During the Triassic Period, before the first dinosaur stepped foot in North America, the
top predator was Carnufex carolinensis, the “Carolina Butcher”.
Although it may have looked vaguely like a dinosaur, this fearsome carnivore was instead
a distant relative of crocodilians, and many aspects of its anatomy have profound implications
regarding the evolution of their shared clade: Crocodylomorpha.
Carnufex’s fossils originate from the Pekin Formation in North Carolina.
The site is two hundred and thirty-one million years old, dating to the Late Carnian Age
of the Triassic Period.
Thus far, only two Carnufex specimens have been discovered.
The holotype was excavated in 2004 and consists of an incomplete skull, a humerus, and a few
ribs and vertebrae.
The second specimen is nothing more than a partial humerus.
While incomplete, what is known about Carnufex has provided exciting insights into the early
evolution of Crocodylomorpha, changing prior assumptions about the emergence of the clade.
Crocodylomorpha is a subclade within Pseudosuchia, which consists of crocodilians and all their
extinct relatives.
During the first period of the Mesozoic Era, the Triassic Period, pseudosuchians were even
more diverse than their close cousins, the dinosaurs.
Some of the first crocodylomorphs were small predators called sphenosuchians.
They do form not a true clade, but were instead ancestral to all later members of Crocodylomorpha.
In stark contrast to their only remaining descendants, sphenosuchians were agile, active
animals and may have even been bipedal.
Crocodylomorpha belongs to a branch of Pseudosuchia called Loricata.
The other loricatans, colloquially called rauisuchians, were all large, terrestrial
predators.
Much like sphenosuchians, rauisuchians do not form a true clade, but are an evolutionary
grade which led to the derived loricatans; the crocodylomorphs.
However, there were no fossils which marked the drastic transition from the huge rauisuchians
to the tiny sphenosuchians.
That was, until the description of Carnufex in 2015.
This new crocodylomorph was far larger than the sphenosuchians and was placed right at
the base of Crocodylomorpha, bridging this critical gap in the lineage which eventually
led to the crocodilians of today.
The skull of the holotype was at least half a meter long, and its total body length is
estimated to have been three meters.
While much larger than most of the later Triassic crocodylomorphs, or any other terrestrial
predator found in the Pekin Formation, this is still much smaller than the apex predators
in most other Late Triassic ecosystems, such as the basal loricatan Saurosuchus.
However, the holotype specimen was not fully grown when it died.
The neural arches are unfused, a trait typical of juvenile pseudosuchians.
Examination of their bone histology has also found both Carnufex specimens were still growing
rapidly when they died.
Based on comparisons with other Triassic loricatans, a fully grown Carnufex could have been as
large as the famous rauisuchian Postosuchus.
Before the discovery of the Carolina Butcher, it was assumed crocodylomorphs first shrank
in size, and then acquired their distinguishing features.
Instead, they initially retained an ecology similar to the other loricatans of the time.
As Carnufex’s skull is not complete, it is unclear exactly what it looked like.
Depending on which early crocodylomorphs are used to fill in the gaps, the shape can change
drastically.
In an event, it is clear the skull was slender and elongate, resembling the shape of the
smaller sphenosuchians more than the bulker skulls of the rauisuchids, the rauisuchians
most closely related to the crocodylomorphs.
However, like the rauisuchids, the Carolina Butcher’s skull was heavily ornamented,
even possessing a raised rostrum in front of its eyes.
Given the immature nature of the holotype, the skulls of adult Carnufex were likely even
more visually striking.
Another basal characteristic was the shape of its antorbital fenestra, the opening before
the orbits.
It was large and elongated, resembling a larger version of Postosuchus’s antorbital fenestra
more than those of other early crocodylomorphs.
However, unlike any of these other pseudosuchians, the rear edge of Carnufex’s antorbital fenestra
was vertical rather than slanted.
