more anteriorly

than the antorbital corner of the frontal).

A wide area of low relief developed along the sagittal axis of

the supraoccipital divides two anterior nuchal fossae. The main

portion of the supraoccipital is almost horizontal but the poster-

ior part, including the foramen magnum and exoccipitals, is

more vertical.

The exoccipitals are laterally round; in dorsal view they are

slightly posterior to the deduced position of the foramen mag-

num. The descending processes of the basioccipital are wide and

triangular, but their ventral corners are badly eroded. Posteriorly

they are transversely orientated, but anteriorly they parallel the

longitudinal axis of the skull. The vomer divides them on the

ventral surface of the skull. The occipital condyles are wide and

form the ventrolateral sides of the foramen magnum; ventrally,

they are largely separated by a deep notch. A condylar neck is

lacking. The foramen magnum is destroyed.

Basicranial observations: pterygoid, petrosal, tympanic. As cur-

rently displayed for exhibition in the Natuurmuseum Brabant,

the holotype skull is on a mobile box and leans on a gypsum

pedestal so that its basicranium is largely inaccessible. Thus, it is

possible to report only significant observations of those portions

of the skull that are free of gypsum (Text-figs 6–7).

The vomer is observed in the posterior portion of the skull

where it divides the descending processes of the basioccipital.

There, it is flat and covers the suture between basisphenoid and

basioccipital, reaching a point very close to the posterior end of

the skull. The vomer is also observed in the rostrum where it is

found between the maxillae. There, it displays a strong ventral

convexity.

The palatines are not visible. The presence of the alisphenoid

in the ventral side of the temporal fossa is uncertain, as it is dif-

ficult to observe the sutural morphology owing to the gypsum

pedestal.

The pterygoids are developed along the dorsoventral axis, and

they are damaged along the posteroventral border. They display

both the medial and the lateral lamina, and a deep pterygoid

fossa. The fossa is a concavity entering the pterygoid so that it is

ventrally bordered by a pterygoid ventral lamina. The lateral

diameter of the pterygoid decreases along the dorsoventral axis.

The pterygoid is laterally bordered by the falciform process of

the squamosal, which does not display a complete infundibulum

for the external opening of the foramen pseudovale.

Only the ventral surface of the petrosal is discussed here

because the bone is articulated with the skull and its dissection

for a detailed description would be too invasive for the general

preservation of the entire skull (Text-fig. 6). Measurements of

petrosal and tympanic bullae are given in Table 2. The posterior

process of the left petrosal is articulated between exoccipital and

squamosal and emerges in the posterolateral corner of the skull.

The posterior process is long and narrow (the lateral diameter of

the pars cochlearis is c. 27 per cent of the length of the posterior

process); its ventral surface is flat, and there is no sulcus or

depression for the posterior exit of the facial nerve (VII).

A B

TEXT -F IG . 6 . Periotic of Balaenella brachyrhynus holotype (NMB 42001). A, periotic as preserved in the skull. B, interpretative

drawing of periotic structures. Scale bar represents 50 mm. See text for explanation of labelling.


Approximately 57 mm anterior to the posterior end of the pos-

terior process, a long and presumably low stylomastoid fossa

begins to develop onward; its posterior border is a depression

on the medial side of the posterior process. The posterior pro-

cess forms a right angle with the anterior process. The latter is

squared and short (the lateral diameter of the pars cochlearis is

c. 61 per cent of the length of the anterior process). A triangular

process 18 mm in length projects from the posterior side of the

anterior process at an acute angle. The pars cochlearis is broadly

round and does not protrude in a cranial direction. It was

impossible to observe the cranial foramina of the pars cochlearis.

The oval window is wide and approximately elliptical; the

groove for the facial nerve is short.

Only the left tympanic bulla is preserved (Text-fig. 7). Its lat-

eral wall is destroyed, including the sigmoid process, but the

conical process is preserved. The tympanic thickness is very

heavy; there is a high thickening along the posterior two-thirds

of the bulla, but the anterior third is narrower. The posterior

tympanic thickening is bisected by a transverse pair of sulci

developed only along the dorsal surface. The peduncles for the

articulation with the petrosal are broken; the posterior peduncle

is close to the posteromedial corner of the bulla and developed

along the anteroposterior axis. The tympanic cavity is relatively

shallow and extends behind the tympanic thickening. The ven-

tromedial keel is pronounced but eroded; the ventral side of the

bulla is strongly concave.

COMPARISONS

Synapomorphic features

Balaenella brachyrhynus has Balaena-like characteristics in

the rostrum, in the sutural pattern of the neurocranial

bones and in the temporal fossa. The most striking fea-

ture shared by Balaenella and Balaena is the lateral pro-

cess of the maxilla located under the supraorbital process

of the frontal (skull in lateral view). The supraorbital pro-

cess is orientated posteriorly in Balenella brachyrhynus,

Balaena montalionis, B. mysticetus and Morenocetus par-

vus. In Eubalaena (including E. belgica), Balaenula astensis

and B. balaenopsis the lateral process of the maxilla and

the anterior portion of the rostrum form a right angle

(skull in lateral view).

As in Balaena mysticetus and B. montalionis, in the

Tilburg skull the supraorbital process of the frontal is dis-

tally flat; this feature is also shared with the fragment of

the frontal of the type material of Balaenotus insignis as

figured by Van Beneden (1878, pl. 27; IRSN Ct.M. 836a).

In Eubalaena, Balaenula astensis and Balaenula balaenopsis

the supraorbital process is prismatic because the ascend-

ing temporal crest is developed along its whole dorsal

surface forming a sharp edge.

Balaena mysticetus and B. ricei have a supraoccipital

with a wide and round anterior border (which is narrower

than that of Balaenula, Eubalaena and Morenocetus),

whereas in Balaenella brachyrhynus and Balaena montali-

onis the anterior portion of the supraoccipital is trans-

versely compressed and the anterior border of the

supraoccipital is transversely straight.

The arrangement of the nuchal fossae of the Tilburg

skull is similar to that observed in Balaenula astensis in

which a pair of anterior fossae are displayed as slight

depressions on the dorsal surface of the supraoccipital.

There is no anterior dome followed by two serial couples

of nuchal fossae as in Eubalaena. Balaena ricei has a later-

ally sloping median ridge, which is not observed in any

other balaenid taxon. In the same region, Balaenula asten-

sis and the Japanese Balaenula sp. display a median con-

vexity that is absent in the Tilburg skull.

Resembling Balaenula astensis, the supraoccipital of

Balaenella brachyrhynus is developed along two planes:

the first plane is approximately vertical and comprises the

foramen magnum and the exoccipitals; the second is

approximately horizontal, and comprises that surface of

A B

TEXT -F IG . 7 . Left tympanic bulla of the holotype skull NMB 42001. A, medial view. B, ventrolateral view. Scale bar represents 60

mm.

BISCONTI : DIMINUTIVE PLIOCENE WHALE 801

the supraoccipital which is anterior to the foramen mag-

num. In Balaenula balaenopsis and B. astensis the supra-

occipital is obliquely bent with an obtuse angle between

the plane across the foramen magnum and exoccipitals,

and the plane of the anterior surface. The supraoccipital

is horizontal in newborns and calves of living giant balae-

nids (Van Beneden and Gervais 1880, 1868–79), so the

pattern observed in Balaenella brachyrhynus and Morenoc-

etus parvus suggests that the verticality or horizontality of

the supraoccipital would depend on the size (and the age)

of the individuals. This hypothesis predicts that small

balaenid taxa have a supraoccipital developed along two

planes, and the anterior portion of the supraoccipital

should be approximately horizontal in all the small forms

independent of their phylogenetic affinities.

Balaenella brachyrhynus shares with Balaena mysticetus

and Balaenotus insignis (Ct.M. 832, 833) the flatness and

general proportions of the posterior process of the petro-

sal. Balaenula astensis also has a flat posterior process, but

in that species the process is markedly shortened (Bisconti

2003a). In Eubalaena glacialis the posterior process

appears different, displaying a complex, somewhat crest-

like dorsal projection from the horizontal ventral portion

of the process (cfr. MSNT 264, USNM 23077). The mor-

phology of the anterior process is shared by the Tilburg

skull, Balaena mysticetus, Eubalaena glacialis and Balaeno-

tus insignis (Bisconti 2003a). Major differences exist

between the anterior process of Balaenella brachyrhynus

and those of Balaenula balaenopsis and B. astensis. In gen-

eral, the differences between the petrosal of B. balaenopsis

and all of the other balaenids are so marked that a mor-

phological analysis should be made to test the hypothesis

that the petrosal does not belong to a balaenid. In fact,

the petrosal of B. balaenopsis (IRSN Ct.M. 858a) was not

found in connection to the type skull (Van Beneden

1880, 1878), and it appears so different that it could eas-

ily belong to a completely different taxon, perhaps to a

cetothere (but see Bisconti 2003a). The differences are in

the morphology of the stylomastoid fossa, which is shar-

ply defined in Balaenula balaenopsis but is not in all of

the other balaenids; in the triangular-shaped cranial open-

ing of the facial canal, which is round in other balaenids;

and in the long and low anterior process, which is usually

short and high in the other balaenids.

