What is driving the differential loss of skull bones throughout tetrapod evolution? Evidence from Temnospondyli

Skull simplification describes the convergent loss of bones in the tetrapod skull over evolutionary time. Much of this trend remains elusive, in particular, the apparent difference in evolutionary conservation across the skull. While some bones (e.g. nasal) have been retained for over 400 million years, others (e.g. tabular) were repeatedly lost. The drivers of this preferential loss remain unknown. We explore whether the discrepancy in evolutionary conservation in the lissamphibian lineage was related to differences in evolvability of skull bones in their stem group, the temnospondyls. Linear morphometrics were applied to temnospondyl skull roof bones to quantify and compare the morphological disparity and evolutionary rates of bones that are lost and retained by lissamphibians. Analyses were conducted at inter- and intraspecific levels to capture the effects of development and natural selection on bone disparity. Bones lost in lissamphibians exhibited greater morphological disparity and evolutionary rates in temnospondyls than retained bones. This indicates that developmental bias may have promoted the retention of conserved bones. The higher disparity and evolutionary rates in bones absent in lissamphibians suggest that they were less constrained by development, potentially facilitating their loss in response to functional demands associated with buccal-pump breathing and eye retraction during feeding.