The helodermatid clan has a rich and diverse evolutionary history that dates back 98 million years across Europe, Asia, and North America to a time well before many dinosaurs had appeared. The fossil record shows that the remaining species of helodermatid lizards are relics of a more diverse lineage that included at least six other genera inhabiting subtropical desert, forest, and savanna habitats. Family members somehow managed to survive the great Cretaceous extinctions, which vanquished the dinosaurs 65 million years ago. Helodermatid lizards have undergone relatively little gross morphological change over this time, and may appropriately be regarded as living fossils. The genus Heloderma has existed since at least the early Miocene (about 23 million years ago).
Today only two species remain: the Gila monster (Heloderma suspectum) and the Mexican beaded lizard or escorpion (H. horridum). The two species are readily distinguished from each other by the Mexican beaded lizard's proportionately longer tail (at least 65% of the body length; no more than 55% in the Gila monster). The escorpion is a longer, lankier, more arboreal lizard than the Gila monster.
Heloderma horridum was first described by Wiegmann in 1829 in Huajintlan, Morelos, Mexico. Four subspecies are recognized. Heloderma suspectum was first described by Cope in 1869, on international boundary between the United States and Mexico, Sierra de Moreno, Arizona. Two subspecies are recognized.
No subfamilies are recognized.
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