During the Permian Period, between three hundred to two hundred and fifty million years ago,
the Age of Reptiles was still far away.
The Permian world was instead ruled ironically by synapsids, the ancestors of modern mammals.
The largest predator of this time was a synapsid named Anteosaurus.
This frightening beast had a massive skull with powerful jaws and large, saber-like canines.
It essentially looked like evolution had combined a bear, a Dimetrodon, a Tyrannosaurus, and
a hippo into a single animal.
Despite how exceptional Anteosaurus was, it remains largely unknown to most of the public.
Worse, for an entire century it was misunderstood, reconstructed as so sluggish that it must
have ambushed prey from the shore like a crocodile.
However, over the years evidence has come to light that suggests Anteosaurus was actually
an active, primarily terrestrial predator.
Anteosaurus was first discovered in 1921 in the South African Karoo Super Group.
Specifically, its fossils are found in the Tapinocephalus Assemblage Zone.
These rocks were formed during the Capitanian Age at the end of the Middle Permian, about
two hundred and sixty-six to two hundred and sixty million years ago.
While Anteosaurus is known from over thirty specimens, a complete skeleton has not yet
been found.
Indeed, even with composites, most of the skeleton remains missing.
Therefore, understanding Anteosaurus first requires understanding where this incredible
beast fits in Synapsida.
Anteosaurus was a therapsid, meaning it was more closely related to mammals than to more
primitive proto-mammals like the sailed backed Dimetrodon.
Specifically, Anteosaurus was a dinocephalian.
Dinocephalians are exclusively known from the Middle Permian and were quite common during
this time.
Some of the most famous dinocephalians are the horned omnivore Estemmenosuchus and the
dome-headed herbivore Moschops.
Anteosaurus belonged to a branch of Dinocephalia named Anteosauria, of which it is the namesake.
The other anteosaurs were carnivores like Anteosaurus, ranging from medium to large
body size.
Of all the anteosaurs, Anteosaurus was the largest.
Based on the fossils that have been found and its robust skeleton, Anteosaurus was about
five to six meters long and weighed about five hundred to six hundred kilograms.
This is larger than any other terrestrial predator known from the entire Permian Period.
Few terrestrial mammalian predators ever grew to a similar size, and most, like short-faced
bear Arctodus, were omnivores.
The second largest anteosaur that grew to similar size was the closely related Titanophoneus.
It weighed about five hundred kilograms, although its size is often underappreciated due to
most Titanophoneus reconstructions being of juveniles.
Some sources state that Anteosaurus was the size of a rhinoceroses or hippopotamus, which
is technically true.
It was larger than the pygmy hippopotamus and about the same size an average Sumatran
rhino, but both the common hippopotamus and most other rhinos were still larger than Anteosaurus.
Of course, a predator the size of even small rhino would have been an absolutely terrifying
prospect.
Anteosaurus’s skull was massive even for an animal of its size, with the largest specimen
measuring eighty centimeters long.
The roof of its skull is very thick and there are two large bosses above the eyes.
From the front this would have given Anteosaurus an appearance reminiscent of Tyrannosaurus
rex.
Also like Tyrannosaurus, Anteosaurus’s skull had attachment points for powerful jaw muscles
and it was reinforced to withstand the bone crunching power they created.
Anteosaurus’s snout was slightly angled downwards relative to its braincase.
However, it was far less extreme than some other dinocephalians such as Moschops.
On the other hand, the end of Anteosaurus’s premaxilla is upturned.
Because of this, the front teeth in the upper jaw point forward.
These long teeth were used to grab onto its prey.
Behind them were its fang-like canines.
While they would have been used when hunting, they probably evolved for fighting other Anteosaurus,
as will be elaborated on later.
The teeth behind the canines were the smallest, and were used for both delivering its bone
crushing bite and for chewing its food.
Compared to the other anteosaurs, adult Anteosaurus had more robust and decorated skulls.
It was also bulkier than most of its cousins.
However, younger Anteosaurus instead resembled their smaller ancestors.
