A. Spotted blindsnake (Typhlops punctatus)—primitive; B. Boa constrictor (Boa constrictor)—fixed teeth; C. Eastern hog-nosed snake (Heterodon platirhinos)—moveable rear teeth; D. Western diamondback rattlesnake (Crotalus atrox)—front-fanged. Primitive skulls have no suspensorium bone, no elongated quadrate bone, and a solid, compact skull. Front-fanged skulls have a suspensorium that is angled up, elongated quadrate that is angled back (allows jaws to open wider), and a less solid skull with more moveable bones. Snake skulls show variations of these traits on a continuum, with primitive and front-fanged at the extremes, and rear-fanged somewhere in the middle.

Michigan Science Art (copyright holder)