Sixty-six million years ago, during the end of the Cretaceous period,
the world was firmly the domain of the dinosaurs. Then their reign suddenly came to a dramatic end
when a massive asteroid stuck the Earth. The devastation it caused was unimaginable,
but the debris the impact created soon surpassed it. As it settled into the atmosphere,
it effectively blocked all sunlight for over a year, killing the world’s remaining plants.
When the sun finally returned, roughly seventy-five percent of all species were gone.
With nearly no food, almost all of the animals that lived through the disaster were tiny,
with the only remaining dinosaurs being a few small birds. However, the largest survivors of
the long, terrible night were not birds or soon to be ascendant mammals, but the dinosaurs’ distant
cousins. Freshwater ecosystems were the least affected by the end-Cretaceous mass extinction,
and crocodilians and their relatives were ectothermic, meaning they needed much less
food than their dinosaurian relatives. Although the largest species perished, those who did live
to see the first dawn of the Cenozoic Era were giants in comparison to their remaining neighbors.
This video aims to give the spotlight to one of these remarkable survivors: Borealosuchus.
Although it may have resembled the crocodilians of today, Borealosuchus
coexisted with the likes of Triceratops, but also thrived in the new, mammal dominated world.
Despite not being the ancestor of modern crocodilians, Borealosuchus is considered
one of the most vital species towards understanding Crocodilia’s early evolution.
Borealosuchus was not a single species but a genus consisting of at least eight species,
although only six have been named. Their fossils have been found throughout North America,
with an impressive range spanning forty million years. Most dinosaur genera only lasted a few
million years before either going extinct or changing enough to warrant a new genus.
The Borealosuchus type species, Borealosuchus sternbergii, is notable for its presence both
before and after the catastrophe at the end of the Cretaceous Period. “Borealosuchus” means “northern
crocodile”, since most fossils belonging to the genus came from Canada and the northern United
States when it was named in 1997. Borealosuchus fossils had been found long before then,
with Borealosuchus sternbergii having been recognized as a distinct species in 1910.
However, at the time they were considered to be species of Leidyosuchus, a genus which had been
treated as a wastebasket taxon, with various unrelated crocodilians being dumped into it.
Today only a single species of Leidyosuchus, Leidyosuchus canadensis, remains in the genus.
It was a basal alligatoroid who lived in Alberta about 75 million years ago,
and it was not particularly closely related to Borealosuchus.
What is unclear is just how closely Borealosuchus was related to the crocodilians of today, or
whether it even was a true crocodilian. Crocodilia is defined as the crown group formed by all the
living species and all of the descendants of their last common ancestor. This excludes a multitude of
close relatives, called neosuchians, who already looked and acted pretty much like crocodilians.
Although the question of which neosuchians belong within Crocodlylia is usually more of an academic
question, it is of particular importance to Borealosuchus. Some phylogenetic trees place
it as the closest relative of Crocodlylia. Others have found it to be a basal member of the clade.
If true, this would also be significant since the oldest Borealosuchus are from the Santonian age,
about eighty-five million years ago, which would make it the oldest known crocodilian.
Either way, Borealosuchus seems to be the closet science has to what the last common ancestor of
the crocodilians looked like. What is certain is that the species of Borealosuchus who survived
into the Cenozoic Era were not the ancestors of today’s crocodilians, as fossils of each major
branch of Crocodlylia have been found dating to the Cretaceous Period. As a side note, although
it is uncertain whether or not Borealosuchus was a true crocodilian or a close relative,
it will be referred to as a crocodilian in this video for the sake of simplicity.
The Mesozoic Borealosuchus had a length between two to three meters, which is on the smaller side
of the range of modern crocodilians. They would have been capable of hunting small dinosaurs
like Leptoceratops, but were well out of the league of the gigantic crocodilian Deinosuchus.
Borealosuchus’s luck changed during the Cenozoic. While mammals slowly took advantage of the demise
of the non-avian dinosaurs, Borealosuchus instead took advantage of the extinction of
larger crocodilians. Although it was an underdog during the Age of Reptiles, found itself as the
local top predator during the Age of Mammals. As the northern crocodile’s prey grew in size,
it did as well. Borealosuchus formidabilis and Borealosuchus wilsoni both grew five meters long,
a size on par with the average saltwater crocodile, the largest living crocodilian.
