Fossils of ancient lizards reveal a fourth eye
Most of us, hopefully, have two eyes. But way back in our evolutionary past we may have had even more. Now some reptiles still have this third eye, it’s called the pineal organ, and is a dot on the top of the head that responds to light. Now, scientists have found an extinct lizard with a fourth eye, something that hasn’t been seen in a land dwelling animal before. So what is this eye, and what does it change about our understanding of eye evolution? Georgia Mills spoke to paleontologist Krister Smith from Senckenberg Research Institute in Germany...
Krister - This lizard belongs to a lineage which entered North America about 56 million years ago, and went extinct there around 34 million years ago. It was a monitor lizard; it would have been active, intelligent, and carnivorous, and well over one metre long. And what greatly surprised us is that it appears to have four eyes rather than the usual three.
Georgia - When you say eyes, I picture a normal human eye. I’m guessing they’re not like that so what were they like, and what did they do?
Krister - They would have appeared to us rather different than out lateral eyes, but in another sense they are, anatomically speaking, well-developed eyes. Your lateral eyes are, in fact, part of your brain and the same here applies to the pineal organs. They are outgrowth of the same part of the brain and, in lizards at least, they acquire a ball-like shape with a lens-like layer, and a cornea, and a retina-like like layer and, in that sense, they are eyes. They also perceive light and they’re indicated on the top of the head by a pale spot, which allows the skin to transmit light to the eye.
Georgia - What about this fourth eye, does that exist in anything around today?
Krister - There is, in fact, only one small group of vertebrates today which have four eyes, and those are the lampreys. They’re jawless fish, so they’re fairly distantly related to us or to any of the bony fish that we’re familiar with, or even to sharks. But, like the lizards, their additional eyes are indicated on the top of the head by a pale spot.
Georgia - Do we know what it’s for?
Krister - It’s a remarkable thing that after some 150 years of study, there’s still a great deal to be learned about the functions of the pineal organs in organisms outside of mammals. They have a number of functions just like in mammals. They tend to help regulate the daily cycles, but in lower vertebrates where the pineal organs form additional eyes, they have other functions as well. There is evidence that they are able to use short wavelength light, in particular the polarisation of short wavelength blue light coming from the sky to orient themselves geographically.
Georgia - This sounds really really useful. Why don’t I still have this pineal eye on top of my head?
Krister - One thing that we know is that an additional eye on the top of the head regressed inside the skull like it is in mammals today. In a great number of lineages independently, just thinking about the land vertebrates, it’s gone as a third eye in many amphibians, in mammals, in turtles, crocodiles, bird; it’s gone, and we don’t really know why.
Georgia - Right. We knew about this third eye before, but you’ve found that the fourth eye was in reptiles, which we didn’t expect. What was the evidence for it; how do you know this was indeed the fourth eye you’re looking at?
Krister - We compared the typical third eye with this new hole that we found, that is ultimately what lead us to our study in this particular part of the brain. We had not just a single hole on the top of the head but rather two holes, which is very unusual, and the first thing that we did, of course, was to examine other specimens to make sure that it’s not simply some strange malformation in a single specimen and, in fact, it does occur in others as well. It seems to have appeared somewhere between 52 and 49 million years ago, and there’s a limited number of structures in that part of the head that could possibly account for this extra hole. So we examined them and came to the conclusion that the pineal organ was really the only explanation.
Georgia - What does your finding tell us about these organs evolved?
Krister - There’s much more dynamism in the evolution of these organs than we have previously given them credit for. We used to think that it was a very simple story: we have a pineal eye and it regresses into the brain in many lineages in parallel; however, we can now say that it’s much more complicated than that. The third eye in lizards is not the same as the third eye in other vertebrates and this leads us to to another point: if in lizards, the parapineal took the place of the pineal, then we have no idea when this happened. Every living member of land vertebrates, we know that our ancestors, the ancestors of mammals had a third eye, but we don’t know which organ it was.