When one thinks of Mesozoic crocodilians and their semi-aquatic relations, what usually comes to mind
is either species very similar to today’s crocodiles or massive, dinosaur eating titans.
However, the Late Cretaceous waters were also home to a very different sort of creature:
the narrow snouted Thoracosaurus. Thoracosaurus lived in North America and Europe at the very
end of the Cretaceous period. Unlike many of the other cotemporary large reptiles,
it survived into the Cenozoic era. Most Thoracosaurus fossils are from individuals
that were about four to five meters long. One specimen from Late Cretaceous North America
is estimated to have been much larger, around eight to nine meters long. Whether this represents
a new species or merely a fully grown individual is unclear. While not as large as Deinosuchus, who
died out before Thoracosaurus evolved, this morph was still larger than any crocodilian alive today.
Even the smaller Thoracosaurus specimens outclassed the other contemporary amphibious
crocodylomorphs alive in Late Cretaceous North America. Of course, Thoracosaurus wasn’t simply
a slightly smaller version of Deinosuchus. It had more in common with species like the gavialoids,
the gharial and the false gharial. With its long, slender snout and interlocking teeth,
Thoracosaurus’s skull was very well adapted for catching fish but less so for dealing with
larger vertebrates. Today’s gavialoids are also specialized fish-eaters, also called
piscivores. They hunt primarily by laying completely still in the water. When fish approach,
they then make a quick sideways strike to catch them in their jaws. Based on its similarities,
Thoracosaurus likely hunted in a comparable manner. By focusing on fish, the gharial is
able to coexist with the mugger crocodile, who is better at hunting larger, terrestrial species.
Dividing resources in such a way is referred to as niche partitioning. Likewise, Thoracosaurus’s
fish heavy diet was allowed it to coexist with Borealosuchus and Brachychampsa. Still, while not
the second coming of Deinosuchus, it is unlikely Thoracosaurus was restricted to aquatic prey.
Though the modern gharial and other slender snouted crocodilians are fish hunting specialists,
they also eat small terrestrial vertebrates. The false gharial is even less specialized,
sometimes even hunting larger species such as deer. With its even greater size,
the large Thoracosaurus morph would have been capable of dealing with even larger prey,
its sheer size somewhat compensating for the inherent weakness of its slender snout. However,
it may have had less opportunities to kill large, terrestrial animals than the false gharial does.
While all of the non-avian dinosaurs perished during the mass extinction at the end of the
Cretaceous period, along with most other living things, crocodilians in general fared better than
most creatures. This was due to both their low metabolisms, resulting in less need for food
than warm blooded creatures like the dinosaurs, and because freshwater ecosystems were some of
the least affected by the K–Pg extinction event. Still, Thoracosaurus was one of the more notable
survivors, living in a marine environment and possessing a fairly large body size.
Even though the eight-meter-long morph doesn’t seem to have survived the catastrophe,
Thoracosaurus is still a contender for the largest survivor of the K–Pg extinction event. Its fish
heavy diet was probably why it survived. While the number of fish Thoracosaurus was dependent
on would have been fewer in number in the aftermath of the meteor, the prey of the other
large crocodilians became completely extinct. Right now, Thoracosaurus’s place in the
crocodilian evolutionary tree is unclear. It has traditionally been placed with a few other
species, who together are informally called the thoracosaurs. One of these, Eothoracosaurus,
lived in eastern North America during the Late Cretaceous period. The others instead lived during
the Cenozoic era and have been found in North America, Europe, and Africa.As of right now,
there is no clade formerly called “Thoracosauria”. The thoracosaurs were originally thought to be a
paraphyletic grade of early gavialoids ancestral to the gharial. However, while the thoracosaurs
were considered to be more closely related to the gharial than to the false gharial,
studies of crocodilian genetics has found the last common ancestor of the two living gavialoids
lived only 40 million years ago, long after the thoracosaurs first appeared. A few years ago,
a study concluded that Thoracosaurus and the other thoracosaurs were not gavialoids,
with their similarities largely being the result of them converging on a piscivorous diet.
Indeed, they may not even be true crocodilians, though these findings remain debated. Even if
Thoracosaurus wasn’t a crocodilian, it mostly a technicality. The clade Crocodilia is formed by
the living species, their last common ancestor, and all of its descendants.
While Thoracosaurus‘s gharial-like skull may have been the result of convergent evolution,
its other crocodilian-like traits were the result to of common ancestry. Still, determining its
evolutionary relationships could be the key to solving lingering questions about the evolution
of the Thoracosaurs, Gavialoidea, and Crocodilia as a whole. Hopefully future research will be able
to solve this mystery. Thank you for watching. I hope you learned something interesting.
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