Much like many other terrestrial, carnivorous pseudosuchians, as well as most meat-eating
dinosaurs, the teeth of the Carolina Butcher were ziphodont, meaning they were compressed,
recurved, and serrated.
Much like the sphenosuchian Dromicosuchus, Carnufex had a notch between its maxilla and
premaxilla, which likely evolved to accommodate the enlarged fourth tooth present in numerous
crocodylomorphs.
Carnufex’s humorous is fairly small, and due to similarities with the humorous of the
bipedal Postosuchus, it seems the Carolina Butcher was also bipedal, although it may
have retained the ability to switch to a four-legged stance.
Since the sphenosuchians are also thought to have been bipeds, this seems to be the
ancestral state of Crocodylomorpha.
Today’s crocodilians have since returned to a purely quadrupedal stance, which is far
from their only evolutionary reversal.
While they and most other living reptiles are ectotherms, pseudosuchians ancestrally
had elevated, mesothermic metabolisms.
Even though they were the ancestors of crocodilians, sphenosuchians ironically had some of the
highest metabolic rates within Pseudosuchia.
Examination of Carnufex’s bone histology found the indictors of its metabolic rate
were more akin to those of sphenosuchians than to other similarly sized pseudosuchians
like the rauisuchians.
Besides allowing it to be more active, a high metabolism would have also enabled a swift
growth rate, and the Carnufex holotype reached its length of three meters in perhaps as little
as three years.
Carnufex is one of the most basal crocodylomorphs, but it is not alone in that distinction.
Its discovery led to the recognition of two others: Redondavenator, who is known from
even less complete remains than those of Carnufex, and a more complete but as of yet undescribed
skeleton from the Carnegie Museum of Natural History labeled “CM 73372”.
Like Carnufex, both of these basal crocodylomorphs were large, terrestrial predators native to
North America.
The limited remains of Redondavenator are comparable in size to the rauisuchid Postosuchus.
Although the Carnegie crocodylomorph was immature when it died, it was already bigger than any
sphenosuchian.
One of the most basal sphenosuchians, Hesperosuchus, was at best a meter and a half long, and the
others rarely grew longer than a single meter.
Whereas Carnufex and more basal loricatans were the Triassic equivalents of wolves and
lions, sphenosuchians have been likened to reptilian foxes.
Carnufex and the other large, basal crocodylomorphs have been described as “invading” the
apex predator niches, but this framing is misleading.
Their rauisuchian ancestors had already been large carnivores for millions of years.
The Triassic loricatans who invaded new niches were the small sphenosuchians.
Since Carnufex possesses numerous unique features, it was an “evolutionary uncle” of the
more derived crocodylomorphs rather than their direct ancestor.
Additionally, fossils of a sphenosuchian have been found in the Pekin Formation, demonstrating
that they had already evolved by Carnufex’s time.
Potential prey of the Carolina Butcher includes the large dicynodont Placerias and the herbivorous
pseudosuchian Revueltosaurus.
Fossils of these two species have also been discovered in contemporaneous fossils sites
in the southwestern United States.
Other animals found in the Pekin Formation, such as the armored aetosaur Gorgetosuchus,
have only been found in eastern North America.
Fossils of the rauisuchian Postosuchus alisonae have also been found in Late Triassic North
Carolina, but it lived ten million years after Carnufex.
Likewise, while early dinosaurs such as Herrerasaurus also lived two hundred and thirty-one million
years ago, they were still restricted to the southern hemisphere.
Thirty million years after the Carolina Butcher, at the very end of the Triassic Period, rauisuchians
and basal crocodylomorphs like Redondavenator were still the top predators on land.
However, nearly all pseudosuchians perished during the end-Triassic mass extinction, allowing
for the rise of the dinosaurs.
Although Carnufex and those like it have been gone for over two hundred million years, their
smaller descendants were the only pseudosuchians which survived into the Jurassic Period, leading
to the pseudosuchians of today.
Thank you for watching, and a thank you to the themattalorian for narrating this video.
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