Repost: How Tylosaurus Lost Its Fringe, and Other Squamate Stories

[This post was originally published on December 2, 2011]

‘Tho the death of a cherished childhood image makes folks twinge
Truth is, the great Tylosaurus had no fringe

As far as my younger, fossil-philic self was concerned, there was never a more terrible marine predator than Tylosaurus. This enormous, sea-going lizard was a true leviathan and the undoubted ruler of the ancient oceans. That impression came almost entirely from a painting renowned paleo-artist Charles R. Knight created for Chicago’s Field Museum.

Suspended within the swell of a Late Cretaceous sea, a grinning Tylosaurus looms over a fleeing marine turtle known to paleontologists as Protostega. A flock of Pteranodon hang in the background – no doubt waiting to pluck bloody turtle bits out of the sea once the inevitable carnage has ended. The violence was all in my imagination, but the sly smile of the enormous mosasaur led me to believe that the poor Protostega had little chance of escape.

I certainly wasn’t the first to imagine such scenes from a time when a shallow sea encroached over what is now the middle of North America. Charles H. Sternberg – a skilled 19th century fossil hunter who uncovered quite a number of marine reptiles from the marine strata of Kansas – daydreamed about the power of mosasaurs in his autobiography Life of a Fossil Hunter. Sternberg envisioned one such creature lolling about in the warm water, only to be rudely interrupted by one of its own kind:

But see! an enemy in the distance is attracting our reptile’s attention. It sets its four powerful paddles in motion, and unrolling its forked tongue from beneath its windpipe, throws it forward with a threatening hiss, the only note of defiance it can raise. The flexible body and long eel-like tail set up their serpentine motion, and the vast mass of animal life, over thirty feet in length, rushes forward with ever-increasing speed through water that foams away on either side and gurgles in a long wake behind.

The great creature strikes its opponent with the impact of a racing yacht and piercing heart and lungs with its powerful ram, leaves a bleeding wreck upon the water. Then raising its head and fore paddles into the air, it bids defiance to the whole brute creation, of which it is monarch.

If only Knight had painted that scene, as well! Yet there was something other than the imposing nature of Knight’s Tylosaurus that struck me. From the back of the head to the tip of the tail, the great mosasaur was decorated by a long ribbon of fringe. This seemed to be standard for the large mosasaurs which swam through the guides to prehistoric life I rapidly devoured in the school and local libraries. Then, without explanation, the fringe was unceremoniously ripped from the mosasaur’s backs. I had to admit that the mosasaurs looked better without the long, flimsy frill, but I had no idea why they had been stripped of their decoration.

Paleontologists already knew that mosasaurs didn’t carry delicate fringe by the time I was flipping through the outdated library books. The meme went back to a simple mistake which was popularized and repeatedly copied despite a relatively quick correction. Marine reptile expert Mike Everhart figured out what happened.

In 1898, Samuel Wendell Williston – America’s foremost authority on fossil marine reptiles – described an exceptional specimen of the mosasaur Platecarpus ictericus which he thought preserved some hitherto-unknown structures. There seemed to be a row of “dermal processes” which stretched from the back of the head to the middle of the body, at the very least, and Williston supposed that these formed “a thick fringe or mane … resembling very much the thongs along the legs of buckskin trowsers.”

American Museum of Natural History paleontologist Henry Fairfield Osborn found similar structures at about the same time. The features had been preserved in a near-complete specimen of Tylosaurus he was in the process of describing and mounting in his New York institution. Osborn correctly identified them as rings of cartilage from the animal’s trachea, but in his 1899 description of the Tylosaurus specimen Osborn – citing Williston’s hypothesis mosasaurs had a “nuchal fringe” – directed artist Charles R. Knight to give the Tylosaurus a mane in a life restoration. Osborn apparently didn’t realize that Williston’s fringe was really a string of cartilaginous throat rings similar to what Osborn himself had just described. Knight carried over the idea for more dramatic paintings of Tylosaurus in action for the AMNH and Field Museum.

Williston soon realized that he made a mistake. In a brief paper “Notes on some new or little-known extinct reptiles” he chided himself for not identifying the tracheal rings for what they were:

In conclusion, I desire to correct an error made by myself. That which I considered the nuchal fringe in the mosasaurs is evidently only the slender cartilaginous rings of the trachea, first described and figured by Professor Osborn. I have no excuse to make for the mistake, which I recognized when too late to correct.

Knight actually took note of this development. As modern-day expert mosasaur-painter Dan Varner has pointed out, Knight updated the painting he created for the AMNH by removing the fringe in a revised version. The trouble was that the image of the maned Tylosaurus was already entrenched and continued to be popularized for as long as his early works were reproduced in books and museum displays.

I don’t especially miss the mosasaur mane – the marine predators look just as fearsome without the ornament – but the dorsal decoration made sense to my younger self. Tylosaurus and other mosasaurs were oceanic apex predators that undoubtedly had to move fast to catch prey. Maybe that fringe stabilized the reptiles while swimming and kept them from rolling while in pursuit, I speculated, like the dorsal fins of sharks do. It’s a moot point now – mosasaurs didn’t have manes – but one would expect creatures that spent their entire lives at sea to have some adaptive accoutrements related to swimming. In fact, paleontologists have recently uncovered a suite of evidence which suggests that some of the last mosasaurs were far more specialized for their aquatic existence than had previously been understood.

