Number and arrangement of extraocular muscles in primitive gnathostomes: evidence from extinct placoderm fishes |
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Authors: | Young Gavin C |
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Institution: | Department of Earth and Marine Sciences, Australian National University, Canberra, Australian Capital Territory 0200, Australia. gyoung@ems.anu.edu.au |
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Abstract: | Exceptional braincase preservation in some Devonian placoderm fishes permits interpretation of muscles and cranial nerves controlling eye movement. Placoderms are the only jawed vertebrates with anterior/posterior obliques as in the jawless lamprey, but with the same function as the superior/inferior obliques of other gnathostomes. Evidence of up to seven extraocular muscles suggests that this may be the primitive number for jawed vertebrates. Two muscles innervated by cranial nerve 6 suggest homologies with lampreys and tetrapods. If the extra muscle acquired by gnathostomes was the internal rectus, Devonian fossils show that it had a similar insertion above and behind the eyestalk in both placoderms and basal osteichthyans. |
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Keywords: | eye muscles jaw evolution gnathostome phylogeny |
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