While the skeletons of prehistoric creatures are impressive, they are not the only type of fossil.
There are also trace fossils, which preserve evidence of the behavior of prehistoric creatures.
Examples include feeding marks, burrows, and footprints. Though informative, trace fossils
rarely reveal the existence of a radically new type of animal in the way skeletons can. However,
a few years ago, the discovery of fossil tracks in Korea turned out to be a rare exception. In the
summer of 2020, nearly a hundred fossil footprints were described from the Korean Jinju Formation,
which dates to roughly 125 to 100 million years ago during the Early Cretaceous Period.
The footprints are twice as long as wide, persevering traces of four toes on each foot.
Some are so well preserved that they even retain skin impressions. These fossilized footprints
were named Batrachopus grandis, which is an ichnotaxon that refers to the type of trace fossil
rather than to the creature that made them. Just as species belong to genera,
Batrachopus grandis, belongs to the larger ichnogenus Batrachopus. Other Batrachopus tracks
had been found across the world since 1843. They weren’t made by dinosaurs but by protosuchians.
Protosuchians belong to the same clade as crocodilians: Crocodylomorpha. Unsurprisingly,
the Batrachopus grandis skin impressions were very similar to modern crocodilians.
The ancestors of today’s crocodilians and most other crocodylomorphs evolved from protosuchians
during the Early Jurassic Period, although some protosuchians lasted into the Late
Cretaceous Period. However, protosuchians were fully terrestrial, with long longs held
directly beneath their bodies instead of the semi-sprawling stance of crocodilians.
While other Batrachopus tracks have been found in Cretaceous Korea, the new Korean tracks were
clearly made by a protosuchian very different from the others. The main two distinguishing
traits of the Batrachopus grandis footprints are how they appear to have been made by a bipedal
animal and the size of the footprints. The tracks are up to twenty-four centimeters long.
Other tracks within the larger Batrachopus ichnogenus are, at most, half that size.
While most protosuchians were about a meter long, based on the length of the Batrachopus grandis
tracks, the authors estimated a length between 2.2 to 3 meters for the crocodylomorph that made them.
The tracks found in 2020 weren’t even the largest. Two additional sets were found
during 2012 at the Haman Formation, which was also formed in Early Cretaceous Korea.
Due to being poorly preserved, they were initially mistaken to be footprints of a large,
bipedal pterosaur. This was strange as all other evidence, including all other pterosaurs tracks,
indicated that these flying reptiles were quadrupeds. It was only with the discovery of
the Jinju tracks that it was realized that these “two-legged pterosaur footprints” were actually
Batrachopus grandis tracks. The Haman Formation footprints are up to 39 centimeters long,
suggesting the crocodylomorph that made them was 4.68 meters long.Such a massive size is
impressive. During the Cretaceous Period and the proceeding Jurassic Period, most large,
terrestrial animals were dinosaurs. The majority of large crocodylomorphs during this time were
amphibious, not unlike today’s crocodilians. However, by all appearances, the maker of the
Batrachopus grandis tracks was, like the other protosuchians, a terrestrial animal.
The footprints lack traces of webbing and are too close to each other to have been made by an animal
with a crocodile-like semi-sprawling posture. While there were some other sizable land crocs
during the Cretaceous and Jurassic periods, only Razanandrongobe exceeded the maker of the
Batrachopus grandis tracks in size. Additionally, most of the other large land crocs were from the
southern, formerly Gondwanan continents. As Batrachopus grandis is from Korea, this shows
that large land crocs were also present in the northern, formerly Laurasia continents as well.
Besides their size, the other major feature that sets the tracks apart from other Batrachopus
tracks is the lack any indication of footprints left by the forefeet, suggesting the track maker
was a biped. This is in contrast to most other crocodylomorphs, including all other
protosuchians, who were obligate quadrupeds. Given how there are close to a hundred of the
Jinju tracks, how they are well preserved, and the presence of other apparently bipedal tracks
in the Haman Formation, the seemingly two-legged nature of Batrachopus grandis doesn’t appear to
be due to a biased sample size. That is not to say that other possibilities haven’t been considered.
