Allometry then locomotor diversification shaped the evolution of lumbar morphology in early placental mammals

Abstract

After the end-Cretaceous mass extinction, placental mammals rapidly diversified in size and locomotor mode, setting the stage for mammals to move into almost every habitat on Earth. Locomotion in extant mammals includes unique sagittal flexion of the trunk primarily driven by lumbar vertebrae, a ribless region of the spine. Consequently, variation in lumbar morphology is associated with a wide variety of locomotor styles. While the origin of this region in early therian mammals in the Mesozoic has been studied, along the therian stem, the later functional diversification of lumbar morphology in placental mammals, the dominant extant group, has been essentially unstudied. We measured the shape of lumbar vertebrae from early placental mammals to test how body size, locomotor specialization, and phylogeny interacted in the diversification of lumbar function after the end-Cretaceous mass extinction. We used 3D geometric morphometrics to quantify shape and compare variation between these Palaeogene mammals and modern mammals. We found that Palaeogene mammals had high disparity in lumbar morphology and that this variation correlated with size and locomotor style. Surprisingly, several ‘archaic’ placentals, like hyaenodontids, showed highly mobile lumbar morphology. These findings show that lumbar vertebrae formed an important and evolvable functional unit at the beginning of the Cenozoic.