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 MaurĂ­lio 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!

Sunday, October 14, 2012

Dino-Eating Crocs Part 1: Introduction to a Croc-Filled World

The notosuchid Baurusuchus, a ten-foot crocodilian which evolved a canine-like body, mammal-like teeth, and was completely terrestrial. Oh, and it was out-competing theropods in their ecological role of top predator in Cretaceous ecosystems.
 No big deal, right?
A few months ago, I was lucky enough to purchase myself the new paleontological book The Complete Dinosaur Second Edition, which I must say, has to be one of the best books about dinosaurs I've read in a long time, and I recommend it to any dinosaur enthusiast like myself. The book is made up of numerous sections and chapters on different dinosaur topics and debates, and leading experts from around the world have sent their research and findings to be presented in the book. Thomas Holtz, one of the world's leading experts on theropods, writes the entire theropod section of the book. Jack Horner, who was the first person to find dinosaur eggs in the Western Hemisphere, helps to write the section on dinosaur eggs and nests. And Darren Naish, who is an expert on fossil vertebrates (specifically birds), writes the section on both living and extinct groups of birds. I was actually very pleased with the bird section, since it's about time we got some living dinosaurs in a dinosaur book.

Ann Darrow and the fictional crocodile Foetodon from Peter Jackson's King Kong
Amazingly, there was a fossil crocodilian that resembles the latter in size and form.
Anyway, I've been spending the last few months reading this very in-depth book, reading through sections by Kristina Curry Rogers, Peter Makovicy, and Gregory S. Paul just to name a few. Finally I came across a section titled Non-Dinosaurian Vertebrates written by Nicholas C. Fraser, whom you might know from his recent book In the Shadow of the Dinosaurs (which I have yet to read). The section reviewed the many fossil vertebrates that lived alongside the dinosaurs, such as plesiosaurs, pterosaurs, mammals, and numerous other fossil groups from the Mesozoic. The section was strangely short, only about 26 pages long compared to the 40 pages some of the other paleontologists wrote (not including references). Still, I was pleased with the chapter, and it taught me many things that I didn't know about non-archosaurian reptiles from the age. So why am I bringing this up exactly? Simple, I was extremely disappointed at the crocodile section of the chapter.

The recently described Kaprosuchus (or Boar Croc) from North Africa, one of
Paul Sereno's newly discovered fossil crocs from the region. 
The section had very little content on the fossil crocodilians during the Mesozoic; less than three paragraphs are dedicated to the entire evolution of the group. And what is covered in the section is largely already known by most dino-nerds like myself, such as the already well-known super crocs of the age and the sea-going metriorhynchids. It's somewhat ironic because Fraser even states at the beginning that crocodiles were as diverse as dinosaurs and pterosaurs, yet he devotes three whole pages on the evolution and diversification of the pterosauria.

This is not meant by any means to be an attack on Fraser, as I said I absolutely loved the section. I'm just disappointed that he devoted such little space to the crocodilians when there is so much to cover, especially since fossil crocs are finally getting some much-deserved media attention for their quirkiness.

Notosuchus was a terrestrial crocodile that lived in South America during the Late Cretaceous.
You might call it the Mesozoic equivalent of a pig, with its hog-like snout and fleshy lips.
The crocodilian group as a whole, by which I mean the Metasuchia, was actually so diverse that I can't do the group justice in a single post, so this will be played out in two separate posts I'm working on. The first will be on the now-extinct, but extremely diverse group called the Notosuchids, which includes many mammal-like members that resemble everything from armadillos to house cats. In fact, they were so successful that they took over all kinds of dinosaur habitat and ruled whole regions where theropods and ornithopods once roamed. The second will be on the still living Neosuchia, which includes living crocodilians, the super-crocs of the Mesozoic, and some rather bizarre members, including some that seem almost Cetacean-like in anatomy and ecological role. They primarily avoided competition with dinosaurs and land animals, preferring an aquatic existence. However, there were certainly many exceptions to this rule.

So the next few topics will likely be on these crocodilians. In fact, I've basically got the next few months of blogs planned out unless I get more requests. Along with crocodiles, I'm hoping to write about spinosaurid skulls, extinct giant birds, dinosaur footprints, and hopefully raptors. Also, since I happened to bring up The Complete Dinosaur Second Edition, I may do a review of it in the future. If a review would be of interest, let me know in the comments.  Until next time, stay sharp!