Snake

Etymology

The word snake comes to English from the Proto-Germanic: *snēk-a- m., originating in the Proto-Indo-European *(s)nēg-o- and has a cognate in the Sanskrit nāgá- m. 'snake' and the English verb sneak.[3] The word serpent comes from Old French, and ultimately from *serp-, "to creep" [4], also ερπω in Greek.

Evolution

The fossil record of snakes is relatively poor because snake skeletons are typically small and fragile, making fossilization uncommon. However 150 million-year-old specimens, readily identifiable as snakes, yet with lizard-like skeletal structures, have been uncovered in South America and Africa.[5]:11 There is consensus, on the basis of comparative anatomy, that snakes descended from lizards.[6][5]:11 Fossil evidence suggests that snakes may have evolved from burrowing lizards, such as the varanids or a similar group during the Cretaceous Period.[7] An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, and was fully terrestrial.[8] One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo, although it also is semi-aquatic.[9] Subterranean forms evolved bodies that were streamlined for burrowing and lost their limbs.[9] According to this hypothesis, features such as the transparent, fused eyelids (brille) and loss of external ears evolved to cope with fossorial difficulies such as scratched corneas and dirt in the ears.[9][7] Some primitive snakes are known to have possessed hindlimbs, but their pelvic bones lack a direct connection to the vertebrae. These include fossil species like Haasiophis, Pachyrhachis and Eupodophis, which are slightly older than Najash.[10]
Fossil of Archaeophis proavus

Primitive groups among the modern snakes, pythons and boas, have vestigial hind limbs; tiny, clawed digits known as anal spurs which are used to grasp during mating.[10][5]:11 Leptotyphlopidae and Typhlopidae are other groups where remnants of the pelvic girdle are present, sometimes appearing as horny projections when visible. The frontal limbs are non-existent in all snakes and this loss is associated with the evolution of the Hox genes controlling limb morphogenesis. The axial skeleton of the snakes' common ancestor, like most other tetrapods had regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic) and caudal (tail) vertebrae. The Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant early in snake evolution and as a result, the vertebrae anterior to the hindlimb buds (when present) all have the same thoracic-like identity (except from the atlas, axis and one to three neck vertebrae), making most of the snake's skeleton being composed of an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. The neck, lumbar and pelvic vertebrae are very reduced in number (only two to ten lumbar and pelvic vertebrae are still present), while only a short tail remains of the caudal vertebrae, although the tail is still long enough to be of good use in many species, and is modified in some aquatic and tree dwelling species.