Geographical variation in skull morphology of the Irrawaddy Dolphin, Orcaella brevirostris (Owen in Gray, 1866)

Analysis of skull morphology of Irrawaddy Dolphin Orcaella brevirostris specimens from throughout its range resulted in significant separation of specimens from Australia (including one specimen from Papua New Guinea) and those from Asian countries. Specimens from Australia were characterised by a range of nodular nasal ossicles (vs. two transversely elongate, antero-posteriorly compressed nasal bones in Asian specimens); a reduced mesethmoid plate, widely separated from the nasal ossicles by exposed frontal bones (vs. well developed mesethmoid plate, adjacent to, or contacting the nasal bones); and narrowly separated pterygoid hamuli (vs. widely separated). The Australian specimens showed a more derived cranial osteology, suggesting an origin from Asian populations - possibly during an episode of lowered sea level - with subsequent genetic isolation of the Australian populations. The results from this study indicate a taxonomic separation of Australian specimens at the subspecies or even species level, but no formal proposal is made, pending further study using both morphological and molecular characters.