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Reptiles are members of the taxonomic group containing the surviving squamates (lizards), snakes, turtles, sphenodonts (the tuatara being the only extant member) and the crocodilians (including alligators) as well as the extinct dinosauria and relatives. Reptilia is a type of evolutionary clade which is considered paraphyletic since does not include all the descendant taxonomic groups from its parent clade of Amniota. These excluded groups are the Aves and Mammalia, which are their own respective monophyletic clades. The ancestral traits of the Amniota are shared by all these groups (Carroll, 1988).

The earliest conclusive reptile is Hylonomus a small lizard-like insectivore from 315 million years ago (Palmer, 1999).  The expansion into open niches from a desiccation resistant egg evolutionary advantage saw an increase of feeding strategies to include herbivory and carnivory (Sahney et al, 2010). As a result the Reptilia were the dominant terrestrial vertebrates of the Mesozoic, after which their prevalence was diminished drastically by a mass extinction event. The four major extant groups have pursued independent evolutionary trajectories since the Triassic (Pough et al, 1998).

The basal state of the amniote skull gave rise to three distinguishable clades which differ in number of temporal fenestrae (Coven, 2000). The varying shapes of these openings was tied to jaw articulation and musculature (Carroll, 1988).  Early on in the evolution of reptiles, there were numerous Anapsids, whose name refers to their distinguishing characteristic of having no openings in their skull. These all went extinct. Turtles & tortoises demonstrate an Anapsid condition but may be evolved from a Diapsid condition (Benton, 2000). The skulls of all other extant reptiles are diapsid, with two openings posterior to the occipital bone (Carroll, 1988). Mammals have a single opening behind the eyes where we can feel the jaw musculature on ourselves.

Reptiles are ectothermic – being dependent upon the temperature conditions of the environment for a significant portion of body heat generation necessary for physiological processes. This restricts their distribution from extremely cold environments but opens up niches unavailable to endotherms (Shine, 2005). The surface area to body volume ratio affects the rate at which bodies lose or gain heat from the environment. This has allowed reptiles flexibility in body form (for example snakes, who being an elongated cylinder shape is far from the ideal heat retention shape of a sphere) which are not available niches for endotherms.

Ectothermy has produced two major consequences of low energy needs and the behavioural control of body temperatures which have both affected life history strategies available to reptiles (Shine, 2005). Small offspring, large litter sizes and infrequent reproduction are three strategies unavailable to endotherms, allowing flexibility and adaptation to broad range of potential niches (Shine, 2005). Squamate reptiles that are ambushing predators tend to have heavyset bodies, whereas active searchers are more elongate and slender-bodied (Vitt & Congdon 1978).

Because ectothermy breaks the temporal link between energy acquisition and expenditure ectotherms can store fat reserves to withstand long periods of starvation (Pough 1980; Shine et al 2002) this allows the opportunity to specialize on a prey type with lower availability (Pough 1980). Many ecotherms rely on only a few prey types (Greene 1997). The deferring of nutritional intake also allows a disjunction of conflict between energy acquisition required for feeding and reproduction allows female to tolerate a greater degree of locomotor impairment and therefore a greater brood size than would be compatible with continued feeding during pregnancy (Shine 2005).

Benton, M. J. (2000). Vertebrate Paleontology (2nd ed.). London: Blackwell Science Ltd. ISBN 978-0-632-05614-9., 3rd ed. 2004

Carroll, R.L. (1988). Vertebrate Paleontology and Evolution, W. H. Freeman and Co. New York

Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 62. ISBN 1-84028-152-9.

Pough, F.H. (1980). The advantages of ectothermy for tetrapods. American Naturalist 115, 92–112

Sahney, S., Benton, M.J. & Falcon-Lang, H.J. (2010). “Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica”. Geology. 38 (12), 1079–1082. doi:10.1130/G31182.1.

Shine R. (2005). Life-History evolution in reptiles. Annual Review Ecology & Evolutionary Systematics. 36, 23–46 doi: 10.1146/annurev.ecolsys.36.102003.152631

Shine R, Sun L, Zhao E, Bonnet X. (2002). A review of 30 years of ecological research on the Shedao pit-viper. Herpetological Natural History 9, 1– 14

Vitt L.J. & Congdon J.D. (1978). Body shape, re- productive effort, and relative clutch mass in lizards: resolution of a paradox. American Naturalist 112, 595–608

Early reptilians in the Permian split into two primary groups which encompass the Reptilia clade today (Colbert & Morales, 2001). The Archosauromorpha infra class is defined by socketed teeth and produced the lineages of turtles, crocodilians and dinosaurs (Carroll, 1988). The Lepidosauromorpha is defined by the teeth being fused to the inner jaw and produced the modern lineages of squamates, snakes and tuataras (Carroll, 1988). Alongside these major branches during the Permian existed the parareptilians and reptiliomorphs which were distinguished by skull characteristics and labyrinthodont teeth. These groups were those who excelled in both size and number as the dominant megafauna, but drastically diminished by the end of this period and eventually went extinct. The Permian-Triassic extinction event marks the transition between these periods of dominance.

