Tag Archives: herpetology

Crocodiles and Cane Toads

You can see them from a helicopter: the white, bloated bellies of dead crocodiles, limply floating down the Victoria River. Australian freshwater crocodiles live hard lives, and most hatchlings are quickly eaten by fish, herons, frogs, turtles, or adult crocodiles. By the time they reach adulthood, at more than 7 feet long, they’ve already proven themselves to be the toughest reptiles around, so finding dead ones didn’t used to be common. Starting in 2002 that began to change (Letnic et al. 2002). Bodies started turning up, floating on their backs, by the hundreds. In their stomachs, researchers found the culprit: cane toads.

Cane toads, an invasive species in Australia, are extremely toxic. Their skin and organs are filled with cardiac glycosides, molecules that induce heart failure. Pets that eat them often die. So do a few humans, who lick the toads hoping to experience the hallucinogenic effects* of toad poison.

A dead freshwater crocodile, after eating a cane toad. Photo by Adam Britton, used with permission.

A dead freshwater crocodile, after eating a cane toad. Photo by Adam Britton, used with permission.

The toads’ natural predators, to varying degrees, have evolved to handle toad poison (also called bufotoxin). Examples include certain army ants and the cat-eyed snakes, which eat the toads and their tadpoles with ease. Even outside the toads’ native range (tropical Latin America), predators are often able to tolerate them because they have already adapted to the toxins of their own local toads.

Australia has a special problem: the country has no native toads. None at all. Since the cane toads’ introduction, scientists have observed dramatic population declines in predatory reptiles, from monitor lizards to pythons to crocodiles. These reptiles are not adapted to living with toads: they don’t instinctively leave toads alone, and when they venture eat one, death by poison is often the result.

Australia is home to two crocodile species. The smaller of the two is the freshwater crocodile (Crocodylus johnstoni), which lives in ponds and the upper reaches of rivers, away from the northern coastline. At 7-10 feet in length, this species is not dangerous to humans unless provoked, instead subsisting on a diet of fish, amphibians, small mammals, and the like.

A freshwater crocodile. Photo by Richard Fisher, licensed under CC BY 2.0.

A freshwater crocodile. Photo by Richard Fisher, licensed under CC BY 2.0.

The larger is the saltwater crocodile (Crocodylus porosus), males of which are the largest crocodiles on earth, reaching lengths up to 20 feet. Although they often live alongside (and sometimes prey upon) freshwater crocodiles, saltwater crocodiles truly thrive in the more coastal habitats: estuaries, mangrove swamps, and sea-bound river deltas.

Both species are opportunists, and will happily snap up a toad if given the opportunity.

A saltwater crocodile. Photo by Lip Kee Yap, licensed under CC BY-SA 2.0.

A saltwater crocodile. Photo by Lip Kee Yap, licensed under CC BY-SA 2.0.

Dr.’s James Smith and Ben Phillips (2006) wanted to find out just how dangerous cane toads were to Australia’s native predators. They harvested the toxins from cane toads and then administered them to various Australian reptiles, including predatory lizards, pythons, and both crocodile species.

When scientists want to know how deadly a toxin is, they calculate LD50. The LD50, or median lethal dose, is simply the amount of poison that will on average cause the death of 50% of victims.

The LD50 depends both on the toxin and on the animal that ingests it. A rat, for example, has a 50% chance of death if it drinks 192 milligrams of caffeine for every kilogram that the rat weighs. Rats typically weigh about 1/3 of a kilogram, so the total LD50 for caffeine is 1/3 of 192, or 64 mg. More toxic substances have lower LD50’s, since it takes less poison to cause death. Caffeine isn’t that toxic. Aren’t numbers fun?

The mangrove monitor, a predator easily poisoned by cane toads. Photo by Jebulon, in public domain.

The mangrove monitor, a predator easily poisoned by cane toads. Photo by Jebulon, in public domain.

