The dinosaurs are easily the most famous prehistoric reptiles, but they were proceeded by equally
remarkable relatives.
One of the deadliest was Erythrosuchus.
This massive, stocky carnivore possessed a powerful, oversized skull.
Despite the seemingly primitive and somewhat strange appearance of this reptile, Erythrosuchus
already had many of the adaptations that made the later dinosaurs so successful.
Erythrosuchus’s fossils are found in the South African Karoo Super Group.
Specifically, they were found in the Cynognathus Assemblage Zone, which was formed during the
Middle Triassic, a time before the oldest known dinosaurs.
Despite the name, the Middle Triassic ended after less than a third of the Triassic Period
had transpired.
Erythrosuchus’s full scientific name, Erythrosuchus africanus, means “Red African crocodile”,
which is in reference to the red color of the first Erythrosuchus fossils that were
discovered.
Erythrosuchus is the namesake of the clade Erythrosuchidae, which consisted of similarly
big-headed carnivores.
Of all the erythrosuchids, Erythrosuchus was the largest.
Erythrosuchidae belongs to the larger clade Archosauriformes, which besides dinosaurs
also includes crocodilians.
While dinosaurs and crocodilians form the crown-group Archosauria, Erythrosuchidae was
an early diverging branch of Archosauriformes.
One of the first archosauriforms was Archosaurus.
It was a medium sized predator from the Late Permian, a time otherwise dominated by synapsids,
the ancestors and relatives of mammals.
The big break for Erythrosuchus’s ancestors came 252 million years ago at the end of the
Permian Period, which was host to the most devastating mass extinction event the world
has yet seen.
While most of the other survivors of the catastrophe were smaller than their immediate Permian
ancestors, some predatory archosauriforms like Proterosuchus were larger and grew proportionally
larger heads.
Many of these archosauriforms quickly diversified into other niches and replaced most of the
incumbent synapsids, but Erythrosuchus essentially continued evolving in the same direction its
ancestors had, only to an almost comedic yet terrifying extreme.
With a body over five meters long and a weight between 1 to 1.6 tons, Erythrosuchus was the
largest predator of its time.
Out of today’s terrestrial carnivores, only the largest, fattest polar bears and saltwater
crocodiles reach the lower estimate.
Indeed, Erythrosuchus was even larger than the previous largest land predator, the fearsome
dinocephalian Anteosaurus from the Middle Permian.
Most of the terrestrial carnivores that would later grow as large or larger than Erythrosuchus
were archosaurs like Fasolasuchus of course the iconic theropod dinosaurs.
Examples of theropods within the same size range as Erythrosuchus include the carnosaur
Neovenator and the famous horned abelisaurid Carnotaurus.
While a normal predator the size of Erythrosuchus would have already had a powerful bite, the
size of Erythrosuchus’s oversized skull dramatically increased its killing potential.
Everything about Erythrosuchus seems to have evolved to ensure this meter long skull was
as large as possible, allowing it to replace the apex predators of the Permian Period.
It may have been a brute force method of adapting to hunt large prey, but it was effective.
Its teeth were 20 centimeters long, about the same size as those of the far larger Tyrannosaurus
rex.
Erythrosuchus’s teeth were recurved and serrated.
While this was similar to the teeth of many dinosaurs and most other terrestrial, carnivorous
archosauriforms, unlike them Erythrosuchus’s teeth were not compressed on the sides.
This may have given it a bone crunching bite like Tyrannosaurus, but this has yet to be
confirmed by computer models.
Erythrosuchus’s robust skull was very deep, although it sloped down between the roof of
the skull and end of the snout.
Unlike the famous Tyrannosaurus rex, Erythrosuchus’s skull was actually narrow.
From the top it had a subtriangular shape, becoming narrower until the snout became a
fraction of the width of the skull roof.
This meant that Erythrosuchus’s head was a lot lighter than first impressions would
indicate, although the ability to support it was still an impressive feat of evolution.
The back of the skull had attachment points for massive neck muscles.
They were so large that it would have been difficult to tell where the head of a live
Erythrosuchus ended and where its neck began.
Erythrosuchus had a notch in the upper jaw between the maxilla and premaxilla, which
is similar to crocodilians.
Because of this notch, some paleontologists have suggested that Erythrosuchus and other
early archosauriforms may have regularly hunted aquatic prey.
The end of the snout was also slightly hooked.
This echoes the more outrageous hooked snouts of more basal archosauriforms like Proterosuchus,
and is one of the few cases where Erythrosuchus could be said to be less extreme than its
ancestors from the Late Permian and Early Triassic.
