Ancient lizards legged it

Among the many things that we think make human beings special is our upright stance and two-legged gait, known as “bipedalism”. However, analysis of a newly discovered set of fossil footprints in South Korea indicates that we came late to the bipedal party: lizards were doing it as early as 125 million years ago.

Bipedalism is most common in birds, but there are more than 50 species of lizard that use it, including the frilled-necked lizard (Chlamydosaurus kingii) and the aptly named Jesus lizard (Basiliscus basiliscus) that manages to run on water.

Just when bipedalism evolved in lizards has been unclear, mostly due to the lack of fossil evidence: fossilisation of lizard bodies is rare, and evidence of tracks even rarer. Owing to the reptiles’ size and light weight, their small and delicate footprints are only preserved in extraordinary circumstances. Until now, there have been only three identified fossil lizard tracks.

However, Hang-Jae Lee of the Korea Institute of Geoscience and Mineral Resources, Daejeon, in South Korea and an international team of researchers may have found a fourth. A 2004 excavation of an abandoned quarry next to Handong power plant, in southern South Korea, unearthed a mudstone slab containing twenty-nine detailed imprints of suspected fossilised lizard tracks.

Lee and colleagues’ current paper, published in the journal Scientific Reports, confirms the footprints as lizard tracks, based on clues about foot morphology. This makes them the oldest lizard tracks ever recorded, dating back to the Aptian-early Albian period, 100 to 125 million years ago.

More importantly, the researchers report that analysis of the tracks indicates that the lizard was in the process of speeding up – going from a walk to a fully bipedal run. Lizards, the scientists note, are the only type of vertebrate “that starts on all limb pairs, then transitions to the bipedal gait by acceleration”.

The find represents the oldest direct evidence of bipedalism in fossil lizards, indicating, Lee and colleagues, say, that running on two legs was deeply rooted in the history of the animals.

So, why was this lizard legging it?

The authors suggest that bipedal locomotion is related to predator avoidance as well as potentially playing a role in enhanced environmental perception and avoiding obstacles, but exactly what happened in that moment captured in the mudstone, will remain unknown.

However, the scientists do note the nearby presence of tracks made by a pterosaur, a flying reptile, and hypothesise a predator-prey relationship between lizards and the larger creatures.

Perhaps then, the little critter was running for its life.