The anterior process of the petrosal in Balaenula

astensis is quite delicate. Differing from the other balae-

nids, it is narrow, and relatively long compared with the

length of the pars cochlearis, but it is high like other

balaenids. In this respect the petrosal of Balaenula asten-

sis is easily identifiable, being characterized by a short,

wide posterior process, and a long, transversely com-

pressed anterior process. In both those features, the pet-

rosal of Balaenula astensis differs consistently from

Balaenella brachyrhynus.

Autapomorphic features

The most striking autapomorphy of the Tilburg skull is

the strong reduction of the nasal bones. The nasals are

extremely short and their morphology is completely

divergent from that of the other balaenid whales. In

Balaenula astensis, B. balaenopsis and Balaena mysticetus,

the nasals are rectangular, their long axis being parallel to

the longitudinal axis of the skull; in Balaena montalionis

and Eubalaena glacialis the nasals are also rectangular but

their long axis is transverse to the longitudinal axis of the

skull. Moreover, the nasals of all balaenids but Balaenella

brachyrhynus display a notch in their anterior wall that is

also observed in dorsal view. The nasals of the Tilburg

skull are delicate and lack the notch. They form a trian-

gular complex that interdigitates with the frontal.

The ratios between the length of the nasals (along their

medial border) and the condylobasal length of skulls

belonging to several fossil and living Balaenidae are as fol-

lows: Balaenella brachyrhynus: 2Æ77; Balaenula astensis:

10Æ71 (data determined by Bisconti 2000); B. balaenopsis:

8Æ71 (estimated from Van Beneden 1878); Eubalaena gla-

cialis: 8Æ6, 7Æ3, 6Æ8 (mean, 7Æ56; data from Tomilin 1967);

Balaena mysticetus: 8Æ9, 8Æ91 (Tomilin 1967; Van Beneden

and Gervais 1868–79); B. montalionis: 7Æ42 (total length

of the reconstructed skull estimated as 2Æ2 m; nasal bone

length from Bisconti 2000).

The nasals of the Tilburg skull are about 60 per cent

shorter than those of the giant balaenids Balaena and

Eubalaena, and about 75 per cent shorter than those of

Balaenula astensis. This suggests that Balaenella brachyrhy-

nus approaches the most advanced stage of nasal reduc-

tion in balaenids and possibly in the entire Mysticeti.

The Tilburg skull bears autapomorphies related to the

feeding apparatus. In particular, the robustness of the

posterolateral portion of the maxilla is remarkable. It is

possible that the strong infraorbital process of the maxilla

is related to the feeding behaviour, but the lack of a com-

plete dentary and postcranial bones prevent a clear inter-

pretation of function in that taxon.

PHYLOGENETIC ANALYSIS

Introduction and discussion of previous work

The phylogenetic relationships of Balaenella brachyrhynus

were investigated through a cladistic analysis of living and

extinct members of the family Balaenidae using an exten-

ded outgroup formed by representatives of the main mys-

ticete radiations. Previous studies on Balaenidae failed in

providing a clear picture of the phylogenetic history of

these whales. McLeod et al. (1993), for instance, did not

include fossil taxa in their analysis and this prevented an


unambiguous interpretation of the evolutionary radiations

of the family. A cladistic study of Balaenidae was made

by Bisconti (2000), who included fossil taxa in the

ingroup. He discovered a clade of small balaenids compri-

sing Balaenula astensis, B. balaenopsis and the primitive

Morenocetus parvus based on the shared features listed in

Table 3. A reinterpretation of these features (presented in

the right-hand column of Table 3) shows that only three

characters proposed by Bisconti (2000) are useful in

supporting a Morenocetus + Balaenula clade, namely: flat

posterior wall of temporal fossa (shared by Balaenula

astensis and B. balaenopsis); supraoccipital bending less

than 60 degrees (shared by all the small balaenids plus

Caperea marginata); convex lateral supraoccipital borders

(shared by B. astensis, B. balaenopsis and Morenocetus

parvus). The length ⁄width ratio of the nasal bones could

be considered as another supporting character for the

Morenocetus + Balaenula clade because it is similar in

Balaenula astensis and B. balaenopsis, but unfortunately

the nasals are not preserved in Morenocetus parvus, Eubal-

aena belgica and Balaena ricei so that ambiguity persists.

The petrosal does not provide supporting evidence for

this clade because of the peculiar morphology of that

bone in Balaenula balaenopsis. Moreover, the petrosal of

Balaenula astensis is particularly difficult to interpret, the

posterior process being very short and the anterior pro-

cess long. In general terms, the character states used by

Bisconti (2000) need to be updated with further morpho-

logical work (see Table 3 and Bisconti 2003a for a discus-

sion of Bisconti’s 2000 dataset).

TABLE 3 . Discussion of potential synapomorphic characters proposed by Bisconti (2000) to support a clade including Balaenula

astensis, B. balaenopsis and Morenocetus parvus.

Character Discussion

Linear mandibular rami Seems a primitive feature for mysticetes in general; many cetotheres and Pliocene

rorquals share this character with small balaenids.

Temporal fossa reduced along

anteroposterior and lateromedial

axes

This state can be demonstrated in Balaenula astensis only; the poor preservation of

Balaenula balaenopsis prevents observation of its status; the Japanese Balaenula sp.

has a wide temporal fossa.

Low parasagittal crest By ’parasagittal crest’, Bisconti (2000) meant the anterior part of the lambdoidal crest; the

state can be demonstrated for Balaenula astensis and Morenocetus parvus only; the

Japanese Balaenula sp. has high crests while it is impossible to observe the feature in

Balaenula balaenopsis.

Flat posterior wall of temporal fossa Character shared by Balaenula astensis, Balaenula balaenopsis and Morenocetus parvus.

Lateral squamosal crest slightly

convex anteriorly

This feature is an autapomorphy of Balaenula astensis, its presence is doubtful in Balaenula

balaenopsis and is impossible to demonstrate in Morenocetus parvus. In the Japanese

Balaenula sp. the squamosal crest is strongly convex as in giant balaenids and the Tilburg

skull.

Rectangular and anteriorly directed

temporal region

Poor preservation of the cranio-mandibular joint makes it impossible to be sure of

this condition in several taxa. The postglenoid process is not preserved in Balaenula astensis,

B. balaenopsis and M. parvus.

Supraoccipital bending

< 60 degrees relative to the

horizontal axis

The feature is observed in Balaenula astensis, Morenocetus parvus and the Tilburg skull. In

the lst of these the bending is approximately 0 degrees, approaching the condition of

newborns of living species.

Lateral borders of supraoccipital

convex

The feature is not observed in the Tilburg skull owing to transverse compression of the

anterior process of the supraoccipital.

Two longitudinal pairs of nuchal

fossae

The feature is observed in Balaenula astensis only. Eubalaena glacialis has two pairs of

nuchal fossae but their arrangement is different from Balaenula astensis. All of the other small

balaenids have one pair of anterior nuchal fossae.

Vertical paroccipital process

with posterior concavity

Autapomorphy of Balaenula astensis.

Tuberosity absent from the posterior

surface of the paroccipital process

Difficult to assess without studying individual variation. However, it might be an

autapomorphy of Balaenula astensis.

Lateral diameter of foramen

magnum wide with respect to the

width of the skull at the level of

exoccipitals

Possible autapomorphy of Balaenula astensis.

Ventral angle between the frontal

wings about 90 degrees

The ‘frontal wings’ correspond to the supraorbital processes of the frontal. The character is

shared by Balaenula astensis and Morenocetus parvus. Poor preservation does not

permit observation of this feature in Balaenula balaenopsis and the Japanese Balaenula


Apart from the failure of previous morphological stud-

ies in depicting a clear interpretation of the phylogeny of

Balaenidae, molecular studies have complicated the situ-

ation further in that they have found very few genetic dif-

ferences between the living genera. This led Arnason and

Gullberg (1994) to propose that living balaenids all

belong to the same genus, i.e. Balaena. This conclusion

was also shared by Gatesy (1998) but is in conflict with

the traditional morphological view in which Balaenidae

includes two living genera (Mead and Brownell 1993).

