The Vicissicaudata revisited - insights from a new aglaspidid arthropod with caudal appendages from the Furongian of China

Abstract

Cambrian marine ecosystems were dominated by arthropods, and more specifically artiopods. Aglaspidids represent an atypical group amongst them, not the least because they evolved and rapidly diversified during the late Cambrian, a time interval between the two diversification events of the Early Palaeozoic. Recent phylogenetic analyses have retrieved aglaspidids within the Vicissicaudata, a potentially important, but difficult to define clade of artiopods. Here we describe a new aglaspidid from the Furongian Guole Konservat-Lagerstätte of South China. This taxon displays a pretelsonic segment bearing non-walking appendages, features as-yet known in all vicissicaudatans, but aglaspidids. A new comprehensive phylogenetic analysis provides strong support for the legitimacy of a monophyletic clade Vicissicaudata, and demonstrates the pertinence of new characters to define Aglaspidida. It also motivates important changes to the systematics of the phylum, including the elevation of Artiopoda to the rank of subphylum, and the establishment of a new superclass Vicissicaudata and a new aglaspidid family Tremaglaspididae. Two diversification pulses can be recognized in the early history of artiopods - one in the early Cambrian (trilobitomorphs) and the other in the late Cambrian (vicissicaudatans). The discrepancy between this pattern and that traditionally depicted for marine invertebrates in the Early Palaeozoic is discussed.

Figure 1

Stratigraphic distribution, palaeobiogeography, and diversity of aglaspidids and other artiopods in the Lower Palaeozoic. Most aglaspidid genera (on the left) are known from Jiangshanian strata of the Upper Mississippi Valley (Laurentia). Quasimodaspis brentsae Waggoner is not represented on this figure due to uncertainties about its age. Aglaspidids are atypical amongst artiopods (on the right), for they first evolved and rapidly diversified during the time interval in-between the Cambrian Explosion and the Great Ordovician Biodiversification Event (GOBE).

Figure 2

Location and depositional environment of the Jiangshanian strata of the Sandu Formation exposed in the Guole area, Guangxi, South China. (a) location of Guole area, c. 40 km N of the Vietnamese border. Map created using Adobe Photoshop CS6 (http://www.adobe.com/products/photoshop.html). (b) late Cambrian lithofacies belts in SE China (modified after ref. 39); note the location of Guole within the Jiangnan area (slope lithofacies belt). (c) depositional environments associated with the upper Cambrian lithofacies belts of SE China; note that the Jiangshanian strata in the Guole area were probably deposited in the uppermost part of the continental slope.

Figure 3

Glypharthrus trispinicaudatus sp. nov. from the Jiangshanian (Furongian) Sandu Formation, Guole area, Guangxi, South China. All specimens are in dorsal view with anterior facing to the top. (a,b,d,e) NIGPAS 165042 (holotype), part and counterpart of an almost complete, articulated dorsal exoskeleton, photographed immersed in dilute ethanol. (a,d,e) counterpart (mostly an external mould). (a) general view. (d,e) detail of posterior trunk region, showing pretelsonic segment bearing a pair of furcal rami (small arrows) and a spiniform telson (large arrow). (b) part (dorsal exoskeleton; mirrored). (c,f) NIGPAS 165043, almost complete, largely articulated dorsal exoskeleton, photographed dry. (c) general view. (f) detail of cephalon showing the eye abutting the glabellar region (small arrow) and merging with the rest of the exoskeleton anteriomedially (large arrow), and the pitted surface. (g,h,i) NIGPAS 165044, part and counterpart of an almost complete, largely articulated dorsal exoskeleton, photographed dry (g,i) or immersed under dilute ethanol (h). (g) counterpart (mirrored). (h) part, detail of T7, showing the articulating ridge separating the articulating platform (arrow) from the posterior part of the tergite. (i) part, general view.

