Lizards are the only vertebrates that, when needing to move fast over a short distance, shift from using all four legs to getting around on two. Ancient footprints reveal this capacity dates back at least to the mid-Cretaceous, when there was even more to run away from than there is today.

Four newly discovered sets of lizard footprints, known as trackways, in the Hasandong Formation, South Korea are the oldest lizard tracks in the world. They were made by a small animal, dubbed Sauripes hadongensis, not thought to match any fossils we know. Nevertheless, the shape of the prints clearly marks the maker as a lizard, probably related to modern iguanas.

Two trackways are entirely made up of hind prints, while the others appear to catch the makers in the processes of shifting from running on all fours to moving upright.

 As the paper in Scientific Reports describing the tracks notes, we don't have a lot of detail on ancient lizards. Although they originated between 213 and 176 million years ago, and are the largest living group of reptiles with 5,800 species, they don't fossilize as easily as larger reptiles. “Unfortunately, fossil footprint records attributable to lizards are even rarer than those of body fossils because of the general light body weight of lizards and their preferred range of habitats,” the paper notes.

Consequently, the 100-125 million-year-old Hasandong tracks, with 29 prints, are a rare and precious resource. Four of these handprints are not only shorter, but have a distinctively longer third digit compared to the others.

The conclusion the lizards were running on two legs, rather than the front tracks simply failing to be preserved, is reinforced by the increasing gap between each print. This is consistent with modern lizards as they accelerate to the point where their front legs come off the ground, taking longer and longer steps as they do so. The prints’ shapes are consistent with the makers running on their toes, rather than a slower, flat-footed, walk. The relatively narrow gap between left and right prints is also consistent with what we see in lizards today, as their feet are much closer together when running upright than when splayed under them.

The footprints are small. ranging from 19 millimeters (0.8 inches) long and wide for the front feet to 22 millimeters (0.9 inches) long and 12 millimeters (0.5 inches) wide for the hind feet. The whole slab, containing all four sets of tracks, is just 70 centimeters (28 inches) by 30 centimeters (12 inches).