title: Number and arrangement of extraocular muscles in primitive gnathostomes: evidence from extinct placoderm fishes.
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.
authors: Department of Earth and Marine Sciences, Australian National University, Canberra, Australian Capital Territory 0200, Australia. gyoung@ems.anu.edu.au
authors: Gavin C
authors: GC
authors: Young
mesh_terms: D000818:Animals; D005075:Biological Evolution; D053476:Extinction, Biological; D005123:Eye / Q000033:anatomy & histology*; D005399:Fishes / Q000033:anatomy & histology* / Q000235:genetics*; D005580:Fossils; D018482:Muscle, Skeletal / Q000033:anatomy & histology*