A Laurentian Iocrinus Hall (Crinoidea, Disparida) in the Dapingian or Darriwilian (Middle...

A LAURENTIAN IOCRINUS HALL (CRINOIDEA,

DISPARIDA) IN THE DAPINGIAN OR DARRIWILIAN

(MIDDLE ORDOVICIAN, ARENIG) OF OMAN

by STEPHEN K. DONOVAN* , C. GILES MILLER� , IVAN J. SANSOM� ,

ALAN P. HEWARD§ and JAN SCHREURS–*Department of Geology, Netherlands Centre for Biodiversity – Naturalis, Postbus 9517, NL-2300 RA Leiden, The Netherlands; e-mail: [email protected]

�Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK; e-mail: [email protected]

�School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK; e-mail: [email protected]

§Petrogas, PO Box 353, PC112, Ruwi, Sultanate of Oman; e-mail: [email protected]

–XGL, Petroleum Development Oman, PO Box 81, Muscat 1000, Sultanate of Oman; e-mail: [email protected]

Typescript received 26 November 2009; accepted in revised form 4 June 2010

Abstract: Early and early Middle Ordovician crinoids are

rare globally, and are best known from North America and

the British Isles. The first Arenig crinoid from the Arabian

Peninsula is Iocrinus sp. cf. I. subcrassus (Meek and Wor-

then), known from two near-complete individuals, and

numerous fragmentary specimens and ossicles. These are the

stratigraphically oldest Iocrinus specimens, and provide an

unexpected extension of the palaeogeographical range of a

genus known otherwise from slightly younger deposits in

Laurentia (North America) and Avalonia (Wales). The Oman

specimens differ from Gondwanan Heviacrinus melendezi Gil

Cid et al., currently classified as a maennilicrinid, but mor-

phologically close to Iocrinus, in having arms that branch at

least seven times instead of four and in having columnals

typical of Iocrinus. Three species from Wales and the Welsh

Borders, previously placed in Iocrinus, but differing in having

smooth, conical dorsal cups, are reclassified as Margoiocrinus

Donovan gen. nov., type species Iocrinus shelvensis Rams-

bottom.

Key words: Arabian Peninsula, Iocrinidae, Heviacrinus,

systematics, palaeogeography, Margoiocrinus.

E arly and early Middle Ordovician crinoids remain

rare. Since the review of Donovan (1988), only Guens-

burg and Sprinkle (2001, 2003, 2007, 2009) have added

significantly to the diversity of taxa of this interval, other

rare records describing single species (e.g. Donovan and

Cope 1989). Furthermore, these crinoids are known

almost exclusively from North America and northern

Europe. A rare exception was provided by a group of cri-

noid arms from the Arenig of Morocco, tentatively

assigned to Ramseyocrinus sp. (Donovan and Savill 1988),

but this remains the only Lower Ordovician crinoid

recorded from the African continent. The specimens

recorded herein are from the early Middle Ordovician of

the Arabian Peninsula and are a significant addition to

the records of these sparse early crinoids.

LOCALITY AND HORIZON

The latex casts described herein came from a decalcified

shell bed in sandstone within a sequence of shell

beds, sandstones and shales of the Amdeh Formation at

Wadi Daiqa, south of Muscat, Oman (23�5¢2.9¢¢N,

58�49¢20.1¢¢E; Text-fig. 1). Isolated crinoid columnals

were recovered from calcareous nodules in shales that

were processed for conodonts (23�5¢5.7¢¢N, 58�49¢17.5¢¢E).

A mixed Skolithos-Cruziana ichnofacies containing trace

fossils of both suspension- and deposit-feeding tracemak-

ers in association with the trilobites Neseuretus and Ogygi-

nus, the bivalved mollusc Redonia, crinoid remains (see

below) and arandaspid agnathan fishes are indicative of

nearshore, shallow water conditions (Lovelock et al. 1981;

Fortey and Morris 1982; Davies et al. 2007; Davies and

Sansom 2009; Sansom et al. 2009).

