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.

”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.

[2] 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.

[2] 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.

These remains cover various elements of the postcranial skeleton, including multiple vertebrae of the neck and tail, some osteoderms, a coracoid and limb bones.

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.

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.

[8] 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.

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.

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.

[2] 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.

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 precise tooth count of the lower jaw is unknown, but Yates and Stein suggest a minimum of 16 mandibular teeth based on the various fossils that have been collected.

The osteoderms forming part of the paratype stem from the dorsal shield, the armour that is situated along the back of the animal, and are moderately robust in morphology with deep pits but lacking a medial ridge.

[3] Initially, Stein and colleagues proposed that Ultrastenos could have been a longirostrine animal, meaning that unlike in any other known mekosuchine, the jaws would have been long and narrow like in modern gharials.

Crocodilians with slender snouts generally possess needle-like caniniform teeth, whereas those in the mandible of Ultrastenos are molariforms (low crowned, elongated mediodistally and with nearly equilateral sides).

In the White Hunter cranium, the ratio is similar to that seen in basal mekosuchines like Australosuchus and Kambara (both are mesorostral generalists) and the modern Orinoco crocodile, which is regarded as the least-specialised of today’s longirostrine taxa.

While the shallow mandible does argue against altirostry, it is pointed out in the 2024 paper that it does not rule out platyrostry, i.e. a flattened skull, while the elongated retroarticular process itself is not diagnostic for longirostry.

This is illustrated well by the orientation of the quadrate and quadratojugal, which following some slight rotation could just as well support a much shallower angle of convergence and thus a less rapid construction of the rostrum, congruent with a brevirostrine morphology.

Reassembly of the mandible further shows that the left, more complete element is much more bent inward than its right counterpart, further exaggerating the constriction which itself is not concrete evidence for longirostry as shown by the clearly brevirostrine Baru iylwenpeny.

[8] Finally, the most damning evidence against the longirostrine hypothesis stems from the fact that Yates and Stein identified one of the White Hunter crania as having most likely been derived from the same individual as the holotype of ”Baru” huberi, which consists of a partial rostrum clearly showing mesorostral and platyrostral morphology, i.e. more generalized anatomy.

The only other potential candidate for this anatomy would be Harpacochampsa from the Middle Miocene, however, while initially placed within Mekosuchinae studies since then have increasingly come to favour the interpretation that it was either a gavialoid or a basal crocodyloid.

[8] As Ultrastenos was regarded as two different forms prior to the 2024 redescription, phylogenies exist for both the rostrum and the cranium separately, both of which do however already showed signs that would later be confirmed by Yates and Stein.

The other Baru species meanwhile occupy a different branch within Mekosuchinae, with Lee and Yates finding them to be allied to Pallimnarchus (now Paludirex) and Kalthifrons, both of which are considered to have been more semi-aquatic forms.

[8] The White Hunter Site is well known for its diverse crocodilian fauna, which besides Ultrastenos also included Mekosuchus whitehunterensis, Quinkana meboldi and Baru wickeni.

While Mekosuchus and Quinkana are two possibly terrestrial animals and thus not in competition with the more semi-aquatic forms, Ultrastenos and Baru wickeni are thought to have been much more similar in niche.

One possible explanation for the two morphologically similar species being able to coexist is the great difference in size, with the smaller Ultrastenos evading the larger crocodilians of its ecosystem.

[2] During the Late Oligocene the Low Lion Site was predominantly covered by open forests but lacks evidence for extensive river systems or wetlands from this region.

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.