Lizards ran bipedally 110 million years ago

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 (N = 25) that are very small, average 22.29 mm long and 12.46 mm wide. 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 is 19.18 mm long and 19.23 mm wide, and 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.

The first possible choristoderan trackway from the Lower Cretaceous Daegu Formation of South Korea and its implications on choristoderan locomotion

“…However, even lizards that swim do not have webbing between toes 22 . In addition, digit V imprint is distinctly separated from other imprints and is connected to the back of the heel trace 19 .…”

“…Besides, amphibians have pentadactyl pes with rounded tips of digits, generally without claw marks 18 . Interestingly, “typical” lizard trackways are characterized by strong heteropody with pentadactyl manus and pes imprints such as Sauripes 19 and Neosauroides 20 , 21 . However, even lizards that swim do not have webbing between toes 22 .…”

“…However, even lizards that swim do not have webbing between toes 22 . In addition, digit V imprint is distinctly separated from other imprints and is connected to the back of the heel trace 19 . Notably, Champsosaurichnus parfeti was named based on two isolated tracks found in the Upper Cretaceous Laramie Formation of Colorado, USA 23 .…”

The relationship between body shape, body size and locomotor mode in extant lepidosaurs

“…The implications of these findings extend beyond observations of extant lepidosaurs. The deep ancestral history of facultative bipedality within squamates provides cause to explore these anatomies beyond the living record (Lee et al, 2018). By exploring the anatomies and mechanical demands associated with this locomotor mode, we can better understand the evolution of facultative bipedality that is exhibited in multiple families throughout the clade Lepidosauria (Clemente et al, 2008).…”

“…In squamates, the mechanisms and evolutionary drivers associated with the adoption of a high-performance, bipedal and locomotor mode remain unclear, despite facultative bipedality having evolved at least 110 million years ago in this group (Lee et al, 2018). Historically, it has been suggested that the velocity at which these animals move, in combination with a posterior shift of the gravitational centre of mass resulted in the ability to rear up on the hindlimbs (Snyder, 1952).…”