The most famous apex predator of the Cretaceous period is Tyrannosaurus rex, while the most
famous of the proceeding Jurassic period was the fellow dinosaur Allosaurus. However, during
the Triassic period, dinosaurs were not the top predators, with the role instead held
by more distant relations. The most famous large predator of this time was Postoschus,
in part due to its appearance in BBC’s Walking with Dinosaurs. Though not a dinosaur, its
appearance was similar to them, perhaps closer to the outdated image many people have of
theropod dinosaurs than the real theropods were. Postoschus instead belonged to Rauisuchidae,
whose other members were also large predators. As Postoschus is known from much more complete
remains than most other rauisuchids, it is key to understanding the rest of the clade.
The sister clade of Rauisuchidae is Crocodylomorpha. While Crocodylomorpha was not very diverse
during the Triassic period, it included the ancestors of crocodilians. Though most Triassic
crocodylomorphs were small, the most basal species were very similar to the rauisuchids,
Therefore, Postosuchus is not only one of the best examples of the predators that ruled
before the dinosaurs rose to prominence, but also offers a glimpse into what the ancestors
of some of today’s large predators were like. Postoschus itself consisted of at least
two species. The type species, Postosuchus kirkpatricki, is found in the southwestern
United States, specifically the Dockum Group and the Chinle Formation. These formations
represent a long period of time, and almost rauisuchid material from the Late Triassic
period in this region has been assigned to Postosuchus kirkpatricki, giving it an official
range of nearly 20 million years. However, the discovery of a new species of rauisuchid
in the region, Vivaron, calls this practice into question. A second species of Postosuchus,
Postosuchus alisonae, is found in the eastern United States. Though small compared to some
of the latter theropod dinosaurs, Postosuchus was the Triassic equivalent of Tyrannosaurus
rex. It had a robust build and was between four to five meters long. Postosuchus kirkpatricki
fossils found in the Placerias quarry site are larger than those found elsewhere and
are a few million years older. However, beyond this there is simply not enough differences
to justify classifying this larger morph as separate species. Even the smaller morph of
Postosuchus kirkpatricki would have matched or outclassed almost any predator alive today,
and when it was alive very few animals would have been safe from its fearsome jaws. The
only contemporary predators large enough to rival Postosuchus were the semi-aquatic phytosaurs.
Postosuchus’s massive, deep skull was filled with numerous openings, which made space for
large jaw muscles, giving Postosuchus a powerful bite. While all of Postosuchus’s teeth were
recurved and serrated, they were heterodont, differing in size and shape from each other,
with four total variants. The largest were almost saber like while another tooth variant
was similar to teeth found in some phytosaurs. Postoschus also had a large, distinctive ridge
over it eyes and a second one above its teeth on the maxillary bone. There has been debate
about whether Postoschus walked on two legs or four. While usually portrayed as a quadruped
in early reconstructions, including Walking with Dinosaurs, more recent research supports
a bipedal Postosuchus. Its hindlimbs were nearly twice the length of the forelimbs,
proportions similar to those of theropods, who were obligate bipeds. However, the proportions
of younger individuals seem to have been different, with arms closer to the length of the hindlimbs.
Therefore, Postosuchus may have hatched as quadrupeds and became more dedicated bipeds
as they grew. This may be why Postosuchus’s pectoral girdle and forelimbs were so robust,
as they were vestiges of their juvenile state. However, these bones were also robust in theropod
dinosaurs. Like them, Postosuchus may have used its arms to hold onto its prey. This
was made easier by a large claw on each hand, which could be used like a meat hook to restrain
struggling prey. Then, Postosuchus could then easily kill its quarry with its deadly bite.
The aetosaurs, one of the major contemporary herbivore clades, seem to have developed a
counter strategy to this. Many pseudosuchian archosaurs, including Postosuchus, had bony
plates called osteoderms on their back. Those of the aetosaurs were massive, which made
it difficult for Postosuchus to find a good place to bite after restraining an aetosaur.
Some aetosaurs also had spikes along the edges of their armor. The aetosaur Desmatosuchus
even had massive shoulder spikes that pointed backwards. An attacking Postosuchus wouldn’t
merely struggle to find a vulnerable place to bite Desmatosuchus, it risked being lethally
impaled during the attempt. Still, aetosaurs were not entirely safe from Postosuchus. An
aetosaur fossil was among the gut contents found in a Postosuchus alisonae specimen.
Other gut contents included the remains of a large dicynodont, a more mammal-like cynodont,
and what might have been an amphibious temnospondyl. In Walking with Dinosaurs, Postosuchus was
portrayed as a primitive reptile outcompeted by the more advanced dinosaurs. While this
was once thought to actually be the case, more recent research has found the truth to
be very different. Though the dinosaurs did see increased diversity as the Triassic period
progressed, predatory dinosaurs remained small until after the Triassic period. Instead,
the boundary between the Triassic and Jurassic periods was marked by a mass extinction, likely
caused by volcanic activity, which wiped out many clades, including the rauisuchids. Rather
than outcompeting relics, the theropod dinosaur merely took advantage of the extinction of
the incumbent large carnivores, much as mammals would later do to them. Even though Postosuchus
itself was likely already extinct before the end of the Triassic period, this shows it
was part of a very successful linage of large carnivore rather than just an obsolete forerunner
of the later dinosaurs. While neither Postosuchus nor any other rauisuchid had descendants that
made it into the Jurassic period, their closest relatives did, and some of those persist today
in the crocodilians. Thank you for watching. I hope you learned something interesting.
Have a great day, and, if you enjoyed the video, please remember to hit the like button.