Meeting Abstract
Birds and crocodylians are each others closest living relatives and, with extinct groups like dinosaurs, form the clade Archosauria. The evolution of the bird skull and how birds develop differently from mammals is well studied, but less is known about crocodylian development. It is still unknown how cartilaginous growth zones in the cranium control the size and shape of the crocodylian face. In mammals, cartilaginous growth plates (synchondroses) in the cranial base expand and ossify the skull. In birds, a growth zone at the anterior end of the beaked face expands and likewise shapes the bones of the skull (apical growth). This apical growth may be unique to birds because of their specialized beak and unique patterning of the face. However, without data from crocodylians, it is possible that apical growth of the rostrum is a developmental feature of Archosauria. We studied the dynamics of cell proliferation in developing embryos of Alligator mississippiensis using in ovo injection of 5-ethynyl-20-deoxyuridine (EdU) at key developmental stages during snout morphogenesis. Identification of EdU-positive cells in medial sections of the developing face allowed us to characterize the pattern of facial growth in A. mississippiensis. During early stages growth was distributed throughout the rostrum and not constrained in an apical growth zone. Later stages with well defined cranial cartilages do not show concentrated growth zones within. This suggests crocodylians have a unique pattern of growth from either mammalian or avian models. Our study adds important data about amniote cranial growth patterns, which is key to understanding the evolution of facial morphogenesis.
