In the 13th century the category of reptile was recognized in Europe as consisting of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, lizards, assorted amphibians, and worms", as recorded by Beauvais in his Mirror of Nature. [6] An older snake may shed its skin only once or twice a year, but a younger, still-growing snake, may shed up to four times a year. [18] The discarded skin gives a perfect imprint of the scale pattern and it is usually possible to identify the snake if this discard is reasonably complete and intact. [9] Arrangement of scales [ edit ] Nomenclature of head scales (top view of head)

Only three reptile species, including the yellow-bellied three-toed skink of Australia, actually combine both eggs and live birthing methods (the rarity suggests that evolution probably does not favor this in-between stage). The skink’s offspring begins life encased in an egg, the same as any other reptile. But as the embryo develops, the egg begins to thin out until all that’s left upon its birth is a small membrane. The main problem with this method is that the thin egg shells don’t contain enough calcium to nourish the offspring. The mothers appear to compensate for this by secreting calcium from the uterus so it can be absorbed by the developing embryo. The evidence suggests that the skink can choose to lay eggs a few weeks early if it seems like there’s less danger to the offspring. In harsher climates, the mother will keep the offspring insider her body for longer to protect them.Lyson, Tyler R.; Sperling, Erik A.; Heimberg, Alysha M.; Gauthier, Jacques A.; King, Benjamin L.; Peterson, Kevin J. (2012). "MicroRNAs support a turtle + lizard clade". Biology Letters. 8 (1): 104–107. doi: 10.1098/rsbl.2011.0477. PMC 3259949. PMID 21775315. Cieri, Robert L.; Craven, Brent A.; Schachner, Emma R.; Farmer, C.G. (2014). "New insight into the evolution of the vertebrate respiratory system and the discovery of unidirectional airflow in iguana lungs". Proceedings of the National Academy of Sciences. 111 (48): 17218–17223. Bibcode: 2014PNAS..11117218C. doi: 10.1073/pnas.1405088111. PMC 4260542. PMID 25404314.

Some snakes have extra sets of visual organs (in the loosest sense of the word) in the form of pits sensitive to infrared radiation (heat). Such heat-sensitive pits are particularly well developed in the pit vipers, but are also found in boas and pythons. These pits allow the snakes to sense the body heat of birds and mammals, enabling pit vipers to hunt rodents in the dark. [b] Goodrich, E.S. (1916). "On the classification of the Reptilia". Proceedings of the Royal Society of London B. 89 (615): 261–276. Bibcode: 1916RSPSB..89..261G. doi: 10.1098/rspb.1916.0012.Farmer, C.G. (2000). "Parental care: The key to understanding endothermy and other convergent features in birds and mammals". American Naturalist. 155 (3): 326–334. doi: 10.1086/303323. PMID 10718729. S2CID 17932602.

a b Sander, P. Martin (2012). "Reproduction in early amniotes". Science. 337 (6096): 806–808. Bibcode: 2012Sci...337..806S. doi: 10.1126/science.1224301. PMID 22904001. S2CID 7041966. Lizard scales vary in form from tubercular to platelike, or imbricate (overlapping). These scales, which on the surface are composed of horny ( keratinized) epidermis, may have bony plates underlying them; these plates are called osteoderms. Excluding the head, snakes have imbricate scales, overlapping like the tiles on a roof. [20] Snakes have rows of scales along the whole or part of their length and also many other specialised scales, either singly or in pairs, occurring on the head and other regions of the body. Reptile skin is covered with scutes or scales which, along with many other characteristics, distinguish reptiles from animals of other classes. They are made of alpha and beta-keratin and are formed from the epidermis (contrary to fish, in which the scales are formed from the dermis). The scales may be ossified or tubercular, as in the case of lizards, or modified elaborately, as in the case of snakes. [1] YOUNG, BRUCE A. (1997). "A Review of Sound Production and Hearing in Snakes, with a Discussion of Intraspecific Acoustic Communication in Snakes". Journal of the Pennsylvania Academy of Science. 71 (1): 39–46. ISSN 1044-6753. JSTOR 44149431.

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Phylogenetic classifications group the traditional "mammal-like reptiles", like this Varanodon, with other synapsids, not with extant reptiles Werneburg, Ingmar; Sánchez-Villagra, Marcelo R. (23 April 2009). "Timing of organogenesis support[s] basal position of turtles in the amniote tree of life". BMC Evolutionary Biology. 9: 82. doi: 10.1186/1471-2148-9-82. ISSN 2730-7182. PMC 2679012. PMID 19389226. article82.