A rare non-trilobite artiopodan from the Guzhangian (Cambrian Series 3) Weeks Formation...

A RARE NON-TRILOBITE ARTIOPODAN FROM THE

GUZHANGIAN (CAMBRIAN SERIES 3) WEEKS

FORMATION KONSERVAT-LAGERST€ATTE IN UTAH,

USA

by JAVIER ORTEGA-HERN �ANDEZ1,2*, RUDY LEROSEY-AUBRIL3*,

CARLO KIER4 and ENRICO BONINO4

1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK; e-mail: [email protected] College, University of Cambridge, St Andrews Street, Cambridge, CB2 3AP, UK3Laboratoire de G�eologie de Lyon: Terre, Plan�etes, Environnement (UMR 5276 CNRS), Universit�e Claude Bernard Lyon 1, 2 rue Rapha€el Dubois, 69622,

Villeurbanne, France; e-mail: [email protected] to the Past Museum, Carretera Canc�un, Puerto Morelos, Quintana Roo 77580, M�exico; e-mails: [email protected], [email protected]

*Corresponding authors

Typescript received 19 July 2014; accepted in revised form 26 September 2014

Abstract: We describe a weakly biomineralized non-trilo-

bite artiopodan arthropod from the Guzhangian Weeks

Formation of Utah. Falcatamacaris bellua gen. et sp. nov. is

typified by a thin calcitic cuticle, broad cephalon without eyes

or dorsal ecdysial sutures, an elongate trunk with distinctively

sickle-shaped pleural spines and a long tailspine with a bifur-

cate termination. The precise affinities of Falcatamacaris gen.

nov. are problematic due to the presence of unique features

within Artiopoda, such as the peculiar morphology of the

pleural and posterior regions of the trunk. Possible affinities

with aglaspidid-like arthropods and concilitergans are dis-

cussed based on the possession of 11 trunk tergites, edge-to-

edge articulations and overall body spinosity. The new taxon

highlights the importance of the Weeks Formation Konservat-

Lagerst€atte for further understanding the diversity of extinct

arthropod groups in the upper Cambrian.

Key words: Arthropoda, non-trilobite Artiopoda, excep-

tional preservation, Laurentia, biomineralization, Cambrian

explosion.

ART IOPODA Hou and Bergstr€om, 1997, is an important

group of extinct arthropods that dominated the Palaeo-

zoic Era due to their remarkable ecological success. By their

abundance and diversity, trilobites are the most emblem-

atic representatives of this clade; however, Artiopoda also

includes numerous non-trilobite taxa described from

Lower Palaeozoic deposits and particularly from sites of

exceptional preservation around the world (Hendricks

et al. 2008; Hendricks 2013). Non-trilobite artiopodans

display a considerable degree of morphological variability,

especially in terms of dorsal tagmosis (Edgecombe and

Ramsk€old 1999) and biramous limb construction (Ortega-

Hern�andez et al. 2013, fig. 4). Various non-trilobite

Artiopoda have been successfully classified into relatively

well-defined groups, which have also been supported by

phylogenetic analyses (e.g. Xandarellida, Nektaspida, Con-

ciliterga; see Hou and Bergstr€om 1997; Paterson et al.

2012; Ortega-Hern�andez et al. 2013). However, a few taxa

are notorious for their unusual morphologies, which

complicate their classification and precise phylogenetic

affinities (e.g. Habelia optata Walcott, 1912; Phytophilaspis

pergamena Ivanstov, 1999), or which occupy an unspeci-

fied basal position relative to better defined groups (e.g.

Retifacies abnormalis Hou et al. 1989; Squamacula clypeata

Hou and Bergstr€om 1997; see Paterson et al. 2012,

Ortega-Hern�andez et al. 2013, Stein et al. 2013). Here, we

describe a new weakly biomineralized artiopodan from

the late Guzhangian (Cambrian Series 3) Weeks Forma-

tion of Utah (USA), which combines a mosaic of mor-

phological characteristics as-yet observed in several

artiopodan groups. The new taxon contributes towards

the increasing knowledge on the diversity of late Cam-

brian non-trilobite arthropods as informed by the Weeks

Formation Konservat-Lagerst€atte.

GEOLOGICAL SETTING

The Weeks Formation consists of a 300-m-thick sequence

of thin-bedded lime mudstones, wackestones and grain-

stones with variable amounts of shale. It crops out in

the North Canyon and neighbouring areas only, in the

© The Palaeontological Association doi: 10.1111/pala.12136 265

[Palaeontology, Vol. 58, Part 2, 2015, pp. 265–276]

vicinity of Notch Peak (Central House Range, Utah,

USA). The Weeks Formation conformably overlies the

shales and argillaceous carbonates of the Marjum For-

mation, which represents relatively deep-water deposits,

and it is conformably overlain by the shallow water car-

bonates of the Orr Formation. The upper part of the

Weeks Formation actually records a substantial trans-

gressive succession to a shallower depositional environ-

ment (Beebe 1990); it marks the end of the House

Range embayment, a fault-controlled trough that devel-

oped within the carbonate platform during Cambrian

Stage 5, as a locus for deep-water sedimentation (Miller

et al. 2012). This upper part of the Weeks Formation

has yielded a rather diverse biomineralized fauna, domi-

nated by trilobites (Robison and Babcock 2011; Lerosey-

Aubril et al. 2012) and inarticulate brachiopods, as well

as exceptionally preserved fossils. These ‘soft-bodied’ or

weakly biomineralized organisms are predominantly

represented by arthropods (Lerosey-Aubril et al. 2013a,

2014; Lerosey-Aubril 2014) and worms (e.g. palaeosco-

lecids), although sponges and a possible ctenophore (i.e.

