Mosasaur Bites and Limpet Scrapes

ByRiley Black
April 11, 2012
13 min read

For a little while, Mosasaurus maximus was my labmate. The sharp-toothed marine predator was an inoffensive neighbor, though — no doubt because only the fossilized bones, coated in beeswax to stop the destructive creep of pyrite disease, remained.

The mosasaur’s skull sat in the prep lab of the New Jersey State Museum. During my time as a volunteer there, I could hardly walk by the skull without slowing my pace to admire the marine lizard’s 66-million-year-old bones. I was especially fascinated by the set of secondary teeth nestled between the mosasaur’s upper jaws – short rows of sharp teeth jutting from the pterygoid bones near the back of the mouth. These small, sharp teeth pierced and held prey as the mosasaur maneuvered its flexible jaws into a better biting position. Given the long rows of larger teeth along the outer jaws, the smaller set seemed like a final insult to whatever prey might find itself sliding down the mosasaur’s gullet.

The skull of Mosasaurus maximus in the collections of the New Jersey State Museum. Photo by the author.

Thanks in no small part to their frightening jaws, mosasaurs have been dubbed the aquatic equivalent of Tyrannosaurus rex. (A bizarre IMAX documentary called Sea Rex makes the connection up front.) And, as with that most famous tyrant dinosaur, the answer to “What did mosasaurs eat?” would seem to be “Anything they wanted.”  As marine reptile expert Mike Everhart cataloged in a tribute to mosasaurs on his Oceans of Kansas website, fossilized gut contents and a few rare, tooth-raked specimens have shown that mosasaurs ate seabirds, fish, plesiosaurs, and even mosasaurs smaller than themselves. Some forms – such as the blunt-toothed genus Globidens – were specialists that crushed clams and other hard-shelled prey, but, for the most part, mosasaurs have been celebrated as predators so powerful and insatiable that they ate just about anything they could catch.

But there was one mosasaur menu item that kicked up a long-running debate among paleontologists. I was reminded of the argument when I visited the San Diego Natural History Museum. William Stout’s fantastic paleo-murals, as well as the life-size dinosaur sculptures, had drawn me to the exhibits, and a smattering of fossils in the gallery represented the various creatures Stout had restored. In one glass case, set below a painting of Cretaceous sea life, there sat a large ammonite with a set of deep punctures running along the shell’s diameter. This coil-shelled cephalopod – a distant, prehistoric cousin of the modern day nautilus – had been killed by a large predator. The two rows of round holes converged at one end to create a triangular shape, and the symmetry of the pattern – the display explained – was the hallmark of a mosasaur.

The left side of a Placenticeras shell with mosasaur bite marks. A line drawing of a mosasaur upper jaw is superimposed to show the details of the eighth detectable bite on the ammonite. Image from Kauffman and Kesling, 1960.

The San Diego shell wasn’t the first of its kind to pique the interest of paleontologists. In 1960, Erle Kauffman and Robert Kesling described a peculiar shell of the Cretaceous ammonite Placenticeras that had been figured in another monograph two years before. A series of circular punctures in the cephalopod’s outer shell, Kauffman and Kesling wrote, showed that “The shell was bitten repeatedly, and bears dramatic evidence of the fatal encounter.”

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There was no doubt about the attacker’s identity. The puncture pattern showed that the predator had a secondary set of pterygoid teeth, and, out of the possible carnivores of the era, only mosasaurs possessed such weaponry. The mosasaur bit the shell up to sixteen times, the paleontologists determined, and the damage probably “disengaged the muscles by which the body of the ammonite was held in the shell”, leaving the soft-bodied invertebrate to be slurped up by the mosasaur. And the constellation of bite marks, Kauffman and Kesling pointed out, suggested that the mosasaur was an experienced ammonite hunter. The mosasaur apparently attacked from above and tried to swallow the ammonite whole. When that failed, the mosasaur crushed the shell to get at the soft inner parts.

As paleontologists continued to collect ammonites from deposits laid down when North America was bifurcated by a warm Cretaceous sea, dozens of similarly-damaged cephalopod shells turned up. For genera such as Prognathodon, at least, ammonites seemed to be a regular part of the mosasaur diet. But, in 1998, paleontologist Tomoki Case and collaborators proposed that mosasaurs might not have been crushing ammonite shells, after all. The numerous holes on multiple ammonite shells were not bite marks, they said, but traces left behind by other mollusks.

Limpets were the culprits. These small, marine snails were found affixed to a large ammonite discovered in South Dakota, and it seemed that these little grazers lived on the discarded cephalopod shell. With their tough, scraping mouthparts, the limpets created depressions in the ammonite shell called “home scars” – holes where the gastropods could return after foraging for algae and be relatively secure. And microscopic scratches, created by feeding limpets, indicated that the small invertebrates really did live on the shell.

To further drive their point home, though, Case and co-authors crushed modern nautilus shells in a set of robotic mosasaur jaws – fashioned out of steel with brass teeth – to see if the marine reptiles were capable of such destruction. Hard bites obliterated the shells, while more restrained bites seemed to superficially produce the same kind of damage seen on fossil shells. Aggregations of limpets, spaced out in rows, seemed to better account for the holes, and the way the ammonite shells were buried and fossilized created the crushed edges around each hole previously attributed to mosasaur teeth. This wasn’t to say that mosasaurs never ate ammonites, though. “We do not deny that mosasaurs may have preyed on ammonites by crushing,” the researchers wrote, and they pointed out that  “Any that escaped swallowing had the potential to be preserved as fossils.” But such specimens had not yet turned up. All the ammonites that had supposedly been bitten by mosasaurs, Case and collaborators asserted, had really been hosts to limpets.

