Saturday, February 10, 2018

Land Crocodiles of the Mesozoic- Director's Cut

Back in December, I wrote another article for the Dinosaur State Park newsletter. This piece was all about Crurotarsi, that sadly overlooked sister group to dinosaurs that dominated the Triassic and early Jurassic. I had to keep the original article short in order to fit it into the alloted space, but here on my blog I've expanded it out and added a couple new entries.

In paleoart crurotarsans are usually portrayed as being very scaley and crocodile-like. Which makes sense considering that crocodiles and alligators are their modern descendants in the same way that birds are the modern descendants of dinosaurs. But dinosaurs were a very diverse group with all sorts of unusual appearances and behaviors, so why wouldn't their cousins be similarly varied? For that matter, why couldn't crurotarsans have a body covering besides scales and thick osteoderms? I've tried to impart some of that diversity to my depictions of the animals here, even giving some of them a covering of pycnofiber "fur".

We usually think of dinosaurs as the dominant animals of the Mesozoic. However, throughout the Triassic and into the early Jurassic, dinosaurs were only a small component of Earth’s fauna. Instead, the positions of “ruling reptiles” were held by a group called the crurotarsans. This taxon, which is also sometimes called Pseudosuchia, is represented today by the twenty-three species of crocodiles and alligators, all aquatic ambush predators. In the past, though, crurotarsans were significantly more diverse and included among their ranks: small, swift terrestrial predators; armored herbivores; fully-marine mosasaur-like specimens; giant, dinosaur-like carnivores and many other varieties. Many crurotarsans possessed a row of thick armored plates along the back, a feature still present in modern-day crocodilians in the form of hardened osteoderms.   

Crurotarsans are the sister taxon to the Avemetatarsalians, the group that includes dinosaurs, modern birds, and pterosaurs. Together Crurotarsans and Avemetatarsalians make up the group Archosauria or “Ruling Reptiles”.

Crurotarsan fossils are found all across the East Coast of North America and many of them likely once lived in Connecticut. These earliest crocodile relatives were primarily terrestrial, possibly because in the Triassic and early Jurassic the large aquatic ambush predator niche was filled by the superficially crocodile-like (but not closely related) phytosaurs and the giant amphibian metoposaurs.

A model of the phytosaur Rutiodon at Dinosaur State Park. Note the nostrils up near the eyes, a feature that distinguishes phytosaurs from crocodilians.
The giant temnospondyl amphibian Metoposaurus from Dinosaur State Park.

As mentioned before, a group of these crocodile relatives was actually herbivorous. This group, known as the aetosaurs, somewhat resembled the more well-known ankylosaur dinosaurs, though they were more lightly than the club-tailed dinosaurs.  It might be better to compare their appearance to that of armadillos or pangolins, though with the armor only along the back.

The Stegomus model at Dinosaur State Park is, unfortunately, hard to get a picture of since it's so small and sits in the middle of a diorama on the far side of the trackway. Instead here's a Stegomus from The Last Days of Pangea exhibit that was displayed at the Bruce Museum in Greenwich, Connecticut until July 2017. 

In 1896 the armored back carapace of a dog-sized aetosaur was discovered in Fairfield, Connecticut by the famous paleontologist Othniel Charles Marsh. This creature, dubbed Stegomus, had an upturned, pig-like snout which it may have used to root around in the soil and pull up tubers and rhizomes. A model of Stegomus can be seen in the Triassic river delta diorama at Dinosaur State Park.

A pair of juvenile Stegomus foraging for seaweed on the shore. One has encountered a curious octopus taking a short stroll on the sand and is flaring it's (hypothetical) oral flaps in a manner similar to the modern Toad-headed Agama Phrynocephalus mystaceus.

