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Fossil Musculature of the Most Primitive Jawed Vertebrates

Science  12 Jul 2013:
Vol. 341, Issue 6142, pp. 160-164
DOI: 10.1126/science.1237275

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From Jawless to Jawed

The earliest vertebrates were jawless. Past reconstructions have assumed that the primitive jawed condition was much like that found in sharks. Trinajstic et al. (p. 160, published online 13 June; see the Perspective by Kuratani) describe fossil musculature from the early jawed placoderms (an extinct class of armored prehistoric fish) that show that the basal structure was distinct from that found in sharks, having a notable dermal joint between the skull and shoulder girdle.

Abstract

The transition from jawless to jawed vertebrates (gnathostomes) resulted in the reconfiguration of the muscles and skeleton of the head, including the creation of a separate shoulder girdle with distinct neck muscles. We describe here the only known examples of preserved musculature from placoderms (extinct armored fishes), the phylogenetically most basal jawed vertebrates. Placoderms possess a regionalized muscular anatomy that differs radically from the musculature of extant sharks, which is often viewed as primitive for gnathostomes. The placoderm data suggest that neck musculature evolved together with a dermal joint between skull and shoulder girdle, not as part of a broadly flexible neck as in sharks, and that transverse abdominal muscles are an innovation of gnathostomes rather than of tetrapods.

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