comb jelly) also occur. Trilobites (e.g. Cedaria minor

Walcott, 1916) indicate that the upper part of the

Weeks Formation was deposited in an open-shelf marine

environment during the late Guzhangian (i.e. late Cam-

brian Epoch 3, formerly early late Cambrian; Robison

and Babcock 2011). A more detailed account of the

structural and environmental contexts of the deposition

of the Weeks Formation is given in Lerosey-Aubril et al.

(2014).

MATERIAL AND METHODS

The studied material consists of the part and counter-

part (BPM 1022a and BPM 1022b, respectively) of a

single individual preserved as a dorsoventral compres-

sion in lime mudstone. Photographs of the dry speci-

mens were taken with a Nikon D3X digital camera

equipped with a Micro-Nikkor AF 60 mm f/2.8 D

macro lens, using low-angle cross-polarized light. The

preservation of the specimens was explored using SEM

(JEOL 310 JSM-6490LV and FEI Verios 460) equipped

with energy dispersive X-ray (EDX) modules (EDAX).

This material is housed at the Back to the Past

Museum (Canc�un, Mexico; BPM). Photographs of a

second specimen, unfortunately held in a private collec-

tion, were also examined. This specimen is fundamen-

tally identical to BPM 1022, except for the presence of

a long tailspine distinguished by a distal bifurcation.

Although this feature is not visible on the material

described herein due to the incomplete preservation of

the posterior part of the body, this aspect of the mor-

phology is briefly mentioned in the description of the

new taxon.

Terminology

The shape of the posterior margin of the cephalon is

described from sagittal axis to genal angle, using the

terms ‘procurved’ and ‘opisthocurved’ when it curved an-

terolaterally and posterolaterally, respectively (Dunlop

and Selden 1997; Lerosey-Aubril et al. 2013b; Lerosey-

Aubril 2014). Abbreviations: exs., exsagittal; sag., sagittal;

tr., transverse; T1–11, trunk tergites 1 to 11.

Preservation

The part (BPM 1022a) and counterpart (BPM 1022b) of

the specimen represent the dorsal exoskeleton of an

almost complete individual associated with possible

remains of internal organs and appendages (Figs 1, 2).

Both parts are almost flat, but the pattern of medial over-

lap of adjacent tergites suggests that the dorsal exoskele-

ton is exposed from the visceral side (i.e. upside-down

position). The specimen is mostly preserved as internal

(BPM 1022a?) or external (BPM 1022b?) moulds of the

dorsal exoskeleton, which exhibit a dark grey to red col-

ouration. The moulds are still partially covered by a thin

layer of light pink material in the medial region of the

F IG . 1 . Falcatamacaris bellua gen. et sp. nov. from the upper part of the Weeks Formation, late Guzhangian (Cedaria Zone), House

Range, Utah, USA. A–I, holotype (BPM 1022a, b), part and counterpart of the dorsal exoskeleton of an almost complete individual

associated with possible remains of internal organs and appendages; photographs of the dry specimens under cross-polarized light with

anterior end facing to the top. A, C–F, I, part (BPM 1022a); A, general view; C, three-dimensional patches of calcium carbonate (black

arrowheads) and iron oxides (white arrowheads) located medially under posterior part of cephalon and trunk tergites T1–3; thesematerials possibly represent the remains of digestive structures; D, three-dimensional patches of calcium phosphate located medially

under trunk tergites T6 and T7; E, left pleural regions of trunk tergites T1 and T2 showing probable remains of cuticle composed of

calcium carbonate; F, detail of the cephalic region showing possible remains of appendages (ap?); I, detail of the posterior trunk

region; note the particular morphology of trunk tergites T9–11, with their abaxial portions strongly curved backwards, mostly consist-

ing of pleural spines. B, G, H, counterpart (BPM 1022b); B, general view; G, detail of the cephalic region showing possible remains of

appendages (ap?); H, detail of trunk tergites T4–6 (right halves), note their straight transverse outline and hook-shaped pleural spines.