In the early 1900s, the fragmented skeleton and armored back plates of what appeared to be another species of Stegomus were discovered in a quarry of Early Jurassic sandstone near East Longmeadow, Massachusetts. These remains were later identified as those of a  predatory crurotarsan and subsequently renamed  Stegomosuchus. While the fossil is too fragmentary to make many guesses about its mode of life, the structure of its legs show that it was a  land-based hunter. It’s size and build also suggest that it might have filled the ecological niche of a small, quick predator similar to a weasel.  Some researchers have suggested that Stegomosuchus may have been the maker of Batrachopus, a type of fossil trackway found throughout the Connecticut Valley.

A pair of Stegomosuchus playing keep-away with a fern frond.
Batrachopus footprints at Dinosaur State Park.

Batrachopus with fingers for comparison.

In 1996 the partial skeleton of Protosuchus, a close relative of Stegomosuchus, was discovered in fossil-rich Early Jurassic sandstones of Nova Scotia. Although it was still a terrestrial animal, the dog-sized Protosuchus displayed many characteristics of modern-day crocodiles, including teeth in its upper jaw that fit into notches on the lower jaw and large muscle anchor points on the back part of the skull to give it a more powerful bite, suggesting that it might have taken down larger, stronger prey than its cousin would have.

Protosuchus relaxing on a pine branch while a couple of Kalligrammatid lacewings drink the salt from its tear ducts. These insects were remarkably butterfly-like in appearance, including having sucking mouthparts and wing scales.

The remains of a cat-sized crurotarsan called Erpetosuchus were discovered in 1995 in Triassic sandstones near Cheshire, Connecticut. The light, lean build and small teeth of this creature indicated that it, like Stegomosuchus,  was also a swift runner dodging through the undergrowth in pursuit of prey. Fossils of Erpetosuchus have also been found in Scotland, indicating that Western Europe and North America would have shared many common fauna when they were pushed together in the great land mass of Pangea during the Triassic.

One of the more intriguing crurotarsans  (to me, anyway) is Euscolosuchus, known only from a few fragmentary back plates, vertebrae and ribs discovered in a Virginia quarry. The armor on this creature had distinctive spines on its sides which would have given it good protection from predators. The most unique features of Euscolosuchus, though, are the spines that project backward from each section of carapace and overlap the plate behind it. When the animal was walking with its back straight, these spines would have laid flat. However, if its carapace were curled up they would have stuck out like the spikes on a hedgehog or an armadillo lizard (Ouroborus cataphractus). Though no purpose for these backward-facing spikes is mentioned in the scientific literature, I can’t help wondering if this animal would indeed have rolled up in a ball like the aforementioned hedgehog, presenting its back spines to any curious predator.

Euscolosuchus curling up into a defensive position with the spines on its carapace projecting out.

Speaking of predators- In 2015 paleontologists described Carnufex, a 9-foot long crocodile relative from North Carolina that walked on its hind legs, giving it a strikingly dinosaur-like appearance.  Carnufex, whose name means “Butcher”, would have been the top predator in its environment. Its head was adorned with numerous bumps and ridges, suggesting that it had some sort of ornamentation. Probably nothing as large and flashy as the double head-crests of Dilophosaurus, but more akin to the blunt horns of Allosaurus. This does, however, provide indirect evidence of social display in Carnufex, either to attract mates or to warn off rivals.

Carnufex getting a tooth-cleaning by a pair of furry (well, pycnofibery) Erpetosuchus.

10 million years after Carnufex, another large crurotarsan named Postosuchus dominated the forests of Triassic North America. Like Carnufex it was also bipedal and would have looked quite a bit like a theropod dinosaur. The skull of Postosuchus was, however, blunter and rounder than its predecessor and had less ornamentation, though it did have prominent ridges over the eyes that might have blocked sun glare from above.  Its large forward-facing eye-sockets indicate that Postosuchus was a visually-based predator, while it’s elongated nostrils suggest that it had a good sense of smell. Postosuchus was first discovered in 1980 in Texas with an eastern species described in 2008 from remains found in North Carolina. This specimen preserved evidence of its last few meals, which consisted of an aetosaur, two different species of mammal-like reptiles, and a medium-sized primitive amphibian.

