Things that are not dinosaurs: Mosasaurs

Discovery of the "grand animal", Mosasaurus hofmani, at Maastricht.  Engraving by G. R. Levillaire, from Wikimedia Commons.

Mosasaurs were a highly successful group of marine squamates that came to rule the seas towards the end of the Late Cretaceous (98-65 million years ago).  Remains of these real life leviathans were some of the first (if not the first) sauropsid fossils ever discovered.  Like their land dwelling cousins, they were air breathers, and secondarily returned to the sea where they became highly adapted to their new marine environment.  They were powerful, streamlined swimmers with long narrow bodies, paddle-like limbs and tails, and gave birth to live young.  Based on an abundance of fossils and a number of different taxa discovered, we have learned a lot about mosasaurs in the more than 200 years since they were first discovered.  This has helped shape a new view of mosasaurs that is radically different from those of the past.

A cladogram showing the relationship between selected diapsid groups.  As you can see, mosasaurs are not dinosaurs.

I give people grief for calling pterosaurs dinosaurs, but at least they are both kinds of archosaurs.  Mosasaurs, on the other hand, are on a completely different branch of the diapsid family tree, as you can see in the obligatory MS Paint cladogram above. Despite being shown alongside dinosaurs on the pages of many books over the years, mosasaurids, are in fact, more closely related to "lizards" and snakes (Squamata), specifically falling within Varanoidea as a close relation to the Komodo dragon and other "monitor lizards". This is a fairly well known fact, and has been for quite a while. Their reduced limbs and loosely-hinged jaws with four rows of teeth caused Edward Drinker Cope to suggest that they were closely related to snakes, and that they shared a concestor and united them within "Pythonomorpha".  While not that closely related, they are both scleroglossans, so I have to give him credit for noticing the family resemblance.  If nothing else, he knew they were not dinosaurs.

A life restoration of Tylosaurus, by Charles R. Knight, from Wikimedia Commons.

The image of mosasaurs that most of us have from our childhood is probably like the one seen above.  Giant sea lizards with dorsal fringes (we now know they were misidentified tracheal cartilage that became something of a meme for their day) who slowly cruised through the warm Cretaceous seas, long bodies undulating laterally as they snatched up anything they came across in their jaws.  Their tails, aside from being highly dorsoventrally compressed, were not all that impressive or spectacular in most regards, and they moved with a simple  anguilliform style of locomotion.  Only the most derived mosasaurs, like Plotosaurus, had a more streamlined fusiform body, the corresponding carangiform style of locomotion, and a crescent shaped caudal fin (like those seen in sharks) that made them much faster swimmers, and generally better suited for an aquatic lifestyle than their other mosasaur cousins (Lindgren et al. 2007).  But a paper was published a little under a year ago that described an extremely well preserved specimen of Platecarpus tympaniticus (LACM 128319) that put Plotosaurus to shame (Lindgren et al. 2010). It preserved skin impressions (with possible pigmentation), a partial body outline, various organ tissues (from the eye, respiratory system, digestive tract, and other viscera), and a downturned section of caudal vertebra with downturned neural spines that indicate it probably had a caudal fluke with two lobes.  Given that Platecarpus lived around 20 million years before Plotosaurus, and is not considered to be as highly derived, it has led many to hypothesize that some (if not most) mosasaurs may have had a tail fluke they inherited from a common ancestor, and were probably faster, more powerful swimmers than traditionally presumed.  If you think that tail flukes in marine diapsids seem strange, well, think again.  They are known to have evolved convergently in two other groups of Mesozoic sauropsids: the ichthyosaurs and the metriorhynchids (two other groups of "not dinosaurs").

An updated life restoration of Platecarpus, using the skeletal reconstruction of  LACM 128319 as a reference.  Created by Dmitry Bogdanov, from Wikimedia Commons.

As if that weren't cool enough, just last Friday, another paper was published in PLos One on mosasaurs.  This time, the mosasaur being discussed was Prognathadon, but no one was really concerned with its tail.  The authors were interested in its proteins.  That's right, I said proteins.  Lindgren was at it again, this time using synchroton radiation-based infared microspectroscopy (a way to study and identify the chemicals in something) on a 70 million year old mosasaur bone (2011). The authors of the paper found that type I collagen, the most abundant protein in bone, managed to survive over the millennia, proving once again that primary soft tissues and biomolecules can be preserved, and in marine sediments at that!  Amazing.

A Mosasaurus skeleton on display at the Maastricht Natural History Museum in the Netherlands. Photo by Wilson44691, from Wikimedia Commons.

So basically, mosasaurs were fast, terrifying, Cretaceous "sea monsters" that came in a multitude of shapes and sizes and had a global distribution.  They were the dominant marine predators in an underwater world that was full of other nasty critters, be they marine reptiles, bony fish, or elasmobranchs.  Whether out in the open ocean, or close to the shoreline, your chances of running into one were pretty good, but your chances of it being a pleasant encounter, were certainly not.  The more fossils that we uncover, and the more research that is done on this fascinating group of animals, the more we learn about them.  But of all the important things to remember about mosasaurs, this one is key: they are not dinosaurs.

(For more on mosasaurs or the other critters from the Late Cretaceous Western Interior Sea, I highly recommend the Oceans of Kansas Paleontology website, or the book Oceans of Kansas, both by Michael J. Everheart.)

References


Lindgren, J., jagt, J.W., & Caldwell, M.W. 2007. A fishy mosasaur: the axial skeleton of Plotosaurus (Reptilia, Squamata) reassessed.  Lethaia, 40: 153-160. doi: 10.1111/j.1502-3931.2007.00009.x


Lindgren, J., Caldwell, M.W., Konishi, T., & Chiappe, L.M. 2010. Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil Mosasaur.  PLoS One 5(8): e11998. doi:10.1371/journal.pone.0011998


Lindgren, J., Uvdal, P., Engdahl, A., Lee, A.H., Alwmark, C. et al. 2011. Microspectrospic Evidence of Cretaceous Bone Proteins.  PLos One 6(4): e19445. doi: 10.1371/journal.pone.0119445