Smith and Phillips calculated that the LD50 for bufotoxin fed to freshwater crocodiles was about 2.76 milligrams. Cane toads, which can weigh up to 2 kilograms, are perfectly capable of killing freshwater crocodiles that eat them.

Here’s the odd thing: while freshwater crocodiles often died as a result of cane toad poisoning, none of the saltwater crocodiles did. To see if the poison was affecting them in other ways, the scientists conducted athletic tests — if the crocodile couldn’t run as fast after poisoning, that was interpreted as a sign the poison was harming the reptile. While the freshwater crocodiles slowed down after ingesting bufotoxin, saltwater crocodiles were just as energetic before as after their toxic meal.

Are saltwater crocodiles immune to bufotoxin? It’s hard to say. The scientists wanted to kill as few crocodiles as possible, and they didn’t have enough crocodiles on hand to test much higher doses. Perhaps extremely high doses of bufotoxin would kill saltwater crocodiles, but the data is lacking.

What we do know is that saltwater crocodiles are much more resistant to cane toad poison than freshwater crocodiles. There are two potential reasons for this, and the most obvious is size. Saltwater crocodiles, males of which can weigh more than 2,000 pounds, are the largest crocodilians and the largest non-marine predators in the world. An adult saltwater crocodile simply cannot eat a toad large enough to reach a lethal dose.

A saltwater crocodile. Photo by fvanrenterghem, licensed under CC BY-SA 2.0.

A saltwater crocodile. Photo by fvanrenterghem, licensed under CC BY-SA 2.0.

Smaller crocodiles are more vulnerable. In 2013, an expedition to remote areas of northern Australia revealed that some populations of pygmy freshwater crocodiles, which only grow to 5 feet, have suffered declines upwards of 60% due to toad poisoning (Britton et al. 2013). The same research team, led by Dr. Adam Britton, is trying to raise money with a crowd-funding campaign to return to these remote sites, to study and help protect pygmy crocodiles. I strongly encourage you to visit the crowd-funding site here, as Britton has prepared a terrific video on pygmy crocodiles and the unique challenges they face.

A pygmy freshwater crocodile. Photo via Adam Britton, used with permission.

An adult pygmy freshwater crocodile. Photo by Adam Britton, used with permission.

The saltwater crocodiles in the Smith and Phillips study were not even close to 2,000 pounds — they were subadults, less than three feet long and closer to five pounds. So a few milligrams of cane toad poison should have killed at least some of them. Instead the walked away un-fazed, without so much as a skip in their gait.

Why? It may have to with the two crocodiles’ evolutionary history. In addition to being the largest, saltwater crocodiles are some of the widest-ranging** crocodiles, distributed from eastern India through Southeast Asia, Indonesia, and New Guinea. Because they can live in saltwater, they have been able to colonize many Pacific Islands (e.g., the Solomons) that are out of reach of other crocodilians.

A cane toad. Photo by Sam Fraser-Smith, licensed under CC BY 4.0.

A cane toad. Photo by Sam Fraser-Smith, licensed under CC BY 4.0.

Throughout their range they encounter a tremendous variety of potential prey. Saltwater crocodiles are not picky eaters, and have been observed feeding on fish (including sharks), frogs, lizards, snakes, turtles, crabs, snails, octopuses (during marine forays), deer, monkeys, pigs, cows, rats, otters, rabbits, porcupines, kangaroos, squirrels, wild cats, jackals, emus, geese, miscellaneous birds, and bats that fly just a little too close to the water.

Also, toads.

Even though Australian crocodiles never encounter toads, they have almost exactly the same DNA as their relatives in Asia and Indonesia. Perhaps they have inherited a tolerance for bufotoxin, while the freshwater crocodile, alone and isolated in Australia, has not.

Freshwater crocodiles might seem like the evolutionary dopes in this story, but there is hope for them. While some populations have been hit hard, others appear to be unaffected, perhaps because cane toads tend to avoid the habitats where freshwater crocodiles do most of their hunting (Somaweera et al. 2012). Research (like the pygmy crocodile project) is continuing to shed light on where cane toads are affecting crocodiles the most, why, and what can be done to protect them.

