Tardigrades as ‘Stem-Group Arthropods’: The Evidence from the Cambrian Fauna

Abstract

The Cambrian fauna can now reasonably be seen as containing many taxa that lie in the stem-groups of the extant phyla. As such, these fossils suggest how both the ‘body plans’ of extant phyla were assembled, and also how various ‘minor’ phyla relate to the larger groupings of today such as the arthropods and annelids. The various arthropod and lobopod taxa of the Cambrian faunas have been controversial and have generally been considered either as lying in the crown or (occasionally) stem groups of the euarthropods, onychophorans and tardigrades. However, phylogenetic analysis strongly suggests that many of even the most euarthropod-like taxa do not lie within the euarthropod crown-group but are more basal. Further, the commonly expressed view that Cambrian lobopods are in effect stem- or crown-group onychophorans also seems not to be well supported. Lobopods in the Cambrian appear to be diverse and not particularly closely related to one another, and certainly cannot be combined in a monophyletic clade. Both these advances offer hope that the tardigrades (placed as the sister group to the euarthropods in many analyses of extant taxa, here collectively named the Tactopoda) may be more closely related to some of these Cambrian taxa than others. The challenge for both neontologists and palaeontologists is to refine the systematic analysis of both living and fossil taxa in order to maximise the usefulness of the (admittedly few) characters that unite tardigrades to their Cambrian forbears.