These are the oldest lizard tracks in the world…
(if you don’t consider Rotodactylus (Early Triassic) strictly a ‘lizard’ (= squamate). One rotodactylid trackmaker, Cosesaurus, is a tiny lepidosaur).
Figure 1. Bipedal lizard tracks from South Korea in situ. They are tiny.
From the abstract
“Four heteropod lizard trackways discovered in the Hasandong Formation (Aptian-early Albian), South Korea assigned to Sauripes hadongensis, n. ichnogen., n. ichnosp., which represents the oldest lizard tracks in the world. Most tracks are pes tracks that are very small. The pes tracks show “typical” lizard morphology as having curved digit imprints that progressively increase in length from digits I to IV, a smaller digit V that is separated from the other digits by a large interdigital angle. The manus track shows a different morphology from the pes. The predominant pes tracks, the long stride length of pes, narrow trackway width, digitigrade manus and pes prints, and anteriorly oriented long axis of the fourth pedal digit indicate that these trackways were made by lizards running bipedally, suggesting that bipedality was possible early in lizard evolution.”
Actually, the lizard was not running.
Typically in running tracks the prints are very far apart and these tracks are sometimes left toe to right heel.
Typically in running tracks the prints are very far apart and these tracks are sometimes left toe to right heel.
Figure 2. Original and new tracings of the bipedal lizard tracks from South Korea. PILs are added. Manual digit 4 and 5 appear to have shifted.
The authors did not venture who made the tracks.
They reported, “based on the palaeobiogeographic distribution of facultative extant families, the lizard that produced S. hadongensis tracks could well have been a member of an extinct family or stem members of Iguania, which was present in the Early Cretaceous.”
They reported, “based on the palaeobiogeographic distribution of facultative extant families, the lizard that produced S. hadongensis tracks could well have been a member of an extinct family or stem members of Iguania, which was present in the Early Cretaceous.”
Actually the closest match among tested taxa
is with Eichstaettisaurus (Fig. 1), a basal member in the lineage of snakes. And this clade is close to the origin of geckos. ReptileEvolutiion.com and the large reptile tree would have been good resources for the authors to use. Lots of lizard pedes were illustrated and scored there.
is with Eichstaettisaurus (Fig. 1), a basal member in the lineage of snakes. And this clade is close to the origin of geckos. ReptileEvolutiion.com and the large reptile tree would have been good resources for the authors to use. Lots of lizard pedes were illustrated and scored there.
Figure 3. Originally imagined as a generic lizard (below), here Eichstattsaurus matched and scaled to the track size walks upright.
Based on a phylogenetic analysis of the tracks
the closest match in the LRT is with Eichstaettisaurus, so a slightly larger relative made them. Distinct from the skeletal taxon, the trackmaker had a longer p2.1 than 2.1 and pedal digit 1 was quite short. Otherwise a good match in all other regards.
So why walk bipedally?
It was walking, not running, so escape from predation can be ruled out. Elevating the upper torso and head, like a cobra, can be intimidating to rivals, or just offer a better view over local plant life. This sort of flexibility could have helped them get into the trees and then to move to higher branches.
References
Lee H-J, Lee Y-N, Fiorillo AR & LÃ J-C 2018. Lizards ran bipedally 110 million years ago. Scientific Reports 8: 2617. doi:10.1038/s41598-018-20809-z
The Pterosaur Heresies 