Hoe een schildpad aan zijn schild komt (evolutie, paleontologie)

YALE DAILY NEWS (New Haven, Connecticut) 22 October 08 How the turtle got its shell - A paleontological mystery solved (Divya Subrahmanyam)
Rudyard Kipling seems to have covered the bases with his “Just So” stories: how the camel got its hump, how the rhino got its skin, how the leopard got its spots.
But it seems that he forgot one. How did the turtle get its shell?
It is one of the oldest debates in paleontology, But two paleontologists, Walter Joyce, collections manager at the Peabody Museum of Natural History, and Spencer Lucas, of the New Mexico Museum of Natural History and Science, have put it to rest. Two decades ago, Lucas and his team unearthed a 210-million-year-old turtle shell in New Mexico.
As far back as the 1800s, paleontologists put forth theories explaining how this bizarre reptile developed armor. One such scientist, George Cuvier, posited that the shell had developed from the ribs, flattening and fusing over time. Eventually, this became the dominant theory. But many modern paleontologists came to believe that the shell, which is now tightly fused with the ribs, was actually separate from the ribs and developed from hardened skin.
“It was an ongoing debate — as old as science is,” Joyce said.
Enter Lucas and Joyce.
Twenty years ago, Spencer Lucas and his team found a fragment of bone at their New Mexico dig site. At first, Lucas said, they thought it was a head spike from a type of dinosaur. But because the fragment seemed to be made up of multiple bones, he was unsure. After comparing it to other Triassic-era turtle fossils in a collection in Germany, though, they concluded that it was probably the neck spike of a turtle, and published a paper saying so.
Then, about three years ago, after returning many times to the site, Andrew Heckert, a member of the Lucas team, found many more fragments. This discovery contained not only more neck spikes, but shards of shell.
Lucas emailed Joyce photographs of the specimen. It didn’t take Joyce long to figure out that they belonged to a turtle fossil, making it the first Triassic-era turtle to be found in North America.
Extrapolating from fragments no more than two inches across, Joyce said that the turtle would have been about 15 inches long in real life, with a shell about one millimeter thick.
More important, the underside of the shell revealed important facts about the turtle’s evolutionary history. Contrary to the beliefs of the paleontologists of yore, the ribs and vertebrae of a turtle are clearly visible and separate from the shell.
Though the modern turtle’s ribs and shell — which is made of dermal bone, just like the human skull — are tightly fused, Joyce said the new evidence makes it clear that the structure is a composite, rather than simply an expansion of the ribs as some have argued.
“Through evolution, through time, the two just came together, and the end product is that they’re fused,” Joyce said. “The further you go back in time, the less associated they are with each other.”
The evidence refutes assertions not only by paleontologists, but by embryologists, who operate under the assumption that embryology recapitulates evolution. Human embryos, for example, have gills and tails, mirroring the early stages of human evolution.
In turtles, the ribs and shell grow together from the beginning, which embryologists believe, by analogy, reflects early turtle evolution.
But Joyce said that the sequence of events in embryology often does not reflect what happened in evolution, an opinion supported by the turtle fossil. An alternative hypothesis would explain turtle embryological development over time: the ribs began to grow towards the skin, and instead of the shell growing off of the skin and then fusing with the ribs, evolution allowed it to save a step and the shell began to grow directly off of the ribs.
Joyce said the finding’s significance is twofold. Not only does it provide solid evidence about the origin of the turtle shell, but it allows paleontologists to search less blindly for more turtle fossils.
Joyce said that prior to this find, Triassic turtle fossils were extremely rare, with only about eight throughout the world and none in North America, while fossils from the Tertiary and Jurassic periods abounded. This perplexed paleontologists, since it is at odds with the idea that turtles have thick shells and large bodies, making them more likely to fossilize. Additionally, as aquatic animals, they are likely to be preserved in sediment, which then turns into sedimentary rock, the only type of rock that contains fossils.
But this fossil has an unusually thin shell and seems to be from a terrestrial mammal, Joyce said. It was more likely to be destroyed in fast-moving river environments, and the fragile bones were more likely to fall apart.
Now, Joyce said, paleontologists know to look for smaller specimens in order to reconstruct the evolutionary history of the turtle.
The next step?
Finding them, Joyce said.
“Where do turtles come from? We just have no idea,” he said.
Turtles are the only vertebrates about whose lineages paleontologists are clueless, due to a lack of transitional fossils, he said. Some hypothesize they are related to arcosaurs, which include dinosaurs, others guess they are related to lizards, and still others think they are the most basal of reptiles on the evolutionary totem pole.
He likened the search for turtles’ history to the debate about the origins of birds. That debate, he said, was settled 10 years ago, when scientists started finding so-called “feathered dinosaurs” in China. The connection between those specimens and modern birds was tight enough that it is now almost universally accepted.
“Nowadays there are more and more people doing paleontology, looking in the Triassic, maybe the Permian, in hopes of finding their ancestors,” Joyce said.
But he said he could not set a timeline for this discovery.
“It took us 200 years to get here!” he said with a laugh. “So it might be next year or it might be 50 years from now. It’ll be cool when it happens, that’s all I can say.”
Joyce only has two weeks left as Vertebrate Collections Manager at the Peabody, he said.
He has accepted a junior professorship in paleontology from the University of Tübingen in Germany. But he will still maintain some of his Yale ties: He will continue to work with Tyler Lyson GRD ’12.
Together, they are performing a comprehensive review of turtle species from the late Cretaceous era, which occurred before the extinction of the dinosaurs, and those from the early Paleocene era, which occurred after their extinction, in order to see how turtles responded to whatever killed off the dinosaurs. Lyson said they are trying to determine the proportion of turtles that survived, and whether there are patterns of response among different species.
As far as they can tell, though, turtles seem to have outlasted the dinosaurs.
“Turtles don’t care!” Joyce said jokingly. “The lame dinosaurs go extinct, but the cool turtles survive.”
http://www.yaledailynews.com/articles/view/25933