The phylogeny of aglaspidid arthropods and the internal relationships within Artiopoda

Abstract

The phylogenetic position of aglaspidids, a problematic group of Lower Palaeozoic arthropods of undetermined affinities, is re-examined in the context of the major Cambrian and Ordovician lamellipedian arthropod groups. A cladistic analysis of ten genera of aglaspidids sensu stricto, six aglaspidid-like arthropods and 42 Palaeozoic arthropod taxa indicates that Xenopoda, Cheloniellida, Aglaspidida sensu lato and Trilobitomorpha form a clade (Artiopoda Hou and Bergstro¨m, 1997) nested within the mandibulate stem-lineage, thus discarding previous interpretations of these taxa as part 'of the chelicerate stem-group (Arachnomorpha Heider, 1913). The results confirm an aglaspidid identity for several recently described arthropods, including Quasimodaspis brentsae, Tremaglaspis unite, Chlupacaris dubia, Australaglaspis stonyensis and an unnamed Ordovician Chinese arthropod. The problematic Bohemian arthropod Kodymirus vagans was recovered as sister taxon to Beckwithia typa, and both form a small clade that falls outside Aglaspidida sensu stricto, thus discarding eurypterid affinities for the former. The analysis does not support the phylogenetic position of Kwanyinaspis maotianshanensis at the base of Conciliterga as proposed in recent studies, but rather occupies a basal position within Aglaspidida sensu lato. The results indicate a close association of aglaspidid arthropods with xenopods (i.e. Emeraldella and Sidneyia) and cheloniellids (e.g. Cheloniellon, Duslia); the new clade ''Vicissicaudata'' is proposed to encompass these arthropods, which are characterized by a differentiated posterior region. The phylogenetic position of aglaspidid arthropods makes them good outgroup candidates for analysing the internal relationships within the groups that form Trilobitomorpha. This work provides a much clearer picture of the phylogenetic relationships among Lower Palaeozoic lamellipedians.

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

“…The characters are broadly based on those of J. Ortega Hernández, D. A. Legg, and S. J. Braddy [19], but the taxonomic scope has been altered, and a number of character codings changed. Shankouia zhenghei Chen, Wang, Maas and Waloszek in Waloszek, Chen, Maas & Wang, 2005 has been used as an out-group rather than Fortiforceps foliosa Hou & Bergström, 1997.…”

“…Whether taxa such as Canadaspis perfecta (Walcott, 1912) are derivatives of the euarthropod stem-lineage [14-16] or represent the crown-group [17] is still a matter of debate. Another open debate is whether Megacheira, including taxa such as Leanchoilia superlata Walcott, 1912 and Yohoia tenuis Walcott, 1912 fall outside Euarthropoda [12,14,16,18,19] or are derivatives of the early chelicerate stem-lineage [20-24]. …”

“…The traditional Arachnomorpha [20,22,25-27] is being increasingly questioned, and an increasing number of studies suggest trilobites to be derivatives of the mandibulatan stem-lineage [16,19,28-31]. Regardless, there is consensus that trilobites belong to a larger group of early Paleozoic arthropods, containing many of the non-biomineralizing taxa attributed to Arachnomorpha.…”

“…Within Lamellipedia, the basal split set off Marrellomorpha from Artiopoda, which comprised the remainder of the chelicerate stem-lineage. Some authors have suggested that Artiopoda could be a monophylum based on the shared presence of a rather similar limb with a bilobate exopod of a proximal lobe bearing the lamellae and a flap-like distal lobe fringed with setae [28] and indeed they were rendered monophyletic in some analyses [16,19,31]. While most analyses yield a number of well supported groups of artiopodans, in particular Helmetiida, Nekatspidida, Trilobita, and Xandarellida, their interrelationships remain unclear [19,22,26,27,31,32].…”

A predatory bivalved euarthropod from the Cambrian (Stage 3) Xiaoshiba Lagerstätte, South China

“…taxa typified by a lobopodian-like construction and a one-segmented head – and the Deuteropoda, which are characterized by complete body arthrodization and a multisegmented anterior region1. Members of Deuteropoda are by far the most diverse and well-known euarthropods represented in exceptionally preserved fossil communities; this clade encompasses several major groups, such as the trilobitormophs (trilobites and their non-biomineralized relatives23), viccissicaudates (aglaspidids, xenopods and cheloniellids145), fuxianhuiids678, megacheirans910, and marrellomorphs11121314. The early diversity of Deuteropoda also includes an enigmatic, and most likely paraphyletic, assemblage of euarthropods typified by the presence of a bivalved carapace that covers the anterior part of the body15161718192021, or may extend to encompass the whole animal (e.g.…”