Evolution and systematics

Fossil snake skulls are rarely recovered because the bones of the skull are small and loosely connected and typically become separated soon after death. Vertebrae are the most common snake fossil. In general, few snake families can be identified in-controvertibly by vertebrae and ribs. Living erycine snakes, however, have several unique vertebral characters, and on this basis numerous fossil vertebrae have been identified as erycine (in the subfamily Erycinae).

The modern Boidae is believed to have descended from basal macrostomatans; it is one of several snake lineages that diverged from the primitive alethinophidians (true snakes) 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.

Boid snakes share many characters with other basal macrostomatan snakes, including fully functional paired lungs, smooth scales (with some exceptions) vestiges of a pelvic girdle, and cloacal spurs. The cloacal spurs of boas are two claw-like structures that are located one on each side of the anal scale. They are usually larger in male boas than in females; the females of some species may not have apparent cloacal spurs. Characters shared with the Pythonidae, the sister taxon of the Boidae, include elliptical pupils and pitted lip scales. The pits in the lips are associated with thermorecep-tion, the ability to detect differences in temperature.

Boas differ from pythons in numerous characters, including: Boid snakes do not have a supraorbital bone (with one exception) while all pythons have a supraorbital bone. Not all boas have labial pits; when present, the labial pits are located between the labial scales while the labial pits of pythons are centered in the labial scales. Two premaxilla are fused together to form a small bone across the front of the upper jaw; the premaxilla of boid snakes is without teeth while the pre-maxilla of most pythons is toothed. Most boas are viviparous, meaning they bear live young; all pythons lay eggs. Three taxa of boid snakes, Charina reinhardtii, Eryx muelleri, and Eryx jayakari, are oviparous and lay eggs.

Undoubtedly, the Boidae is more speciose than is recognized. An analysis of the geographic variation and systematic relationships of populations of Boa constrictor, the most widely distributed boid species, has yet to be accomplished. Likewise, the systematic relationships of the insular populations of all three species of Candoia remain to be investigated fully. There is little doubt that there will be further taxonomic changes within the Boidae.

As of 2002, science recognizes 41 species in seven genera and two subfamilies in the Boidae. The Boinae, the larger subfamily, includes the boas and anacondas, 27 species in five genera. The Erycinae includes the sandboa, rubber boa, rosy boa, and Calabar boa, 14 species in two genera. The division of the Boidae into these two subfamilies is based primarily on osteological characters.

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