Finally, crocodilians are more intelligent than most reptiles. Studies with captive specimens have shown that after just a few encounters, hatchling freshwater crocodiles are able to quickly learn to avoid cane toads. Back in the field, some populations of crocodiles are already showing signs of learning, as cane toads are attacked less often and less enthusiastically than native frogs (Somaweera et al. 2011). As with humans, the best hope for freshwater crocodiles is in the next generation.

A young freshwater crocodile. Photo by Mike Peel, licensed under CC BY-SA 4.0.

A young freshwater crocodile. Photo by Mike Peel, licensed under CC BY-SA 4.0.

*Don’t even think about it.

**Saltwater crocodiles, while secure in Australia, are endangered in Southeast Asia, where many populations have gone extinct.

Cited:

Britton A.R.C., E.K. Britton, and C.R. McMahon. 2013. Impact of a toxic invasive species on freshwater crocodile (Crocodylus johnstoni) populations in upstream escarpments. Wildlife Research 40: 312-317.

Letnic M., J.K. Webb, and R. Shine. 2008. Invasive cane toads (Bufo marinus) cause mass mortality of freshwater crocodiles (Crocodylus johnstoni) in tropical Australia. Biological Conservation 141: 1773-1782.

Smith J.G. and B.L. Phillips. 2006. Toxic tucker: the potential impact of cane toads on Australian reptiles. Pacific Conservation Biology 12(1): 40-49.

Somaweera R., J.K. Webb, G.P. Brown, and R. Shine. 2011. Hatchling Australian freshwater crocodiles rapidly learn to avoid toxic invasive cane toads. Behaviour 148(4): 501-517.

Somaweera R., R. Shine, J. Webb, T. Dempster, and M. Letnic. 2012. Why does vulnerability to toxic invasive cane toads vary among populations of Australian freshwater crocodiles? Animal Conservation 16(1): 86-96.

Scaly and Adorable: Australia’s Pygmy Crocodiles

Yesterday I wrote about crocodile evolution, and some of their amazing extinct relatives (here). I wrote about them partly because prehistoric crocodylomorphs are amazing, and that’s as good a reason as any. But it was also to prove a point: modern crocodilians, 23 species all with similar appearances, might seem like ancient members of a group that has hardly changed at all. This is not so. Crocodilians are instead the only survivors of a vast and hugely diverse lineage of animals, most of which looked nothing like the crocodilians alive today.

Like the finned sea-crocodiles of the Jurassic, modern crocodilians are an off-shoot, just one branch in a massive crocodylomorph tree. Unlike the sea-crocodiles, by some combination of chance and adaptation, modern crocodilians have managed to avoid extinction (so far). Unlike the sea-crocodiles, crocodilians are still evolving.

NileCrocodile--Etiopia-Omo-River-Valley-01.jpg

A Nile crocodile (Crocodylus niloticus). Photo by Gianfranco Gori, licensed under CC BY-SA 4.0.

Dr. Adam Britton, a world-renowned crocodilian biologist, points out that his favorite animals are far more advanced than they look. “I do see them as highly refined survivors of their ancient lineage. The analogy I use when talking about croc evolution is to compare modern crocodiles to Ferraris: they might superficially look and function similarly to a Model T Ford, but they are so much more refined.”

One of Dr. Britton’s favorite species is the Australian freshwater crocodile (Crocodylus johnstoni), the smaller of Australia’s two native crocodiles. Freshwater crocodiles typically reach 7-10 feet in length — impressive, but dwarfed by the saltwater crocodile (Crocodylus porosus), males of which can grow to over 20 feet. The two species share Australia, but because saltwater crocodiles are larger and fiercely territorial, freshwater crocodiles are often relegated to sub-optimal habitats, such as smaller rivers and ponds.

