Trackway evidence for large bipedal crocodylomorphs from the Cretaceous of Korea

Kyung Soo Kim,Jong Deock Lim,Anthony Romilio,Martin G Lockley,Seul Mi Bae

doi:10.1038/s41598-020-66008-7

Abstract

Large well-preserved crocodylomorph tracks from the Lower Cretaceous (? Aptian) Jinju Formation of South Korea, represent the well-known crocodylomorph ichnogenus Batrachopus. The Korean sample includes multiple, narrow-gauge, pes-only trackways with footprint lengths (FL) 18–24 cm, indicating trackmaker body lengths up to ~3.0 m. Surprisingly, the consistent absence of manus tracks in trackways, with well-preserved digital pad and skin traces, argues for bipedal trackmakers, here assigned to Batrachopus grandis ichnosp. nov. No definitive evidence, either from pes-on-manus overprinting or poor track preservation, suggests the trackways where made by quadrupeds that only appear bipedal. This interpretation helps solve previous confusion over interpretation of enigmatic tracks of bipeds from younger (? Albian) Haman Formation sites by showing they are not pterosaurian as previously inferred. Rather, they support the strong consensus that pterosaurs were obligate quadrupeds, not bipeds. Lower Jurassic Batrachopus with foot lengths (FL) in the 2–8 cm range, and Cretaceous Crocodylopodus (FL up to ~9.0 cm) known only from Korea and Spain registered narrow gauge trackways indicating semi-terrestrial/terrestrial quadrupedal gaits. Both ichnogenera, from ichnofamily Batrachopodidae, have been attributed to Protosuchus-like semi-terrestrial crocodylomorphs. The occurrence of bipedal B. grandis ichnosp. nov. is evidence of such adaptations in the Korean Cretaceous.

Highlights

Crocodylomorph tracks are generally rare in the Mesozoic of Asia
Crocodylopodus was reported from the Lower Cretaceous (?Aptian)[11] Jinju Formation of Korea[12,13], where it represents the first Asian occurrence, and adds to the extraordinarily rich Jinju Formation, ichnofauna described as a Konservat-Lagerstätten[14,15,16,17,18,19]
We report on a large, newly-discovered Jinju Formation assemblage with multiple trackways of large crocodylomorph tracks with footprint lengths up to 24.0 cm, from the Sacheon Jahye-ri tracksite, near Sacheon City (Fig. 1)

Summary

Introduction

Crocodylomorph tracks are generally rare in the Mesozoic of Asia. It has been suggested that this is in part due to the lack of sedimentary facies representing suitable habitats for this group of ostensibly aquatic trackmakers[1]. We report on a large, newly-discovered Jinju Formation assemblage with multiple trackways of large crocodylomorph tracks with footprint lengths up to 24.0 cm, from the Sacheon Jahye-ri tracksite, near Sacheon City (Fig. 1) These tracks are more than twice as large as any previously reported batrachopodid tracks and closely resemble Batrachopus with well-preserved pes footprints with clear digital pad impressions and localized skin traces. Most pterosaurian trackways represent quadrupedal[26] not bipedal progression[27], multiple pes-only trackways, from the Haman Formation of Korea have been erroneously attributed to giant bipedal pterosaurs[28] This unexpected evidence of apparently bipedal crocodylomorphs obliges us to investigate the alternative possibility that the trackway configurations represent an unusual mode of preservation, rather than bipedal locomotion, and underscores the need for a reexamination of the Batrachopodidae[29] and other relevant, morphologically-similar ichnotaxa

Discussion

Conclusion