Ultrastenos

Ultrastenos was described in 2016 based on the holotype specimen QM F42665, a posterior cranium and mandible found at the Low Lion Site of the Riversleigh World Heritage Area. While Riversleigh is known to have been home to a variety of other mekosuchine genera, the discovery of Ultrastenos represented a previously unknown morphology of the native Oligocene to Miocene crocodilian fauna. More specifically, Ultrastenos was the first known Riversleigh crocodilian to display what may have been highly elongated jaws similar to those of modern gharials. In addition to the holotype material, multiple fossil remains were assigned as the paratypes. These remains cover various elements of the postcranial skeleton, including multiple vertebrae of the neck and tail, some osteoderms, a coracoid and limb bones. In addition to the Low Lion material, Stein et al. also suggests that material previously collected and described by Willis from the White Hunter Site could have also belonged to this genus.[2][3] This is however contradicted by Adam Yates, who assigns the White Hunter cranium to Baru wickeni.[4] Many of the interpretations made in the type description were later questioned in a 2023 study on mekosuchines, also involving Stein, which makes mention that a re-evaluation of the material is in preparation and that a related form had been discovered.[5]

The name derives from the Latin "ultra" for extreme and the Greek "stenos" for narrow, chosen to reflect the morphology of the animal's mandible. The species epithet honors Dr. Paul Willis, a prolific paleontologist especially involved in mekosuchine research, having participated in the description of over a dozen mekosuchine species.[2]

Aspects of Ultrastenos anatomy resemble today's freshwater crocodile.

Ultrastenos is unique among mekosuchines thanks to its shallow lower jaw and constricted snout, which was very wide at the back and narrowed rapidly towards the base of the rostrum. As only the back of the skull is known, it is not certain what the snout of Ultrastenos really looked like. One possibility is that the snout was relatively short, stopping not far beyond the point of preservation, which may indicate an altirostral snout form akin to Mekosuchus or modern dwarf caimans and dwarf crocodiles. However, in altirostral forms the mandible generally displays very deep mandibles, which clashes with the anatomy of Ultrastenos. Instead, it is thought to be much more likely that the two halves of the mandible fused to form the mandibular symphysis not far beyond what is known, likely continuing onward to form a longirostrine snout. Such a snout form is seen in a variety of modern crocodilians including gharials, the false gharial, slender snouted crocodiles and freshwater crocodiles. Shared traits between these animals and Ultrastenos include the uniform dentition and shallow mandible. The retroarticular process, while not as elongated as in gavialoids, is similar to that of freshwater crocodiles too.[2] However, in a 2023 study authored by multiple prolific mekosuchine researchers including Stein it is argued that the fragmentary nature of the material means that these interpretations are still questionable.[5]

Mandible

Size comparison of Ultrastenos willisi, scalebar = 50 cm

The most striking feature of Ultrastenos is its mandible. It is incredibly gracile and, when looked at from atop, constricts rapidly creating a pronounced curvature in the bone. The back of the lower jaw bulges outwards, but becomes slightly concave towards the front following the constriction. Looking at the lower jaw in profile view also highlights how shallow it is. The dentary is about as tall as the angular and surangular bone, which extend only weakly above and below the dentary following the region where the three bones come into contact. The dentary does grows slightly shallower towards the front, but given the lack of material it is uncertain how that would have continued. The only thing really breaking up this very low profile is the articular process, the connective element between the cranium and lower jaw, which is rather tall. Both angular and surangular become convex at the back of the lower jaw, forming the articular region and an elongated retroarticular process which is notably longer than in any other mekosuchine. The angular and surangular display prominent ridges along their lower and upper ends respectively, the lower ridge being the more developed and forming a buttress, effectively enclosing the ornamented outer surface of the lower jaw. Like that of the skull table, the ornamentation of the mandible is composed of closely spaced pits, although in this region they are much more irregular. The external mandibular fenestra, located at the meeting point of these three major lower jaw bones, appears to have been highly reduced, but not absent.[2]