Westgate and Whitmore (2002) hypothesized that Balaena

included at least five species during the Pliocene and this

implicitly suggests that the genetic differences found in

the living taxa may not completely represent their phylo-

genetic history. This problem is investigated and resolved

through the cladistic analysis of living and fossil Balaeni-

dae reported here.

Material and methods

Eighty-one morphological characters were scored for 19

taxa including 10 balaenids in the ingroup, and seven

non-balaenid mysticetes plus two archaeocete cetaceans in

the outgroup. An annotated character list and the charac-

ter · taxon matrix used in the cladistic analysis are pre-

sented in the Appendix (sections 1 and 2, respectively).

The ingroup included the following taxa: Balaena monta-

lionis, B. mysticetus, B. ricei, Eubalaena belgica, E. glacialis,

Balaenula astensis, B. balaenopsis, the Japanese B. sp.,

Balaenella brachyrhynus and Morenocetus parvus. Poorly

known taxa, such as Balaenotus insignis, ‘Balaena’ etrusca

and ‘Balaena primigenius’ were excluded from the analysis

because they are largely incomplete (Bisconti 2003a).

However, despite the exclusion of these fragmentary taxa,

the analysis represents the most inclusive phylogenetic

study of the family Balaenidae that has been attempted

hitherto. The balaenid taxa included in the cladistic analy-

sis have been described above under comparative data

and much more extensively by McLeod et al. (1993) and

Bisconti (2003a).

In previous studies, Barnes and McLeod (1984),

McLeod et al. (1993) and Bisconti (2000) found that

the Grey whales of the family Eschrichtiidae were sister

to a clade including Neobalaenidae and Balaenidae. This

clade had been previously named Balaenoidea by Gray

(1825). Because of these studies, balaenids were sugges-

ted to be among the most derived mysticetes. A recent

morphology-based analysis of the phylogenetic relation-

ships of the major radiations of living and fossil mys-

ticetes (Kimura and Ozawa 2002), together with new

results from molecular studies (e.g. Arnason and Ledje

1992; Arnason and Gullberg 1994; Gatesy 1998), sugges-

ted that the Eschrichtiidae and Neobalaenidae are more

closely related to the Balaenopteridae than the Balaenidae.

These results conflicted with those of previous analyses in

also suggesting that the divergence of Balaenidae was a

basal event in the evolution of baleen-bearing mysticetes.

Given this uncertainty about the phylogenetic position of

balaenids among the mysticetes, an obvious outgroup to

this family with which to root the cladograms was not

available. Therefore, the outgroup was arranged in such a

way that the major radiations of non-balaenid mysticetes

were included. The following taxa were included within

the outgroup: the ‘cetothere’ Parietobalaena palmeri from

the Lower Miocene of the Calvert Formation of Maryland

(‘cetotheres’ are extinct rorqual-like mysticetes; there is no

general agreement about the systematics of this group; in

fact some authors have claimed that they are para- or po-

lyphyletic whereas others have supported their monophyly:

for further information, see McLeod et al. 1993; Fordyce

and Barnes 1994; Fordyce and De Muizon 1999; Sanders

and Barnes 2002a, b), the Grey whale Eschrichtius robustus

(Eschrichtiidae), the Pygmy Right whale Caperea margina-

ta (Neobalaenidae), the Humpback whale Megaptera no-

vaeangliae (Balaenopteridae, Megapterinae), and the Fin

whale Balaenoptera physalus (Balaenopteridae, Balaeno-

pterinae). Toothed mysticetes were represented by the aeti-

ocetid Aetiocetus polydentatus from the Upper Oligocene

of Kyushu Island (Japan). The phylogenetic analysis used

two archaeocete taxa as the most external outgroups: the

middle Eocene Protocetus atavus and the early Middle–Late

Eocene Zygorhiza kochii. A list of non-balaenid specimens

examined in this work is provided in Table 4, together

with their repositories and the references used to

supplement my personal observations.

Character states were treated as unordered and

unweighted by PAUP 4.0b10 (Swofford 2002) under the

ACCTRAN character state optimization. The search for

the most parsimonious cladograms was made by tree-

bisection-reconnection (TBR) with ten replicates and one

tree held at each step during stepwise addition followed

by bootstrap analysis with 100 replicates. A randomiza-

tion test was performed by PAUP to assess the distance

of the results from 10,000 cladograms sampled equiproba-

bly from the set of all possible trees generated from the

original matrix. Character evolution and morphological

support at nodes were assessed by the evaluation func-

tions of Hennig86 (Farris 1988; Lipscomb 1988).

Results

General patterns. The TBR algorithm found six equally

parsimonious trees, which were 143 steps long. Their

strict-consensus is shown in Text-figure 8A together with

the bootstrap tree that had the same length (Text-

Fig. 8B). Tree statistics are provided in the caption of


Text-figure 8. The randomization test (Text-fig. 9) sugges-

ted that the results of the analyses were significantly dif-

ferent from chance and confirmed the existence of a

phylogenetic structure in the data.

The analyses found a monophyletic suborder Mysticeti

(bootstrap support of 100 per cent) in which the toothed

aetiocetid Aetiocetus polydentatus represented the most

external taxon. TBR and bootstrap analyses converged

toward the monophyly of all the baleen-bearing mysticetes

included (bootstrap support of 99 per cent). The analyses

differed in minor details of the ingroup relationships.

In both the trees, the ‘cetothere’ Parietobalaena palmeri

was monophyletic with a clade including Eschrichtiidae

and Balaenopteridae, and Neobalaenidae was sister to

Balaenidae supporting the superfamily rank taxon named

Balaenoidea by Gray (1825; Text-fig. 8A). This taxon had

been previously reaffirmed by McLeod et al. (1993) and

Bisconti (2000) on morphological grounds and by Gatesy

(1998) based on cytochrome b DNA sequence analysis. The

Balaenoidea received a bootstrap support of 99 per cent.

Ingroup relationships. The six equally parsimonious

cladograms found by TBR and presented as strict-

consensus tree in Text-figure 8A represented the most

parsimonious solutions found in this work. Their strict-

consensus will be considered as the optimal tree in the

next sections. In that cladogram, balaenids are subdivi-

ded into three clades: one clade is formed by Morenoce-

tus parvus; a second includes Balaena and Balaenella;

and a third comprises Balaenula and Eubalaena. These

clades branched from an unsolved polytomy (Text-

fig. 8A). In the bootstrap tree the polytomy was solved

because Morenocetus parvus was attached to the clade

formed by Balaena and Balaenella (Text-fig. 8B). The

monophyly of the family Balaenidae received a boot-

strap support of 100 per cent.

The monophyly of Eubalaena (including E. glacialis

and E. belgica) was confirmed in both the trees receiving

a bootstrap support of 84 per cent. Balaenula (including

B. astensis, B. balaenopsis and the Japanese B. sp.) was

monophyletic and received a bootstrap support value of

94 per cent with its three species branching from an

unsolved polytomy. The monophyly of Eubalaena +

Balaenula was supported by a bootstrap value of 70 per

cent.

Both the analyses converged toward a monophyletic

Balaena (bootstrap ¼ 67%) that included B. montalionis,

B. mysticetus and B. ricei. In the TBR strict-consensus tree

and in the bootstrap tree Balaena montalionis was sister

to a clade including B. mysticetus + B. ricei (supported by

a bootstrap value of 55%).

In all the cladograms, Balaenella brachyrhynus was sister

to Balaena. This sister group relationship received a boot-

strap support of 89 per cent. In the bootstrap tree, More-

nocetus parvus was sister to a clade including Balaenella

and Balaena; the corresponding bootstrap support value

was 65 per cent.

Character support at nodes. The character states were

mapped onto the strict-consensus TBR tree (Text-

TABLE 4 . List of non-balaenid cetaceans examined and references of morphological descriptions.

Taxon Repository No. References

Protocetus atavus SMNS 11084 (holotype) Fraas (1904); Kellogg (1936); Luo and Gingerich (1999)

Zygorhiza kochii USNM 4748, 16638, 449538 Kellogg (1936); Uhen (1998)

Aetiocetus polydentatus Barnes et al. (1994)

Parietobalaena palmeri USNM 10677, 10909, 16570,

24883

Kellogg (1925, 1968b)

Eschrichtius robustus USNM 364969, 364580,

364973, 364970, 364977,

504305, 571931

True (1904); Wolman (1985)

NMB 42002

ZML St 13130, St 20350, 630 True (1904); Gambell (1985); Bisconti (2001)

Balaenoptera physalus MSNT 251, 252, 253, 258, 255,

257 (newborn)

ZMA 14927 (1–2), 14935 (1–2),

14947, 14950 (1-2), 23353

Megaptera novaeangliae MSNT 263 Van Beneden and Gervais (1868–79); True (1904);

Winn and Reichley (1985); Clapham and Mead (1999)

USNM 269982, 486175 (1–2),

13656 ⁄ 16252, 21492ZMA 14952 (1–2), 14953 (1–2),

14964, 14965, 14966, 14967

Caperea marginata IRSN 1536 Beddard (1901); Baker (1985).


