INSIGHTS TO THE EVOLUTION OF THE PINEAL EYE FROM PHYLOGENETICS AND THE FOSSIL RECORD

In tetrapods that have a pineal eye, it is the primary organ that provides information about light intensity to the pineal organ in the brain. The pineal organ then uses that information to entrain daily physiological and behavioral cycles to the actual day-night cycle. Evolutionary loss of the pineal eye represents a major shift in how the brain uses information from the environment. It is also one of the few such transitions that has happened enough times that we can make statistically testable associations between it and other aspects of the animal’s biology. Although lots of work has been done looking at the structure of pineal eye and its taxonomic distribution, it has not previously been examined in a phylogenetic context, except as one character of many in cladistic analyses. I surveyed the literature and found that not only do many lineages independently lose the parietal eye in their evolution, but some also apparently regain the structure. The pineal eye was ancestrally present among early tetrapods, but was independently lost five times among amphibians and regained once, lost twice and regained twice among cynodonts, lost basally in testudines, and lost and regained varying numbers of times depending on which of the published hypotheses of relationships is followed. The parietal eye has a patchwork distribution among squamates and is either fairly conservative in its evolution if fossorial squamates are all closely related, or the evolution is quite rapid if fossorial squamates are not closely related. Using Maddison’s test for clustered changes, and Pagel’s test for correlated evolution, I found that there is a significant relationship between loss of the pineal eye and either fossorial or nocturnal lifestyles, as well as the formation of a bony lateral wall to the braincase. However, there are still groups that have lost the pineal eye but lack any direct evidence for either of those correlated characteristics, and therefore beg further explanation.