Few of the contemporary mammals would have been safe from the larger species of Borealosuchus.
For a crocodilian, the back of Borealosuchus’s skull was unusually wide compared to the
end of its snout, giving it an even more triangular shape than other crocodilians.
Ancestrally, the end of Borealosuchus’s snout was blunt and U-shaped. Those of later forms became
slender and V-shaped. The snouts of some of the Cenozoic species, such as Borealosuchus wilsoni,
were also longer and contained slenderer teeth than those of the older species.
This has led to the suggestion that they were specialized piscivores, or fish eaters.
However, although they do seem to have become better at catching fish than their Late
Cretaceous predecessors, their snouts were still substantially broader than those of specialized
piscivores. The species of Borealosuchus with elongated snouts were also the ones
with the greatest body size. Therefore, they likely remained generalists, hunting both fish
and large terrestrial vertebrates. The orbits of Borealosuchus wilsoni are crescent shaped.
Those of other species of Borealosuchus, and most species of crocodilians, instead have a simpler,
circular shape. The toothrow of all species of Borealosuchus projects outwards after the sixth
or seventh tooth socket. This trait is unique to the genus among crocodilians and their relatives.
Despite being called the “northern crocodile”, Borealosuchus fossils have also been found in
Alabama, Texas, and possibly even New Mexico. The oldest Borealosuchus fossils are from the
Santonian Eutaw Formation in Alabama. With an age of roughly eighty-five million years,
the oldest of these fossils predate the oldest known tyrannosaurid dinosaurs,
predators whom Borealosuchus would outlive by twenty million years. Besides potentially
being the oldest crocodilian fossils found so far, the presence of Borealosuchus in Alabama
is also notable since at the time North America was divided into two landmasses, Laramidia in
the west and Appalachia in the east. Although the Santonian fossils are far from complete,
more extensive fossils from the slightly younger Early Campanian age have confirmed
that these Appalachian Borealosuchus were a new species, although it has yet to receive a name.
The oldest Borealosuchus fossils in Laramidia date to the Late Campanian, and are from the Two
Medicine Formation in Montana. These have yet to be assigned to a specific species.
The type species, Borealosuchus sternbergii, lived during the last age of the Cretaceous,
the Maastrichtian. It is among the crocodilians found in the Hell Creek Formation, alongside
icons such as Triceratops and Tyrannosaurus rex. During the Mesozoic, Borealosuchus had a somewhat
restricted range, but during the Cenozoic, species of Borealosuchus not only became larger,
but to also much more widespread. This was partially because of the extinction of much
of its ecological competition, but also due to the changing climate. Global temperatures
became very high during the Paleocene and parts of the Eocene. This was a great boon
for the ectothermic crocodilians, making it possible for numerous species to coexist.
It was once though that Borealosuchus might be a chronospecies, a single lineage that
changed through time. However, as more fossils of this genus have been uncovered,
this is no longer considered to have been the case. Paleocene fossils of Borealosuchus
sternbergii, who was once assumed to be the ancestor of the Cenozoic species,
have been found alongside Borealosuchus griffithi. Likewise, the Borealosuchus from Campanian Alabama
seems to have been more closely related to some Cenozoic species of Borealosuchus than to others.
It is now clear that the Borealosuchus genus had diversified into multiple cotemporary species,
of which a minimum of three survived into the Cenozoic Era. This shows just how successful
Borealosuchus was, both during both the Age of Reptiles and the Age of Mammals.
Of course, that success did not last forever. The northern crocodile finally
perished forty-five million years ago during the Eocene as the climate began to cool.
Although many groups of animals were negatively affected, crocodilians were particularly hard hit.
Just as the warm temperatures of the early Cenozoic had created an unprecedented period
of crocodilians diversity, the colder climate caused this diversity to crash.
Even though Borealosuchus survived the cataclysmic mass extinction that claimed the non-avian
dinosaurs, it could not survive this smaller and more subtle menace. Besides Borealosuchus,
notable casualties included the marine dyrosaurids and all alligatoriods outside of the Americas.
Crocodilian diversity never recovered throughout most of the world, with Miocene South America
being a notable but rare exception. Still, even though it has no living descendants,
Borealosuchus managed the rare accomplishment of existing for forty million years.
Thank you for watching, and a thank you to the themattalorian for narrating this video.
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