These basal reptiles of archosaurs and lepidosaurs were small and able to take advantage of the conditions following the extinction event associated with Carboniferous Rainforest collapse (Sahney et al, 2010). A large adaptive radiation was possible due to their desiccation-resistant egg physiology that allowed for tolerance of reproduction on drier land relative to the amphibians and other tetrapods of the time. Further radiation of these two extant reptilia lineages continued through Triassic period into the Cretaceous of the Mesozoic era. This was the era of the dinosaurs. The extinction event ending the Mesozoic and leading into the Miocene era reduced the megafauna species of reptiles, and all dinosaurs short of the Aves (Sahney & Benton, 2008).

Being within the Amniota, all reptiles have the basal characteristic of laying eggs (oviparity). Most reproduce sexually, though asexual reproduction is possible in some species (Shine, 2005). Eggs are leathery or shelled with three layers of membranes. Asexual reproduction is via parthenogenisis of diploid genetic clones of the females. This has been observed in seven families within the reptilia (Shine, 2014). Viviparity has evolved independently approximately one hundred times across multiple clades (Shine 2014; Blackburn, 1982, 1985; Shine, 1985) as an adaptation to environmental conditions (Mell, 1929). The mesosaurs of the Permian are the earliest evidence of full viviparity where the egg has evolved into a functional equivalent of the placenta (Piñeiro et al, 2012). Crocodilians and turtles have not evolved viviparity possibly since their embryos do not continue development for very long even if retained in utero (Shine, 1983).

Current geographic distribution of the patterns of viviparity suggested the two potential origins of the selective pressure for this change were to protect them from night time frost events (Mell, 1929; Weekes, 1933) or as a mechanism to increase cumulative incubation warmth (Sergeev, 1940). The ability of a reproducing female to modify her own body temperature allows incubation regimes unavailable at any nesting site in terms of avoiding problematic minimums, maximums and means (Shine, 2005).

Temperature-dependent sex determination affects the gender of the offspring (Burger 1989) and is likely a choice based upon the mother given the predicted environmental conditions that will apply natural selection upon the offspring (Shine, 2005). The incubation effect on offspring gender is observed in all crocodilians, many turtles, sphenodonts, and a phylogenetically diverse set of lizards (Bull & Charnov, 1989; Rhen & Lang, 1999).

Most reptiles have sexual organs which are external only during reproduction. Snakes and lizards differ from the penis present in crocodilians and turtles by the possession of a hemipenes. The Sphenodonts retain the basal state of both genders have a cloaca, which they press together during mating, as Birds do.

Blackburn, D.G. (1982). Evolutionary origins of viviparity in the Reptilia. I. Sauria. Amphibia–Reptilia 3, 185–205.

Blackburn. (1985). Evolutionary origins of viviparity in the Reptilia. II. Serpentes, Amphisbaenia, and Icthyosauria. Amphibia–Reptilia 6, 259–291.

Bull, J.J. & Charnov, E.L. (1989). Enigmatic reptilian sex ratios. Evolution 43, 1561–66

Carroll, R.L. (1988). Vertebrate Paleontology and Evolution, W. H. Freeman and Co. New York

Colbert, E.H. & Morales, M. (2001). Colbert’s Evolution of the Vertebrates: A History of the Backboned Animals Through Time. 4th edition. John Wiley & Sons, Inc, New York

Mell, R. (1929). Beitrage zur Fauna sinica. IV. Grundzuge einer Okologie der chinesischen Reptilien und einer herpetologischen Tiergeogra- phie Chinas. Walter de Gruyter, Germany.

Piñeiro, G., Ferigolo, J., Meneghel, M., & Laurin, M. (2012). “The oldest known amniotic embryos suggest viviparity in mesosaurs”. Historical Biology. 620–630. doi:10.1080/08912963.2012.662230.

Rhen T & Lang J.W. (1999). Temperature during embryonic and juvenile development influences growth in hatchling snapping turtles, Chelydra serpentina. Journal of Thermal Biology 24, 33– 41

Sahney, S. & Benton, M.J. (2008). “Recovery from the most profound mass extinction of all time” (PDF). Proceedings of the Royal Society: Biological. 275 (1636): 759–65. doi:10.1098/rspb.2007.1370.

Sahney, S., Benton, M.J. & Falcon-Lang, H.J. (2010). “Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica”. Geology. 38 (12), 1079–1082. doi:10.1130/G31182.1.

Sergeev, A.M. (1940). Researches in the viviparity of reptiles. Moscow Society of Naturalists (Jubilee Issue):1–34.

Shine R. (1983). Reptilian reproductive modes: the oviparity-viviparity continuum. Herpetologica 39, 1–8

Shine R. (2005). Life-History evolution in reptiles. Annual Review Ecology & Evolutionary Systematics. 36, 23–46 doi: 10.1146/annurev.ecolsys.36.102003.152631

Shine R. (2014). Evolution of an Evolutionary Hypothesis: A History of Changing Ideas about the Adaptive Significance of Viviparity in Reptiles. Journal of Herpetology, 48(2), 147–161

Weekes, C.H. (1933). On the distribution, habitat and reproductive habits of certain European and Australian snakes and lizards, with particular regard to their adoption of viviparity. Proceedings of the Linnean Society of NSW 58, 270–274.

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