Figure 4

Glypharthrus trispinicaudatus sp. nov. from the Jiangshanian (Furongian) Sandu Formation, Guole area, Guangxi, South China. (a–e) interpretative drawings. (a,b) part and counterpart of NIGPAS 165042 (holotype). (c) NIGPAS 165043. (d,e) part and counterpart of NIGPAS 165044. Odd and even numbered trunk tergites are in light grey and dark grey, respectively, to facilitate their identification. Abbreviations: ar, articulating ridge; c, cephalon; e, eye; gr, glabellar region; mr, marginal rim; pbf, posterior border furrow; plf, trunk pleural furrow; pta, pre-telsonic appendages (furcal rami); pts, pre-telsonic segment; T(1, 2, 5, 8 or 11), trunk tergite (1, 2, 5, 8 or 11); t, telson.

Figure 5

Morphological reconstruction of Glypharthrus trispinicaudatus sp. nov.

Figure 6

Strict consensus of six most parsimonious trees under equal weight (EW) parsimony. 252 steps, CI = 0.435, RI = 0.733. Character matrix available as Supplementary Data.

Figure 7

Consensus tree resulting from Bayesian analysis in MrBayes. Setting: Mk model, four runs, 20,000,000 generations, 1/1000 sampling resulting in 20,000 samples, 25% burn-in resulting in 15,000 samples retained. Relationships between non-artiopodan arthropods, and within Cheloniellida, Conciliterga, Nektaspidida, and Trilobita were resolved as in the EW-parsimony analysis (Fig. 6). Character matrix available as Supplementary Data.

Figure 8

Morphology and evolution of the posterior trunk region within the Vicissicaudata. (a) morphology of the posterior trunk regions of aglaspidids (ancestral condition), Emeraldella, and cheloniellids; a morphologically-distinct, pretelsonic segment bearing appendage derivatives (in yellow) occurs in front of the telson (in pink) in all these taxa. (b–d) evolutionary scenarios describing the fusion of the pretelsonic segment and telson into a tailspine, and the relationship between pretelsonic appendages (furcal rami) and postventral plate (pvp) in aglaspidids. (b) aglaspidid ancestor possessed pretelsonic appendages (in yellow) and postventral plate (in purple), but the former structures disappeared in most descendants. (c) aglaspidid ancestor possessed pretelsonic appendages only (in yellow), which were lost in most descendants – the postventral plate (in purple) evolved independently. (d) aglaspidid ancestor possessed pretelsonic appendages only, which later evolved into the postventral plate in most descendants (both structures are in yellow).

Figure 9

Diversity of shape, size, and location of the eyes in aglaspidids and Eozetetes. The eyes abut the typically triangular glabellar region anteriorly, and gradually merge with the rest of the cephalon anteromedially; when occupying a more posterior position, they are prolonged anteriomedially by massive ridges. A, Eozetetes gemelli (SAM P48369a – Cambrian Series 2, Stage 4; Emu Bay Sh., Australia), reinterpreted as showing an eye abutting a triangular glabellar region (arrow heads) anteriorly, courtesy of J. Paterson; B, Aglaspis spinifer (USNM 98912 – Furongian, Jiangshanian; St Lawrence Formation, USA); C, Aglaspella sanduensis (NIGPAS 157028 – Furongian, Jiangshanian; Sandu Formation, China); D, Glypharthrus magnoculus (UA 14332 – Furongian, Jiangshanian; McKay Gr., Canada); E, Chraspedops modesta (MPM no number – Furongian, Jiangshanian; St Lawrence Formation, USA); F, Beckwithia typa (BPM 1060 – Cambrian Series 3, Guzhangian; Weeks Formation, USA); G, Glypharthrus trispinicaudatus (NIGPAS 165042 – Furongian, Jiangshanian; Sandu Formation, China); H, Gogglops ensifera (NIGPAS 160539 – Upper Ordovician, Sandbian; Yaoxian Formation, China); I, Aglaspoides sculptilis (MPM 11168 – Furongian, Jiangshanian; St Lawrence Formation, USA); J, Glypharthrus thomasi (MPM 11163 – Furongian, Jiangshanian; St Lawrence Formation, USA).