This led to many fossils of immature Anteosaurus being misclassified as new species or even
new genera.
A similar fate befell Titanophoneus.
The young of large, robust adult anteosaurs resembling their smaller, more gracile ancestors
is another trait they have in common with the later tyrannosaurid dinosaurs.
Unlike true mammals, Anteosaurus’s legs were held in a semi-sprawling stance.
Its legs were a bit short, which was among the evidence used to support a semi-aquatic
mode of existence.
Long legs help when traversing on land, but create too much drag when trying to swim.
However, Anteosaurus’s legs were still longer than what would be expected of an amphibious
animal, and were notably longer and more powerful than those of most other dinocephalians.
While Anteosaurus may not have been the fastest runner, it would have still been faster than
most of the contemporary large herbivores.
Once it caught its prey, its muscular forelimbs could have easily pinned them down.
No fossils of Anteosaurus’s tail have been found.
It was originally thought its closest relatives had large, crocodile-like tails, which was
used evidence used to a support the idea that Anteosaurus was amphibious.
Such tails make it difficult to walk on land but are excellent sources of propulsion underwater.
The problem with this idea is that it is doubtful Anteosaurus actually had such a tail.
The tails of other anteosaurs, while long, otherwise have little resemblance to the tails
of crocodilians.
The reason for the initial confusion was because at the time Anteosaurus was discovered, only
one other anteosaur, Syodon, had been found, and its tail was also missing.
Because of its thick skull, it has been hypothesized that rival Anteosaurus butted head against
each other, much like rams or pachycephalosaurs.
Similar behavior was present in some other dinocephalians, who belonged to the clade
Tapinocephalidae.
However, while Anteosaurus’s skull roof was thicker than most other anteosaurs, it
was not nearly as thick as tapinocephalid domes.
If it tried to ram into the head of another Anteosaurus, it risked causing significant
damage to itself.
Although Anteosaurus’s skull may have been used to attract mates or ward off rivals,
it would have had too either hit its opponents on their flanks or use its skull purely for
visual display.
However, it had another weapon in its arsenal, its long, saber-like fangs.
Similar teeth are found in some ungulates, such as mouse deer.
These species lack antlers or horns and rivals instead fight each other using their fangs.
If Anteosaurus fought in the same way, this would have been less dangerous than using
its jaws, but such fights would have still been incredibly violent.
This offers some insight into the social dynamics of both Anteosaurus and its prey.
In ungulate evolution, species which shifted from using fangs to blunter weapons to fight
rivals also became more social.
The is because blunt weapons are less likely to cause major harm, reducing the risks of
living with members of the same species.
Since the first dinocephalians had Anteosaurus-like fangs, they seem to have evolved along a similar
path.
This suggests that Anteosaurus was a solitary hunter.
Given their massive size, a single adult would have been capable of taking down even the
largest contemporary animals.
On the other hand, this also suggests that tapinocephalid dinocephalians, who made up
most of its prey, lived in herds.
CT-scans of an Anteosaurus skull have found that the semicircular canals in it inner ear,
which are used for balance, are very long.
A semi-aquatic ambush predator would have little need for such balance but it would
be very helping if Anteosaurus was chasing its prey on land.
Indeed, Anteosaurus’s semicircular canals are proportionately larger than other anteosaurs
and the rest of the Middle Permian carnivores.
This suggests that despite its size, Anteosaurus was an exceptionally agile predator.
In the words of one of the paleontologists behind the study, Dr. Julien Benoit, “Even
though Anteosaurus lived 200-million years before the famous dinosaur Tyrannosaurus rex,
Anteosaurus was definitely not a 'primitive' creature, and was nothing short of a mighty
prehistoric killing machine”.
However, as will elaborated later, Anteosaurus apparently did still have some ties to the
water.
A great deal has also been learned about Anteosaurus from its bone histology.
Like many other synapsids, its bones show evidence of quick growth, proving that it
had a high metabolism, albeit not as high as most modern mammals.
