A newly identified fossil could explain one of evolution鈥檚 biggest mysteries 鈥� the origin of the turtle鈥檚 shell.
Bone fragments from a 210-million year-old, land-dwelling reptile from New Mexico suggest that the earliest turtles didn鈥檛 have much of a shell at all.
Over millions of years, rows of protective armour plates gradually fused together and to the reptile鈥檚 vertebrae, eventually creating a complete shell.
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鈥淭urtles ultimately originated from something that looked like an armadillo,鈥� says lead author , a palaeontologist at the Peabody Museum of Natural History in New Haven, Connecticut.
His colleague , of the New Mexico Museum of Natural History and Science in Albuquerque, discovered a neck-bone fragment of the new reptile more than a decade ago, but its provenance remained debatable because the skeleton was so small, Joyce says.
However, recent erosion revealed enough pieces of Chinlechelys tenertesta 鈥� Latin for thin-shelled turtle 鈥� to remove any doubt.
Unlike turtle fossils dating from the later Jurassic era 鈥� 鈥渢hey鈥檙e so common people stopped collecting them,鈥� Joyce says 鈥� Triassic turtles are few and far between. That鈥檚 probably because they lived on land, where fossilisation is far less likely to happen, he says.
The new animal is about 30 centimetres long, with a shell only a millimetre wide. 鈥淭his one鈥檚 by far the thinnest ever found,鈥� Joyce says.
More importantly, the reptile鈥檚 dorsal ribs aren鈥檛 fully fused to its shell 鈥� or carapace 鈥� as is the case in later fossils and in modern turtles.
鈥淭his is a crucial new discovery,鈥� says , at the University of Louisville in Kentucky, who uncovered the first Triassic turtles in northwest Argentina. These and other early turtles had already gained their carapaces and offered few clues as to its origin.
C. tenertesta, on the other hand, points to the body form that must have given rise to the shell. 鈥淭his new guy is an animal that belong to the lineage of turtles, it鈥檚 a proto-turtle in a way,鈥� he says.
Exactly why turtles evolved their shell remains a mystery, Joyce says. A full shell might offer added protection and stability. And the proof could be in the pudding 鈥� their body plan is the world鈥檚 oldest, changing little over 200 million years. 鈥淔or some reason just being a turtle is an idea that came along and just really works,鈥� he says.
Journal reference: (DOI: 10.1098/rspb.2008.1196)
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