Snake ancestors: Aquatic, burrowing, or terrestrial?

The subject of snake ancestors remains to this day a topic of contention.
There is little argument that snakes evolved from lizards.
The origin and lifestyles of those ancestral lizards provide paleontologists with critical insight into the evolution of snakes.
There are three theories, or hypothesis, that have been suggested.


Aquatic Ancestors

Some paleontologists suggest that snakes evolved from aquatic Varanoids. The theory is not without merit.
Creatures such as Dolichosaurs, Aigailosaurs, and Mosasaurs were all relatives of the modern-day monitor lizards (Varanids). Contrary to popular belief these were not dinosaurs, but lepidosaurs, reptiles with overlapping scales.
Mosasaurs, considered to be the closest relatives to snakes, had several similar features to that of todays snakes.
Like snakes, these animals had a flexible skull and double-hinged jaws which would suggest that prey would, more often than not, be swallowed whole.
The presence of a double row of projecting teeth located on the upper jaw (palate) also suggests that mosasaurs were close ancestors of snakes.
Despite the similarities, there are several characteristics opposing the theory.
As opposed to amniotes Mosasaurs reproduced and gave birth to live young within their aquatic environment. Another significant characteristic is that an aquatic environment and lifestyle has never led to a reduction of limbs in reptiles.

Burrowing Ancestors

A more popular theory is that snake ancestors were in fact burrowing lizards. An elongated streamlined body and a lack of limbs are both ideal adaptations for a sub-terranean existence. A further adaptation suited for burrowing species is the lack of a moveable eyelid. The presence of a transparent scale covering the eye (brille) as opposed to a moveable eyelid would suggest a life more suited to burrowing. Blind snakes (Typhlopidae) are primitive snakes perfectly equipped to deal with their burrowing lifestyle.
Conversely however, a rigid bony skull is better suited to a burrowing existence as opposed to a flexible skull. The physiological structure of a snakes head would suggest that it is incompatible with burrowing.

Terrestrial AncestorsThis hypothesis, that of a terrestrial origin, has been all but dismissed due to the lack of solid physical proof.
The theory is sound however, and is well worth a mention.
The success of any species depends on the evolutionary adaptations that ensure the survival of that particular species. It has been mentioned that early snakes having evolved from terrestrial lizards, "chose" to evolve into burrowing species to avoid the predators that roamed the surface of the earth. Just as the development of the "shelled egg" set the reptiles apart from amphibians, so too the terrestrial lizards evolving into burrowing species eventually would give rise to snakes, distinct from other reptilians.

Taking both the aquatic and burrowing theories into consideration, it is now widely accepted that snake ancestors were in fact semi-aquatic and semi-burrowing. Their habitat would have been predominantly mud on the shores of ancient seas, lakes, swamps, and river banks. Evidence of this is still present today in many limbless amphibians.

Learn more about the evolution of snakes here

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