New Fossil Evidence From the Arctic Could Indicate that Mosasaurs Migrated

Arctic mosasaurs (Squamata, Mosasauridae) from the Upper Cretaceous of Russia

Dmitry V.Grigoriev and Alexander A.Grabovskiy

Summarized by Evan Kruse. Evan Kruse is a senior undergrad student at University of South Florida majoring in geology. He plans on attending graduate school in either paleontology or mineralogy. He enjoys hiking, rock tumbling, identifying the rocks that his friends bring to him, and, secretly, he wishes he could bring back dinosaurs and have his own raptor, hence his paleontology major.

What data were used?  New mosasaur fossils, consisting of a vertebral column, isolated teeth, and a jawbone were found by the researchers in the Zolotaya River in the Anadyr district of Russia. Mosasaurs belong to a superfamily of marine reptiles, all of which have a crocodile-like head, lizard-like body, flippers instead of feet, and a long, paddle-like tail. All three types of fossils were found in roughly the same location, within 5 m of each other. The researchers also used older mosasaur vertebrae fossils from Kotikovo, Russia and the River Lemva, Komi Republic, Russia. 

Methods: This study utilized morphological differences, such as vertebral length-to-height ratios and the distinct facets on the tooth crowns, to classify the newly-discovered mosasaur specimen as a member of the subfamily Tylosaurinae. More data is required to fully classify the specimen and for now they are simply classified as Tylosaurinae indet.(i.e., it is currently an indeterminate species within Tylosaurinae) In addition to the newly discovered fossils, the researchers use the same morphological comparative techniques on two other samples previously discovered elsewhere. These other specimens are very damaged and can only be identified as belonging to the family Mosasauridae indet. The researchers also analyze the geographical location where all three specimens, the vertebrae, jawbone, and teeth, were found in reference to their predicted geographical location during the Cretaceous Period, specifically the Turonian, Santonian, Campanian, and Maastrichtian ages. 

This image shows four images of globes as overhead views looking down at the North pole with red stars marked at various locations as well as dashed and solid white arrows. Each globe has a different continental and oceanic layout and represent the predicted layout during different ages in the Late Cretaceous, one each for the Turonian in the top left, Coniacian-Santonian in the top right, Campanian in the bottom left, and the Maastrichtian in the bottom right. Arrows of cold and warm currents in the ocean are on this chart as well; some of the mosasaur fossils found would have been in the cold water currents.
This diagram shows the predicted geographical location where the mosasaur remains referenced in the paper would have died and been fossilized at during the different ages in the Cretaceous Period. The red stars indicate mosasaur locations. The size of the star indicates the predicted amount of fossil remains at the location. Solid arrows represent warm currents; dashed arrows represent cold currents.

Results:  The mosasaur remains found in the paper were discovered in high latitudes in cold conditions. Researchers have studied the continental movements associated with plate tectonics and have predicted the longitudinal and latitudinal location of the remains during the Late Cretaceous when they were deposited. The fossils retain their position in high latitudes which would have been associated with frigid waters. This means that mosasaurs were much more widespread than previously thought and that they were able to survive in colder climates than previously believed. In the past several years, many studies have come out which discuss and theorize about the thermoregulation (i.e.., how an organism regulates their body temperature) of mosasaurs. Although mosasaurs are a part of the same large group as lizards and snakes (who are ‘cold blooded’, basking in the sun to maintain body heat), these new studies postulate that many or all mosasaurs were endothermic (meaning that they could self-regulate their own body temperature) like mammals are. Being endothermic implies that Arctic conditions were not a problem for mosasaurs and that they would have been able to survive staying part or all of the year in the polar seas. Evidence from this study supports this hypothesis. This study also takes the existence of polar mosasaur fossils as indirect evidence of yearly migration patterns. Today, the high latitudes go through extended periods of constant 24-hour light and darkness, a yearly cycle that does not differ much today from the same cycle during the Cretaceous. This study predicts that the polar regions would have at least two months of solid darkness and at least one month of constant twilight. It is unlikely that mosasaurs had binocular vision for nighttime hunting and it is even less likely that mosasaurs developed echolocation (locating objects by reflected sound, like many whales do) to deal with the absence of light. With this in mind, the presence of mosasaur fossils in the high latitudes can be taken as indirect evidence of mosasaur migration patterns, so it is possible that the mosasaurs did not remain in those high latitudes year-round.

Why is this study important?  This study is important because it hypothesizes that mosasaurs seasonally migrate to and from higher latitudes in much the same fashion as our modern-day whales. If this is true, then we may be able to take the patterns and behaviors of whales and apply them to mosasaur behaviors. 

The big picture:  Predicting the behavior of extinct animals is tough; we often have little to go on, except for what we find in the fossil record. By examining the location and preservation state of fossil assemblages, we are able to make certain predictions about behavior. If mosasaurs migrated in similar fashion to modern-day whales, then we can look at other behaviors whales have and make predictions as to whether or not mosasaurs also shared that behavior. We can look for fossil evidence to support or contradict the hypotheses we put out. This can allow us to consider new behaviors for extinct animals we might not have before, change the way we interpret fossil evidence for other species, recognize new patterns in existing data, and make new predictions that may clue us into new ways to look for fossils. 

Citation: Grigoriev, Dmitry V., and Alexander A. Grabovskiy. “Arctic Mosasaurs (Squamata, Mosasauridae) from the Upper Cretaceous of Russia.” Cretaceous Research, vol. 114, Oct. 2020, p. 104499., 

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