The Derwent River seastar, Marginaster' littoralis (Echinodermata: Asteroidea), has been assessed as critically endangered owing to its highly restricted range within one estuary in Tasmania, Australia. However, there have been concerns about the validity and status of the species. Here, we use non-invasive X-ray computed tomography to review the morphology of the holotype. This reveals the presence of internal ossicles to strengthen the disc margin, a character that requires the taxonomic transfer of this species to the family Asterinidae, genus Patiriella. The presence of relatively long marginal spinelets (0.7-0.9 mm) reliably distinguishes this species from the invasive congeneric Patiriella regularis (0.2-0.35 mm). These findings remove doubt about the validity of the species; the recorded habitat and restricted range of Patiriella littoralis are not exceptional for an asterinid seastar. The weight of evidence suggests that it is an endemic species, rather than a transient exotic introduced to Tasmania via shipping or food importation. Its known habitat has been severely impacted by urbanization, poor water quality and invasive species. Targeted surveys in 1993 and 2010 failed to find this species, and it is highly probable that the Derwent River seastar is now extinct; one of the few documented recent marine invertebrate extinctions.
