The origin and evolution of snakes have been debated throughout palaeontology for years. There seems to be two main theories, which will be explored in depth in this report. The first main theory is that snakes are a sister group to marine varanoid lizards and snakes thus evolved from them. The second is that snakes were once small burrowing lizards and lost their limbs because it was simply easier to dig without them.
First the ‘marine varanoid theory’ will be looked into. First formally suggested by Victorian fossil hunter and evolutionary biologist Edward Drinker Cope (1849-1897), argues that snakes lost their limbs at sea and are closely related to the extinct marine lizards called mosasaurs.
This idea is mostly pushed by MSY Lee. He looked at the relationships between the major lineages of snakes based on phylogentic analysis. Ecological, osteological and anatomical features were examined. According to Lee:
“The marine, limbed Cretaceous snakes Pachyrhachis and Haasiophis emerge as the most primitive snakes”
However, the link between these and advanced snakes are based on very unlikely interpretations that are debateable; even Lee admits this. The view that these large marine snakes were the first primitive snakes directly contradicts with the idea that snake ancestors were small, terrestrial, burrowing creatures.
Below is Pachyrhachis and size in comparison to humans. Note the small hind limbs.
The similarities that snakes and mosasaurs possess are that they both have loose jaws for swallowing large prey. Some snakes have a gular fold that allow stretching in the intramandibular tissues. This is called liberation of the mandibular symphysis and is variable and complex amongst snakes. The only other squamate (scaled reptile) clade which shows this liberation is mosasaurs. They also share with snakes an intramandibular articulation. All these similarities in mandibular mechanics support the theory that snakes and marine reptiles, such as mosasaurs, are very closely related.
Below is Haasiophis swimming in marine waters, supporting Lee’s theory. This primitive snake also appears to have small hind limbs and a dorsal fin-like tail.
Lee believes that mosasaurs were the “intermediate between lizards and snakes”. Lee also states that Pachyrhachis is an excellent example of a transitional taxon. It is true that Pachyrhachis is also a sister-taxon of fairly advanced snakes that are able to dislocate and widen their jaws to swallow their prey. However, there is much debate whether this Cretaceous snake was a ‘missing link’, or just a relative of modern snakes that bears no issue to snake origins. Zaher found Pachyrhachis not to be a basal snake, nor a link between mosasauroids and snakes, but the sister-taxon of advanced macrostomatan snakes instead.
Lee put forward a rebuttal to this by putting all snakes, except Pachyrhachis as a single terminal, thus not allowing an empirical test and a potential negative response of Zaher’s results.
Pachyrhachis is found to be a sister group of Macrostomata, and snakes grouped with the dibamid–amphisbaenian clade instead of with mosasauroids, but it is still up to much debate.
Although this theory is seemed the most unlikely out of the two, it is not impossible that the earliest snakes were marine and only the ancestor of the extant forms was burrowing. There are a possible five major groups of marine lizards in the Mesozoic and they are: aigialosaurs, mosasaurs, dolichosaurs adriosaurs and Aphanizocnemus, which might define a lineage of its own. All these show several similarities with terrestrial varanoid lizards that had already settled into their modern form by the late Cretaceous, such as the giant Gila monster. The primitive mosasaur, Dallasaurus retains several aigialosaur-like features. This suggests that the ancestor of the aigialosaurs and mosasaur at least was an amphibious form, much like many extant varanoids. Therefore, the mosasaurs that we know of that were totally marine, seem to be much further down their evolutionary line. The dolichosaurs, Aphanizocnemus and adriosaurs have much longer bodies and elongate necks, with an increase in a number of dorsal vertebrae and reduced forelimbs to different degrees. An example of this is Adriosaurus microbrachis; it had an extreme reduction of forelimbs that was seen as a tendency towards forelimb loss and the emergence of snakes.
What is also believed from this theory is that the fused, transparent eyelids of snakes evolved to combat marine conditions (corneal water loss through osmosis), and the external ears were lost through disuse in an aquatic environment.
To conclude on this theory, genetic studies in recent years have indicated snakes are not as closely related to mosasaurs or monitor lizards as once believed. However, more evidence links mosasaurs to snakes than to varanids.
Fossil evidence suggests that snakes may have evolved from burrowing lizards during the Cretaceous period. Podophis descouensi gen. et sp. nov. is a new fossil that has been described. It is similar to Pachyrhachis as it is a bipedal snake from the Cenomanian. It confirms that bipedal snakes are basal Ophidia. It is not possible to tell whether these two bipedal snakes make up a sister-taxon to the Serpentes, or a stem group of the Serpentes. Below is Podophis descouensi’s immaculately preserved hind limbs.
Features such as the transparent, fused eyelids and loss of external ears evolved to cope with burrowing difficulties, such as scratched corneas and dirt in the ears, according to this theory.
However, there are many anatomical differences between snakes and lizards, such as their eyes and optic nerves to the brain. This could just suggest different mutations or further evolution and is not such a great hindrance to this argument, especially when lizard and serpent DNA is examined. Scientists compared the DNA of numerous species of lizards and snakes. Their results have shown that snake DNA is significantly different from the DNA of varanid lizards, but is more like the DNA of other land-based lizards. They concluded that this is strong evidence for land-lizard ideas of snake origins.
To add to this credible evidence, a ‘transitional snake’ fossil was described. Coniophis, from the Cretaceous, lived in a floodplain environment and “lacks adaptations for aquatic locomotion”. Nicholas Longrich also describes it as having a lizard-like head with a snake body. It was small, and reduced neural spines suggest a burrowing nature, supporting this theory greatly.
The skull is intermediate between that of lizards and snakes. Hooked teeth and an intramandibular joint indicate that Coniophis fed on relatively large, soft-bodied prey. However, the maxilla is firmly united with the skull, indicating an akinetic rostrum. Coniophis therefore represents a transitional snake, combining a snake-like body and a lizard-like head. Subsequent to the evolution of a serpentine body and carnivory, snakes evolved a highly specialized, kinetic skull, which was followed by a major adaptive radiation in the Early Cretaceous period. This pattern suggests that the kinetic skull was a key innovation that permitted the diversification of snakes.
To conclude, both the varanoid and the burrowing-lizard theory pose compelling evidence. However, due to recent fossil discoveries, such as Coniophis, it would seem that snakes were more likely to have lived on land, not the sea. Their ancient vertebrae suggest a burrowing lifestyle; they were not adapted for aquatic life. To add to this the varanoid argument has many holes in it, as it does not take into account DNA, only phylogenetic similarities. The fossil record has also supported the burrowing theory more than the marine theory. It is true that there are some aquatic snake fossils, but these examples may have just been a sister taxon and may not infringe on the evolution of snakes at all.
From left to right: Coniophis skull adaptation; Erhard’s wall lizard’s head; Ralph the Florida Corn snake’s head. You can see that there are similarities and differences between all and that Coniophis was a ‘mid way’ point between a lizard head and a snake head, such as the shape, teeth and most importantly the jaw structure, which was more like a lizards than modern day snakes’. This leads scientists to believe that the rest of the body would be a ‘transition’ also.