When Giant Bipedal Crocodiles Walked the Earth

Once thought to have been made by giant pterosaurs trotting on two legs, prints in Jinju are revisited and spring a surprise

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Batrachopus grandis
Crocodile on two feet: Batrachopus grandisCredit: Martin Lockley

Mysterious fossil tracks found in Cretaceous South Korea some years ago left paleontologists baffled. The creature that made them was bipedal, but not a dinosaur. The suggestion arose that they had been made by giant pterosaurs walking on their hind legs with their wings folded up.

Not so, says a new paleontological team based on less ambiguous tracks recently found in the Jinju fossil bed in South Korea. Those strange tracks and new, clear ones recently found were made by giant bipedal crocodiles – a towering version of rather wee relatives in a track family called Batrachopus.

Yes, in the Cretaceous about 120 to 110 million years ago, a subset of giant crocs underwent a seemingly unique adaptation for the terrestrial lifestyle – or maybe a throwback to a long-gone ancestral condition – and walked on their hind legs, Kyung Soo Kim, Martin G. Lockley and colleagues reported in Scientific Reports on Thursday. 

The giant two-legged crocs were a towering version of rather wee relatives in a family called Batrachopus. And they were presumably fast: their quadrupedal crocodilian descendants in riverbeds everywhere can be stunningly speedy. These bipedal giant crocs likely dominated the lush, rich Cretaceous coastal ecosystem in Korea, which brimmed with yummy life-forms: from other reptiles to teacup theropods, birds, turtles and prehistoric kangaroo rats, going by other tracks found at Jinju, Gyeongsangnam-do Province.

“Nobody expected such large bipedal crocs,” said Lockley, a University of Colorado professor who has been studying fossil footprints in South Korea for 30 years. Haaretz never expected bipedal crocs, period.

Tracks of Batrachopus grandis in the Jinju Formation, South Korea, and lo, they were not of a pterosaurCredit: Martin Lockley

Local creationists even suggested the bipedal Cretaceous walkers were humans, the team observes. They weren’t.

The quality of the footprints at Jinju leave the paleontologists quite confident of their identification, if surprised. “It shocked us to learn,” stated team leader Prof. Kyung Soo Kim of the Chinju National University of Education, that “the trackways represent bipedal animals 3 to 4 meters long” (up to 13 feet).

The prints of the newly discovered (if long dead) animal whose footprints they named Batrachopus grandis were around 18 to 24 centimeters long, twice the length of previously found batrachopodid tracks, which had typically been in the range of 2  to a rare maximum of 8 centimeters.

OK, it’s no T-rex, but at 3 or possibly as much as 4 meters in length and looming over the head of anybody technologically adept enough to time-travel to Cretaceous South Korea, it would have been demoralizing enough – including to its relatives. “Giant” is relative to other terrestrially oriented crocodylomorphs, most only a meter long, that had transitioned to live on land rather than lurk in water. These smaller croc types were constrained to low life on all fours, near the ground.

However, possibly bipedalism is the early-ancestral position of the crocodilian set. A very early stem species believed to have been ancestral to crocodilians called Poposaurus gracilis, which lived and ate other animals a quarter-billion years ago (in the late Triassic) in what is today the United States – may also have been “a fleet-footed, obligately erect-postured, striding biped,” scientists reported in 2011. But the trait may have been largely lost; none such were known from the later Cretaceous. 

Batrachopus grandis and youCredit: Martin Lockley

Another Cretaceous croc relative in South Korea (and Spain), the crocodylopodus, had a foot length of about 9 centimeters and there’s no evidence it would or could rear up, let alone trot about bipedally.

It bears adding that the prehistoric world had crocodylomorphs that would exceed 9 meters in length, but they dwelled in the land that became South America and Africa, and nobody has suggested they walked on anything but four legs.

Scaly skin, too

How did the paleontologists figure out who made the marks in the Cretaceous mud, and how? By their distinct crocodilian shape; and because the narrow trackways only showed the marks of hind feet, with no sign whatsoever that impressions of front feet had been trodden over by the back ones.

Specific footprints of Batrachopus grandis in the Jinju Formation, showing clear heel and toe impressions typical of crocodylomorphs' rear feetCredit: Martin Lockley

“There is no definitive evidence, either from back footprints covering those made by the front feet or poor track conservation, to suggest the trackways were made by a four-legged crocodylomorph, indicating that B. grandis represents animals that walked on two legs,” the scientists state. “This is a gait not previously seen from trackway specimens belonging to this family,” they spell out.

Also, the paleontologists detected not only complete heel to toe impressions, but even some impressions of the reptiles’ scaly skin. “One track shows well-defined skin (scale) impressions in the heel area that resemble those typical of modern crocodilians,” the team writes.

It bears adding that giant pterosaurs have long baffled paleontological circles, with the range of argument encompassing whether they could even fly or were simply too gigantic to get off the ground, and whether they engaged in bipedal locomotion on the ground. The present study strengthens the now strong consensus that pterosaurs in general were actually obligate quadrupeds when on land, the team says.

Apropos “when on land,” possibly the discovery that Batrachopus grandis walked on two legs could explain the relative paucity of croc tracks in Mesozoic Asia. Once thought to be due to “the lack of sedimentary facies representing suitable habitats for this group of ostensibly aquatic trackmakers,” we can now postulate a different reason entirely. They’d gone onto land – where, among other things, they would have come across a “small hopping mammal,” Koreasaltipes jinjuensis, which has been adorably depicted like a gerbil with feet around 1 centimeter long and a body probably around 10 centimeters in length. Its name means “a new type of hopping trackway found in Jinju,” according to Korea.net.

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