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Traditionally, reptiles are taken to be a paraphyletic group, i.e. a group that does not include all descendants of the last common ancestor. A monophyletic reptile clade would incorporate birds, which are the closest living relatives of crocodiles (and so united with them as Archosauria). Reptiles, birds and mammals are amniotes (characterised by terrestrially adapted eggs with protective extra-embryonic membranes), and reptiles have been described as amniotes without fur or feathers. More than 8000 living species of non-avian reptiles have been described (and once, of course, non-avian dinosaurs roamed the earth). Most familiar are the lizards and snakes (Squamata), turtles and tortoises (Testudines) and crocodiles and alligators (Crocodilia). The ectothermic ('cold-blooded') reptiles share a number of features, the most prominent of which is probably their dry, horny, impermeable skin. In squamates, it consists of scales, overlapping thickenings of the keratinous epidermal layer, whereas turtle and crocodile skin is formed from usually non-overlapping dermal scutes. Reptiles moult, which is most obvious in snakes that shed their skin periodically in one piece. Curiously, some reptiles show temperature-dependent sex determination (TSD), where offspring sex is determined not by chromosomes but by the incubation temperature during a thermosensitive period.
Reptile reproduction is also interesting in terms of evolutionary convergence. While male turtles have evolved an inflatable penis that is strikingly similar to that of mammals, some lizards (e.g. geckos) provide prime examples of parthenogenesis. Parthenogenetic species reproduce asexually and comprise populations consisting entirely of females that produce genetically identical offspring. This reproductive mode is astonishingly common within the 'Australian arid zone', where it has not only evolved in reptiles, but also in insects and plants, probably in response to unusual environmental challenges. In general, reptile reproduction is highly diverse. Although most species lay eggs (oviparity), some lizards and snakes are ovoviviparous, retaining their eggs internally for extended periods of time. This reproductive strategy has evolved many times within this group, but also in other animal groups, such as molluscs and insects. With ovoviviparity, the developing embryo obtains nutrients from the egg yolk, whereas the embryos of truly viviparous (livebearing) animals depend on nutrients from beyond the yolk (i.e. from a placenta). Usually considered the hallmark of placental and marsupial mammals, true viviparity has also arisen in skinks such as Mabuya. Here we find a placenta remarkably similar to that of placental mammals. Fossils of pregnant female sauropterygians, ichthyosaurs and mosasaurs furthermore indicate that live birth also evolved in several disparate lineages of Mesozoic marine reptiles.
Another group of Mesozoic marine reptiles, the placodontids, were aptly nicknamed "Triassic sea cows" (Diedrich 2010, Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 285, p. 293), as they show a number of striking feeding-related anatomical convergences with dugongs. Both groups evolved adaptations to grinding plant material, such as similar canal-like structures in the horny pads or teeth in the upper jaw, possibly to flush out sand and help to expel fluid while crushing food. Reptile dentition offers other intriguing examples of convergence, including the specialised tooth types of aquatic reptiles, the crustacean-trapping teeth of a group of Early Permian marine reptiles known as mesosaurs, the stunningly mammal-like dentition of the skink-like teiids and the Cretaceous Crocodyliformes and the specialised beak structures of turtles and some lizards. Vipers and cobras have independently evolved enlarged, hollow venom fangs in the upper jaw, analogues of which are also found in some lizards and even in a few mammals. On a rather more peaceful note, a Madagascan gecko feeds on honeydew produced by a sap-sucking planthopper, thus showing remarkable parallels with ants that are known for tending honeydew-producing aphids.
Chameleons are famous for capturing insects with their rapidly protrusible tongues. Such ballistic tongue protrusion has also evolved in frogs and plethodontid salamanders. But chameleons are instructive in other ways, too, as their eyes are strikingly similar to those of the sandlance, a small fish that lives in coral sands. Not only can the eyes operate independently in both groups, but they are also equipped with a telephoto arrangement, which allows for fast and highly accurate prey capture.
When it comes to sensory capacities, snakes are particularly intriguing, as some are able to perceive infrared radiation. Thermosensitive pits on the head have evolved at least twice - once in the pit vipers and, given their different arrangement, evidently separately in the more ancient boas and pythons. These pit organs are similar with regard to their ultrastructure and electrophysiological function, but differ in number, location and overall morphology. Even more remarkable is that analogues of these two types of snake infrared receptor are found in two species of beetle that use this sensory modality not for detecting "warm-blooded" prey but forest fires.
Whilst snakes are also well known for their wriggly way of locomotion, a few species do something rather different - they can glide. The ability to glide has evolved repeatedly in a surprising diversity of animals, including not only reptiles, but also mammals, frogs, ants, fish and even some species of squid. These animals employ a range of impressive morphological and behavioural adaptations to generate the required aerodynamic forces. The two species of gliding tree snake possess a uniquely specialised body shape and aerial behaviour, including rib expansion and stereotypic lateral undulations, to produce controlled glides. Several extinct reptile lineages independently evolved feathers for gliding, whereas modern reptiles, for example Draco lizards and some geckos, use different forms of skin membrane (patagia) on various parts of the body. Geckos are, however, more famous for running effortlessly across the ceiling - how do they manage to do this? Their feet are equipped with hairy adhesive pads that are rampantly convergent amongst animals, having evolved several times within the lizards alone.
Reptiles dominate deserts, and several interesting convergences are found amongst desert species. One case involves an Australian agamid, the thorny devil, and the North American horned lizards that have come up with similar solutions to the problems of food scarcity (they eat ants), aridity (they collect and transport drinking water through modified skin structures) and a need for camouflage in their largely open desert habitat. In different parts of the world, members of six lizard families have independently adapted to sand dune habitats, sharing many aspects of morphology and behaviour that allow efficient locomotion and burrowing into loose sand as well as breathing when buried. These different 'morphotypes' are categorised together as unique, sand-dwelling ('psammophilous') ecomorphs. Another example of the recurrent evolution of ecomorphs is found in the anolid lizards inhabiting the Caribbean islands of Cuba, Jamaica, Hispaniola and Puerto Rico. In response to habitat-specific selection pressures, they have independently acquired remarkably similar forms, such as 'twig' anoles and 'trunk-crown' anoles.