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.
Highlights
Preserved Palaeozoic faunas have yielded a plethora of trilobite-like arthropods, often referred to as lamellipedians
We describe a new exceptionally-preserved lamellipedian arthropod from the Sirius Passet Lagerstätte that adds significant new data to the debate, and that has a bearing on the ecology of these early arthropods
The early trilobites, in contrast to taxa with the plesiomorphic articulation type, could enrol, albeit not encapsulated, and it is possible that the initial loss of the large pygidium in Trilobita is linked to the ability of incipient enrolment, potentially coupled with biomineralization in order to strengthen the tergites for protection
Summary
Preserved Palaeozoic faunas have yielded a plethora of trilobite-like arthropods, often referred to as lamellipedians. 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. 1970s, their unfamiliarity prompted the view that they represented the end points of many independent lines of evolution that had convergently undergone arthropodisation [1]. This view was relatively short-lived in the advent of rigorous cladistic analysis [2,3], and the essential similarity between many of these taxa began to be recognised [4]. An important exception to this view were the arthropods of the Orsten fauna, some of which have been shown convincingly to lie in the stem- and crown-group of Crustacea or Tetraconata [6,7,8,9,10]
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