EVOLUTION & DIVERSITY OF SNAKES

Snakes are elongate animals with fragile skulls and skeletons, which may become disarticulated and separated post-mortem. It is therefore no surprise that relatively few complete snake fossils are available, most comprising a few vertebrae and skull fragments.

THE EVOLUTION OF SNAKES

There are two contrasting schools of thought regarding the evolutionary origin of snakes. One theory proposed that they evolved from a now extinct group of large marine reptiles known as mosasaurs, which dominated the Late Cretaceous oceans. The other theory holds that snakes have a terrestrial origin and evolved from within the Anguimorpha, a suborder of lizards that today contains the slow worms, alligator lizards, monitor lizards, and the venomous Gila monster, and beaded lizards. This latter theory is the more widely accepted, but there is still support for an aquatic mosasaur origin.

The earliest snakes are now thought to date from the Middle Jurassic or Early Cretaceous period, 167–140 MYA (million years ago), with fossil examples discovered in England, Portugal, and Colorado, USA. These fossil examples comprise a few vertebrae and fragments of jawbones, but they can be readily identified as snakes by their strongly recurved teeth, a common characteristic of modern and ancestral snakes. Their discovery suggests a much earlier origin for snakes than the previously accepted Late Cretaceous, 95 MYA.

Early snakes are thought to have inhabited warm, wet, well-vegetated habitats, where they existed as terrestrial, nocturnal, wide-foraging, non-constricting stealth hunters, preying on soft-bodied invertebrates and vertebrates of lesser width than their own heads. A modern comparison might be the Asian pipesnakes (Cylindrophis). The greatest explosion in snake diversity appears to have occurred following the Cretaceous–Paleogene extinction event, 66 MYA. This led to the extinction of the dinosaurs, mosasaurs, and 75 percent of all life on Earth, but it also resulted in the rise of the mammals, a potential prey source of early snakes.

Some fossil snakes display hind limbs, including Najash rionegrina, from Late Cretaceous Patagonia, which has a well-developed pelvic girdle and what are believed to have been functional hind limbs. Three Middle Cretaceous marine species — Pachyrhachis problematicus and Haasiophis terrasanctus from Palestine, and Eupodophis descouensi from Lebanon — also had hind limbs. These species are grouped in the extinct family Simoliophiidae, but body elongation and loss of limbs does not necessarily separate snakes from lizards (see "Skeleton and Limbs").

As recently as 2016, an Early Cretaceous fossil from Brazil was described as Tetrapodophis amplectus. It had an extremely elongate body and four short pentadactyl limbs, and was reported worldwide as the first four-legged snake. But this discovery proved extremely controversial, and paleontologists now believe that the fossil is a dolichosaur, an extinct marine lizard-like reptile.

THE DIVERSITY OF MODERN SNAKES

The snakes (suborder Serpentes), along with the lizards (suborder Lacertilia) and the worm-lizards (suborder Amphisbaenia), comprise the order Squamata, the scaled reptiles. The sister clade (group) of the Squamata is the Rhynchocephalia, the beaked reptiles, a once diverse and widely distributed group of lizard-like reptiles that is now confined to New Zealand, where it is represented by a sole extant species, the Tuatara (Sphenodon punctatus). The Squamata and Rhynchocephalia together form the superorder Lepidosauria, the sister clade of the Archosauria, which contains crocodilians, birds, and extinct dinosaurs and pterosaurs.

Modern snakes are divided into two infraorders, the Scolecophidia (worm snakes) and the Alethinophidia (true snakes). The Scolecophidia comprises five families of small fossorial (burrowing) snakes. Although appearing primitive among living snakes, these are actually highly derived, having specialized considerably for their subterranean existence.

The Alethinophidia is divided into the Amerophidia, a small group that has not spread beyond Latin America, and the Afrophidia, which contains the majority of the true snakes. The Afrophidia are the "Out of Africa" clade, because the continent appears to be the group's evolutionary cradle, from where it radiated worldwide. The Afrophidia is further divided into the Henophidia ("old snakes"), which contains the boas, pythons, pipesnakes, shieldtails, and several smaller families of small-mouthed snakes, and the Caenophidia ("recent snakes").

The Caenophidia is divided into two superfamilies. The Acrochordoidea today contains just three species of aquatic filesnakes (Acrochordus), but once included the now extinct Nigerophiidae and Palaeophiidae. The sister clade to the Acrochordoidea is the huge Colubroidea, with its vast and diverse array of ratsnakes, watersnakes, treesnakes, cobras, seasnakes, and vipers. This superfamily comprises 11 families and more than 3,000 species — almost 82 percent of all living snakes.

WHAT IS A SNAKE?

All amphibians, reptiles, birds, and mammals are pentadactyl tetrapods — vertebrates with four limbs, each with five digits. Snakes, as reptiles, are also pentadactyl tetrapods because their lizard ancestors were fully limbed.

SKELETON AND LIMBS

The snake skeleton comprises a skull and a spinal column. Because snakes possess extremely elongate, flexible bodies, they may have up to 500 vertebrae, although 120–240 is more common. Each vertebra is attached to a pair of ribs, which in the absence of a sternum are independent, being interconnected only by powerful intercostal muscles that enable the many modes of snake locomotion. The lack of a sternum allows the rib cage to expand outward so that the body can accommodate large meals, egg clutches, or litters of neonates. The outward expansion and mobility of the ribs is obvious in the dorsoventral flattening of a basking viper, the hooding of a cobra, and the lateral body compression of a swimming sea snake.

All snakes lack front limbs, but the vestiges of the pelvic girdle and hind limbs are present in the boas, pythons, and some other primitive snake groups. Externally, they are represented by a pair of curved horny spurs on either side of the cloaca (genital-excretory opening). Spurs are largest in males, which use them to court the female during copulation.

SKULLS AND TEETH

Unlike the skulls of mammals, turtles, or crocodilians, those of snakes exhibit kinesis, meaning that they are hugely flexible, and the individual bones are capable of the articulation required to manipulate and swallow prey. The large gape of a snake's mouth is achieved because the...