A reinterpretation and taxonomic revision of Ultrastenos willisi Stein, Hand and Archer, 2016, a short-snouted mekosuchine crocodylian from the Oligocene of northern Australia

Ultrastenos willisi from the Oligocene of the Riversleigh World Heritage Area, was described as a mekosuchine crocodylian with a gavial-like, longirostrine morphology. However, the rostrum is not preserved, and the reconstruction rests on a set of inferences that are based on the shape of a mandibular fragment and a cranial reconstruction that was assembled from non-joining fragments. Contrasting with the reconstruction are the proportionally small supratemporal fenestrae and the blunt, molariform posterior teeth, which are discordant with the slender longirostrine morphotype. The issue is resolved by the discovery that QM F31076, a posterior skull fragment ('White Hunter cranial form 1'), which has been referred to U. willisi, is likely to be the missing posterior end of the extremely brevirostrine holotype of 'Baru' huberi. QM F31076 'White Hunter cranial form 1' can be further linked to the holotype of 'B'. huberi via their matching size, preservation and dermal ornamentation. 'White Hunter cranial form 1', in turn, shares a combination of cranial apomorphies with U. willisi and belongs to the same species, indicating that U. willisi is a junior subjective synonym of 'B'. huberi. However, previous phylogenetic analyses have found that 'Baru' huberi is more closely related to other mekosuchine genera than it is to Baru. Consequently, Ultrastenos is retained as a valid genus, and the new combination Ultrastenos huberi is established. With the discovery that Ultrastenos is not a longirostrine taxon there are no longer any known longirostrine mekosuchines, suggesting that the otherwise disparate Mekosuchinae failed to occupy this region of morphospace.