Baru
Baru, sometimes referred to as the cleaver-headed crocodile, is an extinct genus of Australian mekosuchine crocodilian. Its fossils have been found from various Late Oligocene and Miocene localities from across the Northern Territory and Queensland, indicating that Baru was a common genus during the late Paleogene and early Neogene. Three species are recognized, B. darrowi, B. iylwenpeny, and B. wickeni.
Baru was a large and powerful mekosuchine with an incredibly deep and robust skull and long teeth with compressed crowns that in the case of Baru darrowi are furthermore adorned with minute serrations. At a length of around, it was among the largest crocodilians native to Australia at the time and the largest predator of its ecosystem. With dorsally oriented nostrils and eyes and a poor range of head movement, as well as its fossils being associated with freshwater environments, Baru is generally interpreted to have been a semi-aquatic ambush hunter, spending much of its day submerged in water waiting for prey.
While the skull of Baru broadly resembles that of modern crocodiles, its much more robust morphology suggests that it hunted somewhat differently. Willis and colleagues suggest that it inhabited shallower waters that were not suited to drown potential prey items as done by today's crocodiles. Instead, it may have relied on its powerful bite and compressed teeth to quickly incapacitate its prey, minimalizing the risk of it escaping during the ensuing struggle in the shallow waters. The bite of Baru is thought to have been powerful enough to take out even large mammals up to a weight of and even other crocodilians, which were abundant in the environments Baru inhabited.
Although highly successful during the Oligocene and Miocene, Baru eventually fell victim to changes in Australia's climate at some point during the Late Miocene. One hypothesis suggests that the continent was hit by an especially devastating pulse of aridification that was severe enough to destroy the habitat preferred by Baru, before conditions improved again leading into the Pliocene. This may explain the drastic faunal turnover that occurred between the Miocene and Pliocene.
History and naming
Baru is among the first mekosuchines to have been described, with B. darrowi being named in 1990 on the basis of various fossils found in the Northern Territory and Queensland. The holotype, a partial rostrum, was collected from the Bullock Creek Local Fauna in the Northern Territory, with paratypes having been collected from Queensland's Riversleigh World Heritage Area. While small differences between the Bullock Creek and Riversleigh material had been recognized even then, the material was nevertheless assigned to a singular species. A difference in age between the two faunas was also already known, with the Bullock Creek LF dating to the Miocene whereas the specific strata of the Riversleigh WHA were Oligocene in age, but this gap in time was likewise not considered to be especially notably for a crocodilian. In 1997 Willis returned to the Riversleigh material thanks to the discovery of more material, now recognizing that they were distinct from the skull found at Bullock Creek and coining the name Baru wickeni in the process. Willis also described a third species of Baru in the same publication, which he named Baru huberi. The 90s and 2000s also saw the discovery of material at the Alcoota fossil site, but these finds though very complete were initially thought to have simply belonged to an established species, with only later works slowly beginning to recognize its distinctiveness. There had been fossil finds at the site predating even the description of Baru, going as far back as 1962, but given the poor understanding of Australasian crocodilians these isolated early remains were simply referred to Crocodylus at the time. Another major revision of the genus was published in 2017 by Adam M. Yates. Yates noted that the genus was poorly defined in the original 1990 description, revising the diagnosis while also discussing a variety of additional material collected in subsequent years. The paper resulted in two major changes, one being that it did not consider Baru huberi to actually represent a species of Baru. Instead, it and subsequent papers argued that it represents a basal member to an entirely different branch of mekosuchine and a 2024 paper eventually found it to be synonymous with Ultrastenos. The other major change concerned the range of the two accepted species, as material of Baru wickeni was described from Pwerte Marnte Marnte while new material of Baru darrowi was recovered from younger strata of the Riversleigh WHA. Furthermore, the Alcoota material is recognized as likely having belonged to a distinct species, which after initially being slated for a 2022 publication was unveiled in 2023 and given the name Baru iylwenpeny.The word Baru is taken from a figure in the Dreamtime, specifically the believes held by the people of the East Arnhem Region. The figure Baru is described as a crocodile man by Willis and as the totem crocodile by George Pascoe Gaymarani. According to the stories, Baru gave names and laws to the land, the plants, the animals and the people of the East Arnhem Region.
