The iliosacral joint in lizards: an osteological and histological analysis

Abstract

The development of the iliosacral joint (ISJ) in tetrapods represented a crucial step in the evolution of terrestrial locomotion. This structure is responsible for transferring forces between the vertebral column and appendicular skeleton, thus supporting the bodyweight on land. However, most research dealing with the water‐to‐land transition and biomechanical studies in general has focused exclusively on the articulation between the pelvic girdle and femur. Our knowledge about the contact between the pelvic girdle and vertebral column (i.e. the ISJ) at a tissue level is restricted so far to human anatomy, with little to no information available on other tetrapods. This lack of data limits our understanding of the development and evolution of such a key structure, and thus on the pattern and processes of the evolution of terrestrial locomotion. Therefore, we investigated the macro‐ and microanatomy of the ISJ in limb‐bearing squamates that, similar to most non‐mammalian, non‐avian tetrapods, possess only two sacral ribs articulating with the posterior process of the ilium. Using a combination of osteology, micro‐computed tomography and histology, we collected data on the ISJ apparatus of numerous specimens, sampling different taxa and different ontogenetic stages. Osteologically, we recorded consistent variability in all three processes of the ilium (preacetabular, supracetabular and posterior) and sacral ribs that correlate with posture and locomotion. The presence of a cavity between the ilium and sacral ribs, abundant articular cartilage and fibrocartilage, and a surrounding membrane of dense fibrous connective tissue allowed us to define this contact as a synovial joint. By comparison, the two sacral ribs are connected to each other mostly by dense fibrous tissue, with some cartilage found more distally along the margins of the two ribs, defining this joint as a combination of a syndesmosis and synchondrosis. Considering the intermediary position of the ISJ between the axial and appendicular skeletons, the shape of the articular surfaces of the sacral ribs and ilium, and the characteristics of the muscles associated with this structure, we argue that the mobility of the ISJ is primarily driven by the movements of the hindlimb during locomotion. We hypothesize that limited torsion of the ilium at the ISJ happens when the hip is abducted, and the joint is likely able to absorb the compressional and extensional forces related to the protraction and retraction of the femur. The mix of fibres and cartilage between the two sacral ribs instead serves primarily as a shock absorber, with the potential for limited vertical translation during locomotion.

Cranial anatomy of the Galápagos marine iguana Amblyrhynchus cristatus (Squamata: Iguanidae)

“…Most of the studies on this species have focused on physiology or molecular biology to assess phylogenetic relationships (e.g., MacLeod et al, 2015; Miralles et al, 2017; Rassmann, 1997; Rassmann, Tautz, et al, 1997; Rassmann, Trillmich, & Tautz, 1997). There are virtually no studies on the cranial anatomy of Amblyrhynchus , with the few studies available on skeletal adaptations being limited to the post‐cranium (Hugi & Sánchez‐Villagra, 2012; Paparella et al, 2020). To date, the best source of information on the anatomy of Amblyrhynchus is that of de Queiroz (1987), which included a thorough description of the morphological characters he used to assess the relationships amongst iguanids (i.e., iguanines in his work).…”

The appendicular myology of Stegoceras validum (Ornithischia: Pachycephalosauridae) and implications for the head-butting hypothesis

“…Another hypothesis results from comparison with a possibly analogous structure to the iliac flange in squamates, the iliac tubercle or supracetabular process [ 113 ]. This process serves as an attachment area for the iliopubic ligament, iliocostalis, and quadratus lumborum muscles, all of which attach to the dorsal margin and medial surfaces of the process [ 113 ].…”

Plicidentine and the repeated origins of snake venom fangs

“…The decalcified heads were embedded in paraffin wax and then sliced coronally using a microtome and the slides were stained with haematoxylin and eosin or Gomori's trichrome. Staining protocols follow the guidelines published in previous studies [ 30 , 31 ].…”