OLD BONES - SVP'S BLOG

Tortoise: It's What's for Dinner

Tortoises have been on my mind a lot lately. It began when we discovered some giant fossil shell and limb bone remains at one of our sites in Bolivia. My collaborators and I eventually determined they were from a giant tortoise of the genus Chelonoidis, the same genus that includes the famous Galápagos Island tortoises. I knew that smaller species like the red-footed tortoise (Chelonoidis carbonaria) still lived in many areas of South America today, but I was surprised to learn that giants once roamed the continent. I started digging a bit more into the history of these lumbering but lovable megareptiles and came up with some tidbits I thought were worth sharing.


An Aldabra giant tortoise
(Aldabrachelys gigantea) at the Bristol Zoo, one of the few remaining species of giant tortoise. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

The fossil record of South American tortoises extends back about 25-30 million years. The oldest species, Chelonoidis gringorum, was named by none other than the great evolutionary biologist George Simpson. Simpson was usually assiduous about explaining etymologies of his scientific names, but this information is conspicuously absent in this case. One has to conclude that he based gringorum on “gringo” - a word typically used in South America to refer to visitors from the United States - and that he had an impish reason for doing so. But as far as I know, this has never been demonstrated.

The origins of Chelonoidis gringorum are even more interesting than its name, as its ancestors likely floated to South America from Africa. Sound incredible? Check out my last post, which is on trans-oceanic dispersal. A similar scenario explains their presence on many oceanic islands, including Galápagos. Such tortoises are typically quite large (3-5’ long), though it isn’t clear if their large size resulted from or preceded their arrival. Species often evolve larger size on islands, a phenomenon known as island gigantism, but it could also be that giant tortoises are just good at floating to islands. Giant mainland tortoises are known to have inhabited all continents except Australia (and Antarctica) until relatively recently. Then came humans.


A particularly tasty tropical tortoise in Riviera Maya, Mexico. Photo by Dr. Croft. Reuse permitted under
CC BY-NC-SA 2.0.

Being big and armored is generally a good evolutionary strategy, but for tortoises, it lost its effectiveness once humans evolved and spread around the globe. In Africa, giant mainland tortoises went extinct at the end of the Pliocene, about 2.5 million years ago, right about the time humans were becoming proficient at using tools. In what is now Israel, humans were roasting tortoises (albeit medium-sized ones) by more than 400,000 years ago. In mainland South America (as well as North America and Asia), giant tortoises went extinct at the end of the Pleistocene (around 12,000 years ago) along with many giant mammals, probably due in large part to human hunting. And during historical times, ships bringing humans to many oceanic islands signaled the extinction or near-extinction of many insular species.

Interestingly, although tortoises never got a toe-hold in Australia, a completely distinct branch of giant tortoise-like horned turtles known as meiolaniids lived there for tens of millions of years. But like tortoises, these fantastic creatures went extinct in mainland Australia during the Pleistocene and survived on nearby islands until humans arrived a few thousand years ago.

Tomorrow (February 12th) is the birthday of Charles Darwin, a man who was deeply impressed by the giant tortoises of the Galápagos Islands. Thus, it a particularly appropriate time to reflect on these grand creatures and how we can help the remaining subspecies avoid the fate that has befallen so many of their relatives.

-Dr. Darin Croft is an Associate Professor at Case Western Reserve University in Cleveland, Ohio

REFERENCES:

Blasco, R., J. Rosell, K. T. Smith, L. C. Maul, P. Sañudo, R. Barkai, and A. Gopher. 2016. Tortoises as a dietary supplement: A view from the Middle Pleistocene site of Qesem Cave, Israel. Quaternary Science Reviews 133:165-182.

Bonin, F., B. Devaux, and A. Dupré. 2006. Turtles of the World. The Johns Hopkins University Press, Baltimore, 423 pp.

Cadena, E.A., F. Anaya. and D.A. Croft. 2015. Giant fossil tortoise and freshwater chelid turtle remains from the middle Miocene, Quebrada Honda, Bolivia: evidence for lower paleoelevations for the southern Altiplano. Journal of South American Earth Sciences 64:190-198

de la Fuente, M., J. Sterli, and I. Maniel. 2014. Origin, Evolution and Biogeographic History of South American Turtles. Springer Netherlands, Dordrecht, 168 pp.

Harrison, T. 2011. Tortoises (Chelonii, Testudinidae); pp. 479-503 in T. Harrison (ed.), Paleontology and Geology of Laetoli: Human Evolution in Context: Volume 2: Fossil Hominins and the Associated Fauna. Springer Netherlands, Dordrecht.

White, A. W., T. H. Worthy, S. Hawkins, S. Bedford, and M. Spriggs. 2010. Megafaunal meiolaniid horned turtles survived until early human settlement in Vanuatu, Southwest Pacific. Proceedings of the National Academy of Sciences 107:15512-15516.

Zacarías, G. G., M. S. de la Fuente, M. S. Fernández, and A. E. Zurita. 2013. Nueva especie de tortuga terrestre gigante del género Chelonoidis Fitzinger, 1835 (Cryptodira: Testudinidae), del miembro inferior de la Formación Toropí/ Yupoí (Pleistoceno Tardio/Lujanense), Bella Vista, Corrientes, Argentina. Ameghiniana 50:298-318.
Posted: 2/11/2016 12:00:00 AM by croftdarinadmin | with 0 comments
Comments
Blog post currently doesn't have any comments.
 Security code