Lizards may have run on two feet (bipedally) as early as 110 million years ago, according to a study in Scientific Reports.
Many modern lizards can run bipedally on land, however, at what point in their evolution lizards developed the ability to run on their hind legs has not been clear. Hang-Jae Lee and colleagues describe the first direct evidence of bipedal running in fossil lizards: four series of lizard footprints (trackways) discovered in the Hasandong Formation which has been dated back to the Aptian-early Albian period, 125 to 100 million years ago. The authors identified the prints as Sauripes hadongensis, a group of fossilized lizard trackways which represent the oldest crown-group lizard trackways in the world. It is not clear which lizard species made the tracks but the authors hypothesize they may have come from an extinct family of Iguania.
The fossils are very well preserved, allowing the authors to study lizard foot anatomy in detail. They identified twenty-five foot (pes) tracks, showing typical lizard morphology, with curved digits increasing in length from the inside of the foot to the outside, and four shorter hand (manus) tracks with a longer third digit compared to the other four digits.
Bipedality usually occurs when lizards with longer hind than front limbs accelerate to running speed, changing from a four-legged to a two-legged gait. The authors found that the pes tracks were predominant, which is more consistent with a two-legged than a four-legged gait pattern. The pes tracks also showed evidence of increasing stride length, and digitigrade footprints that suggest the lizards which made the tracks were running on their toes. This, together with the narrow width of the pes track - evidence of the hind legs being straighter and closer together as lizards attain bipedal posture - indicates that the tracks were made by lizards running on their hind legs, suggesting that bipedality occurred early in lizard evolution.