Revision of Aglaspidid Phylogeny: Towards a Better Understanding of Arachnomorph Relationships  

Aglaspidids are a group of poorly-understood Lower Palaeozoic arachnomorph arthropods that remain phylogenetically problematic, despite previous efforts to resolve their affinities as either chelicerates, stem-chelicerates, sister taxa to cheloniellids, or triobite-allied arachnomorphs.  Consequently, they are regarded as a “bucket taxon” in which numerous taxa have been included based only on a general resemblance.  They are usually characterized by a semicircular and convex cephalon, devoid of facial sutures; a faint glabellar region; and a pair of anteriorly oriented oval eyes.  Despite the recognition of a labrum in Cambrian representatives and a hypostome in some Ordovician forms, no formal account of all aglaspidids and aglaspidid like arthropods has been undertaken. 

This study presents an extensive re-evaluation of aglaspidid taxonomy and phylogenetic affinities.  Re-study of alleged aglaspidid-like arthropods from the Upper Ordovician Letná Formation (Czech Republic) allows their reinterpretation as representatives of the Trilobita and Xiphosura.  We also describe a new genus of aglaspidid arthropod from the Upper Cambrian of Tasmania.  A phylogenetic analysis indicates that Aglaspidida sensu stricto forms a clade, which includes the Tasmanian aglaspidid, and which is a sister-group to the trilobitomorphs.  Aglaspidid-like arthropods comprise a polyphyletic aggregation of taxa that are usually poorly preserved, and whose morphology is only superficially similar.