Boas have two subfamilies: Boinae or true boas and Erycinae or sand boas. Pythons are sometimes classified as a subfamily of Boidae, but are frequently listed under their own family, Pythonidae. Sand boas are also frequently listed under their own family, Erycidae.
Snakes of the subfamily Boinae are found in Madagascar, Papua, Pacific Islands, and the Neotropics. It has been suggested that genera within each of these particular areas do not form monophyletic groups. True boas are medium to large sized snakes. Females are usually larger than their males. Boas contain many subspecies based on locality. The boas from the Amazon Basin are the most colorful possessing bright cherry red tails. It used to be said that boas were New World Snakes and pythons were Old World Snakes, but, with boas found on Madagascar, Fiji, and Solomon Islands, this is not quite true. Instead, it is possible that boas have survived in evolutionarily isolated areas. South America, until a few million years ago, had a distinct fauna that included marsupial mammals; with the land bridge to North America, boas have migrated north as placental mammals and colubrids have migrated south.
Boa subfamily include:
Compared to true boas, erycines are quite small, with most
members remaining well under a meter in length. Fossil erycines
have been found in rock strata over 50 million years old, and
were once widespread in North America. Now, only two species
remain in North America, as well as the sand boas in Africa,
Asia and southeastern Europe.
At least three erycine species lay eggs: the Calabar Boa, (Calabaria reinhardtii) (once classified as a python for this reason); the Arabian Sand Boa, (Eryx jayakari); and the West African Sand Boa, (Eryx muelleri).
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