The anatomy, affinity and phylogenetic significance of Ilemoraspis kirkinskayae (Osteostraci) from the Devonian of Siberia

Ilemoraspis kirkinskayae is a poorly known genus of Osteostraci (stem-gnathostome) yet is one of the most distinct in terms of its anatomy and biogeography. It is paleobiogeographically isolated from all but one other genus of Osteostraci on the Altaid microcontinents and is one of the few genera to survive into the Middle Devonian. Discovery of a new locality in Khakassia, Russia, has yielded a number of exceptionally well-preserved specimens of the genus and subsequently a wealth of new anatomical information. A new reconstruction of Ilemoraspis is provided, including postcranial anatomy and a sensory line extending onto the trunk. The latter is a novel observation for the Osteostraci and allows the proposal of a scenario for the evolution of trunk lateral lines. New anatomical data for I. kirkinskayae allow testing of established hypotheses on its affinity. Our analyses yield two equally parsimonious solutions that appear unlikely on the basis of comparative anatomy or paleobiogeographic grounds, respectively. This situation is likely to be a consequence of the sporadic sampling of osteostracans in central Asia.