A young freshwater crocodile. Photo by Mike Peel, licensed under CC BY-SA 4.0.

A young freshwater crocodile. Photo by Mike Peel, licensed under CC BY-SA 4.0.

Such habitats include the remote, rocky upstream gorges of the Victoria and Liverpool Rivers (Webb 1985). Here, and at a few other sites in northern Australia, unique freshwater crocodiles live in relative isolation from humans and saltwater crocodiles. Unfortunately, they also live without much food — small streams mean few fish, which make for malnourished crocodiles.

So they evolved. Over time, the crocodiles became smaller to make up for a poor diet, and now they are truly tiny, with the largest reaching 5 feet in length. They became the pygmy crocodiles, small enough that you could (unadvisedly) pick one up and carry it around with you.

A pygmy freshwater crocodile. Photo via Adam Britton, used with permission.

A pygmy freshwater crocodile. Photo by Adam Britton, used with permission.

Stunted growth is one thing — any crocodile, fed a poor diet, will fail to reach its maximum size potential. Pygmy crocodiles are different. They have been growing this way for enough time that they are now genetically predisposed to small size. If you took a pygmy croc from the wild and fed it the same diet as a normal freshwater crocodile, the former would still be much smaller than its cousin.

A pygmy crocodile. Photo by Adam Britton, used with permission.

A pygmy crocodile. Photo by Adam Britton, used with permission.

Pygmy crocodiles aren’t quite distinct enough to be classified as their own species — yet. They haven’t been isolated for very long, so for now they are still considered an unusual population of freshwater crocodiles. They may interbreed with larger crocodiles, in which case they will never fully separate. A more exciting possibility is that they may continue to evolve and diverge in isolation, in which case they may someday become genetically unique enough to constitute a new species.

There is another possibility: pygmy crocodiles may go extinct before they get a chance to evolve any further. Although their habitat is isolated and relatively secure, they are threatened by invasive cane toads, introduced to Australia in 1935. A hungry crocodile will happily snap up a toad, but because cane toads are extremely toxic, they are often the crocodile’s last meal.

Curiosity killed this crocodile -- it tried to eat a poisonous cane toad. Photo by Adam Britton, used with permission.

Curiosity killed this crocodile — it tried to eat a poisonous cane toad. Photo by Adam Britton, used with permission.

Pygmy crocodiles, because they are so small, are especially vulnerable to the poison. Although some populations have been unaffected (Doody et al. 2014). others have declined in abundance by more than 60% since the toads’ introduction (Britton et al. 2013). Why some populations are more vulnerable than others is one of many crucial questions that remain unanswered (Somaweera et al. 2012).

A cane toad. Photo by Sam Fraser-Smith, licensed under CC BY 4.0.

A cane toad. Photo by Sam Fraser-Smith, licensed under CC BY 4.0.

Dr. Britton is leading an effort to study pygmy crocodiles in their natural habitat. The goals of his research are two-fold. First, he means to assess their wild populations to determine if the crocodiles might be endangered. Second, Britton and his team wish to collect DNA from the pygmy crocodiles, to better understand their evolutionary history, and their genetic relationship with larger freshwater crocodiles.

This is an achievable and worthy project, but an ambitious one. Field work is always costly, but pygmy crocodiles live in isolated, hard-to-reach places, and getting there requires use of a helicopter. Dr. Britton and his team have started a crowd-funding effort to raise funds to support pygmy crocodile research — I’ve donated, and if you think pygmy crocodiles are amazing, I strongly encourage you to do so as well. There are some great prizes for donors, including crocodile-themed artwork and jewelry!

You can learn more about the project at its crowd-funding site, here. On the website is a short video in which Dr. Britton discusses and handles pygmy crocodiles. They are positively adorable.*

If crocodiles are Ferraris, then pygmy crocodiles are Smart Cars — tiny and vulnerable, but awesome in an enticingly bizarre sort of way. Pygmy crocodiles are an evolutionary quirk, just like the prehistoric pelican-snouted crocodile Stomatosuchus, or the armadillo-backed Armadillosuchus, or the shark-tailed … you get the picture. There’s just one important difference: pygmy crocodiles are alive. We, as residents of a special time in the history of life, get to appreciate them for the amazing creatures that they are. Let’s try and keep it that way.

