Arthroaspis n. gen., a common element of the Sirius Passet Lagerstatte (Cambrian, North Greenland), sheds light on trilobite ancestry

• Published in: BMC Evolutionary Biology
• Main Authors: Stein, Martin, Budd, Graham E., Peel, John Stuart, Harper, David A. T.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Paleobiologi 2013
• Subjects: Sirius Passet Lagerstatte; Arthropoda; Lamellipedia; Trilobita; Appendages; Pygidium; Functional morphology; Natural Sciences; Naturvetenskap; Greenland; Sirius; North Greenland
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-203302; https://doi.org/10.1186/1471-2148-13-99

Background: Exceptionally preserved Palaeozoic faunas have yielded a plethora of trilobite-like arthropods, often referred to as lamellipedians. Among these, Artiopoda is supposed to contain taxa united by a distinctive appendage structure. This includes several well supported groups, Helmetiida, Nektaspida, and Trilobita, as well as a number of problematic taxa. Interrelationships remain unclear, and the position of the lamellipedian arthropods as a whole also remains the subject of debate. Results: Arthroaspis bergstroemi n. gen. n. sp., a new arthropod from the early Cambrian Sirius Passet Lagerstatte of North Greenland shows a striking combination of both dorsal and ventral characters of Helmetiida, Nektaspida, and Trilobita. Cladistic analysis with a broad taxon sampling of predominantly early Palaeozoic arthropods yields a monophyletic Lamellipedia as sister taxon to the Crustacea or Tetraconata. Artiopoda is resolved as paraphyletic, giving rise to the Marrellomorpha. Within Lamellipedia, a clade of pygidium bearing taxa is resolved that can be shown to have a broadly helmetiid-like tergite morphology in its ground pattern. This morphology is plesiomorphically retained in Helmetiida and in Arthroaspis, which falls basally into a clade containing Trilobita. The trilobite appendages, though similar to those of other lamellipedians in gross morphology, have a unique outward rotation of the anterior trunk appendages, resulting in a 'hard wired' lateral splay, different to that observed in other Lamellipedia. Conclusions: The combination of helmetiid, trilobite, and nektaspid characters in Arthroaspis gives important hints concerning character polarisation within the trilobite-like arthropods. The distinctive tergite morphology of trilobites, with its sophisticated articulating devices, is derived from flanged edge-to-edge articulating tergites forming a shield similar to the helmetiids, previously considered autapomorphic for that group. The stereotypical lateral splay of the appendages of lamellipedians is a ...