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.
This alligator lizard, a member of the lizard suborder Anguimorpha, out of which the Serpentes are believed to have evolved, exhibits body elongation and limbed reduction on the road toward limblessness.
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 may have resembled this modern Boulenger’s Pipesnake (Cylindrophis boulengeri) from Southeast Asia, being small species that preyed on cylindrical prey such as soft-bodied invertebrates or slender vertebrates. The macrostomatan snakes that could feed on broader mammalian prey probably evolved later.
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.
Early snakes were elongate animals with vestigial hind limbs, such as this fossil (Eupodophis descouensi) from the Middle Cretaceous. Vestigial hind limbs are still present in extant boas and pythons.
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.
A NOTE ON CLASSIFICATION AND SCIENTIFIC NAMES
Living organisms are classified using a hierarchical system. For snakes this would be: Kingdom: Animalia; Phylum: Chordata; Class: Reptilia; Order: Squamata; Suborder: Serpentes. Within the Serpentes, snakes are grouped into Superfamilies (ending–oidea), Families (ending–idae) and Subfamilies (ending–inae). Within the families and subfamilies are the Genera that contain the Species. A species name is a binomial, it comprises two words, and it is written in italics with only the generic part receiving a large case initial letter, i.e. Natrix helvetica. Names are not necessarily Latin, but if the name comes from another language it is latinized, i.e. the Sanskrit word “Naia,” for the cobra genus, is latinized to Naja. A trinomial name indicates a subspecies. The name may be accompanied by the name of the describer, and the date of publication. If the name and date are contained in parentheses, this indicates that the name has changed since it was described, usually because it has been moved to another genus, i.e. the Indian Cobra, Naja naja (Linnaeus, 1758) was originally described by Linnaeus as Coluber naja.
A family tree of snakes illustrating the divergence between the burrowing Scolecophidia (worm snakes) and the Alethinophidia (true snakes); the restricted Amerophidia and the much more successful Afrophidia; and the relatively primitive Henophidia (old snakes) and the more advanced Caenophidia (recent snakes). The Caenophidia is divided into the Acrochordoidea, and the diverse and widely distributed Colubroidea, which contains over 3,000 of the more than 3,700 living snake species. Thirty-three snake families are presented, 8 of which contain between 2 and 7 subfamilies, indicated by the vertical brown bars. Representatives from every family and subfamily are included in this book. This is a simplified family tree, the lengths of the arms are not intended to indicate the timelines since divergence between the various families or clades. Due to the constraints of space in the Acrochordoidea, entirely extinct snake families, such as the Nigerophiidae and Palaeophiidae, are omitted.