Trilophosuchus rackhami

Introduction

Trilophosuchus rackhami was a small mekosuchine crocodile from the early Miocene of northern Australia. It had a short, deep head, large eyes and three longitudinal ridges along its skull (giving it its name). Trilophosuchus may have been terrestrial rather than aquatic. Its neck musculature, similar to that of other possibly terrestrial crocodiles, suggests that Trilophosuchus held its head above its body like varanid lizards do.

Identification

Mekosuchines are distinguished from other crocodiles by a large size differential between the alveoli of the largest to smallest teeth, development of a wedge of the supraoccipital bone on the top of the skull, and reduction or loss of the anterior process of the palatines (from Willis 2006).

Trilophosuchus had an unusual short, deep skull. Unique features include three longitudinal crests on the skull roof; and long, narrow supratemporal fossae. The structure of the back of the skull (occiput) suggests that this small crocodile may have held its head above its body, and the muscle insertions on the occiput are more like those of terrestrial crocodiles than more aquatic species.

Trilophosuchus has been described as arboreal, an ability unknown in other crocodiles. At least one palaeontologist that has worked with this material believes that Trilophosuchus was not arboreal. The suggestion made here, therefore, is that Trilophosuchus may have been terrestrial but almost certainly did not climb trees.

Habitat

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

Distribution

Trilophosuchus is known only from the Riversleigh World Heritage Fossil Site, northwestern Queensland.

Related to: Trilophosuchus is a monotypic genus, the sole species being T. rackhami. It is most closely related to other endemic Australian mekosuchines in the tribe Mekosuchini (Baru, Quinkana and Mekosuchus).

Feeding and diet

Judging by its small size, Trilophosuchus would have taken small vertebrate animals such as mammals, turtles, snakes and fish. The anatomy of the back of the skull in Trilophosuchus suggests that feeding might have involved rapid side-to-side, up- and-down and rotational movement of the head. It would not have 'rolled' its prey, like the living Saltwater Crocodile, Crocodylus porosus.

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.

The Riversleigh area during the Miocene would have been cooler and wetter than today. Trilophosuchus would have shared its habitat with larger mekosuchine crocodiles (the massive Baru wickeni). However, because of its small size, Trilophosuchus would have occupied a different niche from Baru. Postcranial material is not described, so further speculation about its lifestyle is probably unwarranted.

Fossils description

The holotype of Trilophosuchus is a relatively complete skull (it lacks the front part of the skull anterior to the orbits as well as the anterior part of the braincase). In spite of the large number of crocodile fossils known from Ringtail Site, where the holotype skull was found, it is apparently not possible to assign any of these other fossils (other skull material, mandibles or postcranial material) to Trilophosuchus.

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.

The biogeography of mekosuchines is intriguing, especially if at least some members were terrestrial. Mekosuchines are now known from the Australian mainland, Vanuatu, New Caledonia, Fiji and possibly New Zealand (a distribution similar in part to that of the horned turtles, or meiolanids). At least two small mekosuchines from Pacific islands have been described as possibly terrestrial: Volia athollandersoni from the Pleistocene of Fiji, and Mekosuchus kalpokasi from the Holocene of Vanuatu (possibly going extinct only 3,000 years ago). Crocodiles are a very old group, and the ancestors of mekosuchines may have been in these areas before a split from Antarctica early in the Cainozoic. Mekosuchines alternatively may have dispersed to Pacific islands over a now-submerged land bridge (the Rennell Ridge running from New Guinea/Australia) later in the Cainozoic. It is also possible that both dispersal and vicariance were responsible for mekosuchine distribution, as argued by Ralph Molnar and colleagues (Molnar et al. 2002).

The relationships of Trilophosuchus appear to be with the tribe Mekosuchini, just basal to the crown group including Quinkana (from Australia), Volia and the widespread genus Mekosuchus. Baru is considered the most basal of the Mekosuchini.

References

• Mead, J. I., Steadman, D. W., Bedford, S. H., Bell, C. J. and Spriggs, M. 2002. New extinct mekosuchine crocodile from Vanuatu, South Pacific. Copeia 2002, 632-641.
• 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.
• Molnar, R. E., Worthy, T. and Willis, P. M. A. 2002. An extinct Pleistocene endemic mekosuchine crocodilian from Fiji. Journal of Vertebrate Paleontology 22, 612-628.
• 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. M. A., 1993. Trilophosuchus rackhami, gen. et sp. nov., a new crocodilian from the early Miocene Limestones of Riversleigh, northwestern Queensland. Journal of Vertebrate Paleontology 13, 90-98.
• Willis, P. M. A. 2001. New crocodilian material from the Miocene of Riversleigh (northwestern Queensland, Australia). Pp. 64-74 in Grigg, G. C., Seebacher, F. and Franklin, C. E. (eds) Crocodilian Biology and Evolution. Surrey Beatty and Sons, Sydney.
• 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., 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.