The lithostratigraphy of the 3500 m thick Amdeh For-

mation was initially established by Lovelock et al. (1981)

who described five successive members, Am1–Am5. This

was subsequently modified following the regional geologi-

cal mapping of Le Metour et al. (1986). Le Metour et al.

(1986) and Villey et al. (1986) placed the Wadi Daiqa expo-

sures of the Amdeh Formation in the Am5 Member, yet the

sedimentary facies and lithologies are more typical of the

sandier Am4 (Sansom et al. 2009). Sansom et al. (2009)

suggested an uppermost Dapingian? to early Darriwilian

[Palaeontology, Vol. 54, Part 3, 2011, pp. 525–533]

ª The Palaeontological Association doi: 10.1111/j.1475-4983.2011.01042.x 525

age (Middle–Late Arenig) for the Wadi Daiqa exposures on

the basis of palynological and trilobite studies. Conodonts

from the samples which yielded the isolated crinoid colum-

nals also indicate a Dapingian to Darriwilian age.

Terminology of the crinoid endoskeleton follows Moore

et al. (1978b), Webster (1974) and Ubaghs (1978). Higher

classification of crinoids follows Simms & Sevastopulo

(1993) and Ausich (1998). Our philosophy of open

nomenclature follows Bengtson (1988). Specimens dis-

cussed herein are deposited in the Department of Palaeon-

tology, The Natural History Museum, London (BMNH).

SYSTEMATIC PALAEONTOLOGY

Class CRINOIDEA J. S. Miller, 1821

Subclass DISPARIDA Moore and Laudon, 1943

Order MYELODACTYLIDA Ausich, 1998

Family IOCRINIDAE Moore and Laudon, 1943

Genus IOCRINUS Hall, 1866

Type species. Heterocrinus (Iocrinus) polyxo Hall, 1866, p. 5, pl.

1, figs 1–4 (=Actinocrinus subcrassus Meek and Worthen, 1865,

p. 148), by monotypy, from the Ordovician (Champlainian to

Cincinnatian) of Ohio, New York and Ontario (Moore et al.

1978a, p. T522; Webster 2003).

Other species. Iocrinus crassus (Meek and Worthen, 1865) (Cin-

cinnatian of Illinois and Ohio); Iocrinus pauli Donovan and

Gale, 1989 (Llanvirn of the Llandrindod Wells area, Powys, mid

Wales); Iocrinus similis (Billings, 1857) (Champlainian of

Ontario); Iocrinus trentonensis Walcott, 1883 (Trentonian of

New York) (Webster 2003).

Diagnosis. (Revised after Donovan and Gale 1989, p. 314)

Dorsal cup conical to slightly bowl-shaped, with promi-

nent radial ridges extending to angles of column, ridges

separated by folds. Five prominent basal plates and five,

commonly larger radial ossicles. An anibrachial, sup-

ported by the C ray radial, in turn supports the anal ser-

ies on the left side and a free arm on the right. Anal sac

complex. Arms with at least four isotomous branches.

Column transversely pentastellate proximally, pentagonal

in the mesistele and circular distally. Permanent attach-

ment by distal, non-planar spiral coil.

Remarks. Donovan and Gale (1989, text-fig. 6) recognized

two morphologically distinct lineages within Iocrinus Hall.

The typical North American species have radial ribs in

the dorsal cup corresponding to the angles of the column.

The proximal column is pentastellate, with the folded

areas extending onto the cup. This includes the Welsh

Iocrinus pauli and the species from Oman described

herein. In contrast, other Iocrinus from Wales and the

TEXT -F IG . 1 . Outcrop map of the

Amdeh Formation in the Saih Hatat

region of northern Oman, showing the

crinoid locality at Wadi Daiqa (after

Sansom et al. 2009, text-fig. 1, modified

after Lovelock et al. 1981, fig. 1).

526 P A L A E O N T O L O G Y , V O L U M E 5 4

Welsh Borders have a conical dorsal cup without ridges

and grooves, and a column that is pentagonal in section

throughout. The latter clade is named Margoicocrinus

Donovan gen. nov. below.

