The evolution of the ‘third eye’ illuminates shifting amniote ecologies through the Late Paleozoic and Triassic

The pineal gland has strong ties to organismal physiology and behaviour, and in several ectothermic clades of tetrapods, it is associated with an eye (i.e. ‘third eye’), which may play a role in thermoregulatory behaviour. Though it is not present in many extant clades of tetrapods, this eye was widespread among tetrapods in the Late Palaeozoic and Triassic, as evidenced by the parietal foramen. Here, we comprehensively reconstruct the evolutionary history of parietal foramen size in Late Palaeozoic and Triassic amniotes for the first time. The size of the parietal foramen, which allows the pineal eye to receive light and regulate metabolic functions, varies significantly across the Permian and Triassic. However, there are similar trends between both major amniote lineages throughout the Permian, hinting at possible convergence in thermoregulatory behaviours between the mammal and reptile lineages during this time. Size trends diverge sharply preceding the mass extinction itself, with the parietal foramen disappearing completely in lineages with presumably higher metabolisms and potential endothermy, including the direct ancestors of mammals and archosaurs, and increasing in size dramatically in dicynodonts and marine reptiles. We discuss these trends and their possible implications for changes in thermoregulation, fossoriality, nocturnality, and aquatic adaptation in the mammal and reptile total clades.