Thursday, November 7, 2019
250 Million Years of Turtle Evolution
250 Million Years of Turtle Evolution In a way, turtle evolution is an easy story to follow: the basic turtle body plan arose very early in the history of life (during the late Triassic period), and has persisted pretty much unchanged down to the present day, with the usual variations in size, habitat, and ornamentation. As with most other types of animals, though, the turtle evolutionary tree includes its share of missing links (some identified, some not), false starts, and short-lived episodes of gigantism. Turtles That Werent: Placodonts of the Triassic Period Before discussing the evolution of genuine turtles, its important to say a few words about convergent evolution: the tendency of creatures that inhabit roughly the same ecosystems to develop roughly the same body plans. As you probably already know, the theme of squat, stubby-legged, slow-moving animal with a big, hard shell to defend itself against predators has been repeated numerous times throughout history: witness dinosaurs like Ankylosaurus and Euoplocephalus and giant Pleistocene mammals like Glyptodon and Doedicurus. This brings us to the placodonts, an obscure family of Triassic reptiles closely related to the plesiosaurs and pliosaurs of the Mesozoic Era. The poster genus for this group, Placodus, was an unremarkable-looking creature that spent most of its time on land, but some of its marine relativesincluding Henodus, Placochelys, and Psephodermalooked uncannily like genuine turtles, with their stubby heads and legs, hard shells, and tough, sometimes toothless beaks. These marine reptiles were as close as you could get to turtles without actually being turtles; sadly, they went extinct as a group about 200 million years ago. The First Turtles Paleontologists still havent identified the exact family of prehistoric reptiles that spawned modern turtles and tortoises, but they do know one thing: it wasnt the placodonts. Lately, the bulk of the evidence points to an ancestral role for Eunotosaurus, a late Permian reptile whose wide, elongated ribs curved over its back (a striking adumbration of the hard shells of later turtles). Eunotosaurus itself seems to have been a pareiasaur, an obscure family of ancient reptiles the most notable member of which was the (completely unshelled) Scutosaurus. Until recently, fossil evidence linking the land-dwelling Eunotosaurus and the giant, marine turtles of the late Cretaceous period was sorely lacking. That all changed in 2008 with two major discoveries: first up was the late Jurassic, western European Eileanchelys, touted by researchers as the earliest marine turtle yet identified. Unfortunately, only a few weeks later, Chinese paleontologists announced the discovery of Odontochelys, which lived a whopping 50 million years earlier. Crucially, this soft-shelled marine turtle possessed a full set of teeth, which subsequent turtles gradually shed over tens of millions of years of evolution. (A new development as of June 2015: researchers have identified a late Triassic proto-turtle, Pappochelys, that was intermediate in form between Eunotosaurus and Odontochelys and thus fills an important gap in the fossil record!) Odontochelys prowled the shallow waters of eastern Asia about 220 million years ago; another important prehistoric turtle, Proganochelys, pops up in the western European fossil record about 10 million years later. This much bigger turtle had fewer teeth than Odontochelys, and the prominent spikes on its neck meant that it couldnt fully retract its head under its shell (it also possessedÃ an ankylosaur-like clubbed tail). Most important, the carapace of Proganochelys was fully baked: hard, snug and pretty much impervious to hungry predators. The Giant Turtles of the Mesozoic and Cenozoic Eras By the early Jurassic period, about 200 million years ago, prehistoric turtles and tortoises were pretty much locked into their modern body plans, though there was still room for innovation. The most notable turtles of the Cretaceous period were a pair of marine giants, Archelon and Protostega, both measuring about 10 feet long from head to tail and weighing about two tons. As you might expect, these giant turtles were equipped with broad, powerful front flippers, the better to propel their bulk through the water; their closest living relative is the much smaller (less than one ton) Leatherback. You have to fast-forward about 60 million years, to the Pleistocene epoch, to find prehistoric turtles that approached the size of this duo (this doesnt mean thatÃ giant turtles werent around in the intervening years, just that we havent found much evidence). The one-ton, southern Asian Colossochelys (formerly classified as a species of Testudo) can pretty much be described as a plus-sized Galapagos tortoise, while the slightly smaller Meiolania from Australia improved on the basic turtle body plan with a spiked tail and a huge, weirdly armored head. (By the way, Meiolania received its nameGreek for little wandererin reference to the contemporary Megalania, a two-ton monitor lizard.) The turtles mentioned above all belong to the cryptodire family, which accounts for the vast majority of marine and terrestrial species. But no discussion about prehistoric turtles would be complete without a mention of the aptly named Stupendemys, a two-ton pleurodire turtle of Pleistocene South America (what distinguishes pleurodire from cryptodire turtles is that they pull their heads into their shells with a sideways, rather than a front-to-back, motion). Stupendemys was far and away the largest freshwater turtle that ever lived; most modern side-necks weigh about 20 pounds, max! And while were on the subject, lets not forget the comparably ginormous Carbonemys, which may have done battle with the giant prehistoric snake Titanoboa 60 million years ago in the swamps of South America.