The fossil history of snakes is very poorly known, since snake skeletons are very delicate and do not fossilize easily. By carefully examining the fossil materials which have been recovered, and by making comparisons of the anatomy of living snakes and their relatives, biologists have been able to reconstruct something of the evolutionary history of snakes.
The earliest known reptiles belong to a group known as the Cotylosaurs, or “stem reptiles”, which are believed by paleontologists to be ancestral to all of the reptile families alive today. The Cotylosaurs first appeared during the Permian, the period of time that immediately preceded the rise of the dinosaurs. During the next few million years, the Cotylosaurs diverged into three distinct groups of reptiles which are distinguished from each other by their differing skull structures. The earliest of the Cotylosaurs were Anapsids, which means that they lacked any arches or openings between their skull bones. The Anapsids eventually went on to produce the modern turtles. Later, another group of Cotylosaurs developed a single arch in the skull, between the postorbital and squamosal bones, through which the jaw muscles passed. These reptiles are known as Synapsids, and they went on to evolve into the modern mammals. The third group of reptiles, the Diapsids, diversified to produce the extinct dinosaurs as well as the modern lizards and snakes. Thus, although snakes are not direct descendants of the dinosaurs, they are distant evolutionary cousins of Tyrannosaurus and Triceratops.
One of the earliest snakes to appear in the fossil record has been given the scientific nameLapparentophis defrenni. It was found in the Saharan Desert and has been dated to the early Cretaceous period, about 130 million years ago. Although the fossil consisted of only a few back bones and was missing all the ribs and the entire skull, the structure of the vertebrae is characteristic of that of snakes. Other fossils, also consisting of just vertebrae, have been found in Spain and northern Africa that may be a few million years older than Lapparentophis. These have been named Coniophis.
One of the most significant fossil snakes to be discovered was Pachyrhachis , found in 100-million year old deposits in the Middle East. It is the most primitive-looking snake to be found: the skeleton has tiny rear legs, complete with hip, knee, and ankle joints, demonstrating that snakes evolved from legged lizard ancestors. Rear limb bones have also been found in other Cretaceous Middle Eastern snakes called Haasiophis and Eupodophis, and in a later South American snake called Najash. The Middle Eastern snakes were found in marine deposits, but the South American Najash was terrestrial.
Another very early snake has been found in marine deposits in North Africa and Europe. This snake, which also lived about 100 million years ago, has been called Simoliophis. Another well-studied snake found in marine deposits is Pachyophis, in the same family as Simoliophis. It has been found in late Cretaceous deposits in Bosnia. It had a small head and a laterally-compressed body that helped it swim, and seems to have spent much of its time in the water, like modern sea snakes, though it was not related to them.
Based on these fossil finds, as well as on anatomical study of modern reptiles, scientists have concluded that the snakes probably evolved from a family of lizards during the time of the dinosaurs. In particular, the Varanid family of lizards, which includes the monitors, are very similar to snakes in their skull structure; both, for instance, possess a movable quadrate bone at the back of the jaw, and both are missing the quadratojugal bone at the rear of the skull.
Based on these similarities, some herpetologists have theorized that an ancient group of monitor-like lizards began to follow a burrowing way of life, tunneling through loose dirt and sand in search of earthworms and other prey, just as some lizards do today. Over a period of millions of years, these burrowing lizards lost their limbs and their external ears–to help them burrow more easily–and also replaced their eyelids with a clear brille or spectacle to protect their eyes while digging. One group of these burrowing lizards then gave up its subterranean lifestyle and emerged to the surface, where they developed a new legless mode of locomotion and rapidly diversified to invade a large number of ecological niches. Today we classify the various descendants of these legless lizards as snakes.