*The crocodiles, I mean. Although if you like listening to British scientists get super-duper excited about wildlife, it’ll be a happy three minutes for you.

A big thank you is owed to Dr. Britton, who graciously allowed me to use his images for this article. Once again, I encourage you to donate to his effort to study these amazing crocodiles. You can learn more about crocodilians at Dr. Britton’s encyclopedic and lavishly illustrated website here.

Cited:

Britton A.R.C., E.K. Britton, and C.R. McMahon. 2013. Impact of a toxic invasive species on freshwater crocodile (Crocodylus johnstoni) populations in upstream escarpments. Wildlife Research 40: 312-317.

Doody J.S., P. Mayes, S. Clulow, D. Rhind, B. Green, C.M. Castellano, D. D’Amore, and C. Mchenry. 2014. Impacts of the invasive cane toad on aquatic reptiles in a highly modified ecosystem: the importance of replicating impact studies. Biological Invasions 16(11): 2303-2309.

Somaweera R., R. Shine, J. Webb, T. Dempster, and M. Letnic. 2012. Why does vulnerability to toxic invasive cane toads vary among populations of Australian freshwater crocodiles? Animal Conservation 16(1): 86-96.

Webb G.J.W. 1985. Survey of a pristine population of freshwater crocodiles in the Liverpool River, Arnhem Land, Australia. National Geographic Society Research Report 1979: 841-852

The Pelican, the Shark, and the Armadillo: Crocodile Evolution Part 1

Crocodiles are very TV-genic – scores of documentaries have focused on them, and most start off with a variant of the following: “Crocodilians are some of the most successful predators on the planet. They survived the dinosaurs, and have remained unchanged for millions of years.”

Crocodylus_acutus_mexico_01.jpg

An American crocodile (Crocodylus acutus). Photo by Tomas Castelazo, licensed under CC BY-SA 2.5.

Today I wanted to write about why that’s not entirely true, and why crocodilian evolution, far from monotonous, is instead dynamic and, like crocodilians themselves, full of surprises.

There are only 23 species of non-extinct crocodilians, compared to 327 turtles and tortoises and more than 9,000 lizards and snakes. The fossil record, however, is full of crocodile relatives, the crocodylomorphs, dating back to at least 225 million years ago (Bronzati et al. 2012; Russel and Wu 1998).

The first crocodylomorphs looked nothing like the crocodiles alive today. Instead they were small, nimble creatures that ran on four long legs. Their sharp teeth reveal a predatory lifestyle, but instead of waiting in ambush at the water’s edge, they were adapted for wandering about over land in search of prey.

These animals, the sphenosuchians, appeared during the Late Triassic period when dinosaurs were just beginning to evolve. With legs pointed down under the body (rather than splayed to the sides), the sphenosuchians were probably terrific at chasing down prey. One such reptile, Hesperosuchus, was about the size of a domestic dog.

Hesperosuchus_BW.jpg

Hesperosuchus. Illustration by Nobu Tamura, licensed under CC BY 2.5.

As the dinosaurs began to diversify, so did the crocodylomorphs. Although many stayed small, some got larger, and began to experiment with different lifestyles. I say “experiment” euphemistically, but evolution is blind — circumstances and natural selection simply allowed crocodylomorphs to fill a wider range of niches. There were large, land-dwelling predators, like Protosuchus:

Protosuchus_BW.jpg

Protosuchus. Illustration by Nobu Tamura, licensed under CC BY 2.5.

