Murgonemys braithwaitei

Introduction

Murgonemys braithwaitei, known from a nearly complete shell (carapace), was a trionychid (soft-shelled) turtle and the oldest trionychid from the Southern Hemisphere. Although not found in Australia today, trionychids are known as fossils from the Eocene to Pleistocene, disappearing about 40,000 years ago. Trionychids might have reached Australia by crossing the seas between the islands north of Australia or may have crossed to Australia from South America.

Identification

Trionychids are soft-shelled, freshwater turtles with a leathery skin over the shell or carapace (rather than a covering of horny epidermal scutes, as in other turtles). Trionychids are all quite similar in form, although they have a wide range of adaptations to aquatic life. The bony shell of trionychids is unlike that of turtles with horny scutes: it is low and rounded, the centre of the carapace has a bony layer, and the outer or peripheral ring of bones and marginal scutes have been lost. In many living species, the lower shell (plastron) has been strengthened to compensate for this loss, with dermal bones embedded in the skin (not attached to the plastron but visible as hard, coarse areas on the shell).

Murgonemys is identified as a trionychid because the carapace lacks scutes, the outside surface of the carapace and plastron are sculptured, the carapace and plastron are unfused at the bridge, there is no suprapygal bone, the eighth pleural bones meet at the midline, peripheral bones seem to be absent, and there is strong contact between rib heads and the vertebral centra (from White 2001).

Most Australian fossil trionychids are identifiable only to family. However, Murgonemyshas been placed in the subfamily Trionychinae based on the articulation of the xiphiplastron (the posterior, or fourth lateral, plate in the plastron) and hypoplastron (the third lateral plastron plate), the anterior process of the xiphiplastron positioned lateral to the posterior process of the hypoplastron. Murgonemys is unique in having an expanded nuchal bone with laterally projecting costiform processes, an expanded first neural bone, and seven neural bones in the carapace (from White 2001).

Habitat

The Murgon fossil site during the early Eocene was a shallow swamp or lake. The vegetation and climate of the period have not yet been determined.

Distribution

Murgonemys is known only from the Murgon area in southeastern Queensland.

Feeding and diet

Living trionychids are almost exclusively carnivorous, feeding on freshwater invertebrates (molluscs, aquatic insects, crustaceans and worms) and small aquatic vertebrates (snakes, frogs and fish). Some species also feed on fruit or aquatic plants. The diet of Murgonemys was probably much the same.

Other behaviours and adaptations

The soft, smooth, flexible shell of trionychids (especially the very flexible rear of the shell) is an adaptation for efficient speed and locomotion through open water and in muddy benthic environments. Some of the larger, coastal trionychids (Pelochelys and Chitra of Asia), both similar in size to Murgonemys, have been found swimming at sea.

The presence of Murgonemys in the Eocene of Australia is of interest biogeographically. One scenario suggests that the ancestors of early Australian trionychids crossed the ocean north of Australia from Indonesia via the islands that now comprise Southeast Asia. There are two living coastal trionychid species from Asia known to swim in marine waters, and it is possible that the ancestors of Murgonemys did the same. However, new Miocene-Pliocene fossil trionychids from South America and Central America indicate a radiation of trionychids in South/Central America, raising the possibility that trionychids may have reached Australia through the 'southern connection' across Antarctica.

Life history cycle

Living soft-shelled turtles often lie partly buried in muddy stream beds, where they are almost invisible to predators. They don't need to rise to the surface to breathe as often as other turtles because their skin absorbs oxygen while underwater (the skin of their throats is well vascularised, and the skin of the carapace can also exchange oxygen). However, they are not adverse to a bit of sunbathing, which trionychids living in the cooler northern regions regularly do. Trionychids also have sharp jaws hidden under swollen lips, a second means of defense. Since these traits are common to all trionychids, it is probable that Murgonemys also had these unusual defense mechanisms and adaptations. Such strategies would have been useful at Murgon, where Murgonemys shared its pond with Kambara, an Eocene crocodile.

