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