Although all modern crocodilians are amphibious, they and their relatives were once much more
diverse.
One of the most exceptional examples was a small land croc named Mekosuchus.
While most land crocs were only distant relatives of the crocodilians, Mekosuchus was an actual
crocodilian who had returned to a terrestrial lifestyle.
Additionally, while most land crocs lived during the Mesozoic Era, Mekosuchus was still
alive just a few thousand years ago, long after the end of the most recent ice age.
Four species of Mekosuchus have been found so far.
Some of these lived on Australia, while the last Mekosuchus populations lived on islands
in the Pacific Ocean.
All four species were small, only between one to two meters long.
The fusion of their vertebrate has confirmed they were fully grown adults when they died,
rather than the juveniles of larger species.
Mekosuchus’s limbs were better suited for terrestrial locomotion than modern crocodilians,
whose exceptional swimming capabilities have left them more sluggish on land.
An intriguing proposal is that Mekosuchus was arboreal, meaning it lived in the trees.
This is a contentious subject.
Some paleontologists are of the opinion that Mekosuchus’s limbs were not any more specialized
for climbing than modern crocodilians.
However, recent research has found that crocodilians are actually far better climbers than they
were once given credit for.
This is especially true for smaller crocodilians and juveniles, who can climb trees vertically.
Therefore, the question now seems to be not whether Mekosuchus could climb trees, but
how often it did.
Since all Mekosuchus species lived in forested environments, it now seems difficult to argue
that it wasn’t at least somewhat arboreal.
While a complete Mekosuchus skull still hasn’t been found, what has been discovered was,
like its legs, very different from the skulls of modern crocodilians.
Their skulls are long and flat.
In contrast, Mekosuchus’s skull was shorter, taller, and rounder.
The shape of its snout would have produced to much drag if it hunted in the water like
its modern relatives.
From the side, its skull resembles the land crocs from the Mesozoic era, such as Morrinhosuchus.
However, Mekosuchus’s skull was still wide like most other crocodilians.
The eyes and nostrils of most crocodilians are on the top of their skulls, which allows
them to keep most of their body hidden beneath the water so at to not alert their victims.
In contrast, Mekosuchus’s eyes and nostrils were instead on the sides of the skull, like
most other terrestrial species.
Among its living relatives, Mekosuchus’s short skull most closely resembles the dwarf
crocodiles and the two species of caiman in the Paleosuchus genus.
They spend more time on land than most other crocodilians and their short skulls are thought
to be adaptations to the dense forests they live in.
Since Mekosuchus also lived in tropical rainforests, and was also fully terrestrial and potentially
arboreal, it would have had even stronger evolutionary pressure for a short snout.
As a drawback, this restricted the size of the animals Mekosuchus has able to hunt.
The back teeth of one species, Mekosuchus inexpectatus, were specialized for crushing.
Since it lived on an island, its teeth are thought have been an adaption for consuming
crabs and mollusks, allowing it to exploit the limited resources on its island home.
Due to a combination of Mekosuchus’s terrestrial lifestyle and its small size, it couldn’t
have used the crocodilian death-roll when feeding.
Examination of its neck vertebrae has found that to compensate for this, it had larger
epaxial neck muscles.
This would have allowed it to make stronger head movements to tear its meals apart while
on land.
The oldest species of Mekosuchus, Mekosuchus whitehunterensis, lived in Australia at the
very end of the Oligocene, about twenty-three million years ago.
The next species, Mekosuchus sanderi, lived during the following Miocene.
During this time Australia was significantly lusher than today, with rainforests covering
most of the continent.
There is a roughly twenty-million-year gap between the Mekosuchus fossils on the mainland
and the later island dwelling species.
Mekosuchus kalpokasi inhabited the in the Vanuatu islands.
The Mekosuchus type species, Mekosuchus inexpectatus, lived on the islands of New Caledonia, the
same islands chosen for the filming of the first episode of Walking with Dinosaurs.
