XRCT SCANS REVEAL UNUSUAL CHARACTERISTICS OF LUMBAR VERTEBRAE IN EXTINCT MAMMALS

Ribless lumbar vertebrae, a characteristic unique to mammals, facilitate dorsoventral flexion of the spine during locomotion. In extant mammals, lumbar vertebrae range from more flexible forms to more stabilizing forms. However, the evolution and range of function in extinct mammals is poorly understood due to the rarity of intact fossils associated with identifiable material, the difficulty of separating the delicate structures from rock matrix, and general lack of study. To examine the changes in lumbar function through time, we acquired X-ray computed tomography (XRCT) scan data of lumbar vertebrae from fossils representing two points in mammal evolution: Adalatherium hui (UA 9030) and Patriofelis ulta (UMNH 550). Adalatherium scans are from Morphosource, and Patriofelis scans were done at the University of Utah hospital. Adalatherium, a mammaliaform from the Cretaceous of Madagascar, was chosen as a representative outside of crown clade Mammalia, and Patriofelis, a placental mammal from the Eocene of North America, was chosen as a representative within the crown clade but from an extinct order. We then compared the shape of the fossil vertebrae to vertebrae from extant mammals to test if the fossils fell within the shape range of modern mammals. We used a NextEngine laser scanner to digitize full lumbar vertebrae columns from 8 species of extant mammals across 6 orders. We quantified shape with 3D geometric morphometrics by placing 16 landmarks on each vertebra. We then performed a generalized Procrustes analysis and principal components analysis (PCA) on the landmark data to visualize major axes of variation. On PC 1 and 2, both Adalatherium and Patriofelis fall within the range of modern mammals. However, this is likely caused by the extreme morphology of armadillo metapophyses dominating these axes. On PC3 and PC4, Adalatherium is separated from extant mammals, while Patriofelis remains in their range. When the armadillo is removed from the analysis, Patriofelis falls within the range of extant mammals, closest to carnivorans, but Adalatherium sits in a separate part of the shape space. The shape of Adalatherium lumbar indicates a more flexible lumbar column, while the Patriofelis lumbar is more stabilizing. This suggests that the locomotor function of spinal flexion was present early in mammalian evolution.