The Evolutionary Dead-End Experiments of Crocodiles

It was at this moment that Steve the fish realised he had made a dire mistake (Image via Andy Murch)

Crocodiles are NOT living fossils... 

They may look ancient, with a smattering of characteristics that make them look like non-avian dinosaurs, but as we dive back throughout the Mesozoic and come up for air in the murky waters of the late Triassic (roughly 230 million years ago), we find that the distant ancestors of the creatures we refer to as the heavily armored  “Crocodylia” (a group that includes all “true” crocodiles, caimans, alligators and gharials) look so completely different from their modern counterparts.

Many early crocodylimorphs (a term that is used to include modern and long-extinct cousins of crocodilians) were completely terrestrial and include the ~230 million year old “Trialestes”, an animal that was originally interpreted as a basal dinosaur, due to its “un-crocodilian” upright stance.

Thanks to the fossil record, we have an incredible sequence of fossil remains that demonstrate how these groups of crocodylimorphs took on many varying ecological niches throughout prehistory: from completely marine forms such as the Thalattosuchians (a variety of crocodylimorphs that could swim in the open ocean with modified “fins” and of which, was undoubtedly an active predator) to the pug faced “Simosuchus”, a small, metre long crocodylimorph that had spatulate-shaped teeth for eating PLANTS.  

Join me as we swim... 

run... 

and jump... 

...through more than 230 million years of evolutionary history to understand the highly successful and altogether weird dead-end experiments of the Crocodylimorphs.

First up, the aforementioned upright crocodylimorph “Trialestes”.

Is it time to run from our feelings? For Trialestes, this was partly true. (Image via Leucona et al. 2016)

The Greyhound Crocodile: “Trialestes” (Sphenosuchia)

At roughly a metre in length, this early South American Sphenosuchian was the greyhound of the Crocodilian lineages. Its gracile form with an erect limb posture would continue well into the Middle Jurassic (roughly 165 million years ago). The sphenosuchians were not large, with most only getting up to a metre in length. 

This guy was the Santas Little Helper of the Crocodile world. Lets hope "The Simpsons" doesn't continue for another 65 million years like the Spenosuchians did (Image via Jaime A. Headden)

The Tiny Walking Crocodile: “Terrestrisuchus” (Sphenosuchia)

With a maximum weight of ~15 kilograms and being half a metre in length, this late Triassic reptile is all kinds of weird. Most Triassic reptiles are odd, but this early crocodylimorph looks like a mix between a skittish lizard and a baby crocodile. Its tail was twice the length of its body; with long thin legs, this creature is hypothesized to run, potentially being capable of rearing up on two legs during sprints like the modern Basilisk lizard.

"The cave is collapsing!... This is no cave!" The Millenium falcon (Pterosaur) escapes the clutch of the Space Slug (Dakosaurus) circa 140,000,000 BC  (Image via Dmitry Bogdanov)

The Marine Crocodile with Flippers: “Dakosaurus” (Metriorhynchidae)

201.3 million years ago, the Triassic/Jurassic extinction event decimated numerous “crocodile-like” lineages. 42% of terrestrial tetrapods were eradicated and more than a quarter of marine fauna disappeared worldwide. Lineages such as the long snouted and presumably semi-aquatic Phytosaurs and the heavily armored Aetosaurs were wiped out in a very short amount of time. This allowed the crocodylimorphs to dominate these respective ecological niches; although the dinosaurs were the dominant force on land, some crocodyliforms dominated the marine niche alongside Pliosaurs and Ichthyosaurs.

One such grouping were the Metriorhynchidae… the most famous representative of these marine crocodiles was the 4.5 metre long “Dakosaurus”. This animal was hydro-dynamically efficient and even had a “finned” tail for greater thrust through the water column. With large serrated teeth, a short “snout” and a deep mandibular symphysis, this was an apex predator that would have been able to twist off large amounts of flesh off a single carcass.