Of course, that paper wasn’t the last word on the subject. In 2001, paleontologists Cameron Tsujita and Gerd Westermann reexamined the evidence and concluded that tooth marks on ammonite shells were much more common than damage done on empty shells by limpets. After examining over 150 Placenticeras shells from Alberta, Tsujita and Westermann found that the various indentations and punctures in the ammonite shells more closely matched what would be expected of large mosasaurs rather than small, algae-eating limpets.

If the holes were limpet home scars, Tsujita and Westermann noted, the punctures should have been more uniform in shape since the limpets would have scraped away at the ammonite shell in a consistent way. The variation from one puncture to the next was more consistent with damage inflicted by different-sized mosasaur teeth. And the idea that the punctures were home scars didn’t match observations of modern limpets. If the limpets scraped depressions in the already-thin ammonite shells, they would have winnowed out the floor from beneath themselves. An ammonite shell was a rather poor substrate to dig a home into. Nor were Tsujita and Westermann taken with the robotic mosasaur tests. The structure of the modern nautilus shells was not as close to that of prehistoric ammonites as Case and colleagues suggested, and the robotic mosasaur model was too simple and inflexible.

Tsujita and Westermann also cataloged the positive evidence that mosasaurs bit ammonites. In almost every injured ammonite specimen, the researchers pointed out, the cephalopod’s body chamber was missing. This pattern of damage was so consistent that it could not have been just a quirk of preservation – the mosasaurs were biting the ammonites in such a way as to expose the tentacled creature’s soft body. More than that, rare ammonite specimens showed signs of healing. These individuals had been attacked by mosasaurs in the same way, but had survived, and the pathological sections of their shells indicated a quick strike by a large predator rather than a gradual scraping by limpets. But the most compelling line of evidence was the simplest – the bite marks were arranged in the same pattern as the mosasaur tooth row. There was no good reason to believe that limpets, time and again, lined themselves up in this V-shaped fashion. A few ammonites were colonized by limpets when the shells fell to the sea floor, but only after those ammonites had already been crushed and killed by mosasaurs.

The skeleton of the mosasaur Plotosaurus, on display at the Natural History Museum of Los Angeles. Photo by the author.

As Kauffman and Kesling had proposed years before, the attacking mosasaurs had a specific ammonite-pulverizing technique. By biting ammonite shells relatively gently – enough to weaken them and break open the body chamber where the cephalopod’s soft parts actually resided – the mosasaurs could extricate their prey while ingesting a minimum of shell pieces. Rather than simply crush ammonites to a pulp, mosasaurs appear to have targeted specific parts of ammonites to access the squishy mollusk bodies.

Then Erle Kauffman drew on a different sort of cephalopod shell to suggest something even more spectacular. Not only did mosasaurs have a particular hunting strategy, but they might have passed down the technique to their offspring. In a 2004 PALAIOS paper, Kauffman called attention to the damaged shell of a nautiloid – another shelled cephalopod more closely related to the modern nautilus. The partial shell appears to have been bitten by two mosasaurs of different size – an adult and a juvenile. Kauffman rejected the idea that the smaller indentations were made by the pterygoid teeth of a big individual. The spacing and shape of the toothmarks was wrong for such a scenario. Instead, Kauffman saw the smaller indentations as the bite of a young mosasaur. The larger specimen bit the shell soon after, since the larger bite overlapped the smaller one.

“The most obvious interpretation is that there were two mosasaurs, one juvenile specimen and one adult specimen belonging to the same species,” Kauffman wrote, “and that the smaller one was being trained that cephalopods were an alternate food source.” A fascinating idea, but not necessarily the most obvious. Perhaps the smaller mosasaur caught the nautiloid, but was forced to give up its prey when a larger mosasaur approached. (After all, big mosasaurs preyed on smaller ones, and so a juvenile would have much to fear from an adult.) And there’s no indication of how much time passed between the time of the bites. The succession was probably quick – the shell doesn’t show any sign  that the nautiloid survived the initial attack and healed. All that we can say for sure is that two mosasaurs bit the same shell soon after each other. What their relationship was to each other, and whether mosasaurs provided any sort of parental care, is unknown. All the same, mosasaurs were not just mindless eating machines which blindly snapped at anything which floated by. These marine equivalents of Komodo dragons developed a deadly strategy to intricately and brutally take ammonites apart.

References:

Case, T., Johnston, P.A., Seilacher, A., Boyce, J.B. 1998. Alleged mosasaur bite marks on Late Cretaceous ammonites are limpet (patellogastropod) home scars.  Geology. 26 (10), 947-950

Kauffman, E.G. 2004. Mosasaur predation on Upper Cretaceous nautiloids and ammonites from the United States Pacific Coast. PALAIOS. 19, 96-100

Kauffman, E.G., Kesling, R.V. 1960. An upper Cretaceous ammonite bitten by a mosasaur. Contributions From the Museum of Paleontology: The University of Michigan. XV (9), 193-248

Seilacher, A. 1998. Mosasaurs, limpets, or diagenesis: How Placenticeras shells got punctured. Fossil Record. 1 (1), 93-102

Tsujita, C.J., Westermann, G.E.G. 2001. Were limpets or mosasaurs responsible for the perforations in the ammonite Placenticeras? Palaeogeographyc, Palaeoclimatology, Palaeoecology. 169, 245-270

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