Scale bars represent 5 mm (C–E) and 1 cm (A, B, F–I). (Colour online)

266 PALAEONTOLOGY , VOLUME 58

A B C

D

E

F H

I

G

ORTEGA-HERN �ANDEZ ET AL . : NON-TR ILOB ITE ARTIOPODAN FROM THE WEEKS FORMATION 267

body (especially in BPM 1022b) that we interpret as

cuticular remains (Fig. 1E). EDX analyses reveal that this

material is predominantly composed of C, O and Ca

(with trace amounts of Si and Al), and thus probably

represents calcium carbonate. These results are surpris-

ing, for previous EDX analyses of the exoskeletons of

non-trilobite arthropods from the Weeks Formation

have shown that they are preserved as iron oxides,

sometimes associated with a cover of an undetermined

material rich in O, Si, Al and Mg (Lerosey-Aubril et al.

2014). The only known exception to date is the phos-

phatic cuticle of the aglaspidid-like arthropod Beckwithia

typa Resser, 1931, and the aglaspidid Tremaglaspis

vanroyi Lerosey-Aubril, Ortega-Hern�andez et al., 2013a;

however, it is likely that this biomineralization repre-

sents a particularity of Aglaspidida sensu lato (Lerosey-

Aubril et al. 2013a, b). Furthermore, the trilobites from

the Weeks Formation mostly retain their original calcitic

composition, although many specimens show traces of

diagenetic silicification (Adrain et al. 2009). Considering

these observations, we believe that the new arthropod

probably possessed a lightly biomineralized (calcitic?)

dorsal exoskeleton.

Three-dimensional patches of yellowish, dark blue and

dark red materials present in the medial region of

the body may represent remains of internal organs

A B

F IG . 2 . Falcatamacaris bellua gen. et sp. nov., interpretative drawings of holotype. A, part (BMP 1022a). B, counterpart (BMP

1022b). Abbreviations: af, furrow separating articulating anteromedian protrusion from rest of tergite; ap?, possible remains of append-

ages; ar, articulating anteromedian protrusion; dg?, possible remains of digestive glands; dt?, possible remain of digestive tract; pc,

cuticular areas rich in calcium phosphate (mostly in BPM 1022b); pp, three-dimensional patches of calcium phosphate (mostly in

BPM 1022a); T1 and T11, trunk tergites 1 and 11. Scale bars represent 1 cm. (Colour online)


(Figs 1A–D, 2A–B). EDX analyses suggest that the yellow-

ish (high peaks of C, O and Ca), dark blue (high peaks of

C, O, P and Ca; minor peaks of Si, F and Al) and the

dark red (strong peaks of C, O and Fe; minor peak of Si)

are mostly composed of calcium carbonate, calcium phos-

phate and iron oxides, respectively. The distribution and

significance of these three-dimensional patches are

addressed below. On BPM 1022b (Figs 1B, 2B), the puta-

tive cuticle displays large areas with a dark blue (rather

than light pink) colouration medially, suggestive of a local

enrichment in calcium phosphate (confirmed by EDX

analyses). The change in colour is gradual, and we believe

that this local phosphatization of the cuticle is secondary

and probably related to the early diagenetic phosphatiza-

tion of internal organs, some of which are now repre-

sented by the three-dimensional patches of calcium

phosphate. Lastly, the matrix is predominantly composed

of C, O, Si and Al (with trace amounts of K, Ca and Mg)

according to EDX analyses, and therefore, it is easily

differentiated from the different materials composing the

fossil.

SYSTEMATIC PALAEONTOLOGY

This published work and the nomenclatural acts it con-

tains have been registered in Zoobank: http://zoobank.

org/References/C27275AC-ED4D-43E0-AE49-75C19FB32C95

Phylum ARTHROPODA von Siebold, 1848

Class ARTIOPODA Hou and Bergstr€om, 1997

Remarks. The new taxon is tentatively assigned to the

Artiopoda based on the presence of a broad cephalon

with genal spines and trunk tergites with well-developed

pleurae and pleural spines, which give it a characteristic

dorsoventrally flattened trilobite-like appearance (see Hou

and Bergstr€om 1997, p. 43). Stein and Selden (2012)

recently proposed a more stringent diagnosis that uses

appendicular characteristics as definitive indicators of art-

iopodan affinities; furthermore, they emphasized that

expanded pleurae in the trunk region occur in several

non-artiopodan groups of Palaeozoic arthropods that

occupy diverse phylogenetic positions (e.g. fuxianhuiids,

Hou and Bergstr€om 1997; Yang et al. 2013; leanchoiliids,

Edgecombe et al. 2011; Haug et al. 2012) and should

therefore be regarded as a plesiomorphic feature.

Although we agree with this definition of Artiopoda,

and despite the paucity of appendicular data, we believe

that the combination of characters exhibited by the

new arthropod from the Weeks Formation supports its

phylogenetic position within this group (see detailed

comparisons below).

Genus FALCATAMACARIS gen. nov.

LSID. urn:lsid:zoobank.org:act:4F40D0F6-C285-48A8-8365-

AF67B7261968

Type species. Falcatamacaris bellua sp. nov. (by monotypy).

Derivation of name. From the Latin ‘falcatam’, meaning sickle

shaped, in reference to the morphology of the genal and pleural

spines, and ‘caris’, meaning crab, a suffix commonly used for

marine arthropods.