Fig. 8A) by the DOS Equis function of Hennig86, which

also provided the reconstructions of ancestral character

states at nodes (Lipscomb 1988). In this way, it was poss-

ible to study patterns of character evolution in different

clades.

Characters 1–3 support the monophyly of the order

Cetacea (see Appendix for a character list); they have

been extensively treated by other authors (Van Beneden

1886; Fraser and Purves 1960; Kellogg 1965, 1968a; Ford-

yce and Barnes 1994; Luo and Gingerich 1999; Bisconti

2001, 2003a) and are not discussed again here. Of course,

the monophyly of the Cetacea is supported by more than

four morphological characters (e.g. Messenger and McGu-

ire 1998; Luo and Gingerich 1999; O’Leary and Geisler

1999) but a discussion on cetacean monophyly is outside

the scope of the present paper.

The monophyly of the suborder Mysticeti is supported

by the following characters: 5(0 fi 1), 6(0 fi 1),

7(0 fi 1), 8(0 fi 1), 12(0 fi 1), 22(0 fi 1) 44(0 fi 1)

and 47(0 fi 1). These characters are related to the devel-

opment of a wide and flat rostrum, lack of the mandibu-

lar symphysis, development of an anterior groove for

the mental ligament, presence of a complex structure (the

infundibulum; see Fraser and Purves 1960) surrounding

the foramen ‘pseudo-ovale’, uniquely derived shape of the

tympanic membrane (which is shaped as a glove finger;

Fraser and Purves 1960), and development of monop-

hiodonty (which is also shared with odontocetes). Char-

acters 9, 10, 21, 25, 54 and 60 are ambiguously

reconstructed and cannot be confidently used to support

the monophyly of Mysticeti.

The baleen-bearing mysticetes are diagnosed by the fol-

lowing characters: 11(0 fi 1), 13(0 fi 1), 14(0 fi 1),

15(0 fi 1), 16(0 fi 1), 18(0 fi 1), 55(0 fi 1) and

73(0 fi 1). From this analysis, baleen-bearing mysticetes

are uniquely characterized by the lack of teeth, the pres-

ence of baleen plates together with their vascular comple-

ment on the ventral surface of the maxilla, the lack of a

close articulation of dentary and squamosal, the presence

of a lateral squamosal crest functioning as an attachment

site for neck muscles, the lack of the hypoglossal foramen

(shared with odontocetes; however, the foramen is pre-

sent in Balaenula astensis; see Bisconti 2000, 2003a), the

straight to slightly concave profile of the glenoid fossa of

the squamosal in lateral view (which is transformed into

a highly concave profile in Balaenopteridae), and a longer

supraorbital process of the frontal.

The clade including Parietobalaena palmeri, Eschrichtius

robustus, Balaenoptera physalus and Megaptera novaeangli-

ae (‘cetotheres’, eschrichtiids and balaenopterids) is sup-

ported by the following characters: 27(0 ⁄ 1 fi 1),

51(0 ⁄ 1 fi 1), 53(0 ⁄ 1 fi 1) and 60(0 ⁄ 2 fi 2). Balaenopte-

rids and eschrichtiids share four characters: 23(0 fi 1),

24(0 fi 1), 25(0 ⁄ 1 fi 1) and 26(0 fi 1). Characters

21(1), 22(1), 26(0 fi 1) and 44(1) are shared by Caperea

marginata and the clade including ‘cetotheres’, eschrichti-

ids and balaenopterids. These characters include the lack

of coalescence of the endocranial opening of the facial

canal into the internal acoustic meatus at adulthood, the

presence of a sagittal crest on the supraoccipital, the pres-

ence of a squamosal cleft and the presence of a flat ros-

trum. The coalescence of the internal acoustic meatus and

the endocranial opening of the facial canal has been des-

cribed in the late ontogeny of balaenopterid whales (Bis-

conti 2001, 2003b); the meatus and facial canal are widely

separated during adulthood in some fossil ‘cetotheres’

such as Parietobalaena palmeri, Diorocetus hiatus and Mes-

ocetus longirostris, and in the neobalaenid Caperea margi-

nata. Among balaenids, a morphological formation that

looks like a sagittal crest has been described in a Pliocene

Eubalaena sp. from Tuscany (Bisconti 2002). The squ-

amosal cleft is a suture observed within the squamosal of

Protocetus atavusZygorhiza kochiiAetiocetus polydentatusParietobalaena palmeriEschrichtius robustusMegaptera novaeangliaeBalaenoptera physalusCaperea marginataMorenocetus parvusBalaenella brachyrhynusBalaena montalionisBalaena riceiBalaena mysticetus

Balaenula sp.

Eubalaena belgicaEubalaena glacialis

Balaenula balaenopsisBalaenula astensis

Protocetus atavusZygorhiza kochiiAetiocetus polydentatusParietobalaena palmeriEschrichtius robustusMegaptera novaeangliaeBalaenoptera physalusCaperea marginataMorenocetus parvusBalaenella brachyrhynusBalaena montalionisBalaena riceiBalaena mysticetus

Balaenula sp.

Eubalaena belgica84

94

70

67

100

100

A

B

100100

5270

99

Balaenidae

Balaenoidea

baleen-bearingMysticeti

Mysticeti

Balaenidae

Balaenoidea

baleen-bearingMysticeti

Mysticeti

6589

55

Eubalaena glacialis

Balaenula balaenopsisBalaenula astensis

TEXT -F IG . 8 . Phylogenetic analysis of the Balaenidae showing

the relationships of Balaenella brachyrhynus. A, strict-consensus

of six equally parsimonious trees found by tree-bisection-

reconnection (TBR). B, bootstrap tree based on TBR results;

bootstrap values are located above the branches. Tree statistics

are the same for both trees: tree length, 143; consistency index

(CI), 0.7552; homoplasy index (HI), 0.2448; CI excluding

uninformative characters, 0.7518; HI excluding uninformative

characters, 0.2482; retention index (RI), 0.8818; rescaled

consistency index (RC), 0.6659.


balaenopterids, neobalaenids and some late ‘cetotheres’;

from the present analysis the presence of a squamosal

cleft should be interpreted as a convergence in different

mysticete clades. Character 44 is concerned with the ori-

entation of the dorsolateral surface of the maxilla in mys-

ticetes: balaenids are unique in having the anterior

portion of the maxilla characterized by a transversely

compressed and dorsoventrally depressed dorsolateral sur-

face; in neobalaenids, the dorsolateral surface of the max-

illa is horizontal and not dorsoventrally depressed as in

balaenids; however, as in balaenids, the maxilla in neoba-

laenids has a transversely compressed dorsolateral surface

(character 44(0 fi 1)).

The monophyly of the superfamily Balaenoidea is

supported by the following characters: 17(0 fi 1),

27(0 ⁄ 1 fi 2), 28(0 fi 1), 31(0 fi 1), 33(0 fi 1),

34(0 fi 1), 35(0 fi 1), 38(0 fi 2), 40(0 fi 2), 43(0 fi 1)

and 56(0 fi 1). These include a suite of morphological

transformations related to hearing and feeding: low and

flattened tympanic bulla with a low sygmoid process, ele-

vated neurocranium and deep skull, well-developed ros-

tral arch, short zygomatic process of the squamosal, low

and round angular process of the dentary, relative posi-

tion of the posterolateral corner of the exoccipital and the

postglenoid process, and superimposition of the supraoc-

cipital onto the parietal (this pattern is different from that

of Balaenopteridae, Eschrichtiidae and ‘cetotheres’ in

which the supraoccipital is developed in between the par-

ietals). Characters 57 and 59 are reverted in the Eubalae-

na + Balaenula clade.

Characters 29(0 fi 1), 45(0 fi 1), 46(0 fi 1), 47(1 fi2), 48(0 fi 1), 49(0 fi 1), 50(0 fi 1), 51(0 ⁄ 1 fi 2),

52(0 fi 1), 53(0 fi 2) and 54(1 fi 2) support the

monophyly of the family Balaenidae. Balaenids are

uniquely characterized by the possession of a wide manus,

anterior torsion in the dentary, mylohyoidal sulcus along

the ventromedial surface of the dentary, incomplete in-

fundibulum around the foramen ‘pseudo-ovale’, location

of the pterygoid near the posterior border of the skull,

presence of a well-developed ventral lamina of the ptery-

goid fossa, palatines partially covering the pterygoids,

long baleen, dorsoventral orientation of the squamosal,

dorsoventrally compressed round window, long and shal-

low stylomastoid fossa, and long and triangular lateral

projection of the anterior process of the periotic.

