#iguanomorpha

Sailfin lizard (Hydrosaurus celebensis)

Bipedal running has convergently evolved multiple times in squamate reptiles, known in over 50 modern species – and fossil evidence shows this is nothing new, with lizards repeatedly developing the ability to sprint on their hind legs for well over 100 million years.

Huehuecuetzpalli mixtecus here lived in east-central Mexico during the mid-Cretaceous, about 105 million years ago. About 25cm long (10"), it was part of an early branch of the iguanomorph lineage, related to the ancestors of modern lizards like iguanas, chameleons, and agamids.

Its limb proportions indicate it would have been a bipedal runner, making it one of the earliest known examples of this type of locomotion in lizards. Its skull also had some features convergent with varanids, suggesting it may have had a similar sort of active-pursuit-hunting ecology.

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Marine iguana (Amblyrhynchus cristatus)

Oklahoma collared lizard (Crotaphytus collaris)

Thorny devil (Moloch horridus)

Rosette-nose chameleon (Rhampholeon spinosus)

Regal horned lizard (Phrynosoma solare)