Like most true lizards, Palaeosaniwa had specialized jaw and skull joints that allowed the skull to flex and bend in different ways and to act as shock absorbers when the lizard ate live and large prey.  From its teeth, we know that Palaeosaniwa was a meat eater, probably feeding on carrion, eggs, and smaller vertebrates, including baby dinosaurs.  The details of its skull and vertebrae indicate that it was a close relative of the modern lizard Varanus, or monitor lizard, of Africa, India, Southeast Asia, Australia, and the islands in between. Palaeosaniwa grew to be about as big as the modern Ora or Komodo dragon (Varanus komodoensis), perhaps 10-12 feet long (3-3.5 meters) and 330 pounds (150 kilograms). Although not as sophisticated a predator as the smaller meat-eating dinosaurs, Palaeosaniwa could probably kill smaller dinosaurs just as the Ora kills pigs, dogs, and deer today.

Palaeosaniwa and Varanus are just two types of the large assemblage of varanoid lizards. Some varanoids, such as the modern Gila monster (Heloderma) of southwestern North America and the larger Cretaceous Estesia of Mongolia, have poisonous fangs. Palaeosaniwa and Varanus show the typical lizard shape; a long snout with a forked tongue, a long body with four legs ending in five claws each, and a long slender tail. However, some Cretaceous varanoids evolved into a very different body shape, like the 10-to-46 foot (3-to-14 meter) mosauroids, which were swimming forms with flippers instead of feet. Another closely related branch of the varanoids evolved without any legs. This specialized group of varanoid lizard is still living today; we call them "snakes."

This is the first time Palaeosaniwa has been shown on a postage stamp!