Scaly amniotes that taxonomists keep redrawing
Reptiles, in ordinary speech, are ectothermic tetrapods with amniotic eggs—traditionally turtles, crocodilians, squamates, and tuatara. Class Reptilia remains disputed; the lineage appears by the late Carboniferous. Living sizes run from a 17 mm gecko to saltwater crocodiles.
The Reptile Database lists around 12,000 living species in four traditional orders: turtles, crocodilians, lizards and snakes, and the tuatara. They range from the Jaragua dwarf gecko, a mere 17 millimetres long, to the saltwater crocodile, which can pass 6 metres and a tonne. The lineage arose in the late Carboniferous from land-adapted reptiliomorphs, and its two great branches, one leading to crocodilians and birds, the other to lizards and their kin, split during the Permian.
Ideas about what counts as a reptile have shifted repeatedly. A thirteenth-century encyclopedia by Vincent of Beauvais lumped together snakes, lizards, amphibians, worms and imaginary monsters. Linnaeus, familiar with Swedish adders and grass snakes that hunt in water, filed reptiles and amphibians together in one class, and the French preferred the word reptile, from the Latin for creeping. J.N. Laurenti first used Reptilia formally, but only in 1825 did Latreille give amphibians their own class, yielding the familiar four tetrapod classes, a scheme T.H. Huxley popularised.
Twentieth-century zoologists tried other cuts. E.S. Goodrich revived Sauropsida and Theropsida in 1916, separating lizard-faced from beast-faced lineages by heart and vessel anatomy, and D.M.S. Watson redrew them in 1956 to exclude birds and mammals. Another system, begun by Henry Fairfield Osborn and spread by Romer's textbook, sorted reptiles by the openings behind the eye: none in turtles, one low opening in the mammal-like synapsids, one high opening in marine groups like ichthyosaurs, and two in lizards, snakes, crocodilians and dinosaurs.
Cladistics upended this. Because birds descend from dinosaurs, a class that leaves them out is not a natural group, whereas mammals and birds each are. Jacques Gauthier in 1988 defined Reptilia as the crown group of turtles, lizards, snakes, crocodilians and birds, and Modesto and Anderson offered a PhyloCode-compliant version in 2004, yet the replacement name Sauropsida never caught on consistently.
Source: Reptile
