The ocular skeleton through the eye of evo-devo.

An evolutionary developmental (evo-devo) approach to understanding the evolution, homology, and development of structures has proved important for unraveling complex integrated skeletal systems through the use of modules, or modularity. An ocular skeleton, which consists of cartilage and sometimes bone, is present in many vertebrates; however, the origin of these two components remains elusive. Using both paleontological and developmental data, I propose that the vertebrate ocular skeleton is neural crest derived and that a single cranial neural crest module divided early in vertebrate evolution, possibly during the Ordovician, to give rise to an endoskeletal component and an exoskeletal component within the eye. These two components subsequently became uncoupled with respect to timing, placement within the sclera and inductive epithelia, enabling them to evolve independently and to diversify. In some extant groups, these two modules have become reassociated with one another. Furthermore, the data suggest that the endoskeletal component of the ocular skeleton was likely established and therefore evolved before the exoskeletal component. This study provides important insights into the evolution of the ocular skeleton, a region with a long evolutionary history among vertebrates.