2) Special Characteristics of the Reptilian Retina

The retina and photoreceptors of "modern" mammals must have evolved from a reptilian system.

Most reptiles have very elaborate color vision, many of them use variations of skin color
for camouflage, warning or even for surprising mating strategies:

			To these lizards color is much more than fashion. Learn more about their surprising Rock-and-Scissors games at the Lizard Land
		These are all male Californian lizards yet they come in three distinct varieties: orange .............................blue......................................and yellow.

We have reason to assume that - as modern reptiles - dinosaurs and mammalian ancestors
had elaborate color vision capabilities.

It is therefore quite justified to colorize the skins of reconstructed dinosaurs as in this beautiful example of a Parasaurolophus by Joe Tucciarone and Jeff Poling. More pictures at the DINOSAUR PICTURE GALLERY ! © Copyright Information This image here is to be assumed copyrighted by the artists and/or the hosting website. Permission to use the illustrations can only be granted by the artist &/or the website of its origin.

What is special with the photosensory cells of reptilian retinas ? Fortunately we can still study the eyes of modern reptiles and make some assumptions on how the eye of the common ancestors was designed.

The retina and photoreceptors of the Red Eared Turtle are a typical example and relatively well studied. As for most reptiles these turtles have a wealth of cone subtypes with sensitivities ranging from the near Ultraviolet (UV A) to Red.

Special "oil droplets" are arranged in the photoreceptors right below the stacks of photopigment membranes of the outer segment (not visible here). This leads to differential filtering of the incoming light. For example the red oil droplet will remove most exept the longest wavelengths. This sequence of filters is assumed to enhance the turtle's color discrimination under conditions of bright (sun)light. What about the "colorless" oil droplets ? They seem to transmit the sum of all visible wavelength resulting in "White" ? Only a few years ago it was discovered that in fact they belong to two cone types - one sensitive for Blue and one for UV.

Thus the input level of the reptilian visual system is doing a "multi-spectral analysis" using narrow- and broader band filtering of the available light. Perhaps turtles and relatives would see a rainbow with more crisp hues and extending into the UV as illustrated in Fig. 16 of a recent article on the visual capacities of extinct vertebrates by M.Rowe.

Similar sophistication of visual sensory equipment can be found in other reptiles and birds. However all these animals have very few rods, the type of photoreceptors with higher sensitivity working under low light conditions. Even the owls or the nocturnal geckos seem to manage low light not by means of "genuine" rods but by "rod-like" cones - apparently adapted secondarily for better sensitivity. At dinner on Mediterranean vacation one may see the cute little geckoes climbing around the dim light bulbs of Greek Tavernas and Italian Trattorias, catching the attracted insects.

See also article by M. Rowe: A general discussion of the evolution of color vision

		Next page: 3) Two main changes in Mammalian Photoreception
			Are there "living fossils" - preserving some features from those early mammalian descendants of diurnal reptiles ?