| The Synapsida is composed of living mammals and all of their extinct ancestors, colloquially known as the “mammal-like reptiles.” These fossils bridge the transition from primitive amniotes to forms that were morphologically similar to modern mammals. A major part of this transition involved the reorganization of the locomotor skeleton from an obligate sprawled posture to one much more upright in which the limbs move in a parasagittal plane. During this transition, we see the appearance of many of the features characteristic of mammals, including endothermy, the ability to sustain aerobic activity, the ability to eat and breathe simultaneously, lactation, and diphyodont dentition, all integrated with postural evolution.; Quadrupedal tetrapod posture has been shown to be highly variant, ranging from sprawling, in which the epipodials are held laterally out from the trunk and move in a horizontal plane, to erect, where the epipodials extend more vertically down from the trunk and move in a plane parallel to the long axis of the trunk of the animal. These extremes mark the endpoints of a continuum of locomotor postures observed in extant organisms. Linear and angular measurements were taken on the limbs and girdles of extant lizards, mammals, and crocodilians. These data were applied in a series of multivariate and bivariate analyses to create a set of morphologic features that correlate with differing postures, while maintaining the continuous nature of postural variation in extant tetrapods. The same linear measurements were taken from a cosmopolitan sample of non-mammalian synapsid postcranial specimens and filtered through the{09} morphological correlates defined by extant taxa, in order to infer posture in the extinct taxa. The result is a quantitative means of constraining previous inferences of postural evolution in the non-mammalian synapsids that were based on qualitative examination of specimens. There are also a number of macroevolutionary hypotheses proposed to be predominant over the evolution of the non-mammalian synapsids. The quantitative data obtained from fossil specimens was used to test some of these hypotheses and set up a framework for future, more detailed exploration of synapsid postcranial evolution. |
