Early tetrapod neck mobility modelled using an extant analogue

Tetrapods arose from lobe-finned fishes with a notochord-dominated axial column^1-3. During this transition, the skull became separated from the shoulder girdle^4-6, giving rise to a neck region. This head-shoulder separation has generated hypotheses suggesting that neck mobility increased during the origin of tetrapods, facilitating the ecological transition towards more terrestrial habitats^4,5,7-9. However, incompletely co-ossified vertebral components¹⁰ have posed a challenge to modelling the mobility of these early tetrapod necks. Here we used material tests on the similarly configured axial columns of sturgeon to predict the capacities of the tetrapod neck region for lateral flexion, extension and rotation across the tetrapod stem. Our model indicates that the soft tissue-dominated axial column was surprisingly rigid, despite lacking co-ossified vertebral components. We estimate that most stem tetrapods could move their heads less than 6° laterally and less than 3° dorsally but could not rotate them. Notably, mobility did not shift in conjunction with the head-shoulder separation that occurred between the origin of tristichopterids (represented by Eusthenopteron) and that of more crownward stem tetrapods. Therefore, mobility in the tetrapod neck did not serve as a functional driver of head-shoulder separation. Instead, forelimb-driven locomotion, implicated in early tetrapods^11-14, provides a better functional explanation for the head-shoulder separation.