Ultrastenos
Ultrastenos is an extinct genus of Australian mekosuchine crocodilian that lived during the Late Oligocene in northwestern Queensland, Australia. Following its discovery, it was speculated that Ultrastenos was a slender-snouted animal similar to modern gharials or freshwater crocodiles due to the seemingly abruptly narrowing mandible. However, a later study found that this was a misinterpretation of the fossil specimen and that Ultrastenos instead had a more generalized lower jaw. The same publication also provided evidence that the fossils of Ultrastenos belonged to the same animal previously named "Baru" huberi, adding further evidence to the idea that the animal was short snouted, contrary to the initial hypothesis. Given that "Baru" huberi was named first, the type species of Ultrastenos changed from U. willisi to U. huberi in accordance with the rules of the ICZN. Ultrastenos was a small mekosuchine, measuring upwards of long.
History and naming
The research history of Ultrastenos is primarily recorded through the independent descriptions of two taxa long thought distinct from each other: Ultrastenos willisi and "Baru" huberi. "Baru" huberi is the older name among the two, coined by Paul Willis in 1997 for an incomplete rostrum found within the Late Oligocene strata of the White Hunter Site within the Riversleigh World Heritage Area. The description of "Baru" huberi was centered around QM F31060, an incomplete rostrum that Willis believed to be similar to that of Baru, thus assigning it to the same genus. Within the same work Willis also named and described a variety of other mekosuchines from the site including Mekosuchus whitehunterensis, Quinkana meboldi and Baru wickeni. Among the described but unnamed material is one fossil that later proved as one of the most significant finds for this taxon's history, several skull fragments including skull tables only referred to as the White Hunter cranial form 1. Though Willis recognized that they could belong to one of the taxa he named, he refrained from assigning them to any species in particular given the lack of overlap.The genus Ultrastenos, meanwhile, was described almost 20 years later 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 had already produced a variety of other mekosuchine genera of varying morphology, the discovery of Ultrastenos was initially thought to represent a unique new morphotype within this group. More specifically, Ultrastenos was believed to be the first Riversleigh crocodilian to display 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. Furthermore, Stein et al. went on to propose that Willis' White Hunter cranial form 1 also belonged to Ultrastenos given some similarities in proportions, assigning the various fragments to Ultrastenos.
Subsequent years saw some of the hypothesis surrounding these two taxa questioned. Research began to increasingly show that "Baru" huberi was not as closely tied to other species of Baru as initially thought. This research included not only the genus revision of Baru published by Adam M. Yates in 2017 but also multiple phylogenetic analysis conducted in 2018, 2021 and 2023. Yates' paper on Baru further questioned the referral of one of the White Hunter crania to Ultrastenos, instead proposing that it belonged to a juvenile Baru wickeni, while Ristevski and colleagues challenged the interpretation of Ultrastenos as a longirostrine form. The team, which included Stein, alluded to a re-description of the taxon being in the works following the discovery of a related form.
This re-description was eventually published in 2024 and tackled both the generic identity of "Baru" huberi as well as the question surrounding the anatomy of Ultrastenos. Authored by both Yates and Stein, the paper found that rather than being two distinct but poorly known taxa, the two animals were actually one and the same. Key in this was Willis' White Hunter cranial form 1. Yates and Stein determined that one of the fossils, QM F31076, perfectly matched the holotype of "Baru" huberi, suggesting that both fossils belong to the same individual. Several lines of evidence are cited in favour of this line of thinking. For example, both are similar in their preservation, despite the fact that material from the White Hunter Site can vary considerably in appearance. The specimens match not only in preservation but also in size, with the parallel edges of the respective fossils interpreted as marking a fracture caused by erosion that separated the elements prior to collection. Another feature that unifies the specimens is the identical skull ornamentation, which is distinct from those of the other White Hunter crocodilians. Finally, both the "Baru" huberi holotype and QM F31076 overlap with material from Bullock Creek, thought to be a related species that shares many of the same apomorphies. The team also argues against Yates' previous referral of QM F31075 to Baru wickeni, in part due to the ontogeny of this genus now being better understood thanks to Baru iylwenpeny, though a juvenile specimen previously held as "Baru" huberi was in turn referred to Baru wickeni. In addition to material previously referred to either Ultrastenos willisi or "Baru" huberi, the team also refers two previously undescribed fossils to Ultrastenos.
The genus name derives from the Latin "ultra" for extreme and the Greek "stenos" for narrow, chosen to reflect the morphology of the animal's mandible as interpreted by Stein and colleagues. Though the species name chosen by them was initially meant to honour Paul Willis, the rules of the ICZN meant that the species name of "Baru" huberi took priority. Consequently, this created the combination Ultrastenos huberi, with the species name honoring Professor Huber, who was employed as rector at the University of Bonn, Germany.
