Coral Snake Ancestory and Evolution
The main clues to the evolution of any creature comes from the preserved remains or fossils of their ancestors, found in the rocks. The earliest fossil remains recognizable as being a snake were found in lower Cretaceous (130 million – year – old) rocks in the sahara, North Africa. The most complete and significant fossil was find was from Upper Cretaceous (80 million year old) sandstone rocks in Argentina. These fossil remains are nearly 6 feet (1.8 meters) long and show similarities to some primitive snakes of today. Another famous fossil is the boalike snake remains found in the Geisel valley, near Halle in Germany. These remains reveal details not only of the snake’s skeleton, but also of skin and scales. The largest snake known from fossil remains is an extinct type of python from Middle Eocene rocks in Egypt. Although only partly preserved, this snake may have been as much as 60 feet (18 meters) in length.
Unfortunately, relatively few fossils of snakes have been found in the rocks. To be preserved, an animal must be covered soon after death in mud or sand that later hardens into rock. This is most likely to happen to creatures that sink to the seabed in areas where sediment is being deposited, and so most fossils are of marine creatures. In addition, it is usually only hard parts, such as bones, shells and teeth that get preserved. Since snakes lived mostly on land, often in forests, and have only delicate bones, few were preserved as fossils.
Despite the rarity of fossils, there is little doubt that snakes evolved from lizards with transitional forms appearing in the cretaceous period. This was the great age of the dinosaurs, when reptiles ruled the earth. It was not until after the dinosaurs had become extinct at the end of the Cretaceous period, about 65 million years ago, that snakes started to diversify significantly and, even today, their evolution is still progressing. In terms of geological time, therefore, the snakes are still a fairly new group, but already there are some 2700 different species, spread all over the world, barring the very cold regions.
Snakes, together with the lizards, and the burrowing amphisbaenids make up a group known as the Squamata. The other groups of living reptiles are the Crocodilia ( crocodiles and alligators), the Chelonia ( turtles and tortoises), and the unique Tuatara of New Zealand. One of the most obvious features of snakes is their lack of limbs, and this feature is shared by other members of the Squamata.
Legless lizards are found all over the world and include the European Glass Lizard, the California legless lizard of North America and New Guinea. Other widely distributed lizards, the skinks, show all stages of limb reduction, from normal legs to their total absence. Examples of skinks with tiny legs are the three-toed Skink from Europe and the Ground Skink of North America. The amphisbaenids are snakelike members of the Squamata that are widely distributed and include the worm lizard, the only species found in North America. There are about 135 species of these burrowing creatures, and nearly all have lost their limbs during evolution.
These limbless lizards and amphisbaenids are in many other ways dissimilar to snakes and so are not thought to have evolved from the same “legless ancestor.” Indeed, the ancestors of the snakes are believed to have had limbs. Instead the common lack of limbs is thought to be an example of parallel evolution. This occurs when groups of animals independently develop similar adaptations to similar environments or ways of life.
Modern Day Look a like
The present day animal that most closely resembles the ancestors of the snakes is the earless Monitor, a rare type of lizard from Southeast Asia. This animal has all four limbs and, on the surface, looks quite different from snakes. The loss of limbs is thought to have occurred during a burrowing phase in the snake’s evolution. It was probably also during this phase that the eyes lost their movable eyelids, reduced in size and become covered by a transparent scale. The senses of smell and taste improved, particularly with the development of Jacobson’s organ. But the ability to hear airborne sound disappeared with the loss of the external ear opening, the entire middle- ear cavity, tympanic membrane, and the Eustachian tube.
Together with the limbs have gone the shoulder blades, collarbones, breastbones and, in most snakes, the pelvic girdle. Only some of the primitive burrowing snakes and the boas and pythons, whose ancestors date back to the Cretaceous period have vestigial hind limbs. The skull and the jaws of most snakes have been modified so that they can swallow the prey whole, the latter often being huge in comparison to the size of the snake.
The evolution of Venom
The evolution of venom is relatively recent. During the course of evolution, the saliva glands of poisonous snakes became poison sacs, and teeth developed into fangs for the injection of poison. In some snakes, the fangs have become grooved for the poison to run down In the most advanced snakes, such as the vipers and the cobras, the fangs have developed into tubes, so that poison can be injected like a hypodermic needle or sprayed at an intruder. This ability to deliver poison developed independently in several different snake families.
The evolution of snakes also produced a large number of differences in form among species. Together, the different species form a distinct and fascinating group within the Squamata order, known as the Serpentes.
Other examples of parallel evolution are found within the snakes. For example, the Emerald Tree Boa from South America and the Green Tree Python from New Guinea and Northern Australia are both very similar looking large, green, arboreal snakes, but they are not closely related. Some unrelated desert species have independently developed “horns.” These can be seen in the Horned Adder and the Desert horned Viper from Africa, the North American Sidewinder Rattlesnake, and several other species.