Although called the “Age of Reptiles”, the first period of the Mesozoic Era, the
Triassic, did not start out that way.
The dominant animals were initially synapsids, the proto-mammals, who had had been the most
common large animals during the proceeding Permian Period.
At the start of the Early Triassic, the synapsid Lystrosaurus was famously the most abundant
animal, and by a wide margin.
However, it didn’t take long for the synapsids to be pushed to the margins by reptiles, and
only mammals and a few other tiny cynodonts remained by the dawn of the Jurassic Period.
Still, a few large synapsids persisted into the Late Triassic.
Among the most famous is Placerias, who lived in North America between 230 to 220 million
years ago.
Even though Synapsida as a whole was on the decline, Placerias weighed over a ton, making
it the largest herbivore in its environment.
Placerias was a member of Dicynodontia, the same clade as Lystrosaurus.
It was not its direct descendent, but belonged to the closely related dicynodont subclade
Kannemeyeriiformes, whose members were also large herbivores.
Like most other kannemeyeriiforms, Placerias’s legs were held in a semi-erect stance.
Combined with its rotund body, Placerias would have been slow compared to the other Triassic
megafauna.
Perhaps the most eye-catching dicynodont trait was their beaks.
They were toothless except for a single pair of teeth, which were usually long and tusk-like.
By dicynodont standards, kannemeyeriiform beaks were relatively long and pointed, which
is thought to have been an adaptation for browsing higher foliage than their Permian
ancestors.
Placerias’s lower jaw was less robust than those of most of its relatives, suggesting
it consumed softer flora, although its bite force would have still been quite formidable.
The back of its skull was short but raised high.
Combined with its beak, Placerias would have looked almost like a mammalian ceratopsian
if its “crest” hadn’t been obscured by its powerful neck muscles.
Like many other kannemeyeriiforms, its postorbital bones had a rugose texture, but Placerias
is distinct for its deep furrows.
Unlike the tusks of most other dicynodonts, which were usually the remaining pair of teeth,
those of Placerias and some other dicynodonts were instead extensions of the maxilla.
The teeth themselves had become much smaller, and those of some Placerias individuals were
located in atypical positions or did not even erupt from the bone at all.
It appears even fairly dramatic mutations no longer affected their fitness, suggesting
the teeth were vestigial.
As for their replacements, they are thought to have been covered with a keratinous sheath,
much like horns often are.
While they faced forwards in Walking with Dinosaurs, the real tusks actually face downwards.
It has been suggested that they were used for digging up roots, but Placerias is thought
to have used its tusks in battles against each in contests over mates.
As will be elaborated later on, the tusks of female Placerias seem to have been smaller
than those of the males.
The first Placerias fossil, a large humerus, was found in 1900, and it was formally named
and described in 1904.
Although not very impressive, many more Placerias fossils have been found since then.
Most come from a single site in Arizona, appropriately dubbed the Placerias Quarry.
Altogether, fossils belonging to at least forty-one individuals of this giant dicynodont
have been excavated there.
However, although Placerias’s post-cranial anatomy is fairly well understood, none of
the skeletons are complete.
The Placerias skeletons seen in museums are either casts or composites made from several
individuals.
Indeed, the most complete skull was not even found in the quarry.
It is far from complete itself, but its discovery did at least did confirm the disarticulated
composites had been put together correctly.
The Placerias Quarry is a part of the well-studied Chinle Formation, but Placerias fossils are
rare throughout the rest of the formation.
It has also been found in the contemporaneous Pekin Formation in the eastern United States,
suggesting this ancient stem-mammal was once fairly widespread.
Currently, there is only one recognized species of Placerias, Placerias hesternus.
A second species, Placerias gigas, was named in 1956, but the supposed differences between
the fossils are now thought to have merely been the result of distortion after fossilization.
Some Moroccan dicynodont fossils were previously been assigned to Placerias, but they have
since been given their own genus: Moghreberia.
The Placerias fossils found at the quarry are divided into two different morphs, one
smaller and one larger, the latter of which also has longer tusks.
These traits cluster with each other consistently, and each morph is about as common as the other,
so their differences cannot simply be the result of individual variation.
As was hinted at earlier, the leading hypothesis is that the two morphs are males and females.
Among species who engage in contests to compete for mates, either violently or through purely
visual display, males often have greater body size and more elaborate display structures
like horns, crests, or tusks.
This perfectly matches the pattern seen in Placerias.
A prior hypothesis was that the smaller morph were those Placerias who were not yet fully
grown, but examination of their bone histology found all Placerias at the Placerias Quarry
were adults.
The smaller morph had a slower growth rate than the larger one, which could have been
the result of developmental plasticity, meaning the smaller morph consisted of Placerias whose
growth had been stunted due to a food shortage, likely the consequence of the droughts common
to the Triassic Period.
Finally, the two Placerias morphs could instead represent two different species.
Since their fossils are disarticulated, it wouldn’t be hard for two closely related
dicynodont taxa to have been accidently been mistaken for a single species.
Furthermore, as will be elaborated on later, Placerias did coexist with at least two other
species of dicynodonts.
However, none of the fossils from the Placerias Quarry match those of the other taxa, and
given the conveniently equal frequency between the two morphs, the sexual dimorphism hypothesis
remains the best explanation.
