Crocodilians are expert swimmers, so much so that it has come at the expense of their
capabilities on land.
During Mesozoic era, distant relatives of the crocodilians called metriorhynchids took
this even further, forsaking land entirely.
While initially mistaken as little more than crocodiles significantly more in tune with
the water, the metriorhynchids are now known to have been active animals more comparable
to dolphins and whales.
One of the most famous of these sea crocs was Dakosaurus.
A large macropredator, Dakosaurus lived during the Late Jurassic and Early Cretaceous periods.
Its skull differed drastically from the other metriorhynchids, resembling that of a theropod
dinosaur more than a crocodilian or any other marine reptile.
So far, two species of Dakosaurus have been discovered.
The type species, Dakosaurus maximus, lived in the seas that covered most of Europe during
the Late Jurassic Period.
The second species is Dakosaurus andiniensis.
It lived is what is now Argentina during both the Late Jurassic and Early Cretaceous.
Fossils of a possible third species of Dakosaurus have also been found in Mexico, which date
to the Late Jurassic Period.
Other metriorhynchids were once considered species of Dakosaurus but have since been
given their own genera.
These include Plesiosuchus and Torvoneustes.
Dakosaurus grew to about four to five meters long.
This is about on par with the larger species of today’s crocodilians.
Its body was completely adapted to the water.
In contrast to early metriorhynchids, like Metriorhynchus itself, Dakosaurus’s limbs
had fully evolved into flippers.
However, like crocodiles, its main source of propulsion underwater was its tail.
Unlike crocodilians, but like mosasaurs and some other marine reptiles, Dakosaurus’s
tail had a fluke.
Crocodilians and most of their extinct relatives have large, bony osteoderms on their skin.
Dakosaurus and the other sea crocs lacked them, which helped to streamline their bodies
and reduce drag when swimming.
Dakosaurus even went so far as to lose most of its scales, a trait convergent with other
marine reptiles like the plesiosaurs.
Overall, Dakosaurus could be more easily mistaken for an off-brand mosasaur than a crocodilian.
Of course, since Dakosaurus came first, it might be more appropriate to call mosasaurs
off-brand Dakosaurus.
Most metriorhynchids had long, low skulls, which allowed for moving them quickly in the
water with minimal resistance.
This speed was ideal since most metriorhynchids specialized in catching fast, nimble prey
like fish and squid.
Dakosaurus’s skull instead evolved in the opposite direction.
It was shorter, wider, and taller, which gave it an almost dinosaur-like appearance.
This was most pronounced in Dakosaurus andiniensis, for which it has been given the nickname “Godzilla”,
after the famous movie monster.
The skull of Dakosaurus maximus was instead transitional between that of Dakosaurus andiniensis
and those of more typical metriorhynchids.
While this deep skull would have allowed for a powerful bite and was more resilient than
any other metriorhynchid’s, it would have also created a lot of drag.
This explains the rarity of such skulls among marine predators.
Dakosaurus’s teeth were also different those of most other metriorhynchids, which were
usually conical like those of crocodilians.
Dakosaurus’s teeth were instead slightly recurved, laterally compressed, and serrated.
Teeth like these are called ziphodont and they were also present in many theropod dinosaurs
as well as in some terrestrial crocodylomorphs.
Ziphodont teeth are efficient at cutting through the flesh of large prey items.
This would have allowed Dakosaurus to easily tear off the flesh of large prey, unlike crocodilians,
who must death roll to tear large carcasses apart.
While a few other metriorhynchids had ziphodont teeth, Dakosaurus is the only one whose serrations
are not microscopic.
Its teeth were also the largest but fewest in number of all metriorhynchids.
All of these traits are typical of species adapted to regularly prey upon large animals.
There were other large, predatory metriorhynchids, all of which belonged to the clade Geosaurinae.
Dakosaurus maximus coexisted with the largest of these: the seven-meter long Plesiosuchus.
A research paper in 2012 found a surprising modern analogue for Dakosaurus and Plesiosuchus:
Orcas.
