Hallucigenia’s onychophoran-like claws and the case for Tactopoda

The claws of the Cambrian lobopodian Hallucigenia resemble the claws and jaws of extant onychophorans, establishing a close relationship between hallucigeniid lobopodians and onychophorans, resolving tardigrades as the closest extant relatives of true arthropods, and showing that the earliest ancestor of the arthropods and their kin would have looked like a lobopodian. Organizing the early arthropods The early fossil record of arthropods, jointed-legged animals, is replete with fossils of lobopodians - wormlike animals with legs and sometimes elaborate body armour. The lobopodians bear a passing resemblance to the velvet worms or onychophorans, predators among the leaf-litter of modern tropical forest floors, but apart from general morphology, no specific links between the two groups were known. Now Martin Smith and Javier Ortega-Hernndez have identified a trait that links them - the tiny claws on the ends of the legs of the Cambrian lobopod Hallucigenia closely resemble the claws and jaws of extant onychophores in their unique method of construction. This allows lobopods and onychophores to be grouped together with tardigrades ('water bears'), the closest extant relatives of true arthropods (including insects, crustaceans and spiders), and suggests that the earliest ancestor of the arthropods and their kin would have looked like a lobopodian. The Palaeozoic form-taxon Lobopodia encompasses a diverse range of soft-bodied ‘legged worms’ known from exceptional fossil deposits 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 . Although lobopodians occupy a deep phylogenetic position within Panarthropoda, a shortage of derived characters obscures their evolutionary relationships with extant phyla (Onychophora, Tardigrada and Euarthropoda) 2 , 3 , 5 , 10 , 11 , 12 , 13 , 14 , 15 . Here we describe a complex feature in the terminal claws of the mid-Cambrian lobopodian Hallucigenia sparsa —their construction from a stack of constituent elements—and demonstrate that equivalent elements make up the jaws and claws of extant Onychophora. A cladistic analysis, informed by developmental data on panarthropod head segmentation, indicates that the stacked sclerite components in these two taxa are homologous—resolving hallucigeniid lobopodians as stem-group onychophorans. The results indicate a sister-group relationship between Tardigrada and Euarthropoda, adding palaeontological support to the neurological 16 , 17 and musculoskeletal 18 , 19 evidence uniting these disparate clades. Thes