Given that only the last four tooth socketss and two teeth are preserved, little is known about the dentition of Ultrastenos. It is clear that the tooth row was constricted and, like the lower jaw as a whole, rather narrow. Based on the anatomy of the sockets, these last four teeth all appear to have been similar in size and the known teeth suggest that they were low, slightly compressed from side-to-side and with cutting edges (carinae).[2]

Stein et al. recovered Ultrastenos as a mekosuchine crocodilian and sister taxa to Trilophosuchus rackhami, with which it shares a vertical exoccipital. Based on prior phylogenetic analysis this would suggest placement within Mekosuchini, however in Stein et al.'s analysis Mekosuchini as previously defined has lost cohesion. This is because in the recovered results, Trilophosuchus was recovered as more basal than Kambara, a platyrostral species from the Eocene. Although Ultrastenos is only fragmentary, the issues with the phylogeny are not caused by it and instead the result of the uncertain placement of Harpacochampsa, which according to some may be a gavialoid.[2] Another study discussing the phylogenetic position of Ultrastenos was that of Rio and Mannion, published in 2021. Their study argued that many previously established mekosuchines were not mekosuchines at all but instead belonged to different groups. Ultrastenos did maintain its status as a mekosuchine, but was recovered in a polytomy alongside the various species of Mekosuchus, with Trilophosuchus being placed as said groups sister taxon.[1]

However, due to its fragmentary nature Ultrastenos is commonly not included in phylogenetic analysis. For instance, Ultrastenos was not included by Lee and Yates in their 2019 study, which recovered Mekosuchinae as being markedly different from both the results of Stein et al. and Rio and Mannion.[6] Similar results were again recovered by Ristevski et al. in 2023, which although discussing Ultrastenos did not include it in its phylogeny. Ristevski and colleagues make specific mention of the uncertain nature of Ultrastenos and the fact that it requires significant revisions, mentioning that material of a close relative had been found.[5]

Baru wickeni and "Baru" huberi, two contemporary crocodilians from the White Hunter Site.

The narrow mandible and possibly longirostrine nature of Ultrastenos suggests it inhabited a niche previously not known from Riversleigh. Typically narrow, elongated jaws in crocodylomorphs are associated with a diet heavily featuring fish, as examplified by the modern gharial. However fish fossils are not commonly found in Oligocene Riversleigh sites and are less common than frogs, snakes and lizards. Subsequently, Stein et al. suggest that rather than being a piscivore, Ultrastenos may have fed on small reptiles and amphibians. Regardless of prey preference, the shallow mandibles of Ultrastenos may have served the same function as in modern longirostrine crocodilians, which is to streamline movement of the rostrum against the surrounding water, while the uniform teeth would be suited to catch, hold and crush small prey. Furthermore, it can be inferred based on how gracile the jaws were that Ultrastenos was incapable of tackling large prey as done by Baru.[2][3]

During the Late Oligocene the Low Lion Site was predominantly covered by open forests. The presence of a longirostrine crocodilian would suggest the presence of bodies of water, but Stein et al. argue that there is no evidence for extensive riversystems or wetlands from this region. Instead, Ultrastenos may have been more at home in forest lakes and ponds that were drained through the underlying karstic limestone. This may find support in the type of environments nowadays inhabited by freshwater crocodiles, which may persist in bodies of water that are only temporary and not available all year round. Following the interpretation that the cranium described by Willis also belonged to Ultrastenos, as suggested by Stein et al., then the genus would have also inhabited the White Hunter Site which is well known for its diverse crocodile fauna. If the White Hunter cranium truly belonged to Ultrastenos, then it may have lived alongside the terrestrial Mekosuchus whitehunterensis and Quinkana meboldi, the small bodied semi-aquatic "Baru" huberi and the much larger macropredator Baru wickeni.[2][3]