Given the presence of two different species of Mekosuchus on two different island chains,
along with the presence of a close relative named Volia on Fiji at roughly the same time,
Mekosuchus was likely once widespread throughout the Pacific islands.
Mekosuchus belonged to its own branch of Crocodilia, Mekosuchinae.
While there is still some debate about where the mekosuchines fit into Crocodilia, it is
clear they were at least more closely related to crocodiles than alligators are.
Mekosuchus’s resemblance to its distant Mesozoic relatives was merely the result of
evolutionary convergence after it lost its ties to the water.
The first mekosuchines were actually very similar to crocodiles, and filled the same
niches in Australia and the surrounding islands for most of the Cenozoic Era.
However, Mekosuchus’s closet relatives were also fully terrestrial.
Volia lived during the Pleistocene and like Mekosuchus lasted even after the last ice
age.
It grew to about three meters long.
The other terrestrial mekosuchines lived on mainland Australia.
Trilophosuchus lived during the Miocene and was about the same size as Mekosuchus.
Although the mekosuchine Quinkana wasn’t very large when it first evolved during the
Oligocene, it eventually grew to become one of the largest carnivores in Australia when
humans first arrived over twenty million years later.
Due to their isolation, both the Vanuatu islands and New Caledonia were home to numerous endemic
species besides Mekosuchus, many of which are unfortunately now extinct.
Of the two island chains, New Caledonia’s extinct fauna are the better studied.
Since the only native mammals on the islands are bats, birds took over many of their same
niches.
These included the flightless Sylviornis.
This large, big-headed bird only lived an average of five-to-seven-years, which Mekosuchus
may have been partially responsible for.
However, Mekosuchus was not the only native carnivore.
A large, unnamed species of monitor lizard was also present on the island.
A more precise estimate of its size has not been given, so it is no clear where it fit
in the food chain in relation to Mekosuchus.
An even stranger reptile was Meiolania.
This late surviving relative of turtles was reminiscent of the ankylosaurian dinosaurs
and the mammalian glyptodonts.
Not only was it heavily armored, but is also had a clubbed tail.
While some species of Meiolania could grow surprisingly large, the shell of the species
native to New Caledonia, Meiolania mackayi, was only about seventy centimeter long.
Therefore, while daunting, it is not inconceivable that Mekosuchus could have occasionally taken
them down.
Fossils that may belong to another species of Meiolania have also been found in the Vanuatu
islands, the home of Mekosuchus kalpokasi, although their identity is disputed.
While it is not known when or why Mekosuchus become extinct on the Australian mainland,
the fate of the island species is less mysterious.
Mekosuchus survived the end of the last ice age without any problem.
But when humans arrived on its island homes around three thousand years ago, Mekosuchus,
and many of its contemporaries, quickly perished.
It is thought they were the victims of overhunting.
This hypothesis is supported by a Mekosuchus kalpokasi fossil which was found among human
refuse at an archeological site.
Other factors likely included humans overhunting Mekosuchus’s prey, habitat alteration, and
the introduction of invasive species such as dogs.
The extinction of Mekosuchus marked the end of Mekosuchinae.
It also marked the end of something even older.
The first crocodylomorphs were ancestrally terrestrial.
Even when the ancestors of today’s crocodilians became semi-aquatic, terrestrial species were
an important part of the fauna for the entire Mesozoic Era, especially in the south.
One group of land crocs, the sebecids, managed to outlive the non-avian dinosaurs and survived
until about twelve million years ago, long after Mekosuchus evolved.
This means that Mekosuchus’s extinction marked the first time there wasn’t a land
croc since Crocodylomorpha first evolved over 225 million years ago.
Still, there are a few crocodilians who spend much of their time on land.
These semi-terrestrial species include the dwarf crocodiles, both species of Paleosuchus,
and the much larger and more deadly Cuban crocodile.
Given time and the right conditions, they could eventually become fully terrestrial.
Unfortunately, the futures of most of these species is currently imperiled by mankind.
If the world is to once again be home to land crocs, humanity will have to give them the
chance it denied Mekosuchus.
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