2. The initial evolution of the vertebrate jaw was followed by rapid biomechanical diversification. Of the roughly 58,000 vertebrate bearing animals living today, 99% have jaws including humans. However, when jaw structures first evolved over 420 million years ago, the world was a very different place full of long extinct animals, many of which have no modern descendents. Having established that biomechanical disparity offered unique insights into the evolution of morphology through time, I used this metric to shine light on the early diversification of the vertebrate jaw. Did the earliest jaw forms stick to a small set of mechanical functions, or show the type of rapid eco-functional diversification usually associated with such innovations? By analyzing the jaw mechanics of the earliest jawed vertebrates (from the Silurian to Devonian periods, ~420-360 million years ago) I found that early, jawed vertebrates went through a rapid functional diversification shortly after jaws first appear in the fossil record (Fig. 2a) (Anderson et al. 2011, Nature). However, these early jaw designs represent a very different kind of world than what we know today. Groups which comprise the vast majority of modern fish diversity, show very humble beginnings, restricted to only a few feeding ecologies, while weird extinct groups (such as placoderms and a surprising number of now extinct lungfishes) show a wide range of feeding ecologies and dominate the jawed vertebrate world (Fig. 2b). Furthermore, when jawed animals first evolved there was a large range of vertebrate animals that lacked jaws. While it has long been assumed that animals with jaws were better adapted, and therefore drove the jawless animals to extinction, faunal comparisons through the Devonian showed that jawless fishes were still a major contributor to vertebrate faunas even as jawed fishes were hitting their peak in feeding function (Fig. 2c).