Diagnosis. Large and weakly biomineralized arthropod

exhibiting the following unique combination of charac-

ters: cephalon wide (tr.), with weakly defined glabellar

region, genal spines inserting laterally, abaxially procurved

posterior margin, and apparently no dorsal eyes or ecdy-

sial sutures; 11 trunk tergites, T1–8 with edge-to-edge

articulations and sickle-shaped pleural spines inserting an-

terolaterally, T9–11 characterized by an abrupt change in

tapering and reduction of the pleural regions into long,

backward-facing pleural spines.

Falcatamacaris bellua sp. nov.

Figures 1–3

LSID. urn:lsid:zoobank.org:act:AC5499E1-B907-453F-A793-

72C44451F59D

Derivation of name. From the latin ‘bellua’, meaning beast or

large animal.

Diagnosis. As for the genus.

Description. Holotype (BPM 1022a, b) consists of a dorsoven-

trally flattened articulated individual with a total length of

126 mm (Fig. 1A–B). Cephalon short (c. 20% total body length;

sag.), but wide (tr.), with length (sag.)/maximum width (tr.) ratio

of c. 0.5. Anterior margin broadly elliptical (Fig. 1F–G). Genalspines insert laterally, approximately at mid-length (exs.) of ceph-

alon, and project posteriorly slightly beyond posterior margin of

head. Posterolateral margin of cephalon possibly opisthocurved

medially, but procurved abaxially, except on a short distance close

to insertion sites of genal spines where it runs laterally only. Sur-

face of cephalon even and apparently devoid of any discrete fea-

tures, such as eyes, furrows or ecdysial sutures. Presence of an

axial region suggested by dark stained area medially, which seems

bilaterally symmetrical (Figs 1F, 2A); this putative axial region is

tongue shaped on its anterior half, but strongly widens (tr.) pos-

teriorly (from approximately a third to a half of maximum cepha-

lic width), and it is separated from anterior cephalic margin by a

distance equivalent to c. 16.5% of cephalic length (sag.). There is

no evidence for the presence of a hypostome on the ventral side.

Trunk composed of 11 tergites (Figs 1A–B, 2A–B). T1 rela-

tively long (approximately a third of sagittal length of ceph-


http://zoobank.org/References/C27275AC-ED4D-43E0-AE49-75C19FB32C95

http://zoobank.org/References/C27275AC-ED4D-43E0-AE49-75C19FB32C95

alon), as wide (tr.) as cephalon, medially protruding above

cephalon, but transverse abaxially; hook-shaped pleural spines,

projecting posteriorly from anterior half of tergite up to mid-

length (exs.) of T2; posterior margin concealed under T2

medially, but otherwise transverse, except for most abaxial portions

that run anterolaterally to form broad notches with posterior

edges of pleural spines. T2–8 essentially similar to T1 (Fig. 1H),

except for moderate and progressive decrease in width (tr.;

c. 75% of maximum width of cephalon only in T8) and gradual

increase in degree of curvature of pleural spines rearwards

(more pronounced sickle shape; Figs 1A–B, 2A–B). T4–8 proba-

bly slightly displaced backwards during flattening of specimen,

resulting in gaps between abaxial parts of tergites; however,

articulations between tergites probably similar from T1 to T8,

consisting of partial overlap of one tergite by an anteromedian

protrusion of the tergite immediately following it (when

observed from the visceral side) and edge-to-edge contacts abax-

ially, as illustrated by T1–T3. T8 slightly differs from more ante-

rior tergites by its longer (sag.) and subtriangular anteromedian

protrusion (Figs 1A, 2A). Narrowing (tr.) of trunk strongly

increasing further backwards, with T9, 10, 11 representing 50%,

35% and 17% of maximum width of the cephalon, respectively

(Figs 1A–B, 2A–B); abaxial portions of these tergites mostly

consist in long pleural spines, which are more and more poste-

riorly directed, with those of T11 being straight and directed

exclusively rearwards (Fig. 1I); there is no evidence of medial

overlap between these tergites; T9–11 account for c. 40% of

total length of trunk. Tailspine not preserved on BPM 1022a, b,

but visible on a specimen held in a private collection (unavail-

able for study); tailspine styliform, approximately twice as long

as the cephalon (sag.), and with bifurcate termination (Fig. 3).

Smooth, light to dark red areas of matrix located close to

the cephalic anterolateral margin may represent the remains of

four appendages (Figs 1F–G, 2A–B). One is located roughly

opposite to abaxial edge of the left posterior portion of the

putative axial region of the cephalon anteriorly. The others

project anterolaterally (the two anteriormost ones) or laterally

(the posteriormost one) from the right side of the cephalon,

suggesting the presence of at least three pairs of stenopodial

cephalic appendages.