Postosuchus surveying her territory.
Beneath the North Carolina Postosuchus researchers also found the nearly complete specimen of a smaller crurotarsan called Dromicosuchus. Bite marks on the skeleton show that it had been attacked by a predator just before it’s death- possibly by the large creature found on top of it- though it is also possible that both bodies were simply washed together in a flood. Though Dromicosuchus was badly crushed, it’s long, slender limbs indicate that it would have been another swift, agile predator perhaps taking up an ecological niche similar to a fox.

A pair of Dromicosuchus play-fighting

While dinosaurs may be the most well-known and popular Mesozoic ruling reptiles, they shared their world with a diverse company of their more crocodilian cousins and would even have, for a time, lived in the shadows of these beasts.

References

Emerson, B. K and Loomis, F. B. 1904 "On Stegomus longipes, a new reptile from the Triassic sandstones of the Connecticut Valley" American Journal of Science 17 (4) 377-380

Fraser, Nicholas and Henderson, Douglas, 2006 Dawn of the Dinosaurs: Life in the Triassic (Life of the Past). Indiana University Press, Bloomington and Indianapolis


Ksepka, Daniel T. and Dzikiewicz, Kate, 2016 "Last Days of Pangea: In the Footsteps of Dinosaurs" Bruce Museum, Greenwich, Connecticut

Liutkus-Pierce, Cynthia M.; Fraser, Nicholas C.; Heckert, Andrew B. 2014 "Stratigraphy, sedimentology, and paleontology of the Upper Triassic Solite Quarry, North Carolina and Virginia" The Geological Society of America, Field Guide 35

Lucas, Spencer G.; Heckert, Andrew B.; Huber, Philip 1998 "Aetosaurus (Archosauromorpha) from the Upper Triassic of the Newark Subgroup, Eastern United States, and its Biological Significance" Paleontology 41 (6) 1215-1230

McDonald, Nicholas G. 2010 Window Into the Jurassic World, Friends of Dinosaur State Park and Arboretum, Inc., Rocky Hill, Connecticut

Olsen, Paul E. 1998 "Paleoecology and Paleoenvironments of the Continental Early Mesozoic Newark Subgroup of Eastern North America": In Manspeizer, W. (ed.) Triassic-Jurassic Rifting and the Opening of the Atlantic Ocean, Elsevier, Amsterdam, 185-230

Olsen, Paul E.; Sues, Hans-Dieter; Norell, Mark A. 2000 "First Record of Erpetosuchus (Reptilia: Archosauria) from the Late Triassic of North America" Journal of Vertebrate Paleontology 20 (4) 633-636

Peyer, Karin; Carter, Joseph G.; Sues, Hans-Dieter; Novak, Stephanie E.; Olsen, Paul E. 2008 "A New Suchian Archosaur from the Upper Triassic of North Carolina" Journal of Vertebrate Paleontology 28 (2) 363-381

Scheyer, Torsten M. and Sues, Hans-Dieter, 2016 "Expanded Dorsal Ribs in the Triassic Pseudosuchian Reptile Euscolosuchus olseni"  Journal of Vertebrate Paleontology e1248768 

Sues, Hans-Dieter, 1992 "A Remarkable New Armored Archosaur from the Upper Triassic of Virginia" Journal of Vertebrate Paleontology 12 (2) 142-149

Sues, Hans-Dieter; Olsen, Paul E.; Carter, Joseph G.; Scott, Diane M. 2003 "A New Crocodylomorph Archosaur from the Upper Triassic of North Carolina" Journal of Vertebrate Paleontology 23 (2) 329-343

Sues, Hans-Dieter; Olsen, Paul E. 2014 "Stratigraphic and Temporal Context and Faunal Diversity of Permian-Jurassic Continental Tetrapod Assemblages from the Fundy Rift Basin, Eastern Canada" Atlantic Geology 51 139-205

Zanno, Lindsay E.; Drymala, Susan; Nesbitt, Sterling J.; Schneider, Vincent P. 2015 "Early Crocodylomorph Increases Top Tier Predator Diversity During the Rise of Dinosaurs" Scientific Reports 5: 9276
 

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