Original paper

Evolution of the iliosacral joint in diapsid phylogeny

Borsuk-Białynicka, Magdalena

Abstract

The biomechanical advantages of the pelvic structure are inspected in terms of the relative position of the iliosacral joint with respect to the acetabulum. Disarticulated bone material from Lower Triassic karst deposits from Southern Poland provides a unique opportunity to study both sides of the ilium in several early diapsids. Extant squamate material provides a good reference source for this study, and permits the reconstruction of a scenario for iliac evolution in the two neodiapsid clades: Lepidosauromorpha and Archosauromorpha. Ancestrally, the iliosacral joint was probably dorsal and posterior to the acetabulum level. Facultative bipedality may evolve when the iliosacral joint is situated relatively close to the acetabulum, as is common in the Iguania. The more postero-dorsal position of the joint in many non-iguanians correlates with a tendency to limb reduction that develops in parallel in several subgroups. This divergence in locomotor adaptation may have been important in the divergence of the squamate clades. The ventral extension of the iliosacral joint, to overlap the medial side of the acetabulum, is considered a possible synapomorphy of the Archosauriformes, and a possible prerequisite for the obligatory bipedality that eventually evolved in crown group archosaurs. The inclusion of the last dorsal vertebra into the sacrum probably preceded this trend in archosauromorph phylogeny.

Keywords

functional anatomypelvisbipedal locomotiondiapsidabasal archosauriformessquamates