Cranial sutures as tools to infer skull function in fossil fish and amphibians: An experimental approach using the extant fish Polypterus

Can functional information be extracted from the morphology of cranial sutures in fossil organisms? Experimental work on extant reptiles and mammals suggests that cranial suture morphology is determined, in part, by the stresses that act on the skull during life. Although some paleontological studies have attempted to use sutures to infer function in fossil fish and amphibians, these works use correlations of suture form and function derived from extant mammals, and do not include experiments on more closely related extant analogues. In this series of studies, I quantify sutural morphology across the fish-amphibian transition, and, using results from strain gage experiments I conducted on the extant fish Polypterus, make functional interpretations of changes in suture shape during this important evolutionary transition.; In Chapter 1, I review the literature on experimental studies directed towards understanding the link between suture form and function. Analyses of extant taxa using in vivo and in vitro methods are discussed, as well as paleontological interpretations of cranial suture shape in fossil vertebrates.; Chapters 2 and 3 focus on suture deformation during feeding and suture shape in the extant fish Polypterus. In Chapter 2, I demonstrate experimentally that sutures in fish exhibit correlations between morphology and function similar to mammalian sutures. Specifically, flat-edged sutures in Polypterus experience tension, while interlocking sutures are loaded in compression. These are the first experiments to measure suture function in fish. In Chapter 3, I present a series of metrics for quantifying suture shape in Polypterus that can be used to rigorously link suture shape and response to loading.; In Chapter 4, I apply this series of metrics that capture suture morphology, and the observed correlations between suture form and function in Polypterus, to fossil taxa that span the fish-amphibian transition. Terrestrial as well as aquatic amphibians are included so that the effect of feeding type on suture morphology can be assessed. These results suggest that the early aquatic amphibian Acanthostega may have been capable of capturing prey on land, based on its sutural morphology.; In Chapter 5, I discuss the implications of this work for paleontological studies of skull function, and present an agenda for future research.