Sunday, October 28, 2012

Dino-Eating Crocs Part 2: Success of the Notosuchia

A depiction of Baurusuchus eating a turtle. These crocodilians were taking up the roles of Dinosaurs near the end of the Cretaceous, and were proving to be deadly adversaries.
When you think of the paleoecology of the Cretaceous period, normally you think of dinosaurs filling up all the niches available, as this period was the height of their success. But this couldn't be further from the truth. During the Cretaceous, many forms of animals took up the intermediate roles between the folds of nature, and while some think these areas were ruled by mammals, that's actually not the case either. It seems that these niches were mostly filled by a group of crocodiles, known as the Notosuchia.

The Notosuchia are a well known group to scientists. We first found their bones more than a hundred years ago, but they remain mysterious and unknown to the public. Their fossils have been unearthed in South America, Africa, Europe, and even as far as East Asia. They were fully terrestrial, as shown by their un-flattened tails, level nostrils and eyes, and long legs, and ranged in size and shape from small little animals no larger than a housecat, to fairly large animals that could've given theropods a run for their livelihood. Their strange teeth and dietary preferences have made them famous in the paleontological world.

The best description of their teeth is that they're mammal-like, although they really have no close resemblance to any animal living or extinct, and were probably more competitive with mammals than dinosaurs ever were, as they occupied the same niches. Some even seem to have given up a diet of meat and evolved towards herbivory, such as Chimaerasuchus from China and Malawisuchus from Malawi. Others were opportunistic omnivores, such as Notosuchus from Brazil and Araripesuchus from all over the Southern Hemisphere. The latter's genus also lasted from 125 - 65 mya, meaning it survived for more than 60 million years, longer than most other land vertebrates from the Mesozoic and certainly longer than any dinosaur genus I know of.

Chimaerasuchus, a 6ft herbivorous croc from Early Cretaceous China.
The location where this fossil was found is the farthest
the group ever got from the Equator.
There were many bizarre members of the Notosuchia, but it's interesting to note that many resemble living mammals. Armadillosuchus has probably the most prominent name of the group, resembling a living Armadillo, and having armor carapaces along its back for defense. Mariliasuchus looked profoundly like a gopher or other burrowing mammal, and has actually been found lying within possible burrows along with associated eggs. Notosuchus had fleshy lips, and might have had either a hog-like snout, or possibly even a trunk like a tapir. Some looked like cats, such as Pakasuchus, which has large eyes and a similar body form to modern felines.Their were even alien-looking animals like Yacarerani, which had a bizarre dentition that looked a lot like a rat or other rodent. All these similarities with mammals, along with the fact that they had upright legs and likely exhibited active lifestyles, has suggested to some researchers that these crocodilians were endothermic, but an examination of this idea has yet to be performed.

These animals were definitely filling up mammal niches, and it's probably due to this group, not the dinosaurs, that most mammals in the Southern Hemisphere stayed small. However, in many places that these small crocs roamed, we're now finding that herbivorous dinosaurs were absent, suggesting that they were also taking up dinosaur roles in the environment. Take South America for example, almost all kinds of hypsolophodont from the northern side of the continent were gone by the Late Cretaceous, but in their place we found these little animals scurrying around. The same is also true of Late Cretaceous Madagascar, we've yet to find one Ornithopod or Ceratopsian in the environment, their niches completely filled with crocs instead. The occurrence of Chimaerasuchus in China also roughly coincides with the disappearance of many small ornithopods in that region. However, the disappearance of Chimaerasuchus also roughly coincides with the diversification of many plant-eating theropods, possibly meaning that once these crocs left the region, theropods filled the niches that both groups had held previously.

The most competitive, and probably my favorite members of the group were the sebecosuchia, which occupied not the niches of small ornithopods and mammals, but were predators taking up the roles that small- to mid-sized theropods had. Along with the mammal-like teeth of other members of the Notosuchia, members of the sebecosuchia also evolved theropod-like teeth, and in some members like Stratiotosuchus, canine-like teeth evolved. They evolved stiff backbones and longer legs, better for actively running after prey, and resembled giant reptilian dogs. These were also the largest members of the Notosuchia, with some species reaching about 15-20ft.

I imagine these animals to be ambush predators, lying in wait along game trails, waiting for an unsuspecting dinosaur to wander by, and possibly even pursuing the animal for a short distance at high speeds until they tired. This behavior, along with their ecological niche, would've put them right at the same level as large theropods, and would've been just as dangerous to the local herbivore populations. In many fossil sites around the world, these animals seem to have even replaced theropods as the top predators as time went on.

A Stratiosuchus preying on some kind of Titanosaur.
Image by Maurílio Oliveira.
This is most strikingly the case in a formation called the Adamantina Formation, which is a layer of rock in Brazil that dates back from 90-83mya. This could be called the Lost Land of the Crocodiles, and the entire fauna is dominated by these animals, with more than 15 species present and virtually no dinosaurs or mammals represented. Instead of mid-sized theropods we see Baurusuchus, Campinasuchus, and Stratiotosuchus filling these roles; in place of ornithomimids and oviraptors, we instead have pig-like animals like Armadillosuchus and Mariliasuchus; and in place of ornithopods and mammals, we have a whole dynasty of herbivorous species present in the fossil layers. The only dinosaurs present are Sauropods, which seem to have been the only dinosaur group these animals couldn't match in ecological role. However, I've recently heard of some unidentified theropod material that's come from the formation, and a fossil Barusuchid apparently has some theropod bite marks preserved on its bones, which might indicate that these dinosaurs were present. But still, the majority of animals in the formation were Notosuchids, and it seems that they took control of this specific region. Why exactly? We may never know.

At the end of the Cretaceous, after all the major dinosaur faunas went extinct, leaving only birds and mammals to take their roles, these crocs didn't go back into the water to join their Neosuchid kin. In fact, the discovery of numerous members of the sebecidae found in Cenozoic rocks proves that these animals survived the disaster at the end of the Cretaceous, and were still as big, powerful, and competitive as ever, and ready to try and take up the roles as predators of mammals. In South America scientists have found the sebecids Sebecus, Bretesuchus, Langstonia, and Lorosuchus in the same fossil rocks as members of the Phorusrhacidae and Sparassodonta, which we believed for a long time were the only large predators on the continent. Not just that, but Bergisuchus from Germany and Eremosuchus from Algeria also shows that they survived in places outside of South America.

Seeing how the group was comprised of large, active, possibly even warm-blooded predators, how did they survive the K-T extinction event? Well, apparently one articulated Baurusuchus specimen suggests the possibility that even these large animals dug burrows, possibly in order to hibernate during tough times as seen in modern day Nile Crocodiles. This behavior could have helped them survive the extinction 65mya that wiped out the dinosaurs, and allowed them to live on through the Cenozoic. But then, why aren't they around nowadays?

There are many mysteries surrounding the extinction of the Notosuchids, however the extinction does coincide with a sharp decrease in global temperatures, known as the mid-Miocene disruption, which might have spelled doom for a group of crocs without any form of insulation to keep them warm. Still, these animals must have been magnificent to see in their glory, and it certainly proves that crocs were definitely not lying down on their lazy bellies by the water's edge during much of their evolution. They were active, powerful, and quite capable of giving even the "Terrible Lizards" a good run when they were alive.

There we go, part one of the Crocs done, and I'll be posting part two within a few weeks, so I'll see you all then!

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