Range. Ordovician: Champlainian to Cincinnatian of North

America; Llanvirn of Wales; Arenig of Oman (revised after Web-

ster 2003).

Iocrinus sp. cf. I. subcrassus (Meek and Worthen, 1865)

Plate 1; Text-figures 2, 3A,B

Material. Five slabs of sandstone preserving incomplete speci-

mens as external moulds, BMNH EE 13822–EE 13824 (Text-

fig. 2), EE 13829, EE 13830; disarticulated ossicles derived from

rock broken down for conodont analysis, BMNH EE 13825 and

EE 13826 (Text-fig. 3A,B); and large latex casts taken from the

field from particularly complete specimens that, unfortunately,

could not be collected, BMNH EE 13820 to EE 13821(1–4)

(Pl. 1).

Locality and horizon. Wadi Daiqa, south of Muscat, Oman;

Dapingian or Darriwilian, Middle Ordovician, upper Arenig (see

above).

Description. Attachment structure not seen. Column xenomor-

phic, pentastellate in section proximally and pentagonal more

distally. The short proxistele gently tapers away from the base

of dorsal cup, grading imperceptibly into the untapered and

longer mesistele (Pl. 1, fig. 2). Mesistele at least five times

longer than proxistele, more rounded pentagonal in section

with less inflated nodal angles. Lower order internodals may be

more pentastellate, giving the column a ladder-like appearance

(Pl. 1, figs 3, 5). Column heteromorphic, N434243414342434.

Nodals highest with nodose angles and convex latera. Interno-

dals progressively lower, all with convex latera except quartin-

ternodals. Articular facet of columnals with a central, rounded

pentagonal lumen (Text-figs 2D, 3A,B). Five areola petals devel-

oped in angles of columnal, each adjacent pair separated by

one or three large crenulae. Articulation symplectial, best devel-

oped as a perilumen and between columnal angles. Raised

circumference of columnal may have lacked a crenularium.

Latera unsculptured.

Dorsal cup poorly seen. Monocyclic, basals deeply infolded as

a continuation of the structure of the proxistele (Pl. 1, fig. 4).

Basals about half height of cup, wide. Radials more convex,

about as high as basals, but wider than high, with broad radial

facets about three quarters of plate width. Plates of crown un-

sculptured.

Tegmen and anal series not preserved.

Arms long, uniserial, apinnulate, branching isotomously prox-

imally, both isotomously and heterotomously more distally.

Arms branching at least seven times, branches becoming more

gracile after each bifurcation. Primibrachials robust, slightly

higher than wide, branching at IBr3. Secundibrachials more slen-

der, branching about IIBr7.

Remarks. Donovan and Gale (1989, p. 317) discussed

how the known species of Iocrinus Hall could be differen-

tiated. Iocrinus pauli and I. subcrassus are broadly similar.

Basals and radials of I. subcrassus are wider than high; ba-

sals are about as wide as high in I. pauli and radials are

slightly higher than wide. Iocrinus trentonensis is similar

to I. subcrassus. Iocrinus crassus has a large dorsal cup

with a radial:basal height ration of 3:2, in contrast to 2:1

in I. pauli. Iocrinus similis is aberrant with an extra plate

between the C radial and the anibrachial; it is only known

from a single specimen. The Oman specimens are classi-

fied tentatively as Iocrinus sp. cf. I. subcrassus; Kelly

(1978, p. 57) considered that Iocrinus subcrassus showed

much variability in overall morphology.

The curved and open coiled fragments of the more dis-

tal mesistele (Pl. 1, fig. 1; Text-fig. 2B) occurred after the

breakage of the crinoid from the dististele, presumed to

have been a distal non-planar spiral coiled attachment

(sensu Brett 1981, table 1, fig. 4E,F) as in other Iocrinus

species (Kelly 1978, pl. 1, figs 7–8). Such an attachment

would have been a much tighter spiral than is seen in the

Oman specimens. It is presumed that individuals were

broken off and transported by the same current that

buried them in sand. Well-preserved Iocrinus commonly

have a more or less straight or gently curved proxistele

and more proximal part of the mesistele (Pl. 1, figs 2–5;

Kelly 1978, pl. 1, figs 4, 9–10; Donovan and Gale 1989,

text-figs 2–3). The curved specimens of the Oman species

are more distal and indicate the flexibility of the column

immediately above the attachment, although the latter, of

course, would have been permanently coiled and made

inflexible by having wedge-shaped columnals (e.g. Dono-

van 1986, text-fig. 12K).

