Baru darrowi

Introduction

Baru darrowi, a massive crocodile from the Miocene of northern Australia, was one of the largest of the mekosuchines, an extinct group of Australasian crocodiles. Although about as large as the living Saltwater Crocodile, Baru may have been more terrestrial in its habits. Baru was probably a semi-aquatic ambush predator, feeding on unwary diprotodontoids or archaic kangaroos.

Identification

Mekosuchines were a diverse group of crocodilians, ranging in size from the very small (e.g., Trilophosuchus at about 1 m in length) to huge (e.g., Pallimnarchus, at over 5m in length). The shape of the head in mekosuchines varied greatly; some mekosuchines had deep, box-like heads; some were 'hatchet-headed' like Baru, and some like Kambara were more generalised (more like other crocodilians).

Baru darrowi, like other Baru species, had a very deep skull with a deeply emarginated tooth row (similar in some respects to the jaws of theropod dinosaurs). Its blade-like teeth were recurved and exceptionally long. Postcranial material has not yet been described.

Habitat

The Riversleigh area from the late Oligocene to early Miocene was mainly forested, with open areas near the forest edges and freshwater streams, lakes and pools in a karst (limestone) environment.

Distribution

Baru darrowi is known only from the Riversleigh World Heritage Fossil Area in northwestern Queensland. Other species of Baru are known from Riversleigh, as well as from Bullock Creek and Alcoota Station in the Northern Territory.

Feeding and diet

Like most large crocodiles, Baru fed on mammals, birds and other reptiles. The first skull of Baru darrowi discovered at Riversleigh had a partial skull of a marsupial ‘lion’ (thylacoleonid) in a corner of its lower jaw. The Riversleigh area had a diverse assemblage of mammals, including many small to mid-sized herbivorous diprotodontoids. The bladelike teeth of Baru suggest that it killed its prey by slicing action. Its teeth were not specialized for fish-eating, as are the teeth of piscivorous crocodiles, and it probably did not include much fish in its diet.

Life history cycle

Crocodiles and alligators (living Crocodylia) are the largest living reptiles, the only truly large reptiles apart from the Komodo Dragon to have survived to the present. Most crocodiles and alligators are restricted to tropical or subtropical regions today. Crocodiles lay up to 60 eggs in a nest near water, and Baru probably did the same. Most crocodiles are aquatic, although species of Baru, along with at least some other mekosuchines, may have been semi-aquatic (spending at least some time on land, to lay eggs or perhaps to feed occasionally).

The Riversleigh area during the Miocene was forested, and cooler and wetter than it is today. The area had numerous fresh water pools, billabongs, streams and lakes. Baruwould have lived alongside smaller mekosuchine crocodiles like Trilophosuchus. However, because of size differences, Trilophosuchus and Baru would have occupied different ecological niches. Postcranial material of Baru is not described, limiting further speculation about its lifestyle.

Fossils description

Baru darrowi (named after the science fiction star Paul Darrow) is represented by a partial skull, lower jaws and other bones. All material was recovered from Riversleigh in northwestern Queensland, where it was relatively common. Fossil material is held by the Queensland Museum in Brisbane.

Evolutionary relationships

Crocodiles are an ancient group of archosaurs, first appearing in the fossil record in the Late Triassic over 200 million years ago. The oldest ‘modern’ crocodile (Eusuchia) may be Isisfordia duncani, from the middle Cretaceous of Queensland. Mekosuchines are an endemic radiation of primitive Gondwanan crocodiles. Mekosuchine fossils are known from Australia and the southwestern Pacific, and there were many unusual types. The oldest mekosuchines are the Eocene Kambara species. Mekosuchines became extinct during the Pleistocene in Australia but survived much longer in New Caledonia and Vanuatu (almost to the present).

Relationships between mekosuchines and living crocodiles, all within the Crocodyloidea, are unclear. Mekosuchines may be members of Crocodylidae but alternatively may belong to their own family. ‘Mekosuchinae’ may in fact not be a natural group; it has been suggested that Harpacochampsa camfieldensis is instead closer to Crocodylidae (Salisbury and Willis 1996). The ancestors of mekosuchines might have reached Australia via South America, although fossil evidence for this is lacking.

Baru is positioned at the base of the tribe Mekosuchini, which includes Trilophosuchus(a small crocodile from Riversleigh), Quinkana (from the Pliocene-Pleistocene of Australia), Volia (from the Plesitocene of Fiji); and Mekosuchus (from both Australia and New Caledonia). Closely related species of Baru (B. wickeni and B. huberi) are known from the late Oligocene-Miocene of northern and central Australia.

References

• Molnar, R. E. 1991. Fossil reptiles in Australia. Pp. 605-701 in Vickers-Rich, P., Monaghan, J. M., Baird, R. F. and Rich, T. H. (eds) Vertebrate Palaeontology of Australasia. Pioneer Design Studio and Monash University Publications Committee, Melbourne.
• Nesbitt, S. The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. Bulletin of the American Museum of Natural History 302, 1-84.
• Salisbury, S. W. and Willis, P. M. A., 1996. A new crocodilian from the Early Eocene of southeastern Queensland and a preliminary investigation of the phylogenetic relationships of crocodyloids. Alcheringa 20, 179-226.
• Salisbury, S., Molnar, R. E., Frey, E. and Willis, P. M. A. 2006. The origin of modern crocodyliforms: new evidence from the Cretaceous of Australia. Proceedings of the Royal Society B: Biological Sciences 273, 2439-2448.
• Willis, P. 2006. Evolution and zoogeography of Australasian crocodilians. Pp. 331-348 in Merrick, J. R., Archer, M., Hickey, G. M. and Lee, M. S. Y. (eds) Evolution and Biogeography of Australasian Vertebrates. Australian Scientific Publishing, Oatlands.
• Willis, P. M. A., Murray, P. F. and Megirian, D. 1990. Baru darrowi gen. et sp. nov., a large, broad-snouted crocodyline (Eusuchia: Crocodylidae) from mid-Tertiary freshwater limestones in Northern Australia. Memoirs of the Queensland Museum29, 521-540.
• Willis, P. M. A., Molnar, R. E. and Scanlon, J. D. 1993. An early Eocene crocodilian from Murgon, southeastern Queensland. Kaupia Darmstädter Beiträge zur Naturgeschichte 3, 25-32.

Further reading

• Archer, M., Hand, S. J. and Godthelp, H. 1994. Riversleigh: The Story of Animals in Ancient Rainforests of Inland Australia. Reed Books, Chatswood.
• Kelly, L. 2007. Crocodile: Evolution’s Greatest Survivor. Allen and Unwin, Sydney.