The Gila monster has four close living relatives, all of which are beaded lizards. They appear strong in their body structure with a stout snout, massive head, and 'little'-appearing eyes, which can be protected by a nictitating membrane. Body mass is typically in the range of 550 to 800 g (1.21 to 1.76 lb). The tail is about 20% of the body size, and the largest specimens may reach 51 to 56 cm (20 to 22 in) in total length.

Its snout-to-vent length ranges from 26 to 36 cm (10 to 14 in). The Gila monster is the largest extant lizard species native to North America north of the Mexican border. The first drawing of a Gila monster by Baird, S. He suspected that the lizard might be venomous due to the grooves in the teeth.

At first, this new specimen of Heloderma was misidentified and considered to be a northern variation of the beaded lizard already known to live in Mexico. Suspectum comes from the describer, paleontologist Edward Drinker Cope. Heloderma means 'studded skin', from the Ancient Greek words helos ( ἧλος), 'the head of a nail or stud', and derma ( δέρμα), 'skin'. states of Arizona and New Mexico, where the Gila monster was once plentiful. The name 'Gila' refers to the Gila River Basin in the U.S. History Evolutionary splitting of the genus Heloderma into species (Reiserer et al) Explanation of the numbers However, it has a fearsome reputation and is sometimes killed in spite of the species being protected by state law in Arizona. The Gila monster is sluggish in nature, so it is not generally dangerous and very rarely poses a real threat to humans. Its venomous close relatives, the four beaded lizards (all former subspecies of Heloderma horridum) inhabit Mexico and Guatemala. It is a heavy, typically slow-moving reptile, up to 56 centimetres (22 in) long, and it is the only venomous lizard native to the United States. The Gila monster ( Heloderma suspectum, / ˈ h iː l ə/ HEE-lə) is a species of venomous lizard native to the Southwestern United States and the northwestern Mexican state of Sonora.

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