The distribution of the calcium carbonate-rich three-dimen-

sional patches on BPM 1022a suggests that they might have

originally represented two pairs of elongate ovoid structures, one

under cephalon/T1 boundary and the other under T1/T2 bound-

ary (Figs 1A–C, 2A–B). One of these patches shows that the cal-

cium carbonate actually surrounds a core of iron oxides, and

this may also be the case for the others. Paired structures in sim-

ilar positions (close to sagittal axis, under tergite boundaries of

the anterior trunk/posterior head regions) have been described

in various Cambrian arthropods and repeatedly interpreted as

fossilized digestive glands (Lerosey-Aubril et al. 2012; Lerosey-

Aubril 2014; Vannier et al. 2014). A similar interpretation is

proposed for the calcium carbonate patches observed on the

specimen described herein based on their taphonomy, position

and overall appearance. A few three-dimensional patches of iron

oxides occur medially and therefore might also represent

remains of internal, possibly digestive organs (Figs 1C–D, 2A–B). However, this is certainly not the case for the patches of iron

oxides associated with pleural spines or appendages. Lastly, the

distribution of the irregularly shaped three-dimensional patches

of calcium phosphate (mostly under adaxial parts of T6–7 on

BPM 1022a; Figs 1D, 2A) does not provide any indication con-

cerning the nature of the structures they represent.

F IG . 3 . Morphological reconstruction of Falcatamacaris bellua

gen. et sp. nov. from the Guzhangian (Cambrian Series 3)

Weeks Formation. Note that the presence of a styliform tailspine

with a bifurcate termination is based on unavailable material

held in a private collection.


Morphological comparisons with Palaeozoic Arthropoda

Cephalic region. A wide (tr.) cephalon with a broadly

elliptical anterolateral margin is a common feature within

Artiopoda (e.g. most aglaspidids, trilobitomorphs and

trilobites; Edgecombe and Ramsk€old 1999), but is also

found in more phylogenetically distant groups (e.g.

xiphosurids; Lamsdell 2013). The overall cephalic shape

of Falcatamacaris gen. nov., however, differs from most

Cambrian arthropods in the presence of an abaxially pro-

curved posterolateral cephalic margin (Fig. 3). Within

Artiopoda, a similar character is also present in represen-

tatives of Conciliterga with freely articulating tergites (e.g.

Helmetia, Kuamaia; see Hou and Bergstr€om 1997; Pater-

son et al. 2012), the aglaspidid-like arthropod Kwanyina-

spis (Zhang and Shu 2005), and also members of the

post-Cambrian group Cheloniellida (Dunlop and Selden

1997). In all these taxa, however, the anteriormost trunk

tergites are anteriorly reflexed abaxially (see Ortega-

Hern�andez et al. 2013, character 50), while in Falcatamac-

aris gen. nov., they have a straight transverse outline;

moreover, almost all of these artiopodans have either

reduced genal spines or these structures are entirely

absent. Only the concilitergan Rhombicalvaria exhibits

well-developed genal spines (Hou 1987), which insert lat-

erally on the cephalon, similar to the new genus. A some-

what comparable condition can also be observed in

various trilobite orders (Redlichiida, e.g. Whittington

et al. 1997; Ptychopariida, e.g. Clarkson et al. 2004;

Odontopleurida, e.g. Whittington and Bohlin 1958). A

differentiated axial region on the cephalon is a widespread

feature among Palaeozoic arthropods (see Ortega-Hern�an-

dez et al. 2013, character 35), as best exemplified by the

glabella of trilobites. However, the weak expression of this

axial region in Falcatamacaris gen. nov. indicates that it

was not delimited by furrows as in trilobites, and unlike

the subtriangular glabellar region of many aglaspidids

(Hesselbo 1992; Ortega-Hern�andez et al. 2010), it is ton-

gue-shaped anteriorly. The lack of dorsal eyes in BPM

1022 offers two possibilities: either these were ventral (e.g.

Nektaspida, Conciliterga, Leanchoiliidae) and not pre-

served in this specimen, or they were genuinely absent/

secondarily lost (e.g. Squamacula, Tremaglaspis). Without

additional data on the ventral morphology of Falcatamac-

aris gen. nov., it is not possible to decide between these

hypotheses. However, it should be noted that there are no

raised bulges that could accommodate a pair of ventral

eyes on the cephalon of Falcatamacaris gen. nov., as it is

typically observed in concilitergans (Hou and Bergstr€om

1997; Edgecombe and Ramsk€old 1999; Paterson et al.

2012). Finally, the absence of dorsal ecdysial sutures is

shared with all non-trilobite Palaeozoic arthropods

(Cotton and Braddy 2004, characters 28 and 29).

‘Anterior’ trunk region. A trunk composed of 11 freely

articulating tergites has been described in various Palaeo-

zoic arthropods. Among non-artiopodans, for instance,

this feature characterizes the megacheiran family Leanch-

oiliidae (Edgecombe et al. 2011; Haug et al. 2012) and

has even been considered as the probable ground pattern

of Xiphosura within Euchelicerata (Lamsdell 2013). In

Artiopoda, however, the presence of 11 trunk tergites is

more typical of the Aglaspidida sensu stricto (Van Roy

2006; Ortega-Hern�andez et al. 2013), being observed in

all but one species of this group (Fortey and Rushton

2009; see also Lerosey-Aubril et al. 2013a). Eleven free

trunk tergites are also found in the artiopodan Squamacula

buckorum Paterson, Garc�ıa-Bellido and Edgecombe, 2012,

and in the problematic Cambrian arthropod Nettapezoura

basilikos Briggs, Lieberman et al., 2008; however, unlike

Falcatamacaris gen. nov. and many aglaspidids, the latter

two species do not possess well-developed pleural spines.

