The lonely eye.

A lthough the dinosaurs are gone, extant vertebrates provide clues to the development of photorecep-tion among the clades that probably preceded and then radiated into the dinosaurs. The ancient order Rhyncho-cephalia, which includes Sphenodon as its only family, with two surviving species, had its heyday between 200 million and 100 million years ago. While this order is reptilian, these creatures were not lizards, but their own order. Approximately 100 million years ago as other members of Rhynchocephalia were succumbing to extinction, Gondwanaland was breaking apart and New Zealand and its islands were breaking free from Australia, thereby isolating this last member of Rhynochocephalia, Sphenodon, presumably without competition. The tuatara survives virtually unchanged, as a fossil relict and provides us with a glimpse of vertebrate evolution from perhaps as long as 200 million years ago. In the early Jurassic (approximately 200 million years ago) tetrapods were radiating into lizards, turtles, crocodiles, dinosaurs, and eventually birds and mammals. Sphenodon was in an order of early reptiles and provides us with a window into those beginnings. The tuatara has a third eye, as do some other reptiles. But this adaptation has been lost in the radiation into later orders such as crocodiles, birds, and mammals, although remnants of this organ can be found in most of these. The third eye, then, represents evolution's earlier approach to photoreception. In the tuatara, as in the other reptiles with the organ, the third eye is a dorsal midline structure just ventral to a parietal plug which is homologous to, and resembles, a cornea. Immediately ventral to the parietal plug is a lens that is surprising similar, at least on a histological basis, to those of the lateral eyes. The vitreous cavity analogue is ventral to the lens and dorsal to a pigment epithelial layer. Ventral to the pigment epithelial cells are the primitive ciliated photoreceptors which are everted (our photoreceptors are inverted with the photopigment on the distal end of the retinal cells). Ganglion cell layers are distal to the photoreceptors, much more like the retina of an octopus rather than that of a vertebrate (see essay BJO July 2003). These structures can be seen in the figure above (Ung CY-J, Molteno ACB, An enigmatic eye: the histology of the tuatara pineal complex. Clin Exp Ophthalmol 2004;32:614–18). The retina is upside down with the retinal pigment epithelium interposed between any incoming light and the photoreceptors! Sphenodon, …