By Tripti Chanda
Can you imagine a reptile that had not two but four eyes? Palaeontologists were stunned to find just such a creature, an ancient lizard, that scuttled about the Earth over 50 million years ago. The now-extinct lizard, the Saniwa ensidens, is the ancestor of the modern-day lizards that we see around us today, and seems to be the only jaw-boned vertebrate ever discovered, to have four eyes.
The fossils of the prehistoric creature was actually discovered in the early 1870s. Although the discovery stumped researchers, when they compared the fossils of the ancient creature to others that were also recovered from the same area, they found that the unusual multiple eyes feature occurred in more than one specimen.
“The fossils that we studied were collected in 1871, and they are quite scrappy — really banged up,” Krister Smith at the Senckenberg Research Institute in Germany, said in a statement. “One would be forgiven for looking at them and thinking that they must be useless. Our work shows that even small, fragmentary fossils can be enormously useful.”
What was the ancient lizard like?
According to the researchers, the prehistoric creature measured around 1.3 to 2.15 metres in size. The lizard fossils were first found in the Bridger Basin in southwest Wyoming. It was named by the American palaeontologist Joseph Leidy, after the first sample was discovered in 1870, by an expedition led by Professor F. V. Hayden. After its initial discovery, there were many other samples found all across the world, including Germany, France, Belgium, and Kyrgyzstan. In the US, researchers found other samples in Wyoming, Utah, California, North Dakota, and Colorado.
However till now, all that was known about the lizard was that it was a prehistoric monitor lizard, which resembled the modern monitor lizard very closely. Researchers also estimated that it was a predator, and depending on its size, likely hunted either invertebrates or vertebrates, and perhaps even small mammals and birds.
The whacky four eyes
Palaeontologists believe that four-eyed lizard sheds light on a new aspect of the evolutionary process. According to the researchers, the lizard’s four eyes seemed to consist of one pair, and two other independently working eyes. The two paired eyes are those that are normally found on monitor lizards and other vertebrates, but the other two eyes appear to possess accessory functions which were not limited to vision and the interpretation of light.
According to the researchers, the third and fourth eyes on the lizard are linked to pineal and parapineal organs, which are basically eye-like photo-sensory structures. Both the third and fourth eyes sit atop the lizard’s head, and play key roles in orientation, and in circadian and annual cycles.
“On the one hand, there was this idea that the third eye was simply reduced independently in many different vertebrate groups such as mammals and birds and is retained only in lizards among fully land-dwelling vertebrates,” Smith said. “On the other hand, there was this idea that the lizard third eye developed from a different organ, called the parapineal, which is well developed in lampreys. These two ideas didn’t really cohere.
“By discovering a four-eyed lizardin which both pineal and parapineal organs formed an eye on the top of the headwe could confirm that the lizard third eye really is different from the third eye of other jawed vertebrates,” Smith added.
Although there are several creatures, such as some species of frogs and fishes, that have three eyes, this ancient lizard is the first instance of the discovery of a creature with four eyes. In this case, the eyes are not ocular orbs, instead they are developed and specialised nodes, that are nestled at the crown of the lizards head.
Although previous studies have described the almost pristinely preserved fossils of the Saniwa, they failed to include observations of the existence of the extra node because of its overlap with the third node. A recent CT scan of the fossils helped palaeontologists build a three dimensional model of the skull, which revealed the presence of gaps in the skull, where the third and the fourth eyes could have existed.
The eye as an evolutionary model
Eyes are extremely complex organs that showcase the success of the evolutionary process. It was a self-assembled organ, which went through many modifications to get where it is today. Due to the complexity of its structure, we can determine the evolutionary paths of various organisms easily by studying the structure of their eye. The eye of an average human is different from a birds eye or that of an octopus. There is even a differentiation in the eyes of various birds, depending on their species. These modifications come through millions of years of adaptations and natural selection.
The presence of four eyes on this lizard, and their functions, could be a perfect model to better understand the species’s evolutionary path. In the modern monitor lizard, both these organs are considerably diminished, and exist deep inside their skull cavity. They have no contact with the atmosphere, and have no sensory exposure. The parapineal organ is extremely reduced and hardly functioning, while the pineal organ has lost its photo sensory abilities.
Why is it gone now?
Evolution does not occur in a straight line. An organ that undergoes mutations, does not necessarily develop in a linear fashion. There is always the possibility of a species’s mutations dying out due to the evolutionary development of a particular organ regressing instead of progressing. Based on the initial study of the fossils and the placement of the parapineal nod, researchers believe that this is what may have happened with the ancient lizard too.
Examination of the impressions of the pineal and the parapineal organ on the wall of the skull show that both the nodes might have had latent photo sensory capabilities. This would mean that they were a reduction of lens-bearing ocular devices, which may have existed in the predecessors of the species. This means that there is a possibility of the existence of an undiscovered ancestor of the monitor lizards, that likely lived much before the Saniwa ensidens.
With changes in the environment and random mutations, another branch of the predecessor species could have sprouted, leading to the apperance of the Saniwa ensidens, with the other branch dying out. However, this is just one of the theories being explored in the preliminary studies, and not much is certain at the moment.
Implications of the discovery
The existence of such a sensory model has not been seen or contemplated before. The new discovery could shed light on the environment and the geographical history of the areas where this ancient lizard once lived. It could also tell us more about the predators and prey that lived alongside the ancient creature, and where it stood on the food chain.
This discovery also provides rare insight into the evolutionary history of eyes, and is expected to pave the way for many more such discoveries. The findings of the study have been published in Current Biology.