There were also marine crocodylomorphs, the thalattosuchians. These, too, were predatory and looked vaguely like modern crocodilians in that they had long jaws, a muscular tails, and laterally flattened bodies (Young et al. 2014). Here, from Europe, are three species of Machimosaurus:

Machimosaurus_illustration.jpg

Machimosaurus species. Illustration by M.T. Young et al. (2014), licensed under CC BY 4.0.

Others looked nothing like any modern animals, and had fin-like feet and tails. Dakosaurus, also from Jurassic Europe (Young et al. 2012), had such an un-crocodile-like skull that their fossils were at first mistakenly attributed to dinosaurs.

Dakosaurus_maximus.png

Dakosaurus maximus. Illustration by M.T. Young et al. (2012), licensed under CC BY 2.5.

Most of the sea crocodiles lived during the Jurassic Period, when dinosaurs on land were becoming truly massive (e.g., spike-tailed Stegosaurus and long-necked Brontosaurus*). Despite an uncanny resemblance, sea crocodiles did not “evolve into” modern crocodiles. Instead they represent an off-shoot of crocodylomorph evolution, a kind of “dead-end” with no modern descendants.

There’s plenty more to say about fossil crocodylomorphs in the Jurassic, but we’ll skip to the Cretaceous, the last period of non-bird dinosaurs with Tyrannosaurus, Triceratops, and the like. Crocodiles, too, were reaching a magnificent crescendo with on of their most diverse lineages, the Notosuchia.

The Cretaceous saw crocodylomorphs occupying all sorts of unusual ecological roles. There were, of course, the usual land-dwelling predators like Baurusuchus:

Baurusuchus_BW.jpg

Baurusuchus. Illustration by Nobu Tamura, licensed under CC BY 2.5.

Others had turned over a new leaf, so to speak, and adopted a life of plant-eating. Simosuchus, one such herbivore (or omnivore — see Buckley et al. 2000), hardly looked like anything you might call a crocodile:

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Simosuchus clarki. Illustration by Smokeybjb, licensed under CC BY-SA 3.0.

There were still weirder crocodylomorphs. Armadillosuchus had armor plates over its back — it looked, and quite possibly behaved, very much like a modern armadillo, only 6 feet long and with sharp, fang-like teeth.

Armadillosuchus.jpg

Armadillosuchus arrudai. Illustration by Smokeybjb, licensed under CC BY-SA 3.0.

Meanwhile, some of the predatory crocodylomorphs had returned to the water, where they began to take full advantage of the lifestyle that would someday become the trademark of crocodilians. Many could be easily mistaken for true crocodiles — they had long snouts, flattened bodies, and spent their time ambushing visitors to the water’s edge. Some were colossal, and scientists have found evidence that some of the largest species (Sarcosuchus and Deinosuchus) preyed on dinosaurs (Boyd et al. 2013), just as Nile crocodiles today pick off unwary wildebeest and zebras.

Size comparison of two extinct and three living crocodylomorphs. Figure by Matt Martyniuk, licensed under CC BY-SA 3.0.

Size comparison of two extinct and three living crocodylomorphs. Figure by Matt Martyniuk, licensed under CC BY-SA 3.0.

Modern-day crocodilians belong to the Neosuchia (“new crocodiles”). This group also evolved in the Mesozoic and includes such strange beasts as Stomatosuchus, a 30-foot-long behemoth with a broad, spoon-like snout. Paleontologists aren’t sure exactly what it ate, but some suspect the lower jaw supported a pouch-like membrane for gulping fish, just like a pelican (Nopsca 1926).

Stomatosuchus2.jpg

Stomatosuchus inermis. Photo by Dmitri Bogdanov, licensed under CC BY 3.0.

Neither Sarcosuchus nor Stomatosuchus were “true” crocodilians, but crocodylians start to appear in the fossil record at around the same time. The oldest crocodilian fossils date back around 80 million years, during the Late Cretaceous (Russel and Wu 1998).

