The origin of Carettochelys insculpta
The widely accepted theory has it that the ancestors of Carettochelys insculpta migrated to Australia (Daly and Alligator rivers) from New Guinea. "The presence of Aboriginal rock paintings of Carettochelys (Cann 1980; Dupe 1980), dating back more than 7 000 years (Chaloupka pers. comm.), indicates that the species has been a long-term resident of northern Australia (Georges, Wombey)". Whether the species inhabits Australia for 7 000 or 15 000 years, the time has been relatively too short for any morphological differences between the two populations to become evident. Furthermore, the exchange of specimens between the two current locations could have been possible through Torres Strait. Therefore, the period of 7 000 years could be decreased by the time which it took for the sea level to rise to the point where any further exchange of specimens between New Guinea and Australia became impossible. One of the support factors of the theory about Carettochelys migrating to Australia from New Guinea is that there is no fossil evidence of Carettochelys in Australia. On contrary, there is a fossil record of Carettochelys which was made in Papua New Guinea. A single fragment of the nuchal bone and an associated external mould (Glaessner 1942) was collected from marine beds at the mouth of Mariana Creek, Vailala river, Papua New Guinea, and was dated as upper Miocene.
The following picture (Figure 1) illustrates a possible migration of Carettochelys insculpta from New Guinea to northern Australia. The red arrow shows a widely accepted migration through Torres Strait. The green arrow suggest my personal suggestion which is based on two facts. One of them poses a question: If the migration occurred through Torres Strait, why are there no signs* of Carettochelys, whether live specimens or fossils, in Cape York Peninsula? The conditions have been more than suitable for turtles species which is clearly proved by the presence of the Chelidae species in this region. If the Carettochelys species, due to some factors that are unknown to me, was not able to get established in Cape York Peninsula, I doubt it would be able to pass through the peninsula in order to reach Northern Territory.
* I have come across one statement which say Carettochelys insculpta has been spotted in the Wenlock River on the west coast of Cape York (Keith Day in pers. comm. with Webb et al., 1986). However, ths record is known now to be false (Thomson, own data; Georges, pers. comm.
Another fact, which could be used to support my suggestion, is that some time ago (20 000 years), when the sea level was approximately 120 meters lower than it is today, the current Arafura Sea and Gulf of Carpentaria were part of the mainland - a continental shell called Sahul (Chapel 1994). To see the Guinea-Australian land (Sahul), please see the figure 2. It would make sense, if the Carettochelys species chose a more direct migration (see the green arrow, figure 1) and explain why the species is not present in Cape York Peninsula.
Of course, there is also one more assumption about the relationship between the Guinean and Australian populations. Both populations could have been evolving together as one single population within the landmass Sahul and split once the sea level rose up to the level when New Guinea and Australia separated from each other.