Fossils description

Murgonemys is known from a complete, semi-articulated shell (carapace) with all but the seventh and eighth right pleurals and the eighth left pleural. There are six thoracic vertebrae attached to the carapace, and an associated left xiphiplastron. The holotype is held by the Queensland Museum in Brisbane. All trionychid fossils from Murgon appear to belong to Murgonemys.

Other trionychid fossils have come from Pliocene and Pleistocene sites in Queensland, and are represented by incomplete, fragmentary material.

Evolutionary relationships

Turtles and tortoises are an ancient group, first recorded from the Triassic (about 220 million years ago). The oldest known turtle, Odontochelys from China, was aquatic and toothed (rather than beaked), with a fully formed plastron but only a partial carapace, possibly the ancestral condition for turtles (to protect the belly from predators) or possibly an aquatic adaptation.

Trionychids are cryptiodires, the largest group of living turtles, characterized by the ability to draw their necks inside their shells. The oldest trionychid may be 'Aspiderites' from the Late Cretaceous of North America. Murgonemys is only known from its shell, and its exact position and relationships within Trionychidae are not clear (diagnostic skull material is needed). The structure of the xiphiplastron suggests that it is a member of the subfamily Trionychinae, the more widespread subfamily of trionychids.

There is now a growing record of trionychids from Gondwana, from the late Miocene to Pliocene of Africa, South America and Central America as well as from the Eocene of Australia. A large trionychid was recently described from Venezuela and a second trionychid from Central America (a species of Apalone, a genus otherwise known from North America). The fossil record of trionychids is now established in both regions, suggesting that trionychids may have successfully colonized Central and South America (rather than arriving as waifs, as previously thought). There was therefore either unexpectedly early Asian immigration to Australia, or dispersal from South America to Australia via Antarctica (although no trionychids have yet been found there). Australian trionychids became extinct in the Pleistocene, when many other (mostly large) animals also became extinct.

References

• Clark, W. B. 1869. Dinornis, an Australian genus. Geology Magazine 6, 383-384.
• Cogger, H. G. and Zweifel, R. G. 1998. Encyclopedia of Reptiles and Amphibians. UNSW Press, Sydney.
• De Vis, C. W. 1894. The lesser chelonians of the Notogherian Drifts. Proceedings of the Royal Society, Queensland 10, 123-127.
• Ernst, C. H. and Barbour, R. W. 1989. Turtles of the World. Smithsonian Press, Washington.
• Gaffney, E. S. and Bartholomai, A., 1979. Fossil trionychids of Australia. Journal of Paleontology 53, 1354-1360.
• Georges, A. and Thomson, S. 2006. Chapter 16: Evolution and zoogeography of Australian freshwater turtles. Pp. 291-308 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.
• Head, J. J., Aguilera, O. A. and Sanchez-Villagra, M. R. 2006. Past colonization of South America by trionychid turtles. Journal of Herpetology 40, 378-381.
• Li, C., Wu, X.-C., Rieppel, O., Wang, L.-T. and Z., L.-J. 2008. An ancestral turtle from the Late Triassic of southwestern China. Nature 456, 497-501.
• Loveridge, A. and Williams, E. E. 1957. Revision of the African tortoises and turtles of the sub-order Cryptodira. Bulletin of the Museum of Comparative Zoology 115, 163-557.
• Meylan, P. A. 1987. The phylogenetic relationships of soft-shelled turtles (family Trionychidae). Bulletin of the American Museum of Natural History 186, 1-101.
• Meylan, P. A. and Gaffney, E. S. 1992. Sinaspiderites is not the oldest trionychid turtle. Journal of Vertebrate Paleontology 12, 257-259.
• White, A. W. 2001. A new Eocene soft-shelled turtle from Murgon, south-eastern Queensland. Memoir - Association of Australasian Palaeontologists 25, 37-44.
• Wood, R. C. and Patterson, P. 1973. A fossil trionychid turtle from South America. Breviora 405, 1-10.

Further reading

• White, A., 1990. 'Murgon's Salad Bowl'. The only thing missing is the dressing. Riversleigh Notes 10, 6-7.