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2008-10-08 – L’origine del carapace delle tartarughe (origin of the turtle’s shell, Chinlechelys tenertesta, New Mexico USA)

Fossil reveals how the turtle got its shell

  • 11:22 08 October 2008
  • NewScientist.com news service
  • Ewen Callaway

A newly identified fossil could explain one of evolution’s biggest mysteries – the origin of the turtle’s shell.

Royal Society)

Chinlechelys tenertesta - The gradual origin of the turtle shell with two hypothetical ancestors, from an animal with isolated lumps of armour, to one with a complete shell (Image: Royal Society)

Bone fragments from a 210-million year-old, land-dwelling reptile from New Mexico suggest that the earliest turtles didn’t have much of a shell at all.

Over millions of years, rows of protective armour plates gradually fused together and to the reptile’s vertebrae, eventually creating a complete shell.

“Turtles ultimately originated from something that looked like an armadillo,” says lead author Walter Joyce, a palaeontologist at the Peabody Museum of Natural History in New Haven, Connecticut.

His colleague Spencer Lucas, 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 – Latin for thin-shelled turtle – to remove any doubt.

Unlike turtle fossils dating from the later Jurassic era – “they’re so common people stopped collecting them,” Joyce says – Triassic turtles are few and far between. That’s 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. “This one’s by far the thinnest ever found,” Joyce says.

More importantly, the reptile’s dorsal ribs aren’t fully fused to its shell – or carapace – as is the case in later fossils and in modern turtles.

“This is a crucial new discovery,” says Guillermo Rougier, 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. “This new guy is an animal that belong to the lineage of turtles, it’s a proto-turtle in a way,” 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 – their body plan is the world’s oldest, changing little over 200 million years. “For some reason just being a turtle is an idea that came along and just really works,” he says.

Journal reference: Proceedings of the Royal Society B (DOI: 10.1098/rspb.2008.1196)

Evolution – Learn more about the struggle to survive in our comprehensive special report.

Source: http://www.newscientist.com/article/dn14892-fossil-reveals-how-the-turtle-got-its-shell.html?DCMP=ILC-hmts&nsref=news4_head_dn14892

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A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell

Proceedings of the Royal Society B
free previewPDF 
Authors
Walter G. Joyce1, Spencer G. Lucas2, Torsten M. Scheyer3, Andrew B. Heckert4, Adrian P. Hunt2

1 Peabody Museum of Natural History, Yale University, New Haven, CT 06520, USA
2 New Mexico Museum of Natural History and Science, Albuquerque, NM 87104, USA
3 Paläontologisches Institut und Museum, Universität Zürich, 8006 Zürich, Switzerland
4Department of Geology, Appalachian State University, Boone, NC 28608, USA

Abstract

 A new, thin-shelled fossil from the Upper Triassic (Revueltian: Norian) Chinle Group of New Mexico, Chinlechelys tenertesta, is one of the most primitive known unambiguous members of the turtle stem lineage. The thin-shelled nature of the new turtle combined with its likely terrestrial habitat preference hint at taphonomic filters that basal turtles had to overcome before entering the fossil record. Chinlechelys tenertesta possesses neck spines formed by multiple osteoderms, indicating that the earliest known turtles were covered with rows of dermal armour. More importantly, the primitive, vertically oriented dorsal ribs of the new turtle are only poorly associated with the overlying costal bones, indicating that these two structures are independent ossifications in basal turtles. These novel observations lend support to the hypothesis that the turtle shell was originally a complex composite in which dermal armour fused with the endoskeletal ribs and vertebrae of an ancestral lineage instead of forming de novo. The critical shell elements (i.e. costals and neurals) are thus not simple outgrowths of the bone of the endoskeletal elements as has been hypothesized from some embryological observations.

Keywords

Triassic, New Mexico, Testudinata, Chinlechelys tenertesta, origin of the turtle shell

ottobre 8, 2008 - Posted by | - Rettili, 3 Triassico, America Northern, Articolo sc. di riferimento, P - Evoluzione, P - Ritrovamenti fossili, Paleontology / Paleontologia | , , , , , , , , , , , , , , ,

2 commenti »

  1. […] yet retract its neck or feet, and its shell was thinner than a modern turtle’s, but Chinlechelys tenertesta was bristled with sharp spines along its neck and […]

    Pingback di Dear Kitty. Some blog :: Turtle shell evolution :: October :: 2008 | ottobre 8, 2008

  2. […] vedi pure: 2008-10-08 – L’origine del carapace delle tartarughe (origin of the turtle’s shell, Chinlechelys… […]

    Pingback di 2008-12-03 - Tartrughe: L’origine del guscio (Odontochelys semitestacea, turtles shell origin) « Paleonews | dicembre 2, 2008


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