The Dinosaurs Killers: “Sarcosuchus” (Pholidosauridae) and "Deinosuchus" (Alligatoroidea)

Representing some of the largest crocodylimorphs to have ever existed, these two genera occupied completely different time frames and evolutionary lineages.

"Night at the Museum" at the Le Museum would be a completely different movie. Im sure it would involve a-lot more death and blood... (Image from the Royal Tyrrell Museum of the "Le Museum" Sarcosuchus)

"Sarcosuchus"

At 11-12 metres in length and weighing 8 tonnes, “Sarcosuchus” lived roughly 112 million years ago in modern day Sahara. Its snout was more gharial-like, comprising more than 75% of the total length of the skull. Most Pholidosaurs (a group that would die out roughly 89 million years ago) were generalist piscivores but Sarcosuchus was a generalist predator, possibly predating large Iguanodontids like Ouranosaurus.

The sun was shining, the seagulls were singing and almost nothing could go wrong for Albert...  Deinosuchus lunging at Albertosaurus (Image via Raul Martin) 

“Deinosuchus”

The 10-12 metre long apex-predator known as “Deinosuchus” was thought to have had one of the strongest bite forces ever measured. This heavily armored “killer-croc” had a bite force anywhere between 18,000 Newtons (of force) to a staggering 102,803 N. This bite was thought to be stronger than even the largest theropods, such as Tyrannosaurus.

This 73 – 80 million-year-old crocodylimorph belongs to the superfamily “Alligatoroidea” aligning it closer to modern Alligators than extant crocodiles.  With huge crushing teeth, it inhabited either side of the Western Interior Seaway and may have fed on large Hadrosaurs such as the 9 metre-long “Kritosaurus”.

Stomatosuchus: Why does my shadow look like that? (Image via: unknown) 

The Baleen Whale Crocodile: “Stomatosuchus” (Stomatosuchidae)

Yep, you heard right. Filter feeding, just like a modern baleen whale. Isn’t it absolutely insane? Stomatosuchus was a 10 metre long filter feeding crocodylimorph that existed in Egypt 100-95 million years ago. Unfortunately the entire holotype of Stomatosuchus was destroyed during an allied bombing raid in 1944 and only pictures remain of the enigmatic crocodile relative. For more information, check out a previous post I made about these bizarre crocodiles: https://a-fools-experiment.blogspot.com/2019/02/giant-filter-feeding-crocodiles-from.html

All the better to eat you with my dear... (Image via Carol Abraczinskas)

The Boar-toothed Crocodile: “Kaprosuchus” (Mahajangasuchidae)

With its huge caniniform teeth that look like tusks, stereoscopic vision (an adaptation made by many big-bodied terrestrial carnivores) and an overall body length of 6 metres, this was a top predator from the Upper Cretaceous of Niger, Africa.

Autobots, ROLL OUT... (Image via Tom Holtz)

The Armadillo crocodile: “Armadillosuchus” (Sphagesauridae)

This creature is absolutely wild. As its name suggests, Armadillosuchus was a late Cretaceous crocodylimorph that convergently evolved to look like an armadillo.  At roughly 2 metres in length and weighing some 120 kilograms, this heavily armored Sphaegesaurian (known exclusively from the Bauru basin of Brazil) was possibly fossorial, using its forelimbs to dig. What sets “Armadillosuchus” apart from other crocodylimorphs were its “heavy body armour, composed of a rigid shield and mobile-banded section as in extant armadillos”, its completely terrestrial lifestyle and an ability to chew like a modern mammal.

I DDNT CHOOoSE THa PUG LYF. THa pug lyf cHoSe mah... (Image via Gordon Robertson)

The Pug Nosed Herbivorous Crocodile: “Simosuchus” (Ziphosuchia)

Another incredibly strange crocodylimorph, this Madagascan from the Maastrichtian (latest Cretaceous) was a blunt snouted herbivore. With spatulate teeth and a relatively small size of 75 centimetres in length, the skulls (adorned with bony projections) of Simosuchus vary considerable, based on ontogeny and sexual dimorphism. It also has a tiny tail (when compared to modern crocodiles), a para-vertebral shield on its back with interlocking osteoderms and short robust legs (which indicate a terrestrial lifestyle).

