Quadrate suspensorium maxilla

quadrate suspensorium quadrate suspensorium

Quadrate Snake Skull

dentary maxilla (rotates forward when jaws open)

dentary

A. Spotted blindsnake (Typhlops punctatus)—primitive; B. Boa constrictor (Boa constrictor)—fixed teeth; C. Eastern hog-nosed snake (Heterodon platirhinos)—moveable rear teeth; D. Western diamondback rattlesnake (Crotalus atrox)—front-fanged. Primitive skulls have no suspensorium bone, no elongated quadrate bone, and a solid, compact skull. Front-fanged skulls have a suspensorium that is angled up, elongated quadrate that is angled back (allows jaws to open wider), and a less solid skull with more moveable bones. Snake skulls show variations of these traits on a continuum, with primitive and front-fanged at the extremes, and rear-fanged somewhere in the middle. (Illustration by Bruce Worden)

drophiinae). Snakes are arboreal, terrestrial, and aquatic and are top predators in almost all natural communities.

Phylogenetic relationships and number of families

A basal split in squamate phylogeny produced Iguania (99 genera and approximately 1,230 species), which retained ancestral traits (visual ambush predators with lingual prehension and poorly developed vomerolfaction), and Scleroglossa (almost 6,000 species), which adopted innovative new methods of finding and eating prey as well as acutely sensitive vomerol-faction and hydrostatic (operated by liquid pressures) forked tongues. Scleroglossa includes dibamids, amphisabaenians, and snakes, but their exact affinities within Scleroglossa remain uncertain. Remaining scleroglossans, in turn, bifurcated into two large clades, Gekkota and Autarchoglossa. Gekkota (about 1,000 species) evolved elliptical pupils and the ability to operate at low temperatures, allowing nocturnal activity. They use their tongues to clean their lips and eye spectacles. Geckos took to the night, where they found a cornucopia of nocturnal arthropods. (Some geckos have reverted to a diurnal way of life.) The largest and most advanced clade, Autar-choglossa (about 4,800 species), is composed of two smaller sister clades, Scincomorpha (seven families of lizards, with about 1,800 species) and Anguimorpha (five lizard families plus 15-18 snake families, with a total of more than 3,000 species). Members of the three clades, Iguania, Gekkota, and Autarchoglossa, differ considerably in morphologic features, physiological characteristics, behavior, life histories, and ecological niches, especially in foraging mode.

Iguanians are sit-and-wait ambush foragers that catch mobile prey as they move past their hunting stations. Most autar-choglossans are more active, foraging widely and searching for prey; as a result, they have access to sedentary and hidden prey items that are unavailable to iguanians. Active foraging is more expensive than ambush foraging, both in terms of energy expended and exposure to predators, but the returns are greater in calories obtained per unit time. Autarchoglossans have evolved flexible joints in their muzzles and skulls (mesokinesis and cranial kinesis), further improving their ability to capture and subdue large and agile prey.

Snake phylogeny has not yet been resolved, but three major groups are recognized: blindsnakes, primitive snakes, and advanced snakes. Blindsnakes (Scolecophidia) include three families (Anomalepididae, Leptotyphlopidae, and Typhlopi-dae). These are specialized burrowers and are considered sister to all other snakes (Alethinophidia), which are organized into four superfamilies (Anilioidea, Booidea, Acrochordoidea, and Colubroidea). Anilioids and booids are considered primitive snakes. As an indication of their lizard origin, boas and pythons still possess vestigial remnants of hind limbs, called "anal claws," which indicate that they are basal members of the ophidian clade. Colubroids, the most diverse snakes, are more advanced. Higher snakes are called Macrostomata, which includes boids, pythons, acrochordids, and the most advanced of all snakes, colubrids, viperids, and elapids. The vast majority of snakes are colubrids.

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