page_title: Onychophora
section_title: Evolution
hierarchical_section_title: Onychophora / Evolution
caption_attribution_description: English: A reconstruction of the earliest known terrestrial Onychophoran, Helenodora inopinata.
context_page_description: Onychophora, commonly known as velvet worms or more ambiguously as peripatus, is a phylum of elongate, soft-bodied, many-legged panarthropods. In appearance they have variously been compared to worms with legs, caterpillars, and slugs. They prey upon smaller animals such as insects, which they catch by squirting an adhesive slime.
  Approximately 200 species of velvet worms have been described, although the true number of species is likely greater. The two extant families of velvet worms are Peripatidae and Peripatopsidae. They show a peculiar distribution, with the peripatids being predominantly equatorial and tropical, while the peripatopsids are all found south of the equator. It is the only phylum within Animalia that is wholly endemic to terrestrial environments. Velvet worms are considered close relatives of the Arthropoda and Tardigrada, with which they form the taxon Panarthropoda. This makes them of palaeontological interest, as they can help reconstruct the ancestral arthropod. In modern zoology, they are particularly renowned for their curious mating behaviour and for bearing live young.
context_section_description: Certain fossils from the early Cambrian bear a striking resemblance to the velvet worms. These fossils, known collectively as the lobopodians, were marine and represent a grade from which arthropods, tardigrades, and Onychophora arose. They are found in the Cambrian, Ordovician (possibly), Silurian and Pennsylvanian periods. Historically, all fossil Onychophora and lobopods were lumped into the taxon Xenusia, further subdivided by some authors to the Paleozoic Udeonychophora and the Mesozoic/Tertiary Ontonychophora; living Onychophora were termed Euonychophora. Importantly, few of the Cambrian fossils bear features that distinctively unite them with the Onychophora; none can be confidently assigned to the onychophoran crown or even stem group. The exceptions are Hallucigenia and related taxa such as Collinsium ciliosum, which bear distinctly onychophoran-like claws. It is not clear when the transition to a terrestrial existence was made, but it is considered plausible that it took place between the Ordovician and late Silurian—approximately 490 to 430 million years ago—via the intertidal zone. The low preservation potential of the non-mineralised Onychophora means that they have a sparse fossil record. Stem-group members include Helenodora (Carboniferous),Tertiapatus dominicanus, and Succinipatopsis balticus (Tertiary);. A Carboniferous fossils from Montceau-les-Mines, France,  Antennipatus possesses clear onychophoran affinities, but its preservation prohibits differentiating between its placement on the stem or crown of the two extant families, or on the onychophoran stem-group. Crown group representatives are known only from amber—there is a single, partial specimen from the Cretaceous, and a more comprehensive record in Eocene deposits from 40 million years ago. However, some of these amber-borne specimens lack slime papillae and separate feet, and thus may belong in the stem group. The vagaries of the preservation process can make fossils difficult to interpret. Experiments on the decay and compaction of onychophora demonstrate difficulties in interpreting fossils; certain parts of living onychophora are visible only in certain conditions. The mouth may or may not be preserved; claws may be re-oriented or lost; leg width may increase or decrease; and mud may be mistaken for organs. More significantly, features seen in fossils may be artefacts of the preservation process: for instance, "shoulder pads" may simply be the second row of legs compressed coaxially onto the body; branching "antennae" may in fact be produced through decay.
caption: Reconstruction of the Carboniferous Onychophoran, Helenodora.
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