Mosasaurs: Masters of the Bronx Cheer

[Author’s Note: After months of fieldwork, museum visits, and other research, A Date With a Dinosaur is finally coming together. And not a moment too soon – my deadline is rapidly approaching. New essays will continue to surface here, but I’m also going to dredge up some favorite posts from years past to help keep things going while I get the book into shape. Today’s essay, originally posted in August of 2011, is about the tongues of the magnificent mosasaurs.]

Fastened to the wall of the College of Eastern Utah’s Prehistoric Museum, there’s an Allosaurus doing an excellent Gene Simmons impression. The bust was created by David A. Thomas – perhaps best known for his Albertosaurus and Pentaceratops mounts at the New Mexico Museum of Natural History – and he gave the Jurassic predator a frozen rictus in which a forked tongue flails over the recurved teeth of the dinosaur’s lower jaw.

All I could think of when I saw the sculpture was “I sure hope that Allosaurus doesn’t bite its tongue!” That’s probably because of Michael Crichton. In his novel Jurassic Park – a techno-phobic fable featuring resurrected dinosaurs run amok – Crichton gave his Tyrannosaurus a similar, prehensile tongue. The terrifying organ comes into play when the theropod has park visitors Lex and Tim hopelessly pinned behind a waterfall:

With a low growl, the jaws slowly opened, and the tongue snaked out. It was thick and blue-black, with a little forked indentation at the tip. It was four feet long, and easily reached back to the far wall of the recess. The tongue slid with a rasping scrape over the filter cylinders.

When sedatives injected into the dinosaur kick in, the razor jaws close down on the tongue and sever it in a spurt of dark blood.

But it’s unlikely that Allosaurus, Tyrannosaurus, or any other dinosaur had a prehensile, forked tongue. All you have to do is look at living birds and crocodylians to understand why. Although all the non-avian dinosaur lineages were extinguished about 66 million years ago, the descendants of dinosaurs (birds) and their distant cousins (alligators and crocodiles) remain. Together these creatures compose what’s known as an “extant phylogenetic bracket” – evolutionary bookends which can be studied to see which traits different lineages shared in common, and, therefore, provide a basis for reconstructing aspects of extinct organisms that can’t be studied directly. Since birds don’t have snake-like tongues, and neither do crocodylians, then there’s no reason to think that dinosaurs did. Sorry, David and Michael.

There was one group of formidable prehistoric predators which probably flicked forked tongues, though. We don’t know this from direct fossil evidence – unsurprisingly, tongues don’t preserve well in the fossil record – but because of the same sort of evolutionary reasoning.

Mosasaurs were among the earliest fossil celebrities. Though their bones were initially confused for those of big fish during the mid-18th century, by the beginning of the 19th mosasaurs had been recognized as enormous seagoing predators closely related to monitor lizards. (It was around this time that the French paleontologist Georges Cuvier was promulgating his idea that species could go extinct and that the life of the past was very different than that of the present era. Mosasaurus, the giant ground sloth Megatherium, the mammoth of Siberia, and the American mastodon gave him some wonderful, alliterative evidence.)

Given the close skeletal similarities between mosasaurs and monitor lizards, as well as the resemblances both groups shared with snakes, it seemed reasonable to reconstruct mosasaurs with forked tongues. In an illustration accompanying Edward Drinker Cope’s 1869 paper “Fossil Reptiles of New Jersey” about the Cretaceous era in the Garden State, a goofy-looking mosasaur dangles a forked tongue out of a toothy grimace. Likewise, art included in the work of marine reptile expert Samuel Wendell Williston often depicted mosasaurs with bifurcated tongues. No giant, aquatic monitor lizard seemed complete without a forked tongue dangling in the Cretaceous air.

Not everyone followed in the footsteps of these early restorations. More recent, late 20th- and early 21st century depictions of mosasaurs may have forked or unforked tongues depending on the vision of the artist. Given this state of disagreement, in 2002 paleontologists A.S. Schulp, E.W.A Mulder, and K. Schwenk took another look at the question to see if they could do any better in determining just what sort of tongues mosasaurs had. This is one of the reasons I adore paleontology – where else can you find scientists seriously considering the tongue anatomy of extinct sea monsters?

The precise relationship of mosasaurs to monitor lizards and snakes is controversial. Fossil evidence and molecular studies of living lizards and snakes have shifted the affinities of the reptilian lineages multiple times. At present, it appears that mosasaurs are more closely related to monitor lizards than to snakes, although both groups contribute to a bracket of fork-tongued reptiles which informs our expectations about mosasaurs.

As pointed out by Schulp and colleagues, almost all lizards and snakes have a notched tongue. What makes the difference is how far that notch goes. In the closest living relatives of mosasaurs, the notch is deep enough to create a fork. So far, so good, but can we get any more specific about the tongue’s anatomy? That’s difficult to do without a living mosasaur, especially since there’s a problematic critter called the earless monitor lizard (Lanthanotus). This lizard is a close relative of true monitors, yet the fact that its tongue is not as deeply forked as in snakes or true monitor lizards may indicate that the similar tongues of both groups evolved independently from each other rather than being a shared trait inherited from a common ancestor. If this is the case, then a detailed anatomical picture of mosasaur tongues becomes especially difficult to draw – the independent evolution of similar characteristics muddies the picture.

Still, the tongues of mosasaurs were probably forked to some degree. Perhaps, Schulp and co-authors argue, they had tongues like the earless monitor or even the infamous Gila monster in which the front of the tongue was a bifurcated chemosensor and the back of the tongue was a thick, papillae-covered organ used to slide food back into the throat. Despite this uncertainty about anatomy, however, the paleontologists were confident that mosasaurs used their tongues to detect chemical traces in their aquatic environment. Monitor lizards and snakes flick their tongues to pick up such signals, and this behavior is associated with a pair of small openings in a set of skull bones called the vomers. Mosasaurs had these fenestrae, too, and brain casts of the marine reptiles appear to indicate that a significant portion of the brain was associated with detecting smells. To pick up these cues, mosasaurs would have flicked their tongues out into the water, which Schulp and colleagues reconstruct like this:

During a tongue-flick, the protruded part of the tongue would have appeared relatively slender because of the independent, extensile nature of the foretongue, but the broad, fleshy base of the tongue would have remained within the mouth.

In other words, mosasaurs perfected the Bronx Cheer over 93 million years ago.

Schulp, Mulder, and Schwenk were thinking about mosasaur tongues in general terms, but I have to wonder how tongue anatomy would have varied from one genus to the next. After all, mosasaurs were a diverse and relatively long-lived group of marine apex predators, and not all of them fed in the same way. The strange mosasaur Globidens, for one, had a mouth full of rounded teeth suited to crushing hard-shelled prey. If Schulp and co-authors are correct that the anatomy of mosasaur tongues may have been a trade-off between detecting chemical signals in the water and retaining a feeding function, would the different diets of disparate mosasaur species have influenced the shape of their tongues? If only there were a few surviving species to study. Then I could simply say “Open wide and say ‘Ahhh’”, though hopefully at a safe distance.

Top Image: Close-up of the Allosaurus sculpture created by David A. Thomas at the CEU Prehistoric Museum in Price, Utah. Photo by author.

References:

Schulp, A.; Mulder, E.; Schwenk, K. (2002). Did mosasaurs have forked tongues? Netherlands Journal of Geosciences, 84 (3), 359-371