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 Bergström, 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.
