The Evolution of Snakes
To pinpoint the origin and evolution of snakes is extremely difficult. First and foremost the delicate skeletal structure of snakes does not fossilize easily. There is little doubt that early snakes were small and therefore rarely show up in the fossil records. Secondly, most fossil snake material found consists of fragments, usually vertebrae, which makes it difficult to distinguish from other close relatives.
In order to begin searching for the origin of snakes it is necessary to look into the history of reptiles. It is only in doing so, that we may begin to find clues into the evolution of snakes.
Most paleontologists agree that reptiles first evolved from amphibians known as Labrythodonts. The name "labrythodont" means "maze-toothed" which describes the distinctive structure and pattern of the enamel and dentine of the teeth. Another distinction which has led paleontologists to believe these were the ancestors of reptiles was the presence of a complex vertebrae as well as a solid heavy skull.
As with amphibians today, the early labrythodonts laid eggs in water. Despite having legs their lifestyle remained primarily aquatic.
These large creatures thrived for more than 150 million years.
Perhaps the most radical evolutionary advancement that set reptiles apart from amphibians was the ability to lay eggs on land. The evolution of the cleidoic egg (shelled egg) in the late Carboniferous (360 to 290 million years ago) was a significant step which allowed the transition of an aquatic to a terrestrial lifestyle. This allowed Amniotes (four-footed vertebrates that have a terrestrially adapted egg) to flourish in a relatively predator-free environment. The ability to reproduce on land without having to return to water was a giant leap into the evolution of reptiles.
Hylonomus "forest wanderer", is the oldest fossil classified as a reptile. The fossil of this small "lizard-like" creature was found in Scotland, in soil dating from the lower Carboniferous (330-340 million years ago).
Hylonomus had a massive Anapsid skull (skull without openings near the temples), and measured about 20cm (8 inches).
Anapsids gave rise to two major reptilian amniotic groups namely Synapsids, which later evolved into mammals and Diapsids.
Diapsids separated into two major groups:
Archosaurs (ruling lizards) were a group of reptiles, which included Dinosaurs and Pterosaurs (flying reptiles), which are still represented today in crocodiles and birds.
Lepidosauromorpha were a group of reptiles more closely related to lizards than to Archosaurs.
Several groups related to lepidosauromorphs included marine reptiles such as plesiosaurs, ichtyosaurs, and placosaurs. Lepidosauria (reptiles with overlapping scales) is another group which evolved from lepidosauromorphs.
It is from this group that we find the order Squamata, (scaled reptiles). This includes all lizards and snakes.
Many groups of squamates have been considered as the ancestors to modern day snakes and since snakes have been around for millions of years it is clear that none of the present day squamates can be regarded as such.
Most paleotologists agree that the evolution of snakes began with lizards. There are two groups of lizards that have been considered as the closest relatives to snakes.
The first group belongs to the superfamily Varaniodea including the family Varaindae. This includes todays monitor lizards, Komodo dragons, and Gila monster.
Based on the skull structure, mandible, tongue, as well as the way the teeth are placed makes the superfamily Varaniodea a likely candidate as the ancestor of snakes.
Pachydrachis and Pachyophis, both serpent-like marine fossils which lived during the Cretaceous (145.5 to 65.5 million years ago), are believed to be the intermediary between snakes and varanoids.
The second group, although fossils have not confirmed this theory, is the group Scincomorphs, or "true" lizards. The argument for this theory is the similarity in the encephalic structure.
Lapparentophis defrenni is one of the earliest snakes to appear in fossil record dating from the Cretaceous (100-96 million years ago).
There are other fossils dating from the Cretaceous such as Similiophis, Pouitella and the aptly named Parachys problematicus which have paleontologists debating whether these were primitive snakes or varanoid lizards.
The Cretaceous period which lasted between 100 and 90 million years seems to be a very pivotal time in the evolution of snakes, and in shaping their history. Many believe that snakes first appeared much earlier during the upper Jurassic (155 - 135 million years ago).
Whilst it is agreed that snakes evolved from lizards, the question that paleontologists are now asking is whether these "lizards" were aquatic, terrestrial, or burrowing?
Once again, there are several points supporting each theory regarding the evolution of snakes.
Click here to make up your own mind.
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