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| Thulaspis tholops Berks, Nielsen, Flannery-Sutherland, Nielsen, Park & Vinther, 2023 DOI: 10.1002/spp2.1495 |
Abstract
Artiopoda was a diverse group of Palaeozoic euarthropods that proliferated in the early Palaeozoic, epitomized by the ubiquitous trilobites. Their possible phylogenetic position outside mandibulates and chelicerates offers the potential for understanding the evolution of euarthropods in more detail. However, this opportunity remains unexploited given that identification of deep-splitting artiopodans remains to be fully explored. Here, we describe a new non-trilobite artiopodan from the lower Cambrian Sirius Passet Lagerstätte, North Greenland. Thulaspis tholops gen. et sp. nov. is a large species with a broad, domed head shield, followed by a trunk consisting of 15 thoracic tergites and a small pygidium, giving the body an ovoid appearance when viewed dorsally. Thulaspis is distinctive with its rounded genae and anterior thoracic pleural tips, as well as short pleural spines posteriorly. A heart-shaped hypostome with an anterior lobe is present. Appendages, partly obscured by the tergal skeleton, have many moderate length gnathobasic spines, and large flap-like exopods with a fringe of small setae. Cladistic analyses recover Thulaspis as the sister taxon to Squamacula, a genus found in the Chengjiang and Emu Bay Shale biotas, in either a polytomy with a number of artiopodan taxa or as a sister group to all other artiopodans, indicating an important role in understanding the roots of artiopodan anatomy and evolution.
Keywords: Artiopoda, Euarthropoda, Sirius Passet Lagerstätte
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| Thulaspis tholops gen. et sp. nov. from the early Cambrian (Series 2, Stage 3) Sirius Passet Lagerstätte (North Greenland). MGUH 34172a, holotype. A, photograph. B, interpretive camera lucida drawing of A. Abbreviations: capp, cephalic appendage; gud, gut diverticulum; hs, head shield; hyp, hypostome; ttg, thoracic tergite. Scale bars represent 20 mm. |
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| Thulaspis tholops gen. et sp. nov. specimen showing phosphatized gut (MGUH 34175a). A, polynomial texture mapping (PTM) image using Static Multi Light rendering mode; outlined area enlarged in C. B, interpretive camera lucida drawing of A with orange colour representing the preserved gut and grey representing diagenetic trace fossils. C, alimentary canal with widened structure associated with head shield interpreted as a crop. Abbreviations: ac, alimentary canal; cr, crop; hs, head shield; ttg, thoracic tergite. Scale bars represent 10 mm. |
SYSTEMATIC PALAEONTOLOGY
EUARTHROPODA Lankester, 1904
ARTIOPODA Hou & Bergström, 1997
Genus THULASPIS nov.
Type species: Thulaspis tholops from the lower Cambrian (Series 2, Stage 3) Sirius Passet Lagerstätte, Buen Formation, Peary Land, North Greenland.
Derivation of name: Greek: Thule, the term given to the northernmost lands, and aspis, shield; with the intended meaning of ‘shield of the northernmost lands’.
Thulaspis tholops sp. nov.
Derivation of name: Greek: thólos, dome, and ops, face; in reference to the reconstructed domed anterior of the head shield.
Diagnosis: Non-mineralized arthropod with ovate outline and 15 thoracic segments. Wide, semi-circular head shield about one-quarter the total length of the animal with small rounded genae. Anteriormost tergite bears slightly rounded spatulate pleural tips while posterior pleura increasingly curve backwards into short spines. Anteromedial expansions are present on the first five thoracic tergites and reduce in sagittal length distally. Tergites have substantial overlap axially and near the pleural tips. Pygidium is small and smooth, one-thirteenth of the total length and one-fifth of maximum width. Biramous limbs have broad, flap-like exopods with marginal setae.
Occurrence: Lower Cambrian (Series 2, Stage 3) Sirius Passet Lagerstätte, Buen Formation, Peary Land, North Greenland.
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| Fragmentary specimen of Thulaspis tholops gen. et sp. nov. showing large exopods (MGUH 34177) A, photograph of specimen with fragmented parts placed together; outlined areas enlarged in B, D–E. B, fragment with exposed exopods bearing setae fringes (white arrows). C, interpretive camera lucida drawing of B. D–E, setae fringes on exopods. Abbreviations: en, endopods; ex, exopod; hs, head shield; ttg, thoracic tergite. Scale bars represent: 10 mm (A–C); 5 mm (D, E). |
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| Reconstruction of Thulaspis tholops gen. et sp. nov., showing post-antennal appendages using the appendages of Squamacula clypeata to help infer attachment of endopods and exopods to basipods (Zhang et al. 2004; Ortega-Hernández et al. 2013): A, dorsal view; B, ventral view. |
CONCLUSIONS:
• We describe Thulaspis tholops, a new genus and species of Cambrian arthropod from the Cambrian Sirius Passet Lagerstätte in North Greenland, thereby expanding the known diversity of this fauna.
• Thulaspis has a broad head, 15 thoracic segments and a small tail shield with biramous appendages bearing gnathobasic protopodites and large flap-like exopods with a margin of small setae.
• We uncover a close relationship between Thulaspis and the enigmatic artiopodan Squamacula and find that these taxa are likely to branch deeply within Artiopoda.
Harry O. Berks, Morten Lunde Nielsen, Joseph Flannery-Sutherland, Arne Thorshøj Nielsen, Tae-Yoon S. Park and Jakob Vinther. 2023. A possibly deep branching artiopodan Arthropod from the lower Cambrian Sirius Passet Lagerstätte (North Greenland). Papers in Palaeontology Papers. 9(3); e1495. DOI: 10.1002/spp2.1495