The sickle-shaped pleural spines of the anterior tergites

(T1–8) in Falcatamacaris gen. nov. are particularly dis-

tinctive in that they project from the anterior portion of

each tergite (Fig. 3). This contrasts with the condition

observed in other artiopodans, such as trilobites, aglaspid-

ids or concilitergans (e.g. Rhombicalvaria Hou, 1987),

where the pleural spines normally extend either from the

entire pleurae or from their posterior portions. To our

knowledge, only a few trilobites (e.g. some Ctenopyge;

Clarkson et al. 2004, figs 17A, 19) exhibit a type of inser-

tion of pleural spines approximating that observed in the

new genus. Tergites of the anterior trunk of Falcatamac-

aris gen. nov. are provided with an anterior medial pro-

trusion apparently separated by a transverse furrow from

the main part of the tergite, which recalls the articulating

half-ring of trilobites. Articulation between two of these

tergites involves the overlap of this structure by the ter-

gite in front, while little- to no-overlap between tergites

occurs abaxially (Fig. 3). This organization deviates from

the typically wide tergite overlap found in most Palaeozo-

ic arthropods, but closely resembles the ‘edge-to-edge’

articulation defined by Edgecombe and Ramsk€old (1999,

character 18), which is primarily observed in the trunk of

concilitergans and trilobites, and to a lesser degree also in

some aglaspidids and aglaspidid-like arthropods (see Ort-

ega-Hern�andez et al. 2013, character 44). Among non-art-

iopodan taxa, a similar anterior extension of the medial

region of the trunk tergites is present in the synziphosu-

rine Willwerathia laticeps Størmer, 1969 (see Anderson

et al. 1998), and further developed into a pseudo-half

ring in the opisthosomal articulation of Xiphosura (see

Lamsdell 2013, fig. 9).

‘Posterior’ trunk region. The posterior exoskeletal mor-

phology of Falcatamacaris gen. nov. is uncommon among


Palaeozoic arthropods; the three posteriormost trunk

tergites (T9–T11) differ significantly from the anterior

ones, being narrower (tr.), longer (sag.) and having ‘pleu-

ral’ regions mostly consist of elongated spines (Fig. 3).

The absence of a pygidium (i.e. an ensemble of fused pos-

terior trunk tergites) is a widespread character within Art-

iopoda; however, in most species, the posterior trunk

shows a gradual change in the morphology of tergites,

which become narrower (tr.) and increasingly curved

rearwards (Hou and Bergstr€om 1997; Edgecombe and

Ramsk€old 1999). The notable and rather abrupt change

in the posterior tergite morphology of Falcatamacaris gen.

nov. has no direct equivalent among Artiopoda and prob-

ably represents a derived condition. There are, however,

some Cambrian arthropods that seem to show a superfi-

cial similarity to the posterior differentiation of Falcata-

macaris gen. nov. For example, the last three trunk

tergites in N. basilikos and Dicranocaris guntherorum

Briggs et al., 2008, are significantly narrower (tr.) than

the anterior ones (Briggs et al. 2008; Hendricks and

Lieberman 2008); the last trunk tergite of D. guntherorum

also bears a pair of well-developed posterior spines

(Briggs et al. 2008, fig. 5), as in Falcatamacaris gen. nov.

The xenopod Sidneyia inexpectans Walcott, 1911, also fea-

tures a discrete anteroposterior trunk differentiation; this

consists of anterior tergites with well-developed pleural

regions and much narrower (tr.) and cylindrical posterior

tergites (see discussion in Ortega-Hern�andez et al. 2013,

character 60). The recently described arthropod Notchia

weugi Lerosey-Aubril, 2014, is also typified by a morpho-

logically distinct posterior trunk region composed of four

tergites that are straight (tr.), rather than abaxially curved

backwards like the anteriormost ones.

Outside of Artiopoda, the peculiar morphology of the

trunk of Falcatamacaris gen. nov. recalls that of the syn-

ziphosurine W. laticeps (see Anderson et al. 1998, fig. 8).

In both taxa, the anterior trunk tergites are characterized

by straight transverse outlines, while the three posterior-

most ones are increasingly curved backwards. However,

the trunk of W. laticeps is composed of ten tergites only,

including a reduced anteriormost one, and its cephalic

shield differs significantly from that of the new genus.

The presence of a long tailspine is particularly common

among Palaeozoic arthropods (e.g. aglaspidids, Emeraldella,

xiphosurids; see Ortega-Hern�andez et al. 2013, character

70); however, a bifurcate termination is only known

in the aglaspidid Glypharthrus simplex (Raasch, 1939;

Hesselbo 1992, figs 11.1, 11.4) and the problematic

artiopodan Habelia optata (Whittington 1981, fig. 61).

