Knob-Scaled Lizards: Xenosauridae

With their flat heads and bodies and lumpy scales, the knob-scaled lizards have an unusual look. The head is usually triangular in shape, coming to a point at the tip of the snout. Some have a very noticeable ridge above the eye and extending forward to the snout and backward to the rear of the head. Often, the females have larger bodies than the males, but the males typically have bigger heads. Their bodies are usually dark brown to black, often with lighter-colored bands or blotches. The largest specimens grow to 4.7 to 5.1 inches (12 to 13 centimeters) long from the tip of the snout to the vent, a slitlike opening on the belly side of the animal at the beginning of the tail. The tail stretches nearly as long as the body.

Until 1999, this family only had four species. Discoveries of two new species—one in 2000 and one in 2002—increased the number to six. The two new species are known only by their scientific names: Xenosaurus penai and Xenosaurus phalaroantheron. Scientists believe additional species are yet to be identified. In particular, they suspect that a closer look at some of the already known knob-scaled lizards may reveal that they should actually be separated into two or more similar-looking species. This type of splitting is especially common in animals that live in small groups that are separated from one another, so the individuals from one group, or population, never see individuals from another population.