Among Balaenidae, the monophyly of Balaenula is sup-

ported by characters 70(0 fi 1), 71(0 fi 1) and

72(0 fi 1); these characters have been described in the

introduction of this paper and in Bisconti (2003a). The

monophyly of Eubalaena (including here only E. glacialis

and E. belgica) is due to the sharing of characters 68

(presence of a dome on the anterior portion of the supra-

occipital: 0 fi 1) and 69 (squared exoccipital in lateral

view: 0 fi 1). Balaenula and Eubalaena form a monophy-

letic group diagnosed by the following synapomorphies:

62 (abrupt depression of the premaxilla in the anterior

half of the rostrum: 0 fi 1), 63 (irregular profile of the

skull due to the development of a distinctive apex

between supraoccipital and frontal: 0 fi 1) and 67

(spreading of the parietal onto the supraorbital process of

0

25

50

75

100

125

150

175

200

225

1 10 19 28 37 46 55 64 73 82 91 100

109

118

127

136

145

154

163

172

181

190

199

208

217

226

235

244

253

262

271

280

289

298

307

316

325

334

343

352

361

370

379

388

397

steps

num

ber

of c

lado

gram

s

TEXT -F IG . 9 . Randomization test. The histogram depicts the tree length distributions of 10,000 cladograms equiprobably sampled

from the set of all possible cladograms based on the character · taxon matrix presented in the Appendix. This result combined ten

separate randomization tests each of which generated 1000 random cladograms. Mean tree length, 351.8; mean standard deviation,

24.53. The arrow indicates the tree length of the most parsimonious cladograms found by TBR whose strict-consensus is presented in

Text-figure 8A.


the frontal medially). Characters 57(1 fi 0) and

59(1 fi 0) are interpreted as unique reversions in the

Balaenula + Eubalaena clade.

The monophyly of Balaena is unambiguously suppor-

ted by two synapomorphies: 65 (raising of the nasals

together with the proximal rostrum: 0 fi 1) and 66

(presence of a crest-like relief on the parietal squama:

0 fi 1). Balaena mysticetus and B. ricei are more closely

related than each is with B. montalionis because of the

rounder morphology of the anterior border of the supra-

occipital (60(1 fi 4)). Balaenella brachyrhynus and Balae-

na form a monophyletic group characterized by the

following unambiguous synapomorphies: 58(0 ⁄ 1 fi 1),

60(0 fi 1), 61(0 ⁄ 1 fi 2) and 64(0 fi 1). These characters

support the view that Balaenella and Balaena share the

posterior orientation of the lateral process of the maxilla

(which, in lateral view, seems to be located under the

supraorbital process of the frontal), distal absence of the

infraorbital plate, and glenoid fossa of the squamosal

located posterior to the posterior apex of the lambdoidal

crest. A marked transverse constriction of the anterior

portion of the supraoccipital is uniquely shared by Balae-

nella brachyrhynus and Balaena montalionis. Characters

57, 59 and 62 are shared by Balaenella, Balaena and Cape-

rea marginata.

DISCUSSION

The discovery of Balaenella brachyrhynus in the Lower

Pliocene of the North Sea region helps our understanding

of the evolution of body size in balaenid whales. In partic-

ular, the phylogenetic analysis presented here suggests that

the origin of the gigantic size typical of the living Right

and Bowhead whales was attained independently in these

different clades. A small size was a common condition in

Pliocene balaenids (such as in Balaenula and Balaenella)

and in the Lower Miocene Morenocetus parvus. Pliocene

species belonging to the extant genera were also commonly

small with respect to the gigantic sizes of the living balae-

nids (Bisconti 2000, 2002; Westgate and Whitmore 2002).

The extinction of all the small balaenids took place poss-

ibly before the end of the Pliocene in all the oceans of the

world, erasing much of the Pliocene diversity of the family.

The reasons for this large-scale (in geographical terms)

extinction are not yet completely understood but theoreti-

cal models are beginning to emerge (Bisconti 2003a) that

deserve further investigation.

One of the most important results of the present paper

is the unambiguous definition of two main balaenid radi-

ations: one includes Balaenella brachyrhynus plus the

genus Balaena, and the other includes the genera Eubalae-

na and Balaenula. The phylogenetic position of Morenoce-

tus parvus is still uncertain, given the low bootstrap

support received by its inclusion in the first clade as sister

to Balaenella + Balaena. The phylogenetic analysis presen-

ted here conflicts with the previous results published by

Bisconti (2000) in which a clade including the genera

Morenocetus and Balaenula was discovered. As discussed

in Table 3, the synapomorphies proposed to support that

clade revealed plesiomorphic character states and are no

longer useful.

The results of the new analysis strongly support a close

relationship of Eubalaena and Balaenula, two genera well

represented in Pliocene and Pleistocene sediments of Eur-

ope, USA and Japan (see Bisconti 2000 and 2002 for

reviews of the fossil records of these genera). They show

the highest morphological distance from Caperea margi-

nata, the sister taxon to Balaenidae. In fact, in both

genera, a complete restructuring of the skull occurred

that was characterized by the interruption of the general

curvature of the premaxilla, the transverse elongation of

the supraorbital process of the frontal, the transverse

expansion of the anterior portion of the supraoccipital,

and a higher elevation of the vertex leading to the inter-

ruption of the continuous curvature of the dorsal profile

of the skull in lateral view. In Balaenula the retention of

the primitive round anterior border of the supraoccipital

(shared with Morenocetus parvus and Caperea marginata)

was paralleled by the development of a suite of derived

character states affecting the temporal and the exoccipital

regions of the skull (see also Bisconti 2003a).

The clade including the genera Balaena and Balaenella

retained the posterior orientation of the supraorbital pro-

cess of the frontal, the distal absence of the ascending

temporal crest from the supraorbital process of the fron-

tal, and a round exoccipital as in Caperea marginata and

Morenocetus parvus, together with a regular profile of the

skull in lateral view, continuously arched rostrum, and

lateral process of the maxilla located under the frontal

in lateral view. The absence of the distal portion of the

infraorbital plate represents an apomorphic feature absent

in the other balaenid clades. Unfortunately, it is imposs-

ible to score this character for Morenocetus parvus because

the maxilla is absent.

Balaenella brachyrhynus and Balaena montalionis share

a transversely constricted anterior portion of the supraoc-

cipital whereas a marked constriction is absent in Balaena

mysticetus and B. ricei (Westgate and Whitmore 2002).

This character is not, however, sufficient to support the

monophyly of Balaenella brachyrhynus and Balaena

montalionis to the exclusion of Balaena ricei and B. mys-

ticetus, and this reinforces the establishment of a different

genus for the Tilburg skull.

The generic differences characterizing Balaenella brac-

hyrhynus are observed in the nasal bones, the rostrum

and the lower inclination of the supraoccipital. While the

last character is independent of the phylogenetic position


of the Tilburg skull seemingly related to the small size of

the whale, the morphology of the nasal bones and the

rostrum strongly support a generic distinction. The max-

illa of B. brachyrhynus is depressed anterior to the frontal

whereas in Balaena it is highly arched and projects dor-

sally, anterior to the frontal (see Bisconti 2003a). The

nasal bones of Balaenella brachyrhynus show the highest

degree of reduction observed among the whole suborder

Mysticeti; these bones are small, short and horizontal,

and differ from the corresponding bones of the other

Balaenidae, which are usually rectangular and bear a

notch on the anterior border (see images in True 1904;

Cummings 1985; see also Bisconti 2000, 2003a). The

function of the small, delicate nasals of Balaenella brac-

hyrhynus, if any, is not yet understood.

The discovery of the Tilburg skull adds important

information on the morphological evolution of Balaeni-

dae and documents an unsuspected diversity among the

small balaenids. Three genera are currently known that

are uniquely characterized by small species: Morenocetus,

Balaenella and Balaenula. The systematic position of the

small genus Balaenotus is not yet clear because it is too

poorly known. Future studies should provide convincing

evidence on the phylogenetic placement of the Lower

Miocene Morenocetus parvus in order to make it possible

to assess the divergence times of the living genera from

their last common ancestor. This information is crucial

for conservation biologists in order to test the estimated

rates of evolution and the genetic health of the living

balaenid populations (Rooney et al. 2001).