This leads to the question of where are the long and

complex anal sacs that are so typical of Iocrinus? None is

apparent on any of the latex casts, even as fragments. It is

conservative to assume that both well preserved crowns

(Pl. 1, figs 2, 4) are in the same orientation with the more

anterior part of the crown preserved. So, the posterior

part of the crown, including the anal sac, would be on

the counterpart of the parent sandstone slab, but this is

not preserved. Alternatively, it may be that the anal tubes

were autotomised as a reaction to disturbance (compare

with Lane 1984) and their hydrodynamic characteristics

led to transport to another site. These are the oldest Iocri-

nus specimens known, but that is no reason to suppose

that the anal sac was any less well developed than in the

North American and Welsh species.

Ausich (in review comments, forwarded to S. K. D. on

18 February 2010) has brought to our attention the simi-

larity of the Oman material to Llanvirn Heviacrinus me-

lendezi Gil Cid, Domınguez Alonso and Silvan Pobes,

1996 (see also Ausich et al. 2002, 2007) from Spain, a

D O N O V A N E T A L . : I O C R I N U S H A L L F R O M O M A N 527

disparid genus lacking an anal sac. We have assumed

above that the anal sac of the Oman specimen was not

preserved (=Iocrinus), whereas perhaps it was not present

(=Heviacrinus). This poses an obvious quandary and the

Gondwanan occurrence of H. melendezi strengthens its

case. Although we are sympathetic to this suggestion, we

consider the systematic position of H. melendezi equivo-

cal. Gil Cid et al. (1996, p. 20) originally assigned the

genus Heviacrinus to the family Iocrinidae, whereas Aus-

ich et al. (2002, p. 989) moved it to the family Maennilic-

rinidae Ausich of the order Maennilicrinida Ausich in

strict accordance with Ausich’s (1998) system of classifica-

tion. Yet, the morphological similarities of Heviacrinus

and Iocrinus are surely too many to dismiss as conver-

gence; Iocrinus is a Heviacrinus with an anibrachial and

an anal sac. To put it more plainly, if these specimens are

Heviacrinus, then Heviacrinus is an iocrinid. We speculate

that Iocrinus may have evolved from a Heviacrinus-like

ancestor by a paedomorphic retention of a larval ⁄ juvenile

anal sac or, vice versa, Heviacrinus is an Iocrinus that has

lost these features (McNamara 1986). We prefer to call

our material Iocrinus (but certainly an iocrinid) and to

suggest that Heviacrinus is a primitive member of the Ioc-

rinidae. Furthermore, the Oman specimens differ from

H. melendezi in having arms that branch at least seven

times (see above) rather than four (Ausich et al. 2002, p.

990) and also have some heterotomous branches, unlike

the Spanish taxon. Less significantly, but in the same

description, Ausich et al. (2002, p. 990) speculated that

the ‘lumen was probably pentalobate,’ although the artic-

ular facet geometry remains unknown; the Oman speci-

mens have a rounded pentagonal lumen.

EXPLANATION OF PLATE 1

Iocrinus sp. cf. I. subcrassus (Meek and Worthen, 1865). Latex casts of specimens that could not be collected and were left in the field.

All specimens coated with ammonium chloride. The scale bar 10 mm applies to all figures.

Fig. 1. BMNH EE 13820, open coiled pluricolumnal, probably coiled post-mortem rather than part of a distal spiral attachment

structure (compare with Kelly 1978, pl. 1, figs 7, 8).

Fig. 2. BMNH EE 13821(1), well preserved crown and proxistele. Column tapering just beneath cup, cup poorly seen, arms uniserial

and branching isotomously at least seven times.

