A Silurian maxillate placoderm illuminates jaw evolution

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Science  21 Oct 2016:
Vol. 354, Issue 6310, pp. 334-336
DOI: 10.1126/science.aah3764

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Jaws from the jawless

Until a fossil called Entelognathus was found to contain a tripartite jaw a few years ago, it was believed that the skeletons of early osteichthyans (bony fish), the ancestors of all vertebrates, were derived independently of those of the earlier placoderms (so-called jawless fish). Zhu et al. now describe a second Silurian placoderm that more securely bridges the jawless toothlike plates of placoderms to the development of the jawed condition that ultimately led to the three-boned jaw in ancestors of modern vertebrates (see the Perspective by Long). This finding upends the traditional belief that the two types of jaw were nonhomologous and sheds light on the evolution of the complex maxilla, a key component of diversification across many modern taxa, including humans.

Science, this issue p. 334; see also p. 280


The discovery of Entelognathus revealed the presence of maxilla, premaxilla, and dentary, supposedly diagnostic osteichthyan bones, in a Silurian placoderm. However, the relationship between these marginal jaw bones and the gnathal plates of conventional placoderms, thought to represent the inner dental arcade, remains uncertain. Here we report a second Silurian maxillate placoderm, which bridges the gnathal and maxillate conditions. We propose that the maxilla, premaxilla, and dentary are homologous to the gnathal plates of placoderms and that all belong to the same dental arcade. The gnathal-maxillate transformation occurred concurrently in upper and lower jaws, predating the addition of infradentary bones to the lower jaw.

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