Sunday, November 17, 2013


     Snakes belong to the suborder Serpentes in the order Squamata (the scaled reptiles). There has been a considerable controversy over their evolutionary origin. The two basic theories are 1)they evolved from terrestrial burrowing reptiles and 2)that they evolved from aquatic reptiles and are closely related to the extinct mosasaurs. In both cases the loss of the legs would aid rather than inhibit mobility. The paleontological evidence is equivocal. In 1997 Michael Caldwell of the University of Edmonton reported a 100-million-year-old fossil of an aquatic snake that retained its hind legs. In contrast others reported a 90-million-year-old fossil of a terrestrial snake that also retained vestigial legs. Molecular studies find little similarity between snakes and living mosasaur relatives such as the Komodo Dragon.

     In a recent brief report in Science Magazine (Science 8 November 2013: Vol. 242 no 6159 p683a- available behind a paywall at Science ) a novel way to approach the question was described. At the 73rd annual meeting of the Society of Vertebrate Paleontology researcher Hong-yu Yi told of how she looked at previously ignored evidence - the anatomy of the inner ear. She CT-scanned the area in question in 10 modern snakes, both aquatic and terrestrial. She also look at 9 modern lizards for comparison.

     In aquatic species the semicircular canal of the ear protruded relatively far from another local structure, the vestibule. The canal protruded far less in land dwelling snakes and lizards. The difference is because of the relative freedom of head movement of the aquatic species.

     She then scanned the preserved skull of a 85-million-year-old fossil snake called Dinilsia. It fell within the terrestrial range. While the evidence isn't definite (criticisms were voiced at the meeting) it adds weight to the hypothesis that the ancestors of snakes were land-dwellers.

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