Evolution of the tetrapod skull: A systematic review of bone loss

Abstract

The simplification of the tetrapod skull occurred convergently in various tetrapod lineages from the Devonian into the Mesozoic, leading to some groups (e.g. lissamphibians) to retain only 19 of the original 41 dermal roof bones present in stem-tetrapods. Despite the potential to shed light on the functional adaptations and developmental mechanisms behind skull simplification, little work has been done on the distribution of bone loss across tetrapod phylogeny. We conducted maximum likelihood ancestral state reconstructions for the presence/absence of temporal and median dermatocranial bones using two large composite trees that placed Lissamphibia either within Temnospondyli or Lepospondyli, reflecting the ongoing debate on lissamphibian origins. Our results indicate that the temporal series did not form a developmental module, as the loss of these bones was quite variable. With the exception of Sauropsida, the intertemporal bone was lost first, followed by the supratemporal, and then the tabular and/or postparietal. In Sauropsida, the tabular and/or postparietal was the second bone to be lost. The supratemporal was lost and regained repeatedly, and was found to be the most variable element, while the nasal, frontal, and parietal were the least variable. Interestingly, the ontogenetic timing of ossification does not correlate with the propensity for a certain bone to be re-acquired or lost. No obvious relationship was found between skull simplification and lifestyle or body size. In summary, the simplification of the dermatocranium is a more complex process than previously thought, and likely involved a mixture of developmental, ecological, and functional drivers.