A number of fossils have been linked to the Onychophora, but whether they really fit into this group is not known with certainty. Aysheaia pedunculata from the Middle Cambrian Burgess shale has many morphological similarities with extant onychophorans, but it differs fundamentally in having a terminal mouth. Another, possibly related, fossil is the Middle Pennsylvanian Mazon Creek Helenodora inopinata, which could have been marine or terrestrial. The connection between Priapulida (based on the fossil Xenusion from the Cambrian sandstone in Germany) and Onychophora is highly speculative. However, the marine Cambrian forms (e.g., Mi-crodictyon, Hallucigenia, and Luolishania) have been definitely assigned to the Onychophora by some authors.
Although first mistakenly described in 1826 as a type of slug, the evolutionary history of onychophorans has long fascinated scientists. Onychophorans were thought to be a "missing link" between the Annelids (segmented worms) and the Arthropods (a group that includes the insects and spiders) because they share morphological characters with both these large phyla. However, molecular studies show that they are, in fact, more closely related to the arthropods. Because of their many unique characteristics, they are considered a separate phylum.
Two families, the Peripatopsidae and the Peripatidae, are recognized. It is assumed that the two families are sister taxa, but whether each family is a monophyletic group (evolved from a common ancestor) has yet to be determined. Compared to many other invertebrates, the gross morphology of the Onychophora is remarkably similar over its wide and disjunct geographical range. The lack of distinguishing features has precluded a satisfactory higher-level classification and also causes difficulties at the species level, with cryptic species being common. No subfamilies are recognized.
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