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

Abstract

BackgroundExceptionally 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.ResultsArthroaspis bergstroemi n. gen. n. sp., a new arthropod from the early Cambrian Sirius Passet Lagerstätte 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.ConclusionsThe 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 homoplastic character shown to be achieved by several groups independently.

The Emu Bay Shale Konservat-Lagerstätte: a view of Cambrian life from East Gondwana

“…Another blind arthropod in the EBS, Australimicola spriggi (Fig. 3c), was originally classified in a lamellipedian group named Conciliterga, with which it shares its style of hypostome attachment , but other phylogenetic analyses have grouped it with Cheloniellida, known from various species in the Ordovician, Silurian and Devonian (Stein et al 2013). …”

Molaria (Euarthropoda) from the Sirius Passet Lagerstätte (Cambrian Series 2, Stage 3) of North Greenland

“…On account of the long telson, specimens from the Sirius Passet Lagerstätte assigned to Molaria steini are unlikely to be confused with contemporaneous arthropod taxa (Blaker 1988;Budd 1995Budd , 1999Budd , 2011Babcock & Peel 2007;Lagebro et al 2009;Stein et al 2013;Peel 2017). Buenaspis Budd, 1999 is of B A similar size, and its head shield and tergites are similar to the anterior portion of M. steini, not least with regard to the concentric crushing pattern of the flattened head shields.…”

“…Fossil arthropods from the Sirius Passet Lagerstätte vary in length of the shield exclusive antennulae from 5 mm to more than 210 mm in Arthroaspis bergstroemi Stein, Budd, Peel & Harper, 2013. Molaria steini is one of the smallest species, with the shield attaining a length of 20 mm, to which may be added a 10 mm long telson.…”

“…The preservational and diagenetic history of the Sirius Passet Lagerstätte is complex (Budd 1997(Budd , 1998(Budd , 2011Conway Morris & Peel 2008;Peel et al 2013;Stein et al 2013;Vannier et al 2014;Strang et al 2016a, b;Peel 2017) and excellent preservation was not extended equally to all. Each of the 10-15 arthropod taxa currently known has its own preservational history, with internal axial structures and distal portions of antennulae and other limbs particularly subject to variable preservation.…”

“…Legg et al (2013) grouped Molaria with Emeraldella within Xenopoda. Edgecombe et al (2017) compared the phylogenetic analyses of Stein et al (2013) and Ortega-Hernández et al (2013) in describing a new early Cambrian aglaspid-like arthropod from South Australia.…”