The New Guinea-Australia Nexus: Using molecular approaches to probe biogeographical relationships between Australian and New Guinean freshwater turtles

A project undertaken at The Institute of Applied Ecology, University of Canberra, and supervised by A Georges

The tropics are important repositories for global biodiversity – half of the earth's species reside there. This appears true also of chelid turtles, as the tropical southern lowlands of New Guinea support the highest species richness for this Gondwanal group in Australasia. Moreover, we have come to suspect that the tropics are also the cradle for biodiversity at higher latitudes. Australasian freshwater turtles provide an exemplary model for exploring these issues.

They total 30 species (more when we consider that some are species complexes), achieve their highest diversity in the lowlands of New Guinea, have populations or closely related species in both New Guinea and northern Australia, and show the classic trend of declining diversity with increasing latitude.

Add to this the recurrent pattern of interconnection and isolation of these land masses, injecting vicariance into the equation, and we have a unique natural backdrop to study the biogeographic drivers of evolution and speciation in our region.

This project has yielded greater understanding of the freshwater turtle fauna of Australia and New Guinea, the diversity of forms, the way this biodiversity is distributed, and the contemporary and historical drivers of diversification. Major influences on the contemporary genetic sub-structuring of freshwater turtles of Australia and New Guinea have arisen from a combination of

  1. the development of the Central Ranges and the Langguru Fold and Thrust Belt (ca 8 Mya);
  2. the opening of Cenderawasih Bay (ca 5 Mya);
  3. the deep waters of the Aru Trough and Cenderawasih Bay that penetrate close to the current coastline to maintain isolation of Vogelkop throughout Pleistocene sea level changes;
  4. the exposure of Torres Strait (ca 7 kya, 12 m bpsl) by sea level change;
  5. the exposure of the Arafura Sill (> 22 kya, 53 m bpsl) and broader plateau extending across to Aru Island;
  6. the development of Lake Carpentaria as an inland freshwater lake and associated swamps (12.2 - 14 kya); and
  7. the diversion of the Fly River by uplift along the Oriomi Rise, from Lake Carpentaria to its present discharge (undated).

Among chelid turtles, the endemic Elseya novaeguineae appears to have been the first to disperse to New Guinea, having come to occupy lowlands that are now to the north and south of the central ranges and the Vogelkop and Bomerai Peninsulas before these were isolated by uplift. The affinities of this species with other Australian species is as yet unclear. Remaining chelids dispersed to New Guinea after the Central Ranges became a barrier, and are today restricted to the southern lowlands. The highest species richness of chelid turtles in the Australasian region today lies in these southern lowlands, and in that sense New Guinea is a repository for turtle biodiversity that has been reduced in Australia by progressive aridification. That New Guinea also serves as a cradle for freshwater turtle biodiversity in northern Australia is in the form of evidence of (a) two independent incursions to northern Australia by Emydura subglobosa via the former Lake Carpentaria (Pleistocene) and Torres Strait (Holocene); (b) dispersal of Carettochelys insculpta to Australia via rivers draining the Arafura Shelf to the west ca 1 Mya; and (c) the evidence of complex interactions within the Chelodina rugosa-burrungandjii-parkeri complex, including cross species introgression. We have resolved issues within the species complexes of Elseya and, identifying major evolutionarily significant units within Elseya latisternum, and two new species within the Elseya dentata group.

In this project, we have clarified the role of the history of interconnection between Australia and New Guinea on diversification of the Australian freshwater turtle fauna, knowledge of benefit to science in both Australia and PNG. In particular, understanding the evolutionary impact of the historical interconnections between the two land masses is essential for understanding the natural history of the Australasian fauna.

Figure 1. Emydura sp.

Figure 2. Chelodina canni

Figure 3. Carpentaria map