While the rest of Erythrosuchus’s body may seem unremarkable compared to its terrifying
skull, its limbs represent an important stage of archosaur evolution.
Most reptiles have sprawling limbs, but those of the first archosaurs were instead erect,
meaning they were positioned directly below their bodies like in most of today’s mammals.
Erythrosuchus’s legs were held in a transitional semi-erect stance.
It is hypothesized that the erect limbs of the archosauriforms were initially an adaptation
to handle the stresses created by their larger body size, which would have been particularly
useful for the gargantuan Erythrosuchus.
In addition to its semi-erect stance, the walls of its limb bones were thick so as to
better support its weight.
Even though some reconstructions, especially older ones, portray Erythrosuchus with short,
stumpy legs, they were long enough for it to have been a fairly competent runner.
This wasn’t the only intermediary trait in Erythrosuchus’s limbs.
A characteristic feature of the archosaurs is the fourth trochanter, a knob on the middle
of the femur which serves as a muscle attachment site.
It is derived from the internal trochanter in other reptiles, a ridge which extends all
the way to the femoral head.
While it may seem like a trivial feature, the evolution of the fourth trochanter was
critical in the evolution of erect limbs and, in a number of later archosaurs, bipedalism.
Erythrosuchus’s internal trochanter was separated from the femoral head, which was
the first step in the evolution of the fourth trochanter.
However, the position of the internal trochanter in the erythrosuchid Garjainia was instead
closer to that of more basal reptiles like lizards.
This suggests that after the erythrosuchids split from the other archosauriforms, their
internal trochanters continued to convergently evolve along a similar path as their archosaurian
cousins.
Despite its seemingly lumbering appearance, Erythrosuchus was not ectothermic, or “cold
blooded” like most reptiles.
Instead, like other archosauriforms, Erythrosuchus burned energy to maintain its own body temperature.
While its metabolism was not as high as the later dinosaurs, it was enough to allow it
to be much more active than most modern reptiles.
The cost of this mesothermic metabolism was that the already massive Erythrosuchus would
have needed a staggering amount of food to support its internal furnace, which is why
crocodilians reverted to a lower, cold-blooded metabolism.
It is hypothesized that one of the main driving forces for the development of a high metabolism
in archosaur evolution was the need to grow quickly, which ectotherms are incapable of.
A rapid growth rate is advantageous in unpredictable ecosystems, like those left in the wake of
the End-Permian Mass Extinction.
If it took too long for an animal to reach adulthood, especially one the size of Erythrosuchus,
the risk of dying due to a devasting drought before it could reproduce was too high.
Much like Erythrosuchus’s anatomy, the Middle Triassic world was very much one in transition.
While synapsids were in decline, the golden age of the archosauriforms was beginning.
Still, the largest herbivores found in the Cynognathus Assemblage Zone are synapsids
called dicynodonts.
These plant eaters had beaks which were toothless except for a pair of large tusks which jutted
out.
The famous and once widely successful dicynodont Lystrosaurus was already extinct by Erythrosuchus’s
time and the only remaining branch of dicynodonts by the Middle Triassic were kannemeyeriiforms.
The largest contemporary kannemeyeriiform was Kannemeyeria itself, but even it was smaller
than Erythrosuchus.
Other potential prey included the two-meter-long mammal-like omnivore Diademodon and the namesake
of the assemblage zone, Cynognathus, a meter-long carnivore.
It also coexisted with the archosauriform Euparkeria, who was more closely related to
archosaurs than to Erythrosuchus.
While their relationship would have offered it no safety from Erythrosuchus, its small
size and speed would have.
Unlike other exceptional predators who were the largest know members of their clades,
such as Anteosaurus and Tyrannosaurus rex, Erythrosuchus was not the last erythrosuchid.
Others, such as Chalishevia terrorized Pangea long after it.
Still, the reign of the erythrosuchids only lasted twelve million years.
They were replaced by the more derived archosaurs, particularly pseudosuchians like the rauisuchids
and ornithosuchids.
Even though these new predators were generally smaller than Erythrosuchus and had proportionally
smaller skulls, the rest of their bodies were more optimized for their role as apex predators.
Of course, while it eventually ended up obsolete, the beginnings of the traits that made its
replacements, and eventually the carnivorous dinosaurs, so successful first appeared in
Erythrosuchus.
Thank you for watching, and a thank you to the themattalorian for narrating this video.
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