The Amazonian Giants: Purussaurus (Caimaninae) and Mourasuchus (Caimaninae)

This skull is the same size as you... (Picture via Aguilera et al. 2006)

Purussaurus

Travel back in time to a world ruled by crocodiles. The Amazon jungle during the Middle – Late Miocene (6-10 million years ago) was a terrifying place. 10-12 metre long caimans such as Purussaurus, weighing more than 5 metric tons, rein supreme. With a heavily reinforced skull (the same size as your average human), it was capable of performing the “death roll” on large vertebrates (based on recently performed biomechanical analyses). It was the undisputed apex predator of Miocene South America.

With its patented flat design, the Mourasuchus surfboard was offering an experience few surfers knew... (Picture via Cidade et al. 2017)

Mourasuchus

Extreme niche partitioning of crocodile types occurred in the Miocene Amazon, but by far the most bizarre that existed within this jungle, was the 12 metre long FILTER FEEDING caiman, the duck faced Mourasuchus. 

For Mourasuchus, its rostrum was broad and dorso-ventrally flattened, with slender mandibles and a short join at the mid line of the jaw (as seen with baleen whales) with small conical teeth that almost seem vestigial. It seems that this weird-ass jaw would be suitable for a “filter-feeding” way of capturing prey, with an envisaged “gular sac” reminiscent of pelicans that would function for this straining technique.  For more information, check out a previous post: https://a-fools-experiment.blogspot.com/2019/02/giant-filter-feeding-crocodiles-from.html

Globidentosuchus looks like it just told a joke and is waiting for your reaction... (Image via University of Zurich)

The durophagous “Globidentosuchus” (Caimaninae)

At less than two metres in length, this Amazonian crocodile has blunted conical teeth that were used for crushing hard-shelled organisms such as clams. This “durophagous” habit has evolved many times in the fossil record, with the 6 metre long mosasaur “Globidens”, the sharks “Ptychodus” and the modern “Heterodontus”, and even in whales with the bizarre “Odobenocetops”, the “Walrus whale”.

The long-term environmental stability of the Amazon would have provided the perfect environment for these bizarre Miocene crocodiles, but monumental change in the earth would soon occur and by the Pliocene, hydrographic changes linked to the Andean uplift would kill off the huge array of crocodylimorphs in the Amazon.

Skippy the short-faced kangaroo and his sidekick, Zippy the Ziphodont. Image via Hodari Nundu on DeviantArt, of a hypothetical situation between the giant short faced kangaroo Procoptodon and Quinkana. 

The Australian Running Crocodile: “Quinkana” (Mekosuchinae)

The latest Pleistocene in Australia was home to a number of bizarre reptiles, none quite as strange as “Quinkana”, the terrestrial crocodile that could gallop on land… 

“Quinkana fortirostrum” was approximately 5 – 6 metres long and weighed somewhere between 700 – 800 kilograms. This species, termed as a “Ziphodont”, was thought to be a terrestrial crocodile. Species belonging to this genus are comparatively rare, with “Q. fortirostrum” being one of the very last Ziphodonts left in existence. The species is renowned for its very broad snout and distinctive dentition, in which its teeth are laterally compressed and serrated. 

Ziphodont crocodilians represent an interesting phase within Australia; whereas most global Ziphodont forms went extinct at the end of the late Miocene (approximately 6 million years ago), Australian forms survived well into the Pleistocene until their disappearance approximately 40, 000 years ago. 