The “burrowing ancestors” theory has, however, come under some criticism recently. Several herpetologists have pointed out that the earliest known snakes do not show any features adapted to a burrowing existence, and most of them are found in marine deposits. Some biologists have therefore theorized that the snake’s unique features are the result of a largely aquatic lifestyle, perhaps evolved from sea-dwelling varanid lizards like the Mososaurs. In this interpretation, the lack of ears, the covered eyes and the long limbless bodies allowed the first snakes to move efficiently through water in search of prey. It was only later, these herpetologists have hypothesized, that snakes moved from an aquatic environment to invade the dry land.
In any case, the first of the modern snakes to appear seem to have been relatives of the living boids, or boas and pythons. One of the most complete skeletons of a large fossil snake was found in Upper Cretaceous rocks in Argentina. Most of the skull was preserved as well as a large number of vertebrae and ribs. The six foot skeleton was named Dinilysia patagonica. Another fossil snake, Gigantophis, that was found in Egypt, had an estimated length of almost fifty feet. A slightly larger boa relative, known as Titanoboa, was found in Colombia and is the largest of all the known snakes. The living boas and pythons all have tiny clawlike toes protruding from either side of their cloaca–these are the remnants of the legs that their ancestors once had, and are thus an evolutionary relic tying the snakes directly to their lizard ancestors.
After the dinosaurs disappeared, the boids were the dominant snake family on earth, and became widespread and very diverse. About 36 million years ago, however, a group of smaller, faster snakes appeared which competed with the boids for food and living space. These were the colubrids, the family which we think of today as “typical snakes”. The colubrids were unable to outcompete the boids and remained a small group of snakes until about 20 million years ago, when the continental plates began to reach their present positions. As the tectonic plates moved away from the equator, the climate cooled dramatically, and the boids, unable to cope with the lower temperatures, disappeared from many areas and were greatly reduced in number and diversity. The colubrids quickly moved into the empty environmental niches that had been occupied by the boids, and soon dominated the snake world. Today, the colubrids make up over two-thirds of all the living species of snakes.
One family of the colubrids, however, added a new twist to the snake’s survival arsenal. About 15 million years ago, snakes began appearing which had a number of greatly enlarged teeth at the rear of their jaw. These teeth had shallow grooves running down one side. Today, such snakes are referred to as opisthoglyphs or “rear-fanged” snakes. In the rear-fanged snakes, the enlarged teeth are used to pierce the skin of prey after it has been seized and partially swallowed, allowing venom (composed of highly modified saliva) to flow out of the Duvernoy’s gland and dribble down the grooved teeth into the wound. Since it is difficult for these snakes to inject their venom until after they have partially swallowed their victim, it is unlikely that the snake’s venom apparatus was originally developed as a defensive weapon. More probably it appeared as an effective way of quickly killing and subduing food. A large number of rear-fanged snakes are still alive today.
Another group of snakes developed a more refined venom apparatus. Instead of having fangs at the rear of the jaws, the Elapids have short fixed fangs at the front of the mouth, where they can be used to bite and strike at enemies as well as food. Living descendants of the Elapids include the cobras and the sea snakes.
In a third group of snakes, known as viperids, the fangs are extremely long, much larger than in the Elapids. In fact, they are so long that the snake cannot close its mouth if they are erected. Thus, the solenoglyphs use a rotating maxillary bone to fold the fangs up against the roof of the mouth, where they are ready to spring into position when the snake bites.
The fossil record of snakes, however, is patchy and incomplete, with large gaps. Newer techniques using molecular biology may give us a more complete picture of snake evolution. The study of snakes using DNA techniques is still in its infancy, but has already revealed a few surprises. New studies indicate that the vipers are not, as was formerly thought, the most recent of the snakes, but instead diverged from the ancestral boid stock at about the same time as both the elapids and the colubrids. And embryological study has shown that the fangs of all three groups appear similar when they are first formed, but migrate to different areas of the jaw according to different growth patterns–and therefore all three fang types may have evolved at around the same time. It appears that the snakes underwent a rapid radiation in their initial burst of evolution, with a number of different lifestyles appearing at once and then developing independently and in parallel afterwards.