Why no land tortoises in Australia? part 1

(writing in progress)

Land tortoises are completely absent from Australia, despite occurring in Eurasia, Africa, and the Americas.

The ‘obvious’ reason for the lack of land tortoises in Australia and New Guinea is ‘Wallace’s Line’ = the sea barrier separating Australia and New Guinea from southeast Asia (https://en.wikipedia.org/wiki/Wallace_Line).
 
This is how I would refute this notion, stepwise.
 
Firstly, tortoises have in fact crossed Wallace’s Line, so long ago that there is a great radiation of several families, many genera and > 30 spp. here. However, all the Australian and New Guinean tortoises are restricted to freshwater, and remain amphibious.

The fact that no species has evolved to be fully terrestrial needs an ecological explanation. If it is true, as sometimes claimed, that the ancestors of the freshwater tortoises of Australia and New Guinea have been here since Gondwana times, then all the more time has been available for them to evolve into fully terrestrial species.
 
Secondly, there has long been a convergent lineage of land testudines in Australia and the western Pacific: the meiolaniids (https://en.wikipedia.org/wiki/Meiolaniidae).

These extinct large testudines (https://en.wikipedia.org/wiki/Turtle), which occurred only in a Gondwana-type distribution (not in the Old World), evolved separately from land tortoises but seem to have been terrestrial. They went extinct in the Pleistocene, as recently as a few thousand years ago in nearby New Caledonia. The question here is why these deeply Australian counterparts for land tortoises failed to survive – again suggesting an ecological, not accidental, explanation.
 
Thirdly, tortoises are known to have crossed Wallace’s Line during the Pleistocene in the form of southeast Asian box turtles, which are more terrestrial than the freshwater tortoises of Australia and New Guinea. This species (Cuora amboinensis, https://www.inaturalist.org/taxa/39893-Cuora-amboinensis) survives today in Timor (https://en.wikipedia.org/wiki/Timor), which is the last island of Wallacea (https://en.wikipedia.org/wiki/Wallacea) short of Australia.

Since box turtles have crossed several sea barriers to reach Timor, and are tolerant of submersion, there seems no reason why box turtles could not have crossed the remaining sea barrier to Australia.
 
Fourthly, land tortoises as strictly defined are known to have crossed Wallace’s Line during the Pleistocene in the form of Megalochelys (https://en.wikipedia.org/wiki/Megalochelys). This, the largest of the giant land tortoises of the world, formerly ranged from the southeast Asian mainland to Timor.

Since Megalochelys must, as in the case of the southeast Asian box turtle, have crossed several sea barriers to reach Timor, there seems no reason why this land tortoise could not have crossed the final strait to Australia. So Wallace’s Line is an unsatisfactory explanation, although there remains some legitimate doubt as to whether the Timor Sea proved too wide to cross.
 
Fifthly, land tortoises, as strictly defined, are also known to have crossed Wallace’s Line during the Pleistocene from Borneo (https://en.wikipedia.org/wiki/Borneo) to Sulawesi (https://en.wikipedia.org/wiki/Sulawesi) – where they coexisted with a cockatoo (Cacatua) and marsupials.

Although Sulawesi is far north of Australia, and separated from it by various Indonesian archipelagoes, the Sulawesi land tortoises again show that sea straits pose questionable barriers for large land tortoises.
 
Sixth, land tortoises are known to have crossed sea barriers far wider than the several hundred kilometres separating Wallacea (specifically Timor) from Australia. Farther west in the Indian Ocean, the Aldabra giant tortoise (https://www.inaturalist.org/taxa/539233-Aldabrachelys-gigantea) is known to have reached the Aldabra archipelago (https://en.wikipedia.org/wiki/Aldabra) at least three times from Madagascar (https://en.wikipedia.org/wiki/Madagascar). Indeed, one individual was recently documented to have floated/swam a minimum of 740 kilometres from Aldabra to the East African coast, where it washed up with barnacles covering its legs and sternum.

So, Wallace’s Line explains little, because the width of its sea barriers is less than that separating the Aldabra archipelago from East Africa, and similar to that separating the Aldabra archipelago from Madagascar.
 
Seventh, land tortoises are known to have crossed sea barriers to reach the Galapagos archipelago (https://en.wikipedia.org/wiki/Gal%C3%A1pagos_Islands), which has always been separated from South America by hundreds of kilometres.

Indeed, land tortoises reached South America in the first place from Africa by crossing the Atlantic Ocean at a time when that ocean was still relatively narrow, but wider than the sea barrier separating Wallacea from Australia and New Guinea in the Pleistocene. The arrival of land tortoises in the Galapagos archipelago actually required two consecutive crossings of wide seas, by two different ancestral land tortoises.

So, Wallace’s Line explains little, because its sea barriers are narrower than either the early Atlantic or the distance from the South American mainland to the Galapagos.
 
With seven different reasons to doubt its validity as a barrier to the arrival of land tortoises in Australia and New Guinea, what is left of any argument invoking ‘Wallace’s Line’?

to be continued in ...

(writing in progress)

Publicado el 29 de junio de 2022 por milewski