Just 15 million more years — a blink of an eye, really — until Earth’s most recent apocalypse. The same mass extinction that wiped out the dinosaurs (except birds), likely caused by a meteor, also wiped out most of the crocodylomorphs. Perhaps owing to their aquatic habits and (relatively) small size, crocodilians were one of the few survivors. A few other groups, like as the sea-faring dyrosaurids, lingered into the so-called Age of Mammals, but these too gradually fell away until only the true crocodiles, alligators, caimans and the gharial remained.

Guarinisuchus munizi, one of the few marine crocodylomorphs after the K-T extinction event. Illustration by Nobu Tamura, licensed under CC BY 3.0.

Guarinisuchus munizi, one of the few marine crocodylomorphs after the K-T extinction event. Illustration by Nobu Tamura, licensed under CC BY 3.0.

So we arrive at the present day, with 23 living species of crocodilians. Those 23 are the survivors survivors, here today because they are adaptable, tough, and a little bit lucky. They bear witness to an ancient lineage of incredibly diverse, versatile, and often bizarre animals, most of which have died. They are the ones that lived through mass extinctions, competition with dinosaurs, and climate change. They may even survive us.

NileCrocodile--Etiopia-Omo-River-Valley-01

A Nile crocodile (Crocodylus niloticus). Photo by Gianfranco Gori, licensed under CC BY-SA 4.0.

*Brontosaurus may or may not be the same as Apatosaurus. If they are the same, then Brontosaurus is not a valid name. I’m not a paleontologist, so I have no opinion on the matter — I used the name Brontosaurus here since I think it is more recognizable, although I may be wrong about that, too. To learn more, check out this article by zoologist and writer Darren Naish.

I’ll be writing more about crocodilian evolution tomorrow, this time with an amazing species that is alive today. If you want to read more about extinct crocodylomorphs, read this article, also by Darren Naish. If this kind of stuff interests you, I recommend following his blog Tetrapod Zoology at the same link.

Finally, if you are interested in a more technical review of crocodylomorph evolution, some good papers to read are Bronzati et al. (2012) and Russel and Wu (1998), cited below.

Cited:

Boyd C.A., S.K. Drumheller, and T.A. Gates. 2013. Crocodyliform feeding traces on juvenile ornithischian dinosaurs from the Upper Cretaceous (Campanian) Kaiparowits Formation, Utah. PLOS ONE 8(2): e57605. doi: 10.1371/journal.pone.0057605

Bronzati M., F.C. Montefeltro, and M.C. Langer. 2012. A species-level supertree of Crocodyliformes. Historical Biology 24(6): 598-606.

Buckley G.A., C.A. Brochu, D.W. Krause, and D. Pol. 2000. A pug-nosed crocodyliform from the Late Cretaceous of Madagascar. Nature 405: 941-944.

Nopcsa F. 1926. Neue Beobachtungen an Stomatosuchus. Centralbl. Min. Geol. Palaontol, B212-215.

Russel A. and X.-C. Wu. 1998. The crocodylomorpha at and between geological boundaries: the Baden-Powell approach to change? Zoology 100(3): 164-182.

Young M.T., S. Hua, L. Steel, D. Foffa, S.L. Brusatte, S. Thüring, O. Mateus, J.I. Ruiz-Omeñaca, P. Havlik, T. Lepage, and M. Brandalise de Andrade. 2014. Revision of the Late Jurassic teleosaurid genus Machimosaurus (Crocodylomorpha, Thalattosuchia). Royal Society Open Science doi: 10.1098/rsos.140222

Young M.T., S.L. Brusatte, Marco Brandalise de Andrade, J.B. Desojo, B.L. Beatty, L. Steel, M.S. Fernández, M. Sakamoto, J.I. Ruiz-Omeñaca, and R.R. Schoch. 2012. The cranial osteology and feeding ecology of the mentriorhynchic crocodylomorph genera Dakosaurus and Plesiosuchus from the Late Jurassic of Europe. PLOS ONE 7(9): e44985. doi: 10.1371/journal.pone.0044985