Placerias and other Triassic dicynodonts have sometimes been envisioned as semi-aquatic,
hippopotamus analogues.
While this might have been true for some species, it is unlikely to have been the case for Placerias.
Even though the layer of the Placerias Quarry which contains its namesake seems to have
once been a pond, the rest of the fauna preserved alongside the Placerias fossils are primarily
terrestrial creatures.
It seems the animals congregated around this pond during the dry season, where they perished
before the rains returned.
Like the other Late Triassic kannemeyeriiforms, Placerias’s pointed beak and the orientation
of its skull were specialized for browsing higher level vegetation, like tree branches.
Therefore, it appears to have been primarily an inhabitant of forests rather than swamps.
It should be no great surprise that the last large Triassic synapsids were restricted to
such dense environments, as their semi-erect gaits would made it difficult for them to
outrun the increasingly dominant archosaurian predators, such as Postosuchus, who had fully
erect gaits.
While they are not the only creatures whose remains have been found at the quarry, the
overwhelming abundance of Placerias at the site has been used to support the idea that
they lived together in herds.
However, it is possible this is instead the result of preservation bias.
Due to their size and robustness, Placerias bones would have been fossilized at a disproportionate
rate.
In its appearance in the first episode of Walking with Dinosaurs, Placerias was portrayed
as an evolutionary relic on the verge of extinction.
This was part of a larger narrative in the episode characterizing the rise of the dinosaurs
as them outcompeting the preceding fauna due to their innate superiority.
In many ways this picture is pretty dated.
The dinosaurs’ ascendancy is now attributed less to them outcompeting their predecessors
and more to mere opportunism following the demise of all other large to medium sized
animals during End-Triassic mass extinction, a disaster which struck twenty million years
after Placerias.
However, the depiction of Placerias itself as on its last legs, evolutionary speaking,
otherwise still reflected the scientific consensus until just a few years ago.
It was the last known dicynodont, and with the exception of the Placerias Quarry, its
fossils are uncommon in spite of the fact that the Chinle Formation is very well sampled.
Today Placerias is viewed quiet differently.
For starters, the Placerias Quarry is atypical of other contemporaneous Chinle Formation
sites because of its more terrestrial fauna.
The sparsity of fossils belonging to this giant dicynodont is likely a consequence of
preservation basis rather than an indication of genuine rarity.
More importantly, Placerias was not the only dicynodont present in its native environment.
In 2013, after a century of debate, it was finally determined that dicynodont fossils
from the contemporaneous Popo Agie Formatio of Wyoming belonged to a distinct genus, Eubrachiosaurus.
Additionally, some dicynodont femurs from both the eastern Pekin Formation and western
Santa Rosa Formation have markedly different morphologies from both Placerias and each
other.
One of these may belong to Eubrachiosaurus, but in any event, Placerias coexisted with
a minimum of two other dicynodonts.
Until recently, it was thought that each Late Triassic environment was home to no more than
a single species of dicynodont.
Far from being on their last legs, it is now apparent that the kannemeyeriiforms were still
doing well during Placerias’s time, albeit not as well as during the Permian and the
first half of the Triassic Period.
Furthermore, in stark contrast to the old perception of Placerias just barely clinging
on, it appears to have been very long lasting.
The Placerias Quarry is dated to 219.4 million years ago, during the Early Norian Age, whereas
the fossils from the Pekin Formation are 231 million years old, dating to the Late Carnian
Age.
Ten million years is longer than most dinosaur genera lasted.
It should be noted that even though Placerias is often portrayed as a contemporary of the
archetypal early dinosaur Coelophysis, its fossils belong to a different stratigraphic
unit of the Chinle Formation, and Coelophysis lived at least several million years after
Placerias’s presumed extinction.
But even if Coelophysis wasn’t neighbors with Placerias, it still shared a world with
other dicynodonts.
A dicynodont from Late Norian South Africa, Pentasaurus, was named in 2018.
It notably coexisted with large sauropodomorph dinosaurs, who had previously been attributed
with the demise of Dicynodontia.
In the following year, the discovery of another late surviving dicynodont was announced.
Named Lisowicia, it lived in what is now Poland at the very end of the Triassic Period.
Although Placerias was the largest herbivore in the American southwest, it was dwarfed
by the six-ton Lisowicia, who was about the same size as an African elephant.
Unlike Placerias, Lisowicia’s limbs were erect like those of mammals, an adaptation
it developed independently from them to support its greater mass.
Although dicynodonts were not as common or diverse as before, Lisowicia demonstrates
that these stem-mammals didn’t just manage to stick around, but were innovating in surprising
and dramatic ways up to the very end of the Triassic Period.
In all likelihood, the last dicynodonts perished along with the rest of the other non-dinosaurian
megafauna during the End-Triassic mass extinction, which was caused by a period of extensive
volcanic activity.
The devastation ranged from the acidification of the oceans, to long term global warming,
which was punctuated by potentially decade long volcanic winters.
Therefore, the dicynodonts only died out due to a cataclysm they could have never prepared
for.
In this light, Placerias should not be seen as the last gasp of antiquated Permian holdovers,
but as an example of the success of Synapsida.
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