Evolution and function of anterior cervical vertebral fusion in tetrapods

Abstract

The evolution of vertebral fusion is a poorly understood phenomenon that results in the loss of mobility between sequential vertebrae. Non-pathological fusion of the anterior cervical vertebrae has evolved independently in numerous extant and extinct mammals and reptiles, suggesting that the formation of a 'syncervical' is an adaptation that arose to confer biomechanical advantage(s) in these lineages. We review syncervical anatomy and evolution in a broad phylogenetic context for the first time and provide a comprehensive summary of proposed adaptive hypotheses. The syncervical generally consists of two vertebrae (e.g. hornbills, porcupines, dolphins) but can include fusion of seven cervical vertebrae in some cetaceans. Based on the ecologies of taxa with this trait, cervical fusion most often occurs in fossorial and pelagic taxa. In fossorial taxa, the syncervical likely increases the out-lever force during head-lift digging. In cetaceans and ricochetal rodents, the syncervical may stabilize the head and neck during locomotion, although considerable variation exists in its composition without apparent variability in locomotion. Alternatively, the highly reduced cervical vertebral centra may require fusion to prevent mechanical failure of the vertebrae. In birds, the syncervical of hornbills may have evolved in response to their unique casque-butting behaviour, or due to increased head mass. The general correlation between ecological traits and the presence of a syncervical in extant taxa allows more accurate interpretation of extinct animals that also exhibit this unique trait. For example, syncervicals evolved independently in several groups of marine reptiles and may have functioned to stabilize the head at the craniocervical joint during pelagic locomotion, as in cetaceans. Overall, the origin and function of fused cervical vertebrae is poorly understood, emphasizing the need for future comparative biomechanical studies interpreted in an evolutionary context.

Unusual cranial and postcranial anatomy in the archetypal ankylosaur Ankylosaurus magniventris

“…The propensity for some ankylosaurs to dig was also suggested by Coombs (1978b) based on the inferred muscle action of the forelimb and shoulder joint morphology. This behaviour is also consistent with the presence of a syncervical in some species (VanBuren and Evans 2017), although the condition is unknown in Ankylosaurus.…”

Head‐turning morphologies: Evolution of shape diversity in the mammalian atlas–axis complex

“…Some species included in our study have fused portions of the cervical vertebral column, resulting in a syncervical anatomy (Vanburen and Evans ). In species with a fused atlas and axis (C1‐C2; the whale Berardius bardii and dolphin Lagenorhynchus obliquidens ), we opted to treat the syncervical as analogous to the atlas, considering it a first functional unit of the cervical spine.…”

Mineralisation of theCallorhinchusvertebral column (Holocephali; Chondrichthyes)

“…The synarcual is a fused element in the anterior vertebral column (Claeson, 2011;Johanson et al, 2013Johanson et al, , 2015Johanson et al, , 2019VanBuren and Evans, 2017) and is one of the better anatomical structures for mineralised tissue characterization, being formed early in development and also mineralising early (Johanson et al, 2015(Johanson et al, , 2019. Synarcual mineralisation progresses from anterior to posterior, and dorsal to ventral, allowing the observation of different mineralisation patterns and stages within a single anatomical structure (Johanson et al, 2015).…”