Sunday, December 11, 2016

Fossil Crocs of the Science Museum

Although the dinosaurs get the most press, the Science Museum of Minnesota's paleontology department has a strongly diversified portfolio of ancient reptiles. In particular, champsosaurs and crocodyliforms (modern crocodilians and their closest extinct relatives) are well-represented in the collections and on exhibit. The Science Museum features mounts of four fossil crocs, each one filling different places in their ecosystems: an apex terrestrial/freshwater croc (Borealosuchus formidabilis), a large estuarine croc with elongate jaws living alongside whales (Gavialosuchus carolinensis), a medium-sized terrestrial/freshwater croc from a dinosaur-dominated setting ("Goniopholis"), and a house-pet-sized terrestrial/freshwater generalist (Wannaganosuchus brachymanus). This diversity shouldn't be too surprising. Crocodylomorphs, with around 225 million years of time on their hands, have done quite a bit of experimentation, including truly marine thalattosuchians and small, terrestrial, occasionally dinosaur-like "sphenosuchians".

Borealosuchus formidabilis

After the end-Cretaceous extinction, the stage was temporarily cleared of large land animals. Crocs, being the largest land animals left, flourished. Borealosuchus formidabilis (aka Leidyosuchus) was one of the beneficiaries. It is the largest known land animal of the Fort Union Formation/Group (we've already mentioned that terminological crack). B. formidabilis is comparable in size to the modern American crocodile or alligator at around 10-13 ft (3-4 m) long. You can see below that it bears a reasonable resemblance to those two, although there are a couple of significant differences. Its arms are relatively long, suggesting it was somewhat more capable on land than most of its modern cousins. Also, although you can't tell it from the photo, this species had very well-developed bony armor. With an apparent lack of other competition and an abundance of B. formidabilis, the scutes may have functioned to protect B. formidabilis from B. formidabilis.

Above: the bones of an unusually common sight in western North Dakota, 60 million years ago.

The museum's supply of Borealosuchus came from Wannagan Creek, where it is particularly abundant; several dozen individuals have been reported from the site, which was also stocked with amphibians, turtles, champsosaurs, a little alligatorid we'll meet below, some ambitious post-Cretaceous mammals, and others, plus over a hundred plant taxa. This community existed about 60 million years ago.

Not pictured: the armor and the other approximately 80 B. formidabilis that came with it.

If you were to propose a fossil mascot for the Science Museum, the Triceratops would probably be the public choice, but B. formidabilis wouldn't be a bad alternative, given its abundance in the collections and the museum's long association with the Wannagan Creek site.

Gavialosuchus (or Thecachampsa) carolinensis

Gavialosuchus carolinensis is the biggest of the mounted crocs, at a little more than 17 ft long (a bit less than 5.5 m). For comparison, this is longer than all but the longest American alligators and crocodiles, but smaller than the longest saltwater crocodiles. It was an estuarine species from the Oligocene of South Carolina, and makes a pretty impressive mount. Our friend comes equipped with long narrow jaws, featuring rather large conical teeth.

It just wants to be pals.

In recent years, Gavialosuchus carolinensis has often been assigned to Thecachampsa, which unfortunately is one of those mid-19th century names given to a single reptilian tooth, so some sort of further reassignment would not be surprising.

If you're disappointed in Thecachampsa, think how it feels for G. carolinensis!

This crocodilian comes from the Chandler Bridge Formation. Many large vertebrates have been found in this formation in pits and depressions at the contact with the underlying Ashley Formation, where phosphatic nodules dissolved. The Chandler Bridge Formation also includes sharks, rays, bony fish, sea turtles, pseudodontorns (false toothed birds), seals, sea cows, and whales.


The Morrison Formation is famous for its dinosaurs, but it is host to many other kinds of animals as well. Crocodylomorphs and turtles are notable components of the fauna. Traditionally, the medium-sized croc of the Morrison has been assigned to Goniopholis, but over the past few years this genus has been restricted to Europe. Amphicotylus has taken over its duties.

Two for the price of one: a skeletal mount and a cast of the original slab, helping to show what went into the work.

The original specimen was a bit of a roadkill, partially articulated and partially not. You can see a long stretch of articulated vertebrae, the underside of the skull, and, near the top, a patch of scutes from the animal's belly.

"Goniopholis" is often a throw-in to Morrison Formation scenes, usually just kind of hanging around in the background doing something appropriately crocodilian. Although it is common to think of ancient crocs as looking basically like the modern varieties, especially if they are thought to have led similar lives, this is not the case. "Goniopholis", for example, not only has the extensive belly armor, but its dorsal armor is arranged in two parallel rows of rectangular scutes along the midline.

You may notice a difference in ambiance; this photo was taken a few years before most of the others on this page, but it certainly illustrates the dorsal armor.

Wannaganosuchus brachymanus

Wannaganosuchus is the other Wannagan crocodilian. You may spot some obvious differences from Borealosuchus. Most notably, it is much smaller. No need to worry about Wannaganosuchus getting too big for the house and having to surreptitiously release it into the wild.

It's also in an inset case, so not the most accessible for photography, either.

Wannaganosuchus is a representative of an early radiation of small generalist alligatorids. The teeth vary depending where they are in the jaws: the anterior teeth are more pointed and the posterior teeth are more blunt. Even as a miniature gator, the broad rounded snout is apparent.

You can get a sense of the broad, snub-nosed face in this photo. Not quite as obvious in this shot are the different types of teeth.

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