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. 

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., https://doi.org/10.1016/j.cretres.2020.104499. 

Computed tomographic analysis of the cranium of the early Permian recumbirostran ‘microsaur’ Euryodus dalyae reveals new details of the braincase and mandible

Bryan M. Gee, Joseph J. Bevitt and Robert R. Reisz

Summarized by Danielle Miller, who is a geology major at the University of South Florida, working towards becoming a paleontologist. She loves watching hockey and listening to music. She has two dogs. She also loves spending time out on the boat, fishing, and hanging out in the Gulf of Mexico.

What data were used? The researchers used two specimens that were identified as Euryodus dalyae from the Richards Spur locality, a fossil site in Oklahoma. Researchers compared the specimens of Euryodus dalyae to the holotype fossils of this species; a holotype is a specimen that is used by paleontologists to define a species. These two specimens are gymnarthrids, which belong to an extinct family of amphibians called microsaurs. These new specimens were compared using data that was collected through a type of CT that is called neutron tomography and x-ray tomography. The researchers also performed a phylogenetic analysis to understand how the newly discovered specimens were related to other microsaurs. The researchers created a number of characters based on the cranial (skull) shapes across the taxa used, as well as on the vertebrate of the taxa. The collected morphological data was put into the matrix (i.e., all of the morphogical characters in total) to be analyzed to determine the evolutionary relationships between the specimens.

Methods: The researchers did tomographic and phylogenetic analyses of the specimens. The phylogenetic analyses were done to see the evolutionary relationship between the specimens and the holotype of Euryodus. During the phylogenetic analysis, the researchers coded the two new specimens into a phylogenetic matrix that was created in a previous study done in 2017. Coding of several other microsaurs were used in the analyses. One of the other microsaurs used in the analyses was from the same genus as the new specimens, while the others were from different genera. The analyses were done using PAUP*, which is a computer program that is used to infer evolutionary trees. For the tomographic analysis of the specimens, the researchers did neutron tomography and X- ray tomography. Tomographic analyses are techniques that are used to represent a cross section of solid objects through X- rays or ultrasound. Neutron tomography is performed by rotating the specimens 180°, then taking neutron radiographs, which are photos produced on film by x-rays, at defined angular positions. This can produce a 3D image of a specimen’s composition. 1200 radiographs of one of the new specimens were produced in this analysis. The other new specimen was analyzed using X- ray tomography. The photos from both tomographic analyses were then analyzed in ImageJ, which is an image processing program that can be used to measure and analyze images. 

Results: The result of this study shows that there is an especially close relationship between one of the recently discovered specimens and the holotype of Euryodus dalyae. The majority of differences between the skeletons are probably due to damage to the specimen over millions of years in the rock record, and not due to biological differences; however, there were some differences between the specimens on the internal portion of the skeleton These two specimens are also very similar to other species of extinct amphibians. The two specimen that were identified as Euryodus dalyae are now described as Euryodus sp. because of this study. Euryodus dalyae and Euryodus sp. look almost the same on the outside, but they are different internally. One such internal difference is the presence or absence of a presphenoid, which is the front part of a bone that is found at the base of the skull of the specimen. The researchers are unsure if these differences are due to ontogeny, the growth of the specimens, or if this is a signal that the two specimens represent different species. This difference is one reason that the researchers encourage more exploration of recumbirostrans, which are the amphibian group that include the family that Euryodus is part of. There was also the presence of an offset partial tooth row in the new specimens. This feature has been seen in a group called the captorhinids, which are lizard-like reptiles. This helps us better identify the broader groups that Euryodus is related to, because the offset teeth are only in certain species. This means that scientists can more confidently say that these groups belong in the same group. However, the offset teeth weren’t identical across the specimens studied here, so further tests need to be done.

Why is this study important? This is important because it provides a new understanding of the Euryodus clade. It can also be used to help determine if microsaurs are really a sister clade (meaning, the most closely related clade) to captorhinids, lizard-like reptiles that ranged from small to large and lived during the Permian, as they have been hypothesized to be before. This study also provides a better understanding of the anatomy of gymnarthrids and other microsaurs. Understanding the anatomy of gymnarthrids and other microsaurs is useful since future researchers can use that data to put other specimen into the group, if they fit that description.

The big picture: If we know the evolutionary relationships between the specimens in this study, then we could start to ask other questions about the group. We could investigate how the group changed across different extinction events or we could better understand the anatomy of these amphibians and see how this anatomy developed in amphibians of today.  

Citation: Gee, B. M., Bevitt, J. J., & Reisz, R. R. (2020). Computed tomographic analysis of the cranium of the early permian recumbirostran ‘microsaur’ Euryodus Dalyae reveals new details of the braincase and mandible. Papers in Palaeontology, 7(2), 721–749. https://doi.org/10.1002/spp2.1304