Internal structures. The putative paired midgut glands in

Falcatamacaris gen. nov. draw comparisons with similar

structures in various Palaeozoic arthropods. Although the

preservation of BMP 1022 does not allow the fine details

of the morphology to be observed (e.g. radiating ridges

and grooves; Butterfield 2002), the paired arrangement of

the midgut glands is similar to that observed in various

trilobite species (Lerosey-Aubril et al. 2011, 2012; Fatka

et al. 2013) and non-trilobite artiopodans (e.g. Chen

et al. 1997; Lagebro et al. 2009; Paterson et al. 2012), par-

ticularly in their position under cephalon/T1 and T1/T2

articulations (Ortega-Hern�andez and Brena 2012).

DISCUSSION

Phylogenetic affinities

The affinities of Falcatamacaris gen. nov. are problematic

due to the scarcity of clear synapomorphic characters

shared with particular clades within Artiopoda (Fig. 4).

However, given the absence of fused body regions into

distinct tagmata (or at least pseudotagmata) and lack of

discrete cephalic structures, it is possible to reject close

affinities with phylogenetically derived groups of Palaeo-

zoic arthropods, such as euchelicerates. The overall mor-

phology of Falcatamacaris gen. nov. may suggest an

association with the Aglaspidida sensu lato (cf. Van Roy

2006), a poorly constrained group composed of the

Aglaspidida sensu stricto and a number of closely related

aglaspidid-like arthropods (Ortega-Hern�andez et al. 2013).

The lack of diagnostic aglaspidid features in Falcatam-

acaris gen. nov. (i.e. postventral plates, anterior tergal

processes), however, rule out any affinities with aglaspid-

ids sensu stricto. A position close to trilobitomorph

arthropods cannot be entirely discarded either, as

Falcatamacaris gen. nov. shares various characters (e.g.

advanced genal spines, pleural spinosity, tergite articula-

tion) with the concilitergan Rhombicalvaria (Hou 1987;

see also Edgecombe and Ramsk€old 1999). Despite the lack

of a fused pygidium and anteriorly flexed trunk tergites,

Falcatamacaris gen. nov. could potentially occupy a posi-

tion within Conciliterga, as the patterns of trunk tagmosis

can be extremely variable even within monophyletic trilo-

bitomorph clades (e.g. Australimicola in Conciliterga, see

Paterson et al. 2012; Luohuilinella in Xandarellida, see

Zhang et al. 2012). A third possibility is that Falcatam-

acaris gen. nov. may occupy an unresolved basal position

within Artiopoda as a whole, similar to other problematic

representatives such as Retifacies and Squamacula (see

Paterson et al. 2012; Ortega-Hern�andez et al. 2013; Stein

et al. 2013). The evolutionary significance of the position

of the gut diverticulae under the cephalon/T1 and T1/T2

articulations in the new arthropod is somewhat inconclu-

sive. Ortega-Hern�andez and Brena (2012) argued that

such organization in trilobites and nektaspidids is indica-

tive of a derived mode of trunk segmentation within Artio-

poda. However, a recent study by Vannier et al. (2014)


clearly showed that the gut diverticulae of lobopodians in

the stem-lineage of Euarthropoda (e.g. Jianshanopodia,

Pambdelurion) also occupies a similar inter-segmental

position that could suggest a plesiomorphic state. This

issue remains unresolved until more information on the

digestive structures of extinct Panarthropoda becomes

available. Finally, the weakly biomineralized cuticle of

Falcatamacaris gen. nov. is uncommon for non-trilobite

artiopodans and carries an uncertain phylogenetic signal.

The phosphatic cuticle of some aglaspidids (Raasch 1939;

Briggs and Fortey 1982; Hesselbo 1992; Lerosey-Aubril

et al. 2013a) or aglaspidid-like arthropods (Waggoner

2003) is arguably the best-known example of biomineral-

ization outside of Trilobita; a similar exoskeletal composi-

tion, however, is also known in the enigmatic arthropod

Phytophilaspis from the lower Cambrian of Siberia

(Ivanstov 1999; Lin et al. 2011). To our knowledge, the

only report of a calcitic cuticle in a non-trilobite artiopo-

dan concerns an undescribed aglaspidid from the Ordovi-

cian of Shanxi Province in China (Fortey and Theron

1994; Ortega-Hern�andez et al. 2013), even though no

direct evidence supporting this claim was provided

(Lerosey-Aubril et al. 2013a).

The non-trilobite arthropod fauna from the Weeks

Formation

The Weeks Formation has yielded a particularly diverse

trilobite fauna composed of more than 35 species, most

of which occur in the upper part of the formation where

exceptionally preserved fossils are found. Examination of

some 150 specimens of non-trilobite arthropod specimens

from the Weeks Formation suggests that they may also

constitute a rather diverse assemblage (c. 15 species).

However, only six species have been described to date:

Beckwithia typa Resser, 1931 (see also Raasch 1939;

Hesselbo 1989), Anabarochilina australis (Hinz-Schallre-

uter, 1993; see also Siveter and Williams 1997), Trema-

glaspis vanroyi Lerosey-Aubril, Ortega-Hern�andez et al.,

2013a, Anomalocaris aff. canadensis (Lerosey-Aubril et al.

