Crocodilians are the sole survivors of a larger group of reptiles called Crocodylomorpha.
While many crocodylomorphs were similar to their modern relatives during the time of
the dinosaurs, others, such as the terrestrial notosuchians, held niches distinctly different
from today’s crocodilians.
Among these was the notosuchian Pakasuchus, who was in many ways more like the small Mesozoic
mammals than its crocodilian cousins.
Its holotype skeleton, which is nearly complete, is one of the best-known vertebrate fossils
known from Late Cretaceous Africa, providing a window into an otherwise poorly known part
of prehistory.
Pakasuchus was first found in the Rukwa Rift Basin in Tanzania during 2008 and was named
in 2010.
Its full scientific name, Pakasuchus kapilimai, means “Kapilima’s cat crocodile”.
Dr. Saidi Kapilima was one of the leaders of the expedition that first found its remains.
“Paka” comes from the Swahili word for “cat”, while “suchus” means “crocodile”,
and is basically the crocodilian equivalent of “saurus”.
Fitting its name, Pakasuchus was cat sized, about half a meter long.
Like many other land crocs, its limbs were long and placed directly beneath its body,
in contrast to the sprawling posture of most other reptiles or the semi-erect stance often
used by crocodilians.
Another trait that separates Pakasuchus from its modern relatives is its osteoderms.
Osteoderms are bony plates on the skin, which are often used as armor and mineral reserves
in crocodylomorphs.
Pakasuchus has less osteoderms than most other crocodylomorphs, and those it does are smaller
than the norm.
The tail is an exception, whose osteoderms are abnormally normal in size and shape.
The osteoderm reduction on the rest of the body is thought to have been a weight saving
measure, allowing Pakasuchus to run faster, though why the tail was unaffected is unclear.
Perhaps the reduction of its osteoderms was to instead increase its agility.
Pakasuchus’s long torso was fairly mobile for a crocodylomorph.
This mobility was likely enhanced by its reduced body osteoderms, since crocodilian osteoderms
serve as attachment points for muscles which stiffen the back.
As a flexible tail would have been less important for an agile land croc, this provides an explanation
as to why evolution left the tail osteoderms untouched.
It should be noted that Pakasuchus would have been less active than the contemporary dinosaurs
and mammals, as notosuchians had an ectothermic, or cold blooded, metabolism.
Still, like today’s monitor lizards, their metabolism was on the high end of the ectothermic
spectrum.
Though crocodilians have elongated snouts, Pakasuchus’s skull was short and compact,
only about seven centimeters long.
Notably, unlike crocodilians, its deep and robust lower jaw was capable of the motions
necessary to chew its food.
Pakasuchus had surprisingly few teeth, only five rows in the upper jaw and eight in the
lower jaw.
While low in number, these were remarkably varied.
The first teeth in Pakasuchus’s jaws are conically shaped and massively enlarged, not
unlike the canines in mammals.
While caniniform teeth are present in many other extinct notosuchians, they have incisor-like
teeth in front of them.
Behind Pakasuchus’s caniniform teeth are smaller conical teeth called premolariforms.
As the name implies, behind these are molariform teeth very similar to those of mammals, with
complex, multicupsed crowns.
Such varied, complex teeth are rare among reptiles and are usually only seen in mammals.
Other notosuchians, such as Mariliasuchus and Chimaerasuchus, also had mammal-like teeth.
However, Pakasuchus took this a step further.
The crowns of its molariform teeth fit together precisely so as to increase their surface
area.
This allowed for some of the most efficient chewing known in Crocodylomorpha.
So, what was Pakasuchus eating with these teeth?
When it was first found, it was assumed to have been a carnivore due compassions with
its molar-like teeth to those of predatory mammals.
However, a study examining the teeth of various crocodylomorphs found Pakasuchus likely to
be a herbivore.
However, this was based on the complexity of just its molariforms.
The average complexity value of all of Pakasuchus’s teeth instead overlapped with both herbivores
and omnivores.
Given the previously mentioned similarities to mammalian carnivores, a mixed diet seems
to be the most likely option.
It has been suggested that notosuchians like Pakasuchus largely replaced the primitive
mammals in the southern continents during the Cretaceous period.
However, mammals have since been found to have coexisted with these notosuchians.
Fossils of the mammal Galulatherium have even been found in the same formation as Pakasuchus
itself, the Galula Formation.
Still, though notosuchians may not have fully replaced mammals, Pakasuchus and its relatives
managed to expand into niches usually occupied by the more active animals, copying many of
their own traits in the process.
While crocodilians are often called “living fossils”, Pakasuchus is an example of just
how diverse and adaptable crocodylomorphs were and can be.
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