Fig. 3. BMNH EE 13821(2), straight length of mesistele (probably distal to that seen in Pl. 1, fig. 2).

Fig. 4. BMNH EE 13821(3), crown and proxistele, cup particularly well seen.

Fig. 5. BMNH EE 13821(4), long, straight length of mesistele (from same region as specimen in Pl. 1, fig. 3).

A B C

D

TEXT -F IG . 2 . Iocrinus sp. cf. I. subcrassus (Meek and Worthen, 1865), latex casts of specimens in BMNH collection. A, BMNH EE

13822, details of more distal parts of arms, Scale bar represents 10 mm. B, BMNH EE 13823, curved pluricolumnal (compare with

Pl. 1, fig. 1) Scale bar represents 10 mm. C, D, BMNH EE 13824, lateral and facetal views, respectively, of a short pluricolumnal, Scale

bar represents 5 mm. All specimens coated with ammonium chloride.


PLATE 1

DONOVAN et al., Iocrinus sp. cf. I. subcrassus

1

4

5

2

3

A finer grained and more calcareous sedimentary rock

was treated in 10 per cent acetic acid for conodont ana-

lysis. The heavy fraction (>2.80 specific gravity) of the

sediment, following sodium polytungstate separation, has

yielded columnals of Iocrinus and a second taxon. The

morphology of the columnals of Iocrinus is well known

and the specimens figured herein (Text-fig. 3A,B) are

comparable to those figured from other species (Kelly

1978, pl. 2, fig. 7; Donovan and Gale 1989, text-fig.

4A,B,D). Other, cyclic columnals probably represent one

or two other crinoid taxa not known from more complete

material (BMNH EE 13827, EE 13828, EE 13831, EE

13832). Although small enough to be radice ossicles from

an attachment structure, Iocrinus did not have a radicu-

lar holdfast for attachment as discussed above. Illustrated

examples include one columnal that is pentagonocyclic

with a synostosial articulation and may well represent

part of an immovable, radicular attachment structure

(Text-fig. 3C). A second is more likely to represent a

columnal from a gracile disparid or other small crinoid,

with a marginal symplectial articulation, circular areola

and central lumen of indeterminate outline (Text-

fig. 3D).

The Oman material is important for the following rea-

sons. These are the oldest Iocrinus specimens to have been

documented and Dapingian ⁄ Darriwilian crinoids continue

A

B

C

D

TEXT -F IG . 3 . A, B, Iocrinus sp. cf.

I. subcrassus (Meek and Worthen, 1865),

articular facets. A, BMNH EE 13825. B,

BMNH EE 13826. C, D, circular

columnals from same deposit. C, BMNH

EE 13827, pentagonocyclic columnal

with planar articular facet (synostosis).

D, BMNH EE 13828, columnal with

marginal symplexy of short crenulae,

circular areola and indistinct lumen.

Scanning electron micrographs of

specimens coated with gold-palladium.

Scale bar represents 0.5 mm.

TEXT -F IG . 4 . Simplified palaeogeo-

graphy of part of the Southern

Hemisphere during the Dapingian ⁄Darriwilian (redrawn after Torsvik

2009). Iocrinus localities are: 1, Oman; 2,

Wales; 3, New York; 4, Ohio and

Illinois; and 5, Ontario. Key to

continental blocks: Av, Avalonia; Ba,

Baltica; La, Laurentia; NC, North China;

SC, South China; Si, Siberia; and U,

Urals. Coastlines stippled.


to be poorly known. It is also the first Dapingian ⁄ Darriw-

ilian crinoid to be described from Arabia; others are

mainly known from North America and northern Europe.

These specimens also add an unexpected extension of pal-

aeogeographical range to that of Iocrinus (Text-fig. 4).

Iocrinus has hitherto been limited to either side of the

Iapetus Ocean between Avalonia (Wales) and, particu-

larly, Laurentia (North America); Iocrinus-like columnals

are also recorded from the Upper Ordovician of Baltica

(Briskeby 1981, pp. 115–116, figs 17, 39, pl. 6, figs 7–9).

