THE ORAL APPARATUS OF MARSUPIALS IS MORE INTEGRATED BUT NOT LESS MORPHOLOGICALLY DIVERSE THAN THAT OF PLACENTAL CARNIVORES

Different clades can display extremely different levels of morphological, taxonomical, and ecological variability. Understanding the factors promoting or decreasing organismal variability is a central question for paleobiologists. To answer this question, researchers have focused on identifying the key innovations that have determined patterns of radiation in specific clades. In this context, developmental constraints can have a major impact on morphological variability (e.g., disparity) and the evolutionary trajectories of shape change. Phenotypic integration, or covariation among traits, has been shown to play an important role in shaping organismal disparity at both micro- and macro-evolutionary scales. A longstanding proposition in the field of mammalian evolution has been that morphological variability among marsupials was limited by developmental constraints, in particular, constraints on the marsupial oral apparatus linked to the need for the embryo to access the mother’s teat early in its ontogeny. In this study, we apply a recently proposed Geometric Morphometric approach (i.e., global integration) to investigate phenotypic integration intrinsic to a particular structure, in order to compare morphological disparity in the oral apparatus of marsupial and placental carnivores. Our results show that the marsupial oral apparatus is significantly more integrated than that of placentals; however, at least when fossil specimens are included, morphological disparity among marsupials was not significantly different to that found in placentals. Our findings underscore the importance of including fossil material in evolutionary studies and show that phenotypic integration alone cannot explain the lower morphological variation of extant marsupials. We suggest that extrinsic variables, such as habitat fragmentation, are likely to have played a key role in limiting marsupial disparity.