Letters From the World of Turtle Evolution – Scientific American (blog)

Im currently deep in the world of turtles its because of the textbook. And long-time readers will know that I suffer from Turtle Guilt anyway and have long aimed to put things right. In view of both of these things combined with the fact that I feel the urge to produce a new Tet Zoo article here are some brief thoughts on turtles that I hope you find interesting. The world of turtle evolutionary history and phylogenetic research is rich and complex, so I thought it would be fun to throw out a small selection of interesting factoids, not to focus on one specialised area. Here we go

Whos on the stem, whos in the crown? If you know anything about the geological history of turtles, youll be aware that a few anatomically archaic Late Triassic and Early Jurassic turtles have been regarded as the oldest representatives of Cryptodira and Pleurodira, the two great turtle groups that exist today. Most notable among these are the Late Triassic Proterochersis (originally described as the oldest known pleurodire) and the Early Jurassic Kayentachelys (originally described as the oldest cryptodire). A Late Triassic pleurodire would mean that the common ancestor of crown turtles was in existence by this time.

But this has been challenged. In a study devoted to phylogenetic analysis of Mesozoic turtles, Joyce (2007) argued that these early turtles are outside the crown group (crown group = the clade that contains living species and all descendants of their most recent common ancestor), and that crown turtles did not, in fact, evolve until considerably later (the Late Jurassic). Joyce (2007) and, later, other authors (Sterli et al. 2013) went further, proposing that a large number of additional taxa among them the remarkable meiolaniids of the Cretaceous and Cenozoic, the fabled Kallokibotion of the Late Cretaceous of Romania and the diverse and abundant baenids of the Cretaceous and Paleogene were stem-turtles too, not archaic cryptodires as long thought. Thisreallocation of taxa and revised view of turtle history has been accepted by some turtle specialists but not by others, and these two schools of thought currently appear to be at an impasse.

You might be thinking that none of this matters much, and perhaps youd be right. But the proposal that those archaic turtle lineages are outside the crown has some important implications: meiolaniids, you see, persisted to relatively recent times, their youngest geological occurrence being from the Holocene of Vanuatuwhere theyve been dated to just c 3000 years ago. We only just missed them, and by missed them I mean that ancient members of our species hunted them to extinction. Had they persisted to the present, we would according to the model proposed by Joyce have especially archaic, early-diverging turtles still with us today, members of a lineage that originated far earlier in the Mesozoic than the other turtle lineages still present. A complication here is that if meiolaniids were still alive our definition of the turtle crown would be far more inclusive, since all or virtually all of those lineages outside the cryptodire-pleurodire clade would now be inside the crown.

Side-necked turtles once lived just about everywhere. Today, pleurodires the side-necked turtles are southern animals of Africa, Madagascar, South America and Australasia. But the fossil record shows that this is absolutely not reflective of their distribution in the past: they were effectively cosmopolitan, with species across North America, Europe and Asia. Most of these animals belonged to groups that are now wholly extinct, like the bothremydids: these were around from the Late Cretaceous until eitherthe Oligocene or Miocene (Lapparent de Broin & Werner 1998, Gaffney et al. 2006). But others belonged to groups that now have a more restricted distribution: Neochelys known from around 8 species that inhabited Europe during the Eocene is a member of Podocnemididae, a group only present today in South America and Madagascar.

Archelon is not the biggest turtle. It has often been said, or at best implied, that certain of the Late Cretaceous marine protostegids in particular the famous Archelon (Archelon! Archelon!, quoth Raquel Welch, 1966) were the biggest turtles ever. This hasnt been true for a while, even though those particular turtles sure were big. Nope, the biggest turtles of all are pleurodires, the record-holder being Stupendemys of the Upper Miocene and Pliocene of northern South America. This giant reached 3.3 m in carapace length and thus must have exceeded 5 m in total length. Incidentally, if youve been to the AMNH in New York and seen the Stupendemys on display there, note that its skull is not actually that of Stupendemys, its an enlarged replica of the skull of another sort of pleurodire: the very deep-faced Miocene podocnemidid Caninemys, named on account of its bulldog-like appearance (Meylan et al. 2009).