Hitherto, it was considered more probable that Iocrinus

was a Laurentian genus that migrated to Avalonia in the

Middle Ordovician (Donovan 1989). But Oman (Gond-

wana) was at a similar palaeolatitude to Avalonia during

the Arenig. Based on what is now known and recognizing

that further, well-dated specimens are needed from inter-

mediate land areas, it is tempting to speculate that migra-

tion from Gondwana to Laurentia would have been via

Avalonia. (For a recent and thorough review of peri-

Gondwanan palaeogeography in the Ordovician, see

papers in Bassett 2009.)

It is also relevant to briefly note the longevity of Iocri-

nus Hall sensu stricto, ranging as it does through most of

the Middle and Upper Ordovician. A Dapingian ⁄ Darriw-

ilian (=lower Middle Ordovician) Iocrinus is not surpris-

ing stratigraphically, although better preserved material is

still desirable to confirm its existence biogeographically

and to answer the Iocrinus ⁄ Heviacrinus question.

Genus MARGOIOCRINUS Donovan gen. nov.

Derivation of name. From Latin margo, margin or border, cele-

brating the occurrence of two out of three species in the Welsh

Borders.

Type species. Iocrinus shelvensis Ramsbottom, 1961, pp. 3–4, pl. 1,

figs 3–8, designated herein; from the Llanvirn (Abereiddian) Wes-

ton Flags Formation (murchisoni Biozone) of the Shelve Inlier,

Welsh Borders (Fortey and Rushton in Fortey et al. 2000, fig. 14).

Other species. Margoiocrinus brithdirensis (Bates, 1965) (Llanvirn

of Dolgellau region, Wales: Bates 1965; Donovan 1986, p. 25,

text-fig. 12K); Margoiocrinus whitteryi (Ramsbottom, 1961)

(Caradoc of Shelve Inlier, Welsh Borders: Ramsbottom 1961, p.

5, pl. 1, fig. 9; Donovan and Gale 1989, pp. 319–320, text-fig. 5).

Diagnosis. (Modified after Donovan and Gale 1989, p.

314) Dorsal cup conical, smooth, composed of five prom-

inent basal plates and five larger radial ossicles. An ani-

brachial, supported by the C ray radial, in turn supports

the anal series on the left side and a free arm on the

right. Anal sac complex. Arms with at least four isotom-

ous branches. Column transversely pentagonal proximally

throughout, becoming circular in section distally. Perma-

nent attachment by distal, non-planar, spiral coil.

Remarks. The smooth, conical cup and pentagonal trans-

verse section of the proxistele of these three species unites

them as a single clade (Donovan and Gale 1989, text-fig.

6) and differentiates them from Iocrinus sensu stricto.

Iocrinus has a broad geographical range – North America,

Wales, Baltica(?) and now Oman – whereas Margoiocrinus

is limited spatially to Wales (I. brithdirensis) and the

Welsh Borders, on the Avalonian plate (Text-fig. 4).

Although with a similar geometry of cup plating to Iocri-

nus sensu stricto, the morphology of the cup and proxis-

tele of Margiocrinus is distinct, and there is no possibility

of confusing the genera on the basis of these diagnostic

features.

Range. Ordovician (Llanvirn to Caradoc) of Wales and the

Welsh Borders of England.

Acknowledgements. This work was supported in part by NERC

grant NE ⁄ B503576 ⁄ 1. We thank the Photographic Unit

(BMNH) for providing the images in Plate 1 and Text-figure 2.

The considered inputs of Martha Richter (BMNH) and Felicity

Heward (Oman) are gratefully acknowledged. S. K. D. thanks

Professor David Harper (University of Copenhagen) for discus-

sions of Ordovician palaeogeography. We gratefully acknowledge

the thought-provoking reviews of Professors William I. Ausich

(The Ohio State University, Columbus) and Thomas E. Guens-

burg (Rock Valley College, Illinois).

Editor. George Sevastopulo

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A Laurentian Iocrinus Hall (Crinoidea, Disparida) in the Dapingian or Darriwilian (Middle...

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