2014), Notchia weugi Lerosey-Aubril, 2014 and now

F IG . 4 . Possible phylogenetic positions of Falcatamacaris bellua gen. et sp. nov. within Artiopoda; topology sensu Ortega-Hern�andez

et al. 2013, fig. 6b.


Falcatamacaris bellua gen. et sp. nov. All of these discov-

eries have resulted in new information about the palaeo-

geographical and stratigraphical distribution of these taxa.

For instance, the bradoriid A. australis is mostly known

from the lower Guzhangian, but its stratigraphical range

extents from the Cambrian Stage 5 to the upper Guzhan-

gian (the youngest occurrence being in the Weeks

Formation; Collette et al. 2011). This arthropod had a

worldwide palaeogeographical distribution, although pos-

sibly restricted to low latitudes, which suggests a pelagic

lifestyle (Collette et al. 2011). Anomalocaris aff. canadensis

has been recently described from isolated frontal append-

ages, which all exhibited particularly small sizes (< 25 mm

in length) compared to previously described frontal

appendages of Anomalocaris species. According to Lero-

sey-Aubril et al. (2014), they belong to a new species of

Anomalocaris phylogenetically close to A. canadensis

Whiteaves, 1892. This latter species is only known with

certainty from the Cambrian Stage 4 (e.g. Kinzers Forma-

tion, Latham Shale) to the Cambrian stage 5 (e.g. Burgess

Shale Formation) of Laurentia, but the genus Anomalo-

caris extends from the Cambrian Stage 3 to the upper

Guzhangian (the youngest occurrence being in the Weeks

Formation), with known occurrences in China (Hou et al.

1995), Australia (Daley et al. 2013) and North America

(Daley and Edgecombe 2014). Until its discovery in the

Cambrian of Utah, the aglaspidid Tremaglaspis was

regarded as a typically Ordovician arthropod, having been

reported from the Tremadocian of Wales (Fortey and

Rushton 2003, 2009) and Morocco (Van Roy et al. 2010)

exclusively. The description of Tremaglaspis vanroyi in the

upper Weeks Formation has extended the stratigraphical

range of the genus back to the Guzhangian and its palaeo-

geographical distribution to Laurentia (Lerosey-Aubril

et al. 2013a). It also illustrated that the Weeks Formation

fauna might provide important insights into the transi-

tion between Cambrian and Palaeozoic Evolutionary

Faunas (Sepkoski 1981).

Unlike Anabarochilina, Anomalocaris and Tremaglaspis,

Beckwithia and Notchia have limited stratigraphical and

palaeogeographical ranges. Beckwithia typa is only

confidently known from the upper part of the Weeks

Formation, where it represents one of the most common

non-trilobite arthropods. Beckwithia? major (Graham,

1931), the only other representative of the genus, was

described from disarticulated and fragmentary sclerites

from the Eau Claire Formation in Wisconsin (Graham

1931; Raasch 1939). However, this material does not per-

mit to distinguish B.? major from B. typa (Hesselbo 1989)

and therefore more likely documents a second occurrence

of B. typa, especially because the Eau Claire Formation and

the Weeks Formation are subcontemporaneous (Babcock

et al. 2014). Beckwithia strongly resembles Aglaspidida

sensu stricto, but a recent phylogenetic analysis has revealed

that it is close to, but definitely outside this clade (Ortega-

Hern�andez et al. 2013). The affinities of Notchia are more

problematical. Only known from the upper Weeks Forma-

tion, this arthropod exhibits a short (sag.) cephalon and a

non-styliform tailspine and, in this regard, somewhat

resembles other Cambrian arthropods, such as Paleomerus,

Strabops or Sidneyia (Lerosey-Aubril 2014). However, care-

ful examination of the only available specimen revealed

many differences compared to these three taxa, which led

Lerosey-Aubril (2014) to question a close phylogenetic

relationship between Notchia and any of them. The

description of Falcatamacaris gen. nov. further exemplifies

the uniqueness of the non-trilobite arthropod fauna from

the Weeks Formation and how it could contribute to a

potentially significant reassessment of the disparity of these

organisms in the late Cambrian.

Acknowledgements. JOH is supported by a Research Fellowship

in Emmanuel College, University of Cambridge (UK). This is a

contribution of the ANR project RALI 197 ‘Rise of Animal Life

(Cambrian–Ordovician) – organization and tempo: evidence

from exceptionally preserved biota’. We thank Jeremy Skepper

(University of Cambridge) for assistance with elemental analysis

of the material and James Lamsdell (Yale University) for discus-

sion on Willwerathia laticeps.

Editor. Xi-Guang Zhang

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A rare non-trilobite artiopodan from the Guzhangian (Cambrian Series 3) Weeks Formation...

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Transcript of A rare non-trilobite artiopodan from the Guzhangian (Cambrian Series 3) Weeks Formation...