Greater than 113 million years in the past, an odd reptile lived in what’s now South Korea. It strode round on two legs like many dinosaurs, but it was not one in every of them. The tracks it left behind point out it was a relative of at the moment’s crocodiles. And the small print of its Cretaceous footfalls resolve one thriller—however open one other.

Footprints like these had been discovered earlier than, although in a lot older rocks. Throughout the Triassic interval (between 252 million and 201 million years in the past) crocodile kinfolk—a part of a gaggle referred to as crocodylomorphs—have been the dominant reptiles on land, and so they included animals that resembled some dinosaurs by strolling on two legs. These varieties went extinct on the finish of the interval, but the geologically youthful tracks from South Korea’s Jinju Formation signify a bipedal crocodylomorph that lived lengthy after the Triassic closed.

Paleontologist Kyung Soo Kim of Chinju Nationwide College of Training in South Korea, fossil footprint skilled Martin Lockley of the College of Colorado Denver and their colleagues describe the puzzling fossil tracks in a paper printed on Thursday in Scientific Studies.

Lockley calls South Korea a “tracker’s heaven” for paleontologists due to the sheer variety of fossil footprints discovered within the nation. Final November, he says, Kim had requested for his opinion on giant Cretaceous tracks uncovered on the web site. The prints seemed just like these attributed to pterosaurs, flying reptiles from the Age of Dinosaurs, that have been strolling on the bottom. However Lockley acknowledged them as one thing else. “I instantly noticed that they have been of the kind referred to as Batrachopus,” or a type of monitor attributed to crocodile kinfolk from the early a part of the Jurassic (201 million to 145 million years in the past). These footprints not solely have been a lot bigger than some other identified Batrachopus tracks, however in addition they indicated that the animals on the web site walked on two legs—and have been current for tens of millions of years extra within the Cretaceous interval.

Pictures of well-preserved tracks of the extinct crocodile relative. Credit score: Kyung Soo Kim Chinju Nationwide College of Training

The presence of bipedal crocs on the fossil web site was surprising. The invention does assist deal with one other fossil thriller, nonetheless. At a unique South Korean location referred to as Acquire-ri, there are tracks that have been additionally beforehand believed to be left by giant pterosaurs. Tracks discovered elsewhere on the planet point out that the flying reptiles folded their wings to waddle on all fours whereas on the bottom. However researchers had thought the prints at Acquire-ri have been made by pterosaurs that moved on two legs to keep away from dragging their wings by means of the muck.

The brand new fossils have modified the evaluation of those tracks—and have undercut interpretations of pterosaurs strolling on two legs at different websites. Pterosaur skilled Liz Martin-Silverstone of the College of Bristol in England, who was not concerned within the new examine, agrees that the presumed “pterosaur” tracks at Jinju look rather more like prints made by crocodile kinfolk than these left by pterosaurs. In truth, she notes, “all beforehand described bipedal ‘pterosaur’ trackways have come out as crocodilian upon reexamination”—which dovetails with skeletal proof indicating these prehistoric flappers tottered round on 4 legs whereas on the bottom.

Such circumstances of mistaken id can occur as a result of the way in which transferring creatures make footprints is rather more advanced than it might initially appear. “The tracks produced can change considerably, relying on issues like what the animal was doing on the time the monitor was made, the character of the substrate, the place the middle of gravity was and what kind of gait it had,” Martin-Silverstone says. This case means one animal can typically make tracks that appear to be these of one other.

With the brand new discover, paleontologists now know of at the least two fossil websites that apparently document the bipedal footsteps of Cretaceous crocodylomorphs—not these of pterosaurs appearing unusually. “This new footprint proof exhibits we’ve to rethink the crocs of yesteryear and regard some as agile land dwellers,” Lockley says. Though crocodiles are sometimes forged as “residing fossils” which have modified little since their origin within the Triassic, skeletal and monitor proof has proven that crocs within the Age of Dinosaurs have been different, energetic animals that always seemed very totally different from the swimming ambush predators we’re aware of at the moment. However what precisely did those that made the Jinju tracks appear to be?

Thus far no skeletons of crocodylomorphs able to strolling this manner have been present in the identical rocks because the tracks within the new examine. However older fossils might give paleontologists an concept of what sort of fossils to search for. Throughout the Triassic interval, a bipedal crocodile relative named Postosuchus roamed North America and was able to making tracks just like these present in South Korea. Both a long-lost consultant of this group survived—or crocodylomorphs advanced to stroll on two legs a second time. If the fossil document is sort to paleontologists, they might sometime have the ability to match footprints to bones.

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