The Evolution of Snakes Evolution of Snakes by Lenny Flank -------------------------------------------------------------------------------- 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. Snakes, like all living things, are the product of the process of evolution, which allows species to change over time in response to environmental factors to produce entirely new species. The engine of evolution is "natural selection", in which those individual animals that possess superior survival traits tend to live longer than others and reproduce, in turn passing those same traits on to their offspring. According to most paleontologists, reptiles evolved from the large group of ancient amphibians known as Labrynthodonts, which received their names from the distinctive structure of their teeth. In the Labrynthodonts, the enamel of the tooth was folded in on itself to form a complex mazelike pattern. The evolutionary advance that set the reptiles apart from the amphibians was the development of the amniote or shelled egg which could be laid on land, freeing the reptiles from the necessity of returning to the water as adults for reproduction. The oldest known fossil egg was found in Texas, and dates to the lower Permian period of the earth's history, over 275 million years ago. It is not known from which particular group of Labrynthodont amphibians the reptiles developed; several different families of ancient amphibians seemed to have been developing characteristics at the time similar to those of reptiles. The oldest fossil which can be definitely recognized as a reptile is a small lizardlike animal known as Hylonomus, whose skeletons have been found inside petrified tree stumps in Nova Scotia. During the period of time in which Hylonomus lived, the earth was a different place than it is now. The continents were all joined into one large super-continent near the equator, known as Pangea ("all earth"), and even such places as Antarctica and northern Canada had warm, humid climates with lush tropical forests. Since then, the Pangea land mass has broken up into a series of "plates" which move slowly atop the earth's mantle, a process known as "plate tectonics". As we shall see, the breakup and movement of these plates has had noticeable consequences for the evolution and distribution of modern snakes. Hylonomus was a member of a group of very ancient reptiles 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 evolutionary cousins of Tyrannosaurus and Triceratops. (Modern birds are also descended from Diapsid reptiles, and are thus distant evolutionary cousins of the modern snakes.) One of the earliest snakes to appear in the fossil record has been given the scientific name Lapparentophis 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. Recently another fossil, consisting of just two vertebrae, have been found in Spain that are a few million years older than Lapparentophis. This fossil has not yet been named. Another very early snake has been found in marine deposits in North Africa and Europe. This snake, which lived about 100 million years ago, has been called Simoliophis. Although it appears to have been at least partially aquatic, Similiophis does not appear to be related to any of the modern sea snakes, and may not be related to any living snakes. Both Lapparentophis and Similiophis appear to have gone extinct some time before the end of the Cretaceous. The most complete skeleton of a 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, and it shares many anatomical characteristics with the modern boas and pythons, which are usually considered to be the most primitive of the living snakes. Another fossil snake, Gigantophis, that was found in Egypt, had an estimated length of over fifty feet, and is the largest of all the known snakes. It was also related to the modern boids. One of the most interesting snake fossils is the extinct boid Paleryx, found in Germany. Fossils of this ancient snake have been found which still contain the impressions of the scaled skin. 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. Snakes and lizards share a number of distinct features in the structure of their skull; both, for instance, possess a moveable quadrate bone at the back of the jaw, and both are missing the quadratojugal bone at the rear of the skull. In particular, the Varanid family of lizards, which includes the monitors, are very similar to snakes in their skull structure. The most snake- like of the living monitors is the Earless Monitor, a burrowing semi-aquatic lizard found in Borneo. The Earless Monitor has movable eyelids, but the lower lid sports a clear "window" which allows the monitor to see even when its eyes are closed, protecting it from water and dirt. This is very reminiscient of the snake's brille or eyecap, which is formed in embryonic snakes when the transparent upper and lower eyelids fuse together. The Earless Monitor also has a number of snakelike features in its skull architecture and, as the name implies, it lacks any trace of an external ear, just as in snakes. It is probable that the Earless Monitor more closely resembles the saurian ancestor of the snakes than any other living lizard. 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. At about the time that the dinosaurs reached their apex, 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 attack recently. Several herpetologists have pointed out that the Dinilysia skull does not show many features adapted to a burrowing existence. Some biologists have theorized that the snake's unique features are the result of a largely aquatic or semi-aquatic lifestyle, as illustrated by the Earless Monitor. In this interpretation, the lack of ears, the covered eyes and the long limbless bodies allowed the first snakes to move efficiently through water or wet marshy areas in search of prey. It was only later that snakes moved from an aquatic environment to invade the dry land. During the time that snakes developed, the Varanid family did contain a number of semi-aquatic and marine species, including the giant Mososaurs. In any case, the first of the modern terrestrial snakes to appear seem to have been relatives of the living boids, or boas and pythons. These were large heavy-bodied snakes with a rather primitive and heavy skull structure. 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. Shortly after the opisthoglyphs appeared, another group of snakes developed a more refined venom apparatus. These snakes are known as proteroglyphs, and are classified as the Elapids. Instead of having fangs at the rear of the jaws, the proteroglyphs have short fixed fangs which have migrated (by reducing the size of the maxillary bone) to the front of the mouth, where they can be used to bite and strike at enemies as well as food. The grooves in the fangs have become deeper and meet at the edges to form a hollow tube. These hollow fangs are connected to venom glands in the cheeks, which can inject venom through the fangs like hypodermic needles when the snake bites. Living descendants of the Elapids include the cobras and the sea snakes. By about 10 million years ago, the most highly specialized of the snakes appeared in the fossil record--the solenoglyphs, commonly known as vipers. In the vipers, 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. A short time after the vipers appeared, a group known as the pit vipers developed a number of heat-sensitive pits on the front of the face, which they used for finding their warm-blooded prey at night (this feature has also been independently developed by the venerable old Boid family). Finally, just a few million years ago, a group of pit vipers developed a structure at the end of their tail, made up of interlocking pieces of unshed skin, which could be loudly rattled and used as a warning device against predators. The rattlesnakes are generally thought to be the most specialized of all the living snakes. 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. Using methods such as immunological responses and DNA-DNA hybridization, the precise genetic "distance" between living species can be determined, and a rough picture of when and in what order they evolved can be drawn. The study of snakes using DNA techniques is still in its infancy, but has already revealed a few surprises. Preliminary results indicate that the vipers are not, as was formerly thought, the most recent of the snakes, but instead diverged from the ancestral boid stock before both the elapids and the colubrids. If this finding is confirmed, it means that we have to completely re-think our view of how snakes evolved. 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. Much work remains to be done on the evolution of snakes. -------------------------------------------------------------------------------- Excerpted from "Snakes: Their Care and Behavior". (c) copyright 1997 by Lenny Flank, Jr. 1