Thanks to their kinship with monitors and komodo dragons, mosasaurs have often been portrayed as giant, seagoing lizards with feet adapted into paddles and a relatively broad tail. As a result, paleontologists hypothesized, mosasaurs probably moved slow most of the time and relied on brief bursts of speed to ambush prey. Thanks to some exceptionally-preserved specimens of the mosasaurs Plotosaurus and Platecarpus, however, it now seems that some mosasaurs had kinks near the ends of their tails. Paleontologists have seen this before – ichthyosaurs have downward kinks in their tails, too, and sharks have similar upward kinks. These tailbends are support structures for big, fleshy fins, and so it appears that the tails of Plotosaurus and Platecarpus actually supported upper fin lobes akin to those seen among ichthyosaurs and some marine crocodiles. This might mean that these mosasaurs were capable of open-ocean cruising and even greater spurts of speed – mosasaurs were not constrained to only being near-shore ambush hunters.

A new paper published in PLoS One by paleontologists Johan Lindgren, Michael Everhart, and Michael Caldwell add a few more wrinkles to the story. This time, the focus is mosasaur skin. Remnants and impressions of mosasaur skin have been found before – the Platecarpus ictericus Williston studied, for one, was preserved with scale impressions and showed the outline of one of the animal’s paddles. But most of these soft tissue traces are very patchy, cover a small part of the body, and some have even destroyed in the process of preparing specimens. Our understanding of what the scales of mosasaurs looked like is relatively limited, but the specimen of Ectenosaurus described by Lindgren, Everhart, and Caldwell illustrations that the aquatic adaptations of mosasaurs went down to the creature’s body covering.

The mosasaur at the center of the new study is not a new specimen. This Ectenosaurus was discovered among the Upper Cretaceous Niobrara Chalk of Kansas in 1953 by George F. Sternberg, one of the sons of Charles. The front half of the skeleton – including a beautiful, elongated skull – was intact, and previous work on the fossil noted some vestiges of the animal’s skin. Until now, though, no one knew exactly how intricately this mosasaur’s integument had been preserved. Not only were there remnants of the outer layer of scales which had become phosphatized through fossilization, but parts of the underlying skin were also etched in the chalk.

Lindgren and co-authors state that the soft tissue fossils were found among 23 slabs collected from around the neck area of the Ectenosaurus. For the most part, the two millimeter-wide scales are rhomboid or oval in shape, but what is truly remarkable is that each scale is effectively split into two halves by a small ridge which runs along the midline. Furthermore, there are curious bundles of preserved tissue associated with the scales which they researchers interpret as being fiber which formed part of the underlying layer of skin.

Compared to other mosasaurs, Ectenosaurus had small scales. At the moment, though, it is unclear whether the entire animal was covered in these small structures or whether the size has something to do with their placement near the neck. Even so, the fact that the scales are clearly keeled may contribute to the general picture of how mosasaurs became adapted as aquatic predators.

Lindgren, Everhart, and Caldwell characterize Ectenosaurus as a fish-eater on the basis of the animal’s “narrow, elongate skull and slender teeth.” Such a predator would have to move quickly to catch fast-moving prey, and the keeled scales would have channeled water in such a way as to reduce drag on the animal’s body (much the way that the dermal denticles which cover the bodies of sharks do). More than that, the researchers propose that the presumed fiber bundles found among the scales would have strengthened certain parts of the skin and kept it from creasing, thereby keeping a smooth, streamlined surface. In turn, Lindgren and collaborators suggest that these strengthening, streamlining features indicate that Ectenosaurus kept the front of its body rigid while swimming – the mosasaur may have primarily beat its tail to propel itself rather than undulating like an eel, catshark, or snake.

Some of these hypotheses will require further evidence to test. If Lindgren and colleagues are correct that Ectenosaurus kept the fore-part of its body rigid, then we would expect that soft tissue structures from the upper torso would show similar strengthening in the skin and that the tail should have some sort of tail-bend to accommodate a broad fin for propulsion. Frustratingly, the back half of the Ectenosaurus in question was not recovered and the soft tissue portions appear to come from just one section of the body.

Nevertheless, the keeled scales of Ectenosaurus – as well as those previously found with Plotosaurus and other mosasaurs – indicate that mosasaurs were not just big lizards that could swim. Millions of years in the sea shaped their anatomy, from their paddle-shaped limbs to the detailed anatomy of their scales. Their ecological role as top oceanic predators has always been apparent, but we are only just starting to understand how evolutionary pressures fine-tuned these frightening reptiles.

[For more on mosasaurs, see my previous posts on their peculiar mode of swimming and the question of whether they had forked tongues.]

Top Image: A fringed Tylosaurus as envisioned by Charles R. Knight. From Osborn 1899.

References:

Everhart, M. Origin of the Dorsal Fringe on Mosasaurs. Accessed 12/2/2011

Lindgren, J., Caldwell, M., Konishi, T., & Chiappe, L. (2010). Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil Mosasaur PLoS ONE, 5 (8) DOI: 10.1371/journal.pone.0011998

Lindgren, J., Everhart, M., & Caldwell, M. (2011). Three-Dimensionally Preserved Integument Reveals Hydrodynamic Adaptations in the Extinct Marine Lizard Ectenosaurus (Reptilia, Mosasauridae) PLoS ONE, 6 (11) DOI: 10.1371/journal.pone.0027343

Osborn, H. 1899. A complete mosasaur skeleton, osseous and cartilaginous. Memoirs of the American Museum of Natural History. I (IV) 165-188

Sternberg, C. 1909. The Life of a Fossil Hunter. New York: Henry Holt and Company. p. 49

Williston, S. 1898. Editorial Notes. Kansas University Quarterly 7(4): 235.

Williston, S. 1902. Notes on some new or little-known extinct reptiles. Kansas University Science Bulletin, 1(9): 247-254, 2 pl.