Pentastomids and Onychophorans

Pentastomids are obligatory parasites of vertebrate respiratory systems (figure 43e.8). There are about 100 described species, and they all infest various tetrapods, including two cosmopolitan species that infest humans. The blood-sucking adults inhabit respiratory tracts of their hosts, where they anchor themselves by means of their hooklike head appendages. For years it was believed that pentastomids were allied with the onychophorans as vermiform, pre-arthropod creatures. However, several recent independent molecular studies (using 18S gene sequences) have revealed the pentastomids to be highly modified crustaceans. Corroborative independent work over the past few years has come from cladistic analyses of sperm and larval morphology, nervous system anatomy, and cuticular fine structure. Furthermore, Müller and Walossek's work on the Swedish Orsten fauna proves that the pentastomids (and also the tardigrades) had appeared at least by the Upper Cambrian, long before the land vertebrates had even evolved. So, we must ask what the original hosts of these parasitic crustaceans might have been. Walossek, MŸller, and even Stephen Jay Gould have noted that Conodont fossils are common in all the Cambrian localities that have yielded pentastomids, and thus the conodonts (also long a mystery, but now widely regarded as parts of early fish-like vertebrates) might have been the original hosts of the pentastomids.

As with pentastomids, onychophorans too were part of the amazing, early-Cambrian, explosive marine diversification (figure 43e.9). They have been found in Burgess shale-type faunas at several localities, in Cambrian deposits from China and Siberia, and in the Swedish Orsten fauna. And, we now know that Conway Morris' original reconstruction of Hallucigenia (from the Burgess Shale, see figure 43e.8) had the animal turned upside-down. Ramsköld and Hou recently turned Hallucigenia over and found a second pair of legs, concluding it was an onychophoran with long dorsal spines. And there is now an onychophoran known from the Chengjiang deposits of China with side plates and spines. Aysheaia (also from the Burgess Shale, see figure 43e.7) was originally described by Walcott as an annelid, but it too is now regarded as an early marine onychophoran.

Although usually grouped with the onychophorans, the pentastomids may in fact be more related to crustaceans. Onychophorans have been found in many of the new and old fossil discoveries.