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Abstract
The vertebrate skull serves critical functions and is likely under multiple evolutionary pressures (phylogenetic history constraining diversification and producing conserved phenotypes; allometry constraining phenotypes due to body size; adaptive response to selective pressures related to microhabitat driving traits to converge). The diverse New World toads (NWT) of the genera Anaxyrus (North America), Incilius (Middle America) and Rhinella (South America) form an ideal clade for investigating the evolutionary processes promoting skull diversity given they occupy varied microhabitats and elevational ranges. Forty-three species are on the IUCN red list and two are Extinct. Although their overall external morphology is relatively conserved, they exhibit remarkable cranial diversity: variation in size and skull shape, including cranial crests and skull ossification. Using CT scans, 3D geometric morphometrics and phylogenetic comparative methods, we quantified the diversity of skull shape and addressed four questions: Is phylogenetic history posing an intrinsic constraint on skull diversification? Is allometry influencing skull diversification? Are ecological pressures posing a selective pressure on skull evolution? We demonstrate substantial variation in skull diversity, with a significant phylogenetic history on skull shape and size. We found three bioclimatic variables associated with NWT skull shape: Temperature Seasonality, Mean Temperature of Wettest Quarter and Precipitation of Coldest Quarter. There is no support for significant correlation between skull shape and skull size, elevation, latitude, longitude, microhabitat and the remaining bioclimatic variables. Our results indicate that Rhinella has the higher disparity in skull shape and that Rhinella disparity is significantly different than in Incilius. Our findings suggest that NWT might be under different selective pressures compared with anurans in general. Although NWT skull shape presents significant influence of phylogenetic history, bioclimatic variables, most taxa possess intermediate shapes, which indicates that they might have been influenced by the interaction of these factors, or others not explored in this study. Furthermore, we highlight that it is necessary to consider that bioclimatic variables might be exerting an indirect effect on NWT skull shape, whether by influencing the abiotic or biotic aspects of their habitat. We highlight the importance of taking into consideration multiple evolutionary processes that may influence the diversification of phenotypes.
