1. One difficulty in the evolution of gnathostomes (that we completely ignored) is the fact that
the earliest jawed placoderms did not have a jaw structure that can be directly matched with modern gnathostomes. Specifically, these ancestral placoderms had a upper and lower jaws each with a single element made of endochondral bone with no teeth. Why would this situation be a problem for discussing the evolution of modern gnathostome jaws? Apply the phylogeny and development course themes to propose a hypothesis to explain how the modern gnathostome jaw could have evolved from these precursors. For each theme, what type of evidence would you need to support this hypothesis and why would that evidence support your ideas?