CONCLUSIONS

In this paper, a new genus and species of a balaenid

whale has been described and compared with all the liv-

ing and fossil members of the family Balaenidae. The spe-

cimen concerned is from the Lower Pliocene near

Antwerp (Belgium) and is named Balaenella brachyrhynus.

This whale is characterized by being the smallest represen-

tative of the Balaenidae, and in having a depressed max-

illa anterior to the frontal, horizontal supraoccipital,

anterior border of the supraoccipital transversely constric-

ted, and supraorbital process of the frontal directed poste-

riorly. The most intriguing feature is that it shows the

highest degree of nasal reduction among the mysticetes.

The phylogenetic relationships of Balaenella brachyrhy-

nus investigated through a cladistic analysis of an ingroup

of several balaenid and non-balaenid mysticetes resulted

in the discovery of two main balaenid radiations: one

including the genera Eubalaena and Balaenula, and the

other including Balaena and Balaenella. The phylogenetic

position of the Early Miocene Morenocetus parvus was not

confidently established but it seems more closely related

to the Balaena + Balaenella clade than to the Eubalae-

na + Balaenula clade. The analysis also confirmed the

position of Caperea marginata as sister to the family Bala-

enidae.

Balaenella brachyrhynus is sister to Balaena. Its mor-

phology, together with its phylogenetic position, docu-

ments an unsuspected diversity among the small

balaenids of the Pliocene and suggests that small size has

been a common condition among the balaenids just a few

million years ago.

Acknowledgements. I am indebted to Frans Ellenbroek (direc-

tor of NMB, Tilburg, Holland) who allowed me to study the

specimen, and Marie-Cecile Van De Wiel (NMB) who assisted

me during my work and provided help on several occasions. I

thank Klaas Post (Natuurmuseum Rotterdam, Holland) very

much for making it possible for me to study the Dutch col-

lections of fossil mysticetes, providing literature, making useful

comments on an early draft of the manuscript, and kindly

assisting me during my visits to Holland. Thanks are also due

to Olivier Lambert for his help during my study of the IRSN

collection and to David Bohaska who assisted me during the

study of the USNM collection. Albert Sanders (The Charleston

Museum, Charleston, South Carolina) provided a thorough

and insightful review of the manuscript: his help is gratefully

acknowledged. Mark Uhen (Cranbrook Institute of Science,

Bloomfield Hills, Michigan) and Oliver Hampe (Museum fur

Naturkunde, Berlin, Germany) carefully reviewed the manu-

script, providing suggestions that significantly improved the

quality of the paper. This work has been supported by

MURST funds, being a contribution of the Pisa Unit (Unit

coordinator Walter Landini, Dipartimento di Scienze della

Terra, University of Pisa) to the research project ‘Palaeobioge-

ography of Central Mediterranean from Miocene to Quater-

nary’ (national coordinator Danilo Torre, Dipartimento di

Scienze della Terra, University of Florence).

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APPENDIX

Morphological characters used in the phylogenetic analysis

The following is an annotated list of the character states used

in the cladistic analysis. A brief discussion of the characters is

provided only for characters described here for the first time

or not used in previous cladistic analyses of the mysticetes.

Otherwise, the characters are referred to the appropriate litera-

ture.

1. Suprameatal area of petrosal: 0, low; 1, high. Luo and Ginge-

rich (1999) gave accurate descriptions of the petrotympanic

complexes of early cetaceans. They showed that in basilosau-

rids the suprameatal area of the petrosal is high and flat-to-

slightly concave. Sanders and Barnes (2002a, b) described this

same condition in Oligocene baleen-bearing mysticetes. A

high suprameatal area of the petrosal is present in several

Miocene ‘cetotheres’ (see Kellogg 1965, 1968a; Van Beneden

1886) and some balaenids (Bisconti 2003a). In balaenopterids

the suprameatal area of the petrosal is comparatively lower

(Bisconti 2001).

2. Air sinus: 0, absent; 1, present around the tympanic bulla

(Fraser and Purves 1960; Luo and Gingerich 1999).

3. Ascending temporal crest: 0, absent; 1, present. This crest is

located on the supraorbital process of the frontal in mystic-

etes; it can be located on the posterodorsal edge of the pro-

cess (Oligocene toothed and baleen-bearing mysticetes), at

the middle of the process (‘cetotheres’, eschrichtiids, balae-

nids, and neobalaenids) or along its anterior border (balaeno-

pterids) (Fordyce and Barnes 1994).

4. Parietal and squamosal are bulged into the temporal fossa: 0,

no; 1, yes. The bulging of the parietal and squamosal into the

temporal fossa is observed in basilosaurid archaeocetes, ‘ce-

totheres’, and eschrichtiids.

5. Plate-like infraorbital process of the maxilla: 0, absent; 1, pre-

sent (Messenger and McGuire 1998; Sanders and Barnes

2002b).

6. Mandibular symphysis: 0, present; 1, absent (groove for men-

tal ligament present) (Messenger and McGuire 1998; Sanders

and Barnes 2002b).

7. Tympanic membrane: 0, present; 1, modified into glove finger

(Fraser and Purves 1960; Messenger and McGuire 1998).


8. Foramen ‘pseudo-ovale’: 0, absent; 1, present and perforating

styliform process of squamosal (Barnes et al. 1994).

9. Sternum: 0, formed by manubrium and several seternebra; 1,

formed by manubrium only. In archaeocetes and odontocetes,

the sternum is formed by manubrium and some sternebra

(Kellogg 1936) whereas in mysticetes it includes the manubri-

um only (Tomilin 1967).

10. Number of ribs attached to the sternum: 0, several pairs; 1,

one pair (True 1904; Tomilin 1967).

11. Teeth in the adult: 0, present; 1, absent (Messenger and

McGuire 1998).

12. Dental generations developed during embryology: 0, poly-

ophiodonty; 1, monophiodonty (Messenger and McGuire

1998).

13. Baleen plates: 0, absent; 1, present (Messenger and McGuire

1998).

14. Lateral squamosal crest: 0, absent; 1, present. The lateral

squamosal crest is observed on the dorsal edge of the zygo-

matic process of the squamosal and anterior to the caudal

apex of the lambdoidal crest in mysticetes; the term is used

by Kellogg (1965, 1968a).

15. Cranio-mandibular joint: 0, dentary and squamosal closely

articulate each other; 1, dentary and squamosal are not clo-

sely articulated. In baleen-bearing mysticetes, the glenoid

fossa of the squamosal is a wide concavity, which is not as

closely articulated with the dentary as in terrestrial mammals

and in archaeocete and odontocete cetaceans (Sanderson

and Wassersug 1993); however, it seems that a close articu-

lation of dentary and squamosal was present in the toothed

mysticete Aetiocetus polydentatus (Barnes et al. 1994).

16. Angular process of dentary: 0, high; 1, low; 2, very low. In

archaeocetes, toothed mysticetes, and eschrichtiids the angu-

lar process of the dentary is well developed. In balaenids

and ‘cetotheres’ it is lower (Bisconti 2000). In balaenopte-

rids, the angular process is strongly reduced (Lambertsen

et al. 1995).

17. Angular process of dentary in lateral view: 0, squared,

robust, pterygoid groove absent; 1, round, slightly-built,

pterygoid groove absent; 2, squared, slightly-built, pterygoid

groove present. A round angular process is present in Bala-

enidae (Bisconti 2000, 2003a). In balaenopterids, the angu-

lar process is rectangular and is separated from the

condylar area through a pterygoid groove Lambertsen et al.

1995).

18. Supraorbital process of frontal: 0, short; 1, long. In archae-

ocetes and some ‘cetotheres’ (such as the skull USNM

187416 of Cetotherium megalophysum) the supraorbital pro-

cess of the frontal is short. In the mysticetes included in this

work, it is longer (Bisconti 2003b).

19. Supraorbital process of frontal: 0, broad; 1, slender; 2, very

broad. In archaeocetes, the supraorbital process of the fron-

tal is broad; in mysticetes it is usually slender; in balaenopte-

rids it is very broad, allowing for the attachment of the

anterior portion of the muscle temporalis (Lambertsen et al.

1995; Zeigler et al. 1997).

20. Supraorbital process of frontal: 0, horizontal; 1, gently des-

cending from the interorbital region of frontal; 2, abruptly

depressed. The supraorbital process of the frontal is abruptly

depressed from the interorbital region of the frontal in bala-

enopterids, eschrichtiids, and related taxa. In ‘cetotheres’

and balaenids it gently descends from the interorbital region

(Zeigler et al. 1997).

