The varanid lizards had their origins in Aisa and North America during the last days of the dinosaurs. Some of these early monitor-like lizards were unusual in that they had a characteristic which is only present in the modern day helodermid lizards - venom! Estesia mongoliensis and Paleosaniwa canadensis had skulls with grooves in their teeth for venom ducts. Other details of their lifestyle are unclear, although they undoubtedly ate the mammals and smaller dinosaurs of their day and age.