Stand-Up Croc

In South Korea, fossil footprints reveal a crocodile that walked on 2 legs

Discovery overturns previous belief about some fossilized prints in South Korea.

Looking at a living crocodile might feel like getting a glimpse into the past but, according to new research, crocs today can't exactly live up to their "living fossil" reputation. That's because over the course of the 200 million years crocodiles have been on Earth, they've changed.

According to a study published Thursday in the journal Scientific Reports, some ancient crocodiles walked on two legs. The study details fossilized footprints discovered in South Korea that belong to a previously unknown crocodile relative.

This is the first evidence from this time period of a bipedal crocodylomorph. Crocodylomorpha is a branching, diverse group that includes crocodilians and their extinct relatives.

The finding also overturns researchers' understanding of what the footprints really show. Previously, scientists thought that these kinds of footprints were left by a pterosaur — the flying reptile group that includes pterodactyls — walking on two legs.

This new analysis suggests that, actually, they belonged to a bipedal crocodile — a creature that was walked on two legs because it was semi-adapted to land.

Artist rendering of Batrachopus grandis.

The researchers named the new species Batrachopus grandis. The well-preserved fossils, discovered in South Korea's Jinju Formation, date back to the Lower Cretaceous, which spanned 145 to 105 million years ago.

Sized 18 to 24 centimeters long, the prints suggest that the croc bodies were almost 10 feet long. They seem to have been left only by the back limbs of this crocodile relative, showing a clear heel-to-toe walking pattern.

This implies the existence of a bipedal crocodile — but it isn't the earliest instance. A study published in 2015 showed at least one such crocodylomorph lived in North Carolina 230 million years ago. That terrifying croc had blade-like teeth it used to attack prey — researchers named it Carnufex carolinensis, meaning "Carolina Butcher."

New findings from Jinju — The Lower Cretaceous Jinju Formation has given researchers insight into what life was like more than 100 million years ago.

Located near Jinju City, South Korea, the formation is home to other discoveries: It's there that researchers discovered the first tracks of a small hopping mammal, known as Koreasaltipes jinjuensis, and the first-known turtle tracks in Korea.

Batrachopus grandis tracks.Kyung Soo Kim, Chinju National University of Education, Kyungnam, South Korea

The new finding gives some clues about prehistoric life in the area even beyond bipedal crocodiles.

Since the previous understanding of the fossils has changed, the finding also tells researchers something about pterosaurs: They may not have walked on two legs at all, the researchers say.

"Rather, they support the strong consensus that pterosaurs were obligate quadrupeds, not bipeds," the study authors write.

Today, crocodiles live in tropical areas in Asia, Africa, Australia, and in the Americas.

Some species of crocs are smaller than the ancient Batrachopus grandis. Cuvier's dwarf caiman is the smallest, with male crocs averaging around 4.9 feet long and females around 3.9 feet.

But modern crocodiles can also get a whole lot bigger. The largest recorded crocodile, a saltwater croc, clocked in at more than 20 feet long. Sadly the crocodile, named Lolong, died in 2013 in the Philippines — but not before claiming the Guinness World Record for biggest croc.

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) 18–24cm, indicating trackmaker body lengths up to ~3.0m. 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-onmanus overprinting or poor track preservation, suggests the trackways were 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 2–8cm range, and Cretaceous Crocodylopodus (FL up to ~9.0cm) 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.
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