In the North Atlantic, there are two types of killer whales, which may be different species
or subspecies.
The smaller type is a generalist, preying upon both fish and large animals, while the
larger type instead specializes in large prey.
Focusing on different prey allows them to coexist, which is called niche partitioning.
Just as the larger Orca type is two meters longer than the smaller type, Plesiosuchus
was two meters longer than Dakosaurus.
Likewise, the teeth of the smaller killer whale type exhibit much more wear, which is
also true for Dakosaurus’s teeth.
Dakosaurus’s tooth wear is not as extreme, which is likely because, unlike mammals, it
replaced its teeth throughout its life.
Therefore, the niche partitioning of these two types of killer whales seems to mirror
that of these two ancient sea crocs.
If true, Dakosaurus would have eaten both fish and large marine reptiles, while Plesiosuchus
would have been higher in the food chain.
However, “Godzilla” seems to have evolved another trait convergent with killer whales:
suction feeding.
Suction feeders create negative pressure inside their mouths so that when they open them,
the nearby water is pulled in, along with their hapless prey.
This leads to the fish, and their scales, coming into contact with the whale’s teeth
as they get sucked in.
This is partially responsible for the tooth wear present in the smaller killer whales.
Like them, Dakosaurus’s short, circular mouth would have been ideal for creating the
required negative pressure for suction feeding.
Besides Plesiosuchus, both species of Dakosaurus coexisted with species of Pliosaurus.
Although not as large as the inflated Liopleurodon in Walking with Dinosaurs, they were still
massive enough to not only compete with “Godzilla”, but also to prey on it.
Metriorhynchids had chambers in their skulls which looked like a second pair of nostrils.
Instead, these contained salt glands, which excreted excess salt.
Since Dakosaurus lived entirely in saltwater, these were critical for its survival.
Modern crocodiles also have salt glands, except theirs’ are smaller and located on their
tongues.
It is highly doubtful that Dakosaurus’s salt glands were inherited from a common ancestor
with crocodilians.
The first members of both of their clades lived in fresh water.
Indeed, it is currently unclear whether the metriorhynchids and their branch of Crocodylomorpha,
Thalattosuchia, even share a semi-aquatic ancestor with the crocodilians.
They may have instead evolved their affinity for water, along with some of their other
crocodilian-like traits, independently from a terrestrial crocodylomorph.
The first crocodylomorphs were endothermic, producing their own body heat like mammals
and dinosaurs.
On the other hand, whether it was the result of evolutionary convergence or common ancestry,
most thalattosuchians were ectothermic like crocodilians and most other reptiles.
This makes it all the more surprising that analyses of the isotopes of metriorhynchid
fossils have found that their body temperatures were higher and more stable than their relatives,
suggesting they were endothermic.
Reverting to endothermy would have allowed Dakosaurus to be more active than crocodilians
and ectothermic thalattosuchians like the teleosauroids.
However, metriorhynchid metabolisms were still lower than those of other marine reptiles
like the plesiosaurs and ichthyosaurs.
There is a hypothesis that the high metabolism of Dakosaurus and its relatives may be why
they survived into the Cretaceous Period.
Global oceanic temperatures fell during end of the Jurassic Period, which is suspected
to have been responsible for the subsequent decline of the ectothermic teleosauroids.
With its higher body temperatures, Dakosaurus would have been less vulnerable to climate
change.
While it has yet to be tested, it has been speculated that Cretaceous metriorhynchids
like Dakosaurus andiniensis may have evolved even higher metabolisms.
The sea crocs died out before the end of the Early Cretaceous.
Since their extinction, other crocodylomorphs have adapted to marine life.
However, none of Dakosaurus’s crocodilian cousins have taken to the ocean to the same
extent as it and the other metriorhynchids, let alone adopted Dakosaurus’s unorthodox
marine adaptions.
Instead, the legacy of the real “Godzilla”, is carried on by the mammalian orcas.
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