Species
- B. darrowi
- B. iylwenpeny
- B. wickeni
Description
The skull of Baru was exceptionally broad and robustly built, with deep jaws and strongly curved, wave-like jaw. The skull is trapezoid in cross section and described as altirostral by some researchers, setting it apart from the platyrostral snout shape typically associated with generalist crocodilians such as Paludirex, Kambara and Australosuchus. Viewed from the side the skull can be described as slightly wedge-shaped, although the cranium is almost as high just behind the nostrils as it is just before the eyes with a slight concavity between these regions. In dorsal view the skull is triangular, as typical for many crocodiles.The premaxilla, the very tip of the snout, is a short, wide and robust element matching the proportions of the rest of the skull. The width of the nares stands out especially in Baru iylwenpeny, in which the width across the snout tip is over half the length of the maxillary section of the rostrum. However, most notable across all species is the profile of the premaxilla between the teeth and the nostrils, as it is very blunt and nearly vertical. This trait seems to have developed as the animal aged, with younger individuals still having more rounded snout tips. Originally, Baru wickeni was differentiated on the basis of this until material of older individuals was found, showing that the vertical premaxillary tip was found across both species. As in many other crocodilians, the contact between the premaxilla and maxilla is marked by a deep notch that receives the enlarged fourth tooth of the lower jaw. This notch not only marks the end of the premaxilla in side view, but also gives the element a D-shape when viewed from above. The nares allow it to quickly tell apart Baru darrowi from the other two species. While the external opening of the nares is roughly trapezoid in all species, in Baru darrowi it is entirely surrounded by the premaxilla. In contrast, the nares of Baru wickeni and Baru iylwenpeny are contacted by the paired nasal bones, which penetrate the upper surface of the premaxilla and contact the external nares. As expected from a semi-aquatic animal, the nares open dorsally, meaning upwards, which makes them well suited for keeping the head underwater over extended periods of time. In Baru iylwenpeny a small projection sits just before the nares, tho the size of it appears to vary between individuals.
The maxilla matches the premaxilla in its robust morphology, while also displaying a greatly festooned lower margin just behind the notch that separates it from the premaxilla. This region is described as a semilunate convexity by Yates, which essentially highlights that this part of the maxilla produces a very prominent rise that houses the first five maxillary teeth. In Baru darrowi and Baru wickeni the remaining teeth behind this first festoon of the upper jaw are placed along a much lower ridge that is much less prominent than those of the premaxilla and anterior maxilla. The same however does not apply to Baru iylwenpeny. While the first festoon is still prominent and in no way reduced, the teeth of the posterior maxilla sit on a festoon that is almost equally as prominent, making the previous section appear shallower by contrast and somewhat closing the gap seen in other species. Both Baru darrowi and Baru iylwenpeny show that the posterior branch of the dorsal alveolar nerve exits through an opening that is directed upwards near the upper margin of the maxilla, but it is uncertain if the same applies to Baru wickeni.
A series of ridges, crests and bosses are present across the skull of Baru. This includes a ridge located before the eyes atop the lacrimal bone. This preorbital ridge is best developed in Baru wickeni, in which it its flanked by a depression in the skull that runs along the side of said ridge. The same ridge can also be observed in Baru darrowi, however, in the more recent species it is not nearly as pronounced as in the Oligocene form, with Baru iylwenpeny lacking it altogether. Similarly, a deep ridge runs across the jugal and maxilla of Baru wickeni, which forms an overhanging flange. Much like the preorbital ridge, this flange is notably less prominent in Baru darrowi and absent in Baru iylwenpeny. While these ornamentations are most prominent in the oldest form, this does not necessarily mean that they were lost throughout the animals evolution, at least when following the interpretation that Baru iylwenpeny was actually the most basal species of the genus as recovered by Yates, Ristevski and Salisbury. On the inverse to these ridges, which are most prominent in Baru wickeni, the squamosal bone of Baru iylwenpeny and Baru darrowi features a sideways-facing boss that is absent in the oldest species. Baru iylwenpeny also possesses unique cranial sculpting, notably a pair of pits present atop the suture between prefrontal and frontal bone.
The supratemporal fenestrae are proportionally small, making up only around 12% of the length of the skull table. They are usually described as D-shaped, which sets them apart from those of Kalthifrons and Trilophosuchus which have elongated oval fenestrae, those of Kambara which are oval or those of Australosuchus and Volia which have more rectangular openings. The fenestrae are noted for being highly reduced in large individuals of Baru iylwenpeny, in which the surrounding bones of the skull table begin to overhang these openings more and more until they resemble a comma. The contribution of the supraoccipital bone to the skull table varies among species. It is a prominent contributor in Baru wickeni, unknown in Baru darrowi and barely present, sometimes even absent, in Baru iylwenpeny.
Much like the cranium, the lower jaw was a large and robust element with pronounced festooning. Although no complete mandible of Baru darrowi is known to science, there are some more fragmentary remains that allow to find similarities between it and the better understood Baru wickeni. As typical, the fourth tooth of the dentary is the largest and is followed by a depression in the jaw until the peak of the next festoon. In Baru iylwenpeny this depression, situated between the fourth and eleventh tooth, varies in dimensions, being deeper in some and shallower in other specimens. The sides of the mandibular ramus of Baru is heavily sculpted. On the surangular bone, the edge of this sculpted area is bordered by a ridge that projects towards the side in both Baru darrowi and Baru wickeni. This trait is not completely unique to Baru, as the same condition was independently evolved by Mekosuchus, a more distant relative among mekosuchines. Similar to the upper jaw, the mandible displays pronounced festooning. The mandibular symphysis, the region where both halves of the lower jaw fuse, extends beyond the fifth tooth of the lower jaw.