Description
Skull
Ultrastenos, contrary to its name, had a snout described as platyrostral and brevirostrine skull, meaning its snout was flattened and short rather than elongated as initially proposed by Stein and colleagues and bearing a slight resemblance to more generalized mekosuchines such as Baru.The external nares are positioned rather closely to the very tip of the snout, which makes the premaxilla appear steep when seen in profile view, which is similar to what is observed in Baru with its almost vertical premaxillary tip, though not nearly as robust as in these larger taxa. Overall the naris is longer than it is wide and directed anterodorsally, meaning it opens slightly towards the front of the skull rather than facing purely upwards as is typically seen in many semi-aquatic crocodilians. As is common among crocodilians, a prominent notch is present between the final tooth of the premaxilla and the first tooth of the maxilla, allowing for a smooth insertion of an enlarged dentary tooth. In Ultrastenos, this notch is bordered by two prominent ridges. The nasal bones stretch across the rostrum and come into contact with the nares. Willis compared this condition to Baru wickeni and used it to differentiate Ultrastenos from Baru darrowi when he still considered these animals to be congeneric. While the nares and nasals contact, the nasals do not extend into the former. The lacrimals feature a subtle ridge that divides the surface and its ornamentation into two sections, one anteromedial close to the nasals and prefrontals and another posterolaterally, near the lateral edge of the eye sockets. This ridge, dubbed the canthus lacrimalis to distinguish it from similar structures in caimans and saltwater crocodiles, is described as being similar though weaker than what can also be seen in Baru wickeni. The quadrate and quadratojugal are curved inward, especially towards the front where they contact the jugal, which is similarly constricted. Furthermore, the jugal as a whole is gracile and short. The jugal forms the base of the postorbital bar and bears a foramen in this region of the skull. The distance between the eye sockets is narrow and leads into the trapezoid skull table. The supratemporal fenestrae are prominent and teardrop-shaped, as noted by both Stein and colleagues in 2016 and Yates and Stein in 2024. Through these fenestrae the supratemporal fossae are visible, as the surrounding bones of the skull table do not overhang these openings.
The occiput, the back of the head, is wide yet shallow, forming a concave surface that runs perpendicular to the quadrates. The squamosals are not well expressed at the back of the head and the posterolateral processes are only weakly developed, not extending as far back as in other crocodilians. Something similar can be observed with the paroccipital process, a part of the exoccipital, which is likewise poorly developed and rounded, but still clearly set apart from the quadrate and squamosal. The basioccipital, which forms the base of the occiput, is a tall element that's significantly wider at the top and narrows towards the bottom with a prominent keel running down its vertical surface. The foramen magnum, through which the spinal cord passes, appears as an inverted triangle just above the occipital condyle that articulates with the first neck vertebra.
The lower surface of the jaw shows a distinct premaxillary fenestra, a hole on the opposite side of the nares, that is located behind the first pair of premaxillary teeth. While the dorsal surface of the premaxilla extends back to the position of the third maxillary teeth, the ventral surface ends much sooner, contacting the maxillae at the level of the large notch and forming a nearly straight transverse suture. The paired suborbital fenestrae are notably larger than the single premaxillary fenestra and extend backwards roughly from the position of the seventh or eight maxillary tooth socket depending on the specimen, though in both cases this places the beginning of these openings before the beginning of the orbits. The front-most parts of the suborbital fenestrae are comparably narrow thanks to an expansion of the palatines, which were otherwise narrow. The pterygoids are large elements that form part of the palate. In Ultrastenos the corners of the pterygoids extend beyond the back of the choanae, which itself is surrounded by ridges.
Yates and Stein describe Ultrastenos as having had four premaxillary teeth in adults, stating that the number seen in hatchlings is not known. This contrasts with Willis' older claim that juveniles had five premaxillary teeth, which was based on a specimen now referred to Baru wickeni. The precise number of maxillary teeth is uncertain, but was likely somewhere between 14 and 15 teeth on either side. The latter interpretation is followed by Yates and Stein on the basis of specimen QM F61097, but it cannot be ruled out that Ultrastenos only had 14 teeth as suggested by Willis, which is still a pair less than in Baru. There is little space between the first six alveoli and no signs of reception pits for the dentary dentition. Among these first six alveoli the fifth is the largest, outsizing even the fourth premaxillary alveolus, and forming the peak of a mild festoon. From the sixth to the ninth alveolus the teeth become more widely spaced, though the individual gaps are uneven in size. This area also preserves some reception pits between the individual teeth, most prominently both before and after the seventh maxillary tooth and to a lesser degree further back. This leads into a second series of closely spaced teeth following the tenth tooth, which also coincides with a second festoon that peaks around the tenth or eleventh tooth. Overall most teeth of the maxilla are described as subcircular, meaning that they lacked significant labiolingual compression.