All material gathered at the Low Lion Site is interpreted to have belonged to a single individual that is set apart from all other crocodilians of the Oligocene to Miocene Riversleigh fauna by the distinctive constriction of the mandible. The skull material indicates a relatively small bodied animal similar in size to the extant Freshwater crocodile and notably larger than the known mekosuchine dwarf forms such as Trilophosuchus and Mekosuchus. The cranium is approximately 16 to 18 cm wide between the lateral margins of the quadrates, however there is no indication that the holotype specimen was a mature individual. The mandible could have either been truncated just anterior to the end of the preserved bone or narrowed further towards the midline, potentially meeting an elongated mandibular symphysis, however preservation does not allow for a certain answer. Since an elongated mandibular symphysis is the defining trait of a longirostrine morphology, this skull shape can only be assigned to Ultrastenos tentavively. However the mandible still shares a number of characters seen in extant longirostrine taxa such as the low mandibular profile also seen in gharials, Tomistoma, Mecistops and Freshwater crocodiles. In these animals a shallow rostrum serves to streamline the skull for rapid rostral movement against the water column. Homodonty is another trait associated with a longirostrine morphology, however the presence of only four dentary alveoli does not allow for a certain attribution of this trait to Ultrastenos.