The relative absence of large marsupial predators in the Tertiary on mainland Australia allowed for “Q. fortirostrum” to assume the role as a large terrestrial predator, similar to the niche in which “Megalania” (the giant monitor lizard) partook…

A crocodile on a branch (Image via Kent MacElwee)

The three primary branches of modern Crocodylia began separating roughly 95 – 80 million years ago and included the likes of modern Crocodiles, Alligators and Gavialids (such as the modern gharials). There is little doubt that these three groups had already diverged by the Cretaceous/Paleogene Mass Extinction Event. Alongside two other groups of crocodylimorphs, (the long snouted Dryosauridae which would become completely extinct some 35 million years ago and the terrestrial and primarily carnivorous Sebecosuchians that lived until the Mid Miocene) they thrived throughout the Cenozoic.

From swimming in the open ocean to galloping on land, the evolution of the Crocodylimorphs demonstrates how adaptable the crocodile design is. These animals are apex predators in their own right and have changed radically in their 230 million year history…

So if you hear someone say:

“Crocodiles have remained unchanged for millions of years”

You know what to say…  

References

Lecuona A, Ezcurra MD and Irmis RB 2016. Revision of the early crocodylomorph Trialestes romeri (Archosauria, Suchia) from the lower Upper Triassic Ischigualasto Formation of Argentina: one of the oldest-known crocodylomorphs. Papers in Palaeontology (advance online publication). DOI: 10.1002/spp2.1056

Crush, P. J. "A late Upper Triassic sphenosuchid crocodilian from Wales." Palaeontology 27.1 (1984): 131-157.

Pol, Diego, and Zulma Gasparini. "Skull anatomy of Dakosaurus andiniensis (Thalattosuchia: Crocodylomorpha) and the phylogenetic position of Thalattosuchia." Journal of Systematic Palaeontology 7.2 (2009): 163-197.

Sereno, Paul C., et al. "The giant crocodyliform Sarcosuchus from the Cretaceous of Africa." Science 294.5546 (2001): 1516-1519.

Schwimmer, David R. King of the crocodylians: the paleobiology of Deinosuchus. Indiana University Press, 2002.

Aguilera, Orangel & Riff, Douglas & J, Bocquentin-Villanueva. (2006). A new giant Purussaurus (Crocodyliformes, Alligatoridae) from the Upper Miocene Urumaco Formation, Venezuela. Journal of Systematic Palaeontology. 4. 221-232. 10.1017/S147720190600188X.

Cidade GM, Solórzano A, Rincón AD, Riff D, Hsiou AS. 2017. A new Mourasuchus (Alligatoroidea, Caimaninae) from the late Miocene of Venezuela, the phylogeny of Caimaninae and considerations on the feeding habits of Mourasuchus. PeerJ 5:e3056 

Sereno, Paul, and Hans Larsson. "Corrigenda: Sereno PC, Larsson HCE (2009) Cretaceous Crocodyliforms from the Sahara. ZooKeys 28: 1–143." ZooKeys 29 (2010): 73.

Marinho, Thiago S., and Ismar S. Carvalho. "An armadillo-like sphagesaurid crocodyliform from the Late Cretaceous of Brazil." Journal of South American Earth Sciences 27.1 (2009): 36-41.

Turner, Alan H., and Joseph JW Sertich. "Phylogenetic history of Simosuchus clarki (Crocodyliformes: Notosuchia) from the late cretaceous of madagascar." Journal of Vertebrate Paleontology 30.sup1 (2010): 177-236.

Aguilera, Orangel A., Douglas Riff, and Jean Bocquentin‐Villanueva. "A new giant Purussaurus (crocodyliformes, alligatoridae) from the upper Miocene Urumaco formation, Venezuela." Journal of Systematic Palaeontology 4.3 (2006): 221-232.

Scheyer, T. M., Aguilera, O. A., Delfino, M., Fortier, D. C., Carlini, A. A., Sánchez, R., ... & Sánchez-Villagra, M. R. (2013). Crocodylian diversity peak and extinction in the late Cenozoic of the northern Neotropics. Nature communications, 4, 1907.

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