Ophidia

Ophidia was defined as the 'most recent common ancestor of Pachyrhachis and Serpentes (modern snakes), and all its descendants' by Lee and Caldwell (1998: 1551). The latter author has used Ophidia in a manner inconsistent with this definition, using it to incorporate other more basal stem-snakes, such as the Late Cretaceous Najash rioegrina or the Jurassic Diablophis and Portugalophis. The clade name Ophidia derives from the Ancient Greek word ὀφίδιον (ophídion), meaning 'small snake'. Modern snakes are thought to have evolved from either burrowing or aquatic lizards during the mid-Cretaceous period, and the earliest known fossils date to around 112 Ma ago. However, the relationship between modern snake and more primitive snake ancestors, many of which retained hind limbs, is less clear. While many of these 'stem-snakes' are known from Mesozoic fossils, some of them may be descendants of the earliest true snakes rather than more primitive lineages. Below is a cladogram modified from a study by Wilson et al. (2010), which found many stem-snakes of other studies to be true snakes instead. Below is different phylogenetic overview of ophidians, following the study by Caldwell et al. 2015. The fossil record of snakes is relatively poor because snake skeletons are typically small and fragile, making fossilization uncommon. Fossils readily identifiable as snakes (though often retaining hind limbs) first appear in the fossil record during the Cretaceous period. The earliest known snake fossils come from sites in Utah and Algeria, represented by the genera Coniophis and Lapparentophis, respectively. These fossil sites have been tentatively dated to the Albian or Cenomanian age of the late Cretaceous, between 112 and 94 Ma ago. However, an even greater age has been suggested for one of the Algerian sites, which may be as old as the Aptian, 125 to 112 Ma ago. Based on comparative anatomy, there is consensus that snakes descended from lizards.:11 Pythons and boas—primitive groups among modern snakes—have vestigial hind limbs: tiny, clawed digits known as anal spurs, which are used to grasp during mating.:11 The Leptotyphlopidae and Typhlopidae groups also possess remnants of the pelvic girdle, sometimes appearing as horny projections when visible.