Evolution and systematics

Two genera of bolyeriid snakes survived into the late twentieth century on Mauritius and several closely associated small islands. This family is believed to be the modern descendant of an early lineage of macrostomatan snakes that diverged from the alethinophidians at the end of the Cretaceous. It is distinguished by a character that is unique among all vertebrate animals: the maxilla is divided into separate anterior and posterior parts by a movable joint. The joint is located beneath the eye; it allows the front of the upper jaw to bend up or down independently of the rear portion of the jaw.

For many years the Bolyeriidae family was classified as a subfamily within the Boidae. For that reason bolyeriid snakes most often were referred to as "boas." It now is recognized that the Bolyeriidae is a unique lineage not closely allied with the Boidae. In recognition of the uniqueness of the modified maxilla of the Bolyeriidae, the snakes in this family are identified as "splitjaw snakes." The sole member of the Bolyeriidae to survive into the twenty-first century is the keel-scaled splitjaw, Casarea dussumieri.

The smooth-scaled splitjaw, Bolyeria multocarinata, is the second bolyeriid species known to science. One specimen, identified by a unique scar and believed to be the last survivor of the species, was found in faunal surveys of Round Island in the 1960s and early 1970s. It was last seen in 1974, and the species is considered extinct.

Splitjaw snake (Casarea dussumieri). (Illustration by Marguette Dongvillo)

No fossils of this family exist. Other common names of the splitjaw snake include Round Island boa, splitjaw boa, and keel-scaled boa. No subfamilies are recognized.

The taxonomy of the keel-scaled splitjaw is Casarea dus-sumieri Schlegel, 1837, Round Island, Mauritius.

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