The crocodilians of today are large, amphibious ambush predators. However, they are descended
from terrestrial animals, many of which were very different. Among the first was the diminutive
Hesperosuchus agilis. While perhaps not as impressive as some of their later descendants,
it was small carnivores like these that gave give rise to modern crocodilians and their
relatives, collectively referred to as crocodylomorphs. Hesperosuchus agilis lived during the Late
Triassic period in what is now the Chinle Formation in the southwestern United states.
The first fossils of Hesperosuchus were found in 1929. Among the team that excavated these
fossils was Barnum Brown, the paleontologist that also found the first skeleton of Tyrannosaurus
rex. During this time, it was thought to be an early theropod dinosaur, a common misclassification
for early crocodilian relatives as the time. It was not until the 1980s that its true identity
as a basal crocodylomorph was realized. While Hesperosuchus did not have to wait quiet as
long to receive a name, it would not be until 1952 that was given its scientific name Hesperosuchus
agilis. Its name means “agile Evening Star crocodile”. “Agile” is certainly an
appropriate moniker for this creature. Hesperosuchus was far smaller and less cumbersome than modern
crocodilians. Instead, it had the long legs and compact feet of a swift runner. Whereas
crocodilians have a low metabolism, Hesperosuchus and other basal crocodylomorphs were endothermic,
or warm, blooded. This means they maintained their own internal body temperature, allowing
them to be more active, like dinosaurs and mammals. Hesperosuchus is thought to have
filled a niche similar to that of modern foxes, hunting small vertebrates while also using
its speed to avoid larger predators. Given its long hindlegs, it was likely a facultative
biped, able to switch from a quadrupedal and bipedal stance, much like dinosaurs such as
the hadrosaurids, some sauropodomorphs, and possibly Spinosaurus. While likely spending
time in both stances, Hesperosuchus is thought to have spent more time in a bipedal stance.
This is because its forelimbs were adapted for grasping as in theropod dinosaurs. The
bones of Herperosuchus or a close relative were found in the stomach of Coelophysis,
perhaps the most famous Triassic dinosaur of all, confirming Herperosuchus was preyed
upon by these early dinosaurs. Before it was realized the bones were from Herperosuchus,
it was thought the bones were from juvenile Coelophysis, leading to Coelophysis being
mistaken as a cannibal. While Coelophysis is known from numerous skeletons, in general
dinosaurs were still a minority during the Late Triassic period and the Chinle Formation
was no exception. Instead, Herperosuchus lived in the shadows of more distant relations of
crocodilians. These included the armored aetosaur Desmatosuchus, the dinosaur-like Effigia,
and the deadly predator Postosuchus. While many of the other pseudosuchians dwarfed Hesperosuchus,
it represented the future. It many ways, Hesperosuchus and its relatives can be thought as the crocodylomorph
equivalent of Archaeopteryx. The direct ancestors of crocodylomorphs were large, facultatively
bipedal predators, a body plan that was preserved in the closely related rauisuchids like Postosuchus
and continued in the most basal crocodylomorphs, like Carnufex. Hesperosuchus is the one of
the most primitive known crocodylomorphs to break away from this trend, remaining small
throughout adulthood. That said, Hesperosuchus probably isn’t the ancestor of today’s
crocodilians just as Archaeopteryx probably isn’t the direct ancestor of today’s birds.
In paleontology, it is more likely to found the evolutionary equivalent of an uncle than
the direct ancestor, and harder to tell the difference. Just as birds were the sole surviving
dinosaurs as the end of the Cretaceous period, small crocodylomorphs similar to Hesperosuchus
were the only pseudosuchians to survive the mass extinction at the end of the Triassic
period. While these crocodylomorphs survived at least until the Late Jurassic period, fully
quadrupedal crocodylomorphs evolved from them not long after the mass extinction. These
small reptiles then diversified into a staggering array of species, some of which are still
alive today. In this regard, despite its unassuming size, Hesperosuchus holds a very important
place in the Tree of Life. Thank you for watching. I hope you learned something interesting.
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