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2009-06-18 – Limusaurus inextricabilis and the evolution of dinosaurs hands

per informazioni in italiano vedi:

BLOG THEROPODA – Limusaurus inextricabilis Xu et al. (2009) – Prima Parte: Un Ceratosauria senza denti dal Giurassico Superiore della Cina!

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New dinosaur gives bird wing clue

Limusarus fossil (James Clark)
The Limusaurus fossil sits among small crocodile fossils

A new dinosaur unearthed in western China has shed light on the evolution from dinosaur hands to the wing bones in today’s birds.

The fossil, from about 160 million years ago, has been named Limusaurus inextricabilis.

The find contributes to a debate over how an ancestral hand with five digits evolved to one with three in birds.

The work, published in Nature, suggests that the middle three digits, rather than the “thumb” and first two, remain.

Theropods – the group of dinosaurs ancestral to modern birds and which include the fearsome Tyrannosaurus rex – are known for having hands and feet with just three digits.

It’s a really weird animal – it’s got no teeth, had a beak and a very long neck, and very wimpy forelimbs
James Clark, GWU

It has been a matter of debate how the three-fingered hand developed from its five-fingered ancestor. Each digit among the five was composed of a specific number of bones, or phalanges.

Palaeontologists have long argued that it is the first (corresponding to the thumb), second, and third fingers from that ancestral hand that survived through to modern birds, on grounds that the three fingers in later animals exhibit the correct number of phalanges.

However, developmental biologists have shown that bird embryos show growth of all five digits, but it is the first and fifth that later stop growing and are reabsorbed.

The remaining three bones fuse and form a vestigial “hand” hidden in the middle of a bird’s wing.

‘Weird animal’

James Clark of George Washington University in Washington DC and Xing Xu from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing hit an palaeontologist’s gold mine in the Junggar Basin of northwestern China.

Previous digs have unearthed the oldest known fossil belonging to the tyrannosaur group and the oldest horned dinosaur among several others.

Limusarus fossil (Portia Sloan)
The dinosaurs had beaks and may have had feathers

This time, the ancient mire has yielded a primitive ceratosaur, a theropod that often had horns or crests, many of whom had knobbly fingers without claws.

“It’s a really weird animal – it’s got no teeth, had a beak and a very long neck, and very wimpy forelimbs,” Professor Clark told BBC News.

“Then when we looked closely at the hand, we noticed it was relevant to a very big question in palaeontology.”

The fossil has a first finger which is barely present, made up of just one small bone near the wrist. The fifth finger is gone altogether.

It is a fossil that appears to offer a snapshot of evolution, proving that the more modern three-fingered hand is made up of the middle digits of the ancestral hand, with the outer two being shed.

The third finger is made up of the four phalange bones that the second should have, and it is presumed that the second would lose one bone to become like the first finger that was missing in the fossil.

This process of shifting patterns of gene expression from one limb or digit to another is known as an “identity shift”, and was again caught in the act – making the conflicting theories of bird hand origin suddenly align.

“This is amazing – it’s the first time we’ve seen this thing actually starting to disappear,” Jack Conrad, a palaeontologist at the American Museum of Natural History, told BBC News.

“There’s been this fundamental rift – there was no way to make peace between the good data we were seeing from the developmental biologists and the palaeontological evidence that showed with every fossil we found we were seeing [fingers] one, two and three.”

from: http://news.bbc.co.uk/1/hi/sci/tech/8105513.stm

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Other links:

Fossil Solves Mystery of Dinosaur Finger Evolution

FOXNews – ‎17/giu/2009‎

By Jeanna Bryner Bird wings clearly share ancestry with dinosaur “hands” or forelimbs. A school kid can see it in the bones.

Fossil Catches Dinosaur Red-Handed, Evolving Into Bird Wired News

NEW DINOSAUR: Fossil Fingers Solve Bird Wing Mystery? National Geographic

giugno 18, 2009 - Posted by | - R. Dinosauri, - Teropodi, An. Vertebrates, Articolo sc. di riferimento, Asia, Italiano (riassunto), P - Evoluzione, P - Ritrovamenti fossili, Paleontology / Paleontologia, X - Nature | ,

1 commento »

  1. Vargas, AO, Wagner GP, and Gauthier, JA. Limusaurus and bird digit identity. hdl.handle.net/10101/npre.2009.3828.1

    Here is our response to the Limusaurus paper. It was recently rejected by nature, not for any technical reason but because it was considered not to be of sufficient interest/importance.

    We have uploaded it at the nature precedings citable archive, because we think it is important there is a quick and citable reply that unlike Xu’s proposal, is consistent with the view of the larger community of theropod paleontologists, namely, that tetanuran digits still are I, II, III. We are preparing a longer paper on this topic.

    Commento di thearchosaur | ottobre 10, 2009


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