21. Internal opening of facial canal coalesces into the internal

acoustic meatus during early ontogeny: 0, yes; 1, no. Biscon-

ti (2001) demonstrated that the internal acoustic meatus

and the internal opening of the facial canal are separate in

the early ontogeny of Balaenoptera physalus. Subsequently,

Bisconti (2003b) extended this observation to Balaenoptera

edeni and B. borealis. These formations coalesce during the

late ontogeny of balaenopterids. In archaeocetes, balaenids

and ‘cetotheres’, the internal opening of the facial canal does

not coalesce and both are included in a common cavity at

least during the latest ontogeny (Geisler and Luo 1996; Luo

and Gingerich 1999; Bisconti 2003a). The coalescence is

absent in Caperea marginata.

22. Sagittal crest on the anteriormost portion of the supraoccip-

ital: 0, absent; 1, present. A sagittal crest is observed in

aetiocetids, some ‘cetotheres’ (including Parietobalaena

palmeri), neobalaenids, and in an Eubalaena sp. from the

Pliocene of Tuscany (Bisconti 2002).

23. Ascending process of the maxilla: 0, absent; 1, definite and

long. State 1 is typical of Balaenopteridae, Eschrichtiidae,

and some advanced ‘cetotheres’ including Cetotherium rath-

kei (Tomilin 1967; Pilleri 1986, his fig. 15, p. 25). In other

‘cetotheres’ (including Parietobalaena palmeri), the ascending

process of the maxilla is tapered and broad (Sanders and

Barnes 2002b). In balaenids and neobalaenids, the ascending

process of the maxilla is short and not as definite as in the

above taxa (Tomilin 1967).

24. Interorbital region of frontal: 0, present; 1, absent. In bala-

enopterids and some late ‘cetotheres’ (such as Cetotherium

rathkei), the ascending processes of the maxillae superim-

pose onto the interorbital region of the frontal; therefore,

this region is obliterated. In the other mysticetes and archae-

ocetes this region is not obliterated because the ascending

processes of the maxillae are short or absent (Bisconti

2003b).

25. Posterior apex of the ascending process of maxilla: 0, anter-

ior and far to postorbital corner of frontal; 1, close to post-

orbital corner. In early ‘cetotheres’, such as Parietobalaena

palmeri, Diorocetus hiatus and Pelocetus calvertensis, the

ascending process of the maxilla is slightly superimposed

onto the anteromedial portion of the interorbital region of

the frontal and its posterior apex does not move far posteri-

orly. In balaenopterids and later ‘cetotheres’ (such as Ceto-

therium rathkei), the posterior apex of the ascending process

of the maxilla is very posterior on the dorsal wall of the

skull.

26. Squamosal cleft: 0, absent; 1, present. The squamosal cleft is

a widely distributed character observed in several mysticete

taxa such as Neobalaenidae, Balaenopteridae and Eschrichtii-

dae.

27. Shape of zygomatic process of squamosal in lateral view: 0,

elongated, slender and subtle; 1, crescent-shaped; 2, slightly

pointed or round; 3, sharply triangular. In archaeocetes and

early ‘cetotheres’ (such as Parietobalaena palmeri) the zygo-


matic process of the squamosal is usually slender (see, e.g.,

Kellogg 1936, fig. 3, p. 23 and fig. 31a, p. 108). In balaeno-

pterids, the zygomatic process of the squamosal is curved

ventrally and this gives the process a crescent shape (True

1904, pls 3–4, 26–27, 31). This shape is also shared by the

aetiocetid Aetiocetus polydentatus (Barnes et al. 1990) and

the Oligocene baleen bearing mysticetes Micromysticetus rot-

hauseni and Eomysticetus whitmorei, together with an

unnamed Oligocene mysticete from Japan (Sanders and Bar-

nes 2002a, b; Okazaki 1994). In balaenids and neobalaenids

the zygomatic process of the squamosal is stocky (see True

1904, pls 43–44; Baker 1985). In eschrichtiids the zygomatic

process of the squamosal is triangular in lateral view (True

1904, pl. 47).

28. Position of the posterolateral corner of the exoccipital rel-

ative to the posterior border of the postglenoid process in

ventral view: 0, close and posterior; 1, close and medial; 2,

far and posterior (Bisconti 2003b). In archaeocetes and Oli-

gocene baleen-bearing mysticetes the posterolateral corner

of the exoccipital is close and posterior to the posterior

border of the postglenoid process of the squamosal. In esc-

hrichtiids and some ‘cetotheres’, such as Cetotherium rath-

kei and Mixocetus elysius (Kellogg 1934), this condition is

emphasized being the posterolateral corner of the exoccipi-

tal located far to the posterior of the posterior border of

the postglenoid process (state 3). In the other mysticetes

the posterolateral corner of the exoccipital is located closer

(state 1).

29. Manus: 0, short; 1, long; 2, wide. A long manus is typical of

Balaenopteridae whereas a wide manus is observed in the

living Balaenidae (Tomilin 1967).

30. Shape of the anterior process of the petrosal in dorsal view:

0, round; 1, squared; 2, triangular. Bisconti (2001, 2003a),

Van Beneden (1878, 1880, 1886), Kellogg (1936, 1965,

1968a), and Sanders and Barnes (2002a, b) gave adequate

descriptions of the morphology of the anterior process of

the petrosal in mysticetes and archaeocetes.

31. Parietal exposition at the cranial vertex: 0, absent; 1, present,

parietal under the supraoccipital (Balaenoidea); 2, present,

parietal divided posteriorly by the interposition of the supra-

occipital (Balaenopteridae, Eschrichtiidae, ‘cetotheres’). In

balaenids and neobalaenids the supraoccipital superimposes

the parietal and the posteromedial portion of the interorbital

area of the frontal (Bisconti 2002). In balaenopterids, esc-

hrichtiids and ‘cetotheres’ the supraoccipital is interposed in

between the parietals.

32. Position of coronal suture: 0, anterior to the anterior border

of the supraoccipital; 1, posterior. As a consequence of the

superimposition of the supraoccipital onto the parietal and

the posteromedial region of the interorbital area of the fron-

tal, the coronal suture is posterior to the anterior border of

the supraoccipital in balaenids and neobalaenids (Bisconti

2002, 2003b).

33. Main orientation of the squamosal: 0, horizontal; 1, dorso-

ventral (McLeod et al. 1993; Bisconti 2000, 2003a).

34. Dorsoventral compression of tympanic bulla: 0, absent; 1,

present (generates a shallow tympanic cavity) (McLeod et al.

1993; Bisconti 2003a).

35. Extremely low conical process of tympanic bulla: 0, no; 1,

yes. State 1 observed in balaenids and neobalaenids only

(Bisconti 2003a).

36. Tympanic bulla transversely enlarged: 0, no; 1, yes. State 1

observed in balaenids and neobalaenids only (Bisconti

2003a).

37. Internal opening of facial canal: 0, wide, broad and shallow;

1, small, cylindrical and deep (Geisler and Luo 1996).

38. Length of the zygomatic process of squamosal: 0, long; 1,

short; 2, very short. A short zygomatic process of the squ-

amosal is observed in eschrichtiids and such ‘cetotheres’ as

Cetotherium rathkei. In these taxa, however, a zygomatic

process of the squamosal can be easily distinguished. In

balaenids and neobalaenids the zygomatic process of the

squamosal is even shorter and somewhat vestigial (McLeod

et al. 1993).

39. Cervical vertebrae: 0, free; 1, fused. State 1 observed in

balaenids and neobalaenids only (McLeod et al. 1993).

40. Rostral arch: 0, absent; 1, rostrum slightly arched; 2, rostrum

highly arched. State 1 is observed in eschrichtiids only

(McLeod et al. 1993). The rostrum as a whole is highly

curved in Neobalaenidae and Balaenidae; a detail of the cur-

vature (the curve of the premaxilla) is treated in character

62.

41. Coronoid process of dentary: 0, present; 1, absent. The coro-

noid process is absent in eschrichtiids, balaenids, and neoba-

laenids (McLeod et al. 1993).

42. Mandibular condyle: 0, articular surface dorsal; 1, articular

surface posterior. State 1 is observed in eschrichtiids, balae-

nids, and neobalaenids (McLeod et al. 1993).

43. Height of neurocranium: 0, low; 1, high. In balaenids the

neurocranium is high due to the enormous curvature of the

rostrum. This character is absent in other mysticetes

(McLeod et al. 1993).

44. Rostrum transversely compressed: 0, yes; 1, no, rostrum flat


45. Mylohyoidal sulcus along the ventromedial surface of the

dentary: 0, absent; 1, present (McLeod et al. 1993).

46. Anterior torsion of the dentary: 0, absent; 1, present


47. Infundibulum: 0, absent; 1, complete; 2, incomplete. State 2

is observed in balaenids only (Fraser and Purves 1960).

