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 4 m (13 ft), 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 300 kg (660 lb) 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.

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.

Previously, "Baru" huberi was thought to have been a species of this genus as well, but later studies have found that it was not especially close to Baru and instead was a synonym of Ultrastenos willisi, creating the combination Ultrastenos huberi. Ultrastenos is now thought to be closer to Mekosuchus and Trilophosuchus.

The skull of Baru was exceptionally broad and robustly built, with deep jaws and strongly curved, wave-like (festooned) 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.