Giant tortoises were formerly widespread, and not just on islands. Today we associate giant tortoises with oceanic islands, most famously the Galpagos but also the Seychelles. If youre up to speed on recently extinct animals youll also be aware of the recently extinct Cylindraspis tortoises of the Mascarenes, and perhaps of the big tortoises that also once occurred on the Caribbean islands. The impression you get from these animals is that giant size in tortoises was an island thing, and that tortoises are only able to achieve giant size when evolving in isolation from continental predators. But the fossil record paints a different picture.

Giant tortoises those with a carapace length exceeding 70 cm were a widespread presence in continental habitats too, and in fact have been since the Oligocene at least. Taraschelon an Oligocene form from France reached c 80 cm in carapace length. The biggest tortoise of all Megalochelys atlas (carapace length 2.1 m, mass c 1000 kg) inhabited southern Asia between the Miocene and Pleistocene and lived alongside a typical assortment of big continental mammals, and similarly big tortoises (they may be additional specimens of Megalochelys) also inhabited eastern Europe during the Pliocene (Boev 2008). Another giant Cheirogaster was present in Greece during the Miocene, Pliocene and Pleistocene. Some Cheirogaster specimens exceed 1.5 m in carapace length; the skull alone can be 23 cm long. Europe was also home to several species of Titanochelon during the Miocene and Pliocene: this animal occurred from Portugal all the way to western Asia and seems to have had a carapace length of about 1.2 m. North America was home to Hesperotestudo during the Pleistocene, some species of which rivalled Galpagos giant tortoises in size. This brief listing is far from complete, but you get the point: there were really big, fully terrestrial tortoises in many continental environments during the Cenozoic.

Its also worth noting that giant continental tortoises still exist today: there are some big South American Chelonoidis species, and the very large Centrochelys and Stigmochelys species of Africa. Another assumption that these animals were limited to tropical, frost-free places is also challenged by the fossil record, since some of these very large tortoises (thinking here of the North American Hesperotestudo) appear to have been able to dig deep burrows and avoid the cold surface temperatures sometimes present in the places where they occurred (thanks to Mark Gelbart for this idea).

That will do for now. We will revisit turtles again soon. While on the subject of this group, my review of Olivier Rieppelnew book Turtles As Hopeful Monsters has recently been published (Naish 2017). For previous Tet Zoo turtle articles, see

Refs - -

Bakker, R. T. 1986. The Dinosaur Heresies. Penguin Books, London.

Boev, Z. 2008. First finds of giant land tortoises discovered in Bulgaria. Science News April 2008, 2-4.

Gaffney, E. S., Tong, H. & Meylan, P. A. 2006. Evolution of the side-necked turtles: the families Bothremydidae, Euraxemydidae, and Araripemydidae. Bulletin of the American Museum of Natural History 300, 1-700.

Joyce, W. G. 2007. Phylogenetic relationships of Mesozoic turtles. Bulletin of the Peabody Museum of Natural History 48, 3-102.

Lapparent de Broin, F. de & Werner, C. 1998. New late Cretaceous turtles from the Western Desert, Egypt. Annales de Palontologie 84, 131-214.

Meylan, P. A., Gaffney, E. S. & Campos, D. de A. 2009. Caninemys, a new side-necked turtle (Pelomedusoides: Podocnemididae) from the Miocene of Brazil. American Museum Novitates 3639, 1-26.

Naish, D. 2017. Review of Turtles as Hopeful Monsters: Origins and Evolution. Palaeontologia ElectronicaVol. 20, Issue 2; 1R: 3p.

Sterli, J., de la Fuente, M. & Cerda, I. A. 2013. A new species of meiolaniform turtle and a revision of the Late Cretaceous Meiolaniformes of South America. Ameghiniana 50, 240-256.

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Letters From the World of Turtle Evolution - Scientific American (blog)