48. Posterior extension of the palatine: 0, anterior to posterior

border of the skull; 1, very close to the posterior border of

the skull. State 1 observed in balaenids only (Fraser and Pur-

ves 1960).

49. Ventral lamina of the pterygoid: 0, absent; 1, present. State 1

observed in balaenids only (Fraser and Purves 1960).

50. Pterygoid partially covered by the palatine: 0, no; 1, yes

(Fraser and Purves 1960).

51. Stylomastoid fossa: 0, absent; 1, deep as a notch; 2, long and

shallow. The stylomastoid fossa is shaped as a deep notch in

several ‘cetotheres’ (e.g. Parietobalaena palmeri, Diorocetus

hiatus) and balaenopterids (Bisconti 2003b). In balaenids it

occupies a long portion of the posterior process of the peri-

otic and is shaped as a shallow concavity (Bisconti 2003a).

52. Oval window: 0, elliptical; 1, dorsoventrally compressed. In

all the balaenids included into this analysis, the oval window


is markedly compressed along the dorsoventral axis (Bisconti

2003a).

53. Lateral projection of the anterior process of petrosal: 0,

absent; 1, present and short; 2, present, triangular and long.

The triangular projection of the anterior process of the pet-

rosal is well marked in living and fossil Balaenidae. It is not

preserved in Balaenula astensis and B. balaenopsis (Bisconti

2003a; see also Geisler and Luo 1996).

54. Baleen plate length: 0, baleen short; 1, long (Eschrichtiidae,

Neobalaenidae); 2, very long (Balaenidae) (McLeod et al.

1993).

55. Glenoid fossa of squamosal: 0, short and concave; 1, flat-to-

slightly concave; 2, highly concave and long. State 1 is

observed in balaenids, neobalaenids, eschrichtiids, and ‘ce-

totheres’. State 2 is typical of Balaenopteridae.

56. Relative position of zygomatic process of squamosal and

postglenoid process: 0, postglenoid process lower than the

zygomatic process; 1, postglenoid process on the same hori-

zontal plane as zygomatic process. In balaenopterines

and megapterines the postglenoid process is lower than the

zygomatic process of the squamosal (see True 1904, pls 3–4,

26–27, 31).

57. Distal presence of the ascending temporal crest on the

supraorbital process of the frontal: 0, yes; 1, no. The crest is

absent in Caperea marginata, Morenocetus parvus and in

Balaenella and Balaena (see also Bisconti 2003a).

58. Lateral process of maxilla in lateral view: 0, anterior to the

supraorbital process of frontal; 1, under the supraorbital

process of frontal (Bisconti 2003a).

59. Supraorbital process of the frontal: 0, transverse to the long

axis of the skull; 1, directed posteriorly (McLeod et al.

1993).

60. Anterior process of the supraoccipital: 0, round and wide; 1,

squared and transversely constricted; 2, squared; 3, triangu-

lar; 4, narrowly rounded. This character accounts for the

morphological diversity of the anterior portion of the supra-

occipital in mysticetes. State 1 is observed in Balaenella and

Balaena montalionis; state 2 is present in balaenopterids;

state 3 is found in ‘cetotheres’; state 4 is seen in Balaenula.

61. Position of the glenoid fossa relative to the posterior apex of

the lambdoidal crest: 0, glenoid fossa located below the

lambdoidal crest; 1, anterior; 2, posterior. The character is

related to Bisconti’s (2003a) diagnosis of Balaenula.

62. Curvature of premaxilla: 0, no curvature; 1, regular curva-

ture; 2, irregular curvature. In Balaenula astensis and Eubal-

aena the anterior 25 per cent of the premaxilla is directed

ventrally, interrupting the regular curvature of the rostrum

in that region (Bisconti 2000, 2003a).

63. Curvature of the dorsal surface of the skull: 0, skull mainly

straight; 1, regular curvature; 2, irregular curvature. A regu-

lar curvature is observed in Balaena and Balaenella. Irregular

curvature is present in Eubalaena and Balaenula (McLeod

et al. 1993).

64. Distal portion of the infraorbital plate of the maxilla: 0, pre-

sent; 1, absent (Bisconti 2003a).

65. Orientation of the nasals and the proximal rostrum: 0,

onward; 1, upward. State 1 is present in Balaena mysticetus,

B. montalionis and, presumably, in B. ricei (Bisconti 2003a).

66. Relief on the parietal squama: 0, absent; 1, present (Bisconti

2000, 2003a).

67. Spreading of the anterolateral portion of the parietal onto the

emergence of the supraorbital process of the frontal: 0,

absent; 1, present. The spreading of the parietal onto the

emergence of the supraorbital process of the frontal is

observed in Balaenula and Eubalaena among the Balaenidae

(Bisconti 2003a).

68. Dome on the supraoccipital: 0, absent; 1, present (Bisconti

2002).

69. Posterior outline of the exoccipital in lateral view: 0, round;

1, squared. Squared exoccipitals are observed in Eubalaena

skulls (Bisconti 2000, 2003a).

70. Position of the glenoid fossa of the squamosal relative to the

orbit: 0, posterior; 1, under the orbit. State 1 is diagnostic of

Balaenula (Bisconti 2003a).

71. Height of the ventral surface of the exoccipital: 0, higher

than the orbit; 1, at the level of the orbit. State 1 is diagnos-

tic of Balaenula (Bisconti 2003a).

72. Height of the ascending temporal crest: 0, low; 1, high. A

high ascending temporal crest is observed in Balaenula (Bis-

conti 2003a).

73. Hypoglossal foramen: 0, present; 1, absent. The presence of

the hypoglossal foramen is known in Balaenula astensis (Bis-

conti 2000, 2003a).

74. Olecranon: 0, present; 1, absent.

75. Acromion: 0, present; 1, absent.

76. Coracoid: 0, present; 1, absent.

77. Shape of the scapula: 0, high and short; 1, high and long.

78. Antibrachium: 0, shorter than humerus; 1, longer than

humerus. See Westgate and Whitmore (2002) for a discus-

sion of this character.

79. Radius: 0, straight; 1, highly convex. The radius is anteriorly

convex in living balaenids and in Balaena ricei (see Pilleri

1987; Benke 1993; Westgate and Whitmore 2002).

81. Pars cochlearis: 0, transversely short; 1, protruded cranially.

A cranial protrusion of the pars cochlearis is observed in

balaenopterids and in the living Eubalaena glacialis (Bisconti

2001, 2003a).

82. Groove for the tensor tympanic muscle: 0, present; 1, absent

(Luo and Gingerich 1999).


Character · taxon matrix used in the phylogenetic analysis

Protocetus atavus ?000000000000000000000000000?000000000000000000000000-00--00000-0000000-000000000

Zygorhiza kochii 111100000000000000000000-0000000000000000000000000000-000-00100-000000010??????00

Aetiocetus polydentatus ?1111111??0100000000?11000000000000000000001001000000-000003100000000001000000000

Parietobalaena palmeri 111111111111111101111100-00000000000000001010010001010100003100000000000100000000

Eschrichtius robustus 111111111111111001111011113301000000010110010010001011110002000000000000100000000

Balaenoptera physalus 111111111111111221221111111022200000000001010010001010200002000000000000100011011

Megaptera novaeangliae 111111111111111221221111111022000000000001010010001010200002000000000000101111011

Caperea marginata 111111111111111111001100-12101111111121210110010000001111010111000000000100011011

Morenocetus parvus ?1101??11111111??101?000-021??111????21??01?112??????21111100???000000001????????

Eubalaena glacialis 111011111111111111111100-02111111111121210101121112122110000022000111000100000110

Eubalaena belgica ?1101?11??11111??111?000-021??111111?212?01???2111???211000002200011100010???????

Balaenula balaenopsis 111011111111111111110000-021?1111111121210101121112102110000022000100111100000000

Balaenula astensis 11101111??11111111110000-021?1111111?2?2101011211121?21100000220001001110??????00

Balaenula sp. (Japan) 111011111111111111110000-021?1111111121210101121112122110000022000100111100000000

Balaenella brachyrhynus ?1101?11??11111??1110000-021?1111111?2?2?010??211121221111112111000000001??????00

Balaena montalionis ?1101?11??11111??111?000-021??111????2?2?010??2111???21111112111110000001????????

Balaena ricei 11?0?1111?11111?????1000-021??111???1?12??1011211121?2???1?4211??10000??100000100

Balaena mysticetus 111011111111111111110000-021111111111212101011211121221111142111110000001?1100100


SKULL MORPHOLOGY AND PHYLOGENETIC RELATIONSHIPS OF A NEW DIMINUTIVE BALAENID FROM THE LOWER PLIOCENE...

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