This is not the only insight that has been gleamed from Anteosaurus’s internal bone
structure.
Other clues about this ancient beast have been uncovered from its bone density.
Animals that spend much of their time in the water have denser bones than their terrestrial
counterparts.
This reduces their buoyancy, making it easier for them to sink underwater.
Notably, the discovery that the famous dinosaur Spinosaurus had dense bones has been used
to support the idea that it hunted fish underwater.
Anteosaurus’s bones, especially its ribs, were denser than what would be expected from
a fully terrestrial animal, suggesting it was adapted to spend some time in the water.
However, its bone density is still not on par with most semi-aquatic animals.
The oxygen isotypes of its bones tell a similar story.
Terrestrial and aquatic animals have a different ratio of the oxygen isotopes Oxygen 18 and
Oxygen 16.
An analysis of the fossils of Anteosaurus and other fauna from the Tapinocephalus Assemblage
Zone found that five of the Anteosaurus specimens had results consistent with a terrestrial
lifestyle.
Despite this, the oxygen isotope ratio of one Anteosaurus was between that of an aquatic
and terrestrial animal.
Therefore, although it seems to have been primarily terrestrial, the evidence indicates
it still spent more time in the water than most other terrestrial carnivores.
The reason for this is answered by the environment Anteosaurus lived in and its favored prey.
There were two different types of large prey animals in the Tapinocephalus Assemblage Zone.
The first, two species of the reptilian pareiasaur genus Bradysaurus, seem to have been fully
terrestrial based on their oxygen isotopes.
The other group consisted of other dinocephalians, including the Moschops and the namesake of
the assemblage zone, Tapinocephalus.
Their oxygen isotopes indicate they spent more time in and around the water than the
pareiasaurs.
Notably, another group of large predators from the region, the more mammal-like therocephalians,
also had isotopic values identical to those of the pareiasaur Bradysaurus.
Therefore, Anteosaurus’s limited affinities for the water seem to have been the result
of niche partitioning.
Therocephalians focused on the more inland dwelling pareiasaurs while Anteosaurus specialized
in hunting the more water loving herbivorous and omnivorous dinocephalians.
So ironically, the initial hypothesis was that Anteosaurus was tied to the water was
not completely wrong, if not for any the reasons initially given.
Still, it would have primarily hunted on land, and would not have ambushed its prey from
the water like a crocodile.
A better ecological comparison for Anteosaurus would be a strange cross between a tiger and
a polar bear.
While the Permian is famous for ending with the infamous Great Dying, the most devasting
mass extinction event, Anteosaurus was a victim of a different mass extinction.
Named the Capitanian mass extinction, it transpired two hundred and sixty million years ago, ten
million years before the Great Dying.
Only recently recognized as a mass extinction, the Capitanian mass extinction is thought
to have been caused by volcanic activity, much like the Great Dying and the later mass
extinction at the end of the Triassic Period.
For Anteosaurus, one of the most important consequences of this volcanic activity was
increased aridification in South Africa.
This destroyed the habitat of its primary favored prey, driving it, and all other dinocephalians,
to extinction.
Anteosaurus and the other anteosaurs were replaced by more mammal-like predators, the
therocephalians and the more well-known gorgonopsians.
However, even the largest gorgonopsian, Inostrancevia, was dwarfed by Anteosaurus.
Carnivores capable of rivaling or exceeding Anteosaurus’s size wouldn’t evolve until
the Mesozoic era.
The first to do so were the erythrosuchids, who had massive skulls even by the standards
of their already incredible size.
They were followed by the loricatans, the relatives and ancestors of the crocodilians,
and then by the dinosaurs during the Jurassic Period.
Just like how dinosaurs went from being seen as lumbering, cold-blooded monsters to more
active and dynamic animals, Anteosaurus also shows how much our understanding of extinct
creatures has improved over time.
Although the Permian Period was filled with a variety of amazing creatures, Anteosaurus
was one of the most impressive.
This proto-mammal was not only the largest predator of its time, but the largest terrestrial
predator the world had yet seen.
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