Evolution and systematics

There are few snake fossils that can be incontrovertibly identified as pythons; these fossils are relatively recent, and all have been assigned to extant genera. Snake fossils from the mid- to late Miocene in Australia have been identified as Lia-sis and Morelia (formerly Montypythonoides). Fossils assigned to the genus Python are known from the Pliocene of eastern Africa.

Pythons are considered to be basal macrostomatans, one of several ophidian lineages that diverged from the primitive alethinophidians near the end of the Cretaceous. Macrostomatan snakes are distinguished by characters of the skull and musculature that allow them increased jaw flexibility, a greater gape, and the ability to consume larger prey. The pythons likely evolved in the Australo-Papuan region where today they exist with the greatest diversity. Seven of the eight genera of pythons occur in Australia and New Guinea. The genus Aspidites, an Australian endemic, is the sister taxon to all other pythons. It is assumed that Python, the most widespread genus, evolved in the Indo-Papuan region and dispersed north to Southeast Asia and then west into Africa.

Pythons share many characters with other basal macros-tomatan snakes, including fully functional paired lungs, smooth scales (with some exceptions), vestiges of a pelvic girdle, and cloacal spurs. A cloacal spur is a claw-like structure used in courtship that is found on either side of the anal scale;

male pythons tend to have larger cloacal spurs than female pythons. Characters shared with the Boidae, the sister taxon of the Pythonidae, include elliptical pupils and pitted lip scales that are associated with thermoreception. Thermoreception is the ability to sense differences in temperatures.

Pythons differ from boas in numerous characters, including: The supraorbital bone of pythons is in contact with the prefrontal. The supraorbital bone is a small bone located at the top of the eye socket. Only one taxon of boa possesses a supraorbital bone, and it does not contact the prefrontal. Python labial pits are located in the centers of the lip scales; when present in boas, pits are located between the lip scales. Two premaxilla are fused together to make the small bone at the front of the upper jaw: the premaxilla of most adult pythons is toothed, while the premaxilla of boid snakes is without teeth. All pythons are oviparous and lay eggs, while all but three taxa of boid snakes give live birth.

The Pythonidae is a widely distributed lineage that is undoubtedly more species-rich than is recognized currently. Mainly because of the practical problems involved in preserving and caring for large specimens, there is a surprising paucity of representative material from most python populations in museum collections. In 2000 and 2001, taxonomic revisions resulted in an increase in the number of python species from 25 species to the currently recognized 32 species. There is little doubt that future investigations will identify more taxa. No subfamilies are recognized.

Burmese python (Python molurus bivittatus) laying eggs. (Photo by Animals Animals ©Jim Tuten. Reproduced by permission.)

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