The penetration depth of the forefeet and hindfeet of some sauropod dinosaurs differed
from each other, occasionally creating tracks only preserving traces of the forefeet. While
the opposite scenario could create the illusion of bipedal tracks, the penetration depth of the
forefeet in other Batrachopus tracks don’t seem to have differed much from the hindfeet. Another
possibility is seen in modern crocodilians, who sometimes only leave impressions of their
hindfeet when moving underwater. However, this is still an unlikely explanation,
as the crocodilians footprints produced in such a manner are unlikely to preserve traces of as much
of the foot as seen in the Batrachopus grandis tracks in the Korean Jinju Formation. Therefore,
the Batrachopus grandis footprints probably were left by a genuinely bipedal crocodylomorph.
As strange as it may sound, the crocodylomorphs who left the Batrachopus grandis footprints
weren’t the only bipedal crocodylomorphs. During the Triassic period, the first crocodylomorphs,
sometimes called sphenosuchians, were also at least facultatively bipedal, able to switch
from walking on two or four legs. Most of these were small, though the first sphenosuchians,
such as Carnufex, were large predators. Like the protosuchians, the sphenosuchians do not form
a true clade. Instead, they are ancestral to the protosuchians, and through them,
all other crocodylomorphs. Some of the other members of the larger clade crocodylomorphs
belong to, Pseudosuchia, were also bipeds. These included Poposaurus and Postosuchus, who was very
close to the ancestry of the crocodylomorphs. Given its apparent protosuchians affinities,
the Batrachopus grandis crocodylomorph doesn’t seem to have inherited its bipedalism from any
of these Triassic species. Rather it is more likely the maker of the Korean tracks reacquired
bipedalism from a Protosuchus-like ancestor. Unfortunately, footprints can only tell so much.
Without a skeleton, what the maker of the tracks looked like is a mystery. The reconstructions made
by the paleontologists who studied the footprints portray it as basically a protosuchian mimicking
Postosuchus, but this is only speculation. For instance, it is not clear what it ate.
While protosuchians were generally carnivores, some, such as Edentosuchus, were plant eaters,
as were some other crocodylomorph such as Simosuchus. Likewise, during the Triassic Period,
there was clade of bipedal, herbivorous pseudosuchians called the shuvosaurids.
Fragmentary remains of one shuvosaurid, Sillosuchus, suggest it could grow even larger
than the Batrachopus grandis crocodylomorph. Surprisingly, the trackways may offer some insight
into the lifestyle of their makers. One of the sites contained five Batrachopus grandis trackways
that were sub-parallel to each other. This suggests that the trackmakers lived in groups
or at least tolerated each other. While it is possible that they were following a shoreline or
another obstacle, given how the tracks were almost evenly spaced out from each other, it seems the
tracks were made at roughly the same time. While the idea of a social crocodylomorph may seem odd,
it is not without precedent. Fossils of distant relatives of crocodilians, such as aetosaurs, have
been in groups, which have been interpreted as herds. Furthermore, today’s Cuban crocodiles have
been observed working together when hunting. If the Batrachopus grandis trackmaker was a predator,
with a length reaching over four meters, it would have been capable of hunting fairly
large dinosaurs. All in all, Batrachopus grandis is an exciting discovery. It provides evidence of
bipedalism in Crocodylomorpha after the Triassic. It was also one of the few large, non-dinosaurian
terrestrial animals from the Cretaceous Period. It has even offered some evidence of a potentially
pack hunting crocodylomorph. Hopefully, a skeleton will soon be found so more can be learned about
this mysterious reptile. Thank you for watching. I hope you learned something interesting.
This video is a remake of the very first video I did. While that video is now unlisted,
you can still view it by using the link in the description if you wish to compare it to this
one. Have a great day, and, if you enjoyed this video, please remember to hit the like button.