Mosasaurs from the Arctic Circle: Part 2

The Western Interior Seaway has occupied a space in almost every museum that I’ve been to, and it’s no question why. Most well-known prehistoric sea creatures lived here, and their fearsome appearance and abundance of remains lend them frequent roles in popular books, movies, and video games. However, while this shallow sea of the interior harboured familiar monsters, the deeper Pacific held a whole different series of beasts.

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The Western Interior Seaway has occupied a space in almost every museum that I’ve been to, and it’s no question why. Most well-known prehistoric sea creatures, such as Tylosaurus, Mosasaurus, Elasmosaurus, Xiphactinus, Dolicorhynchops, and the massive turtle Archelon lived here, and their fearsome appearance and abundance of remains lend them frequent roles in popular books, movies, and video games. However, while this shallow sea of the interior harboured familiar monsters, the deeper Pacific held a whole different series of beasts.

The Puntledge River Razor-Tooth

Kourisodon puntledgensis is the only species of Mosasaur that is only known from the Arctic Circle. Named for the flat, shear-like teeth that helped it catch prey, Kourisodon was named in 2002 on the basis of a partial skeleton discovered along the banks of the Puntledge River, which consisted of a partial torso and set of jaws(1). Remains have also been found from the Izumi Group of Japan, but likely belong to a new species due to morphological and temporal differences(2). Kourisodon was originally described as a member of the “Leiodontini”, a clade that named for the mysterious “Leiodon” (now known as Liodon)(1). However, this genus has recently been the state of some controversy; Liodon‘s type species has been found to be a nomen dubium, and the interpretations of its evolutionary relationships vary considerably. Some authors prefer to group all remaining species within Prognathodon(3), some prefer to keep them as close relatives(4), and others interpret some species as relatives of Mosasaurus while others as species of Prognathodon(5). Whatever the classification, it seems that Kourisodon, if this original conclusion is valid, was a close relative to Prognathodon (and maybe Liodon and Mosasaurus), which brings forth an interesting puzzle.

While Prognathodon was a massive Mosasaur, reaching lengths of up to 10 metres(6), Kourisodon had an adult length of about 3.75 meters(1), about a third of Prognathodon‘s. If these two were such close relatives, why did one shrink to such measly proportions? Here are a couple possible suggestions:

  1. Intense competition from other mosasaurs caused Kourisodon to evolve smaller body sizes to cope with less food. Other Mosasaurs have also been found from the Pender Formation and nearby(1), so perhaps Kourisodon was a latecomer to the Pacific and the larger forms were outcompeted by the other large Mosasaurs, and instead scavenged or hunted small fish. I find this unlikely, however, as the number of other Mosasaurs in this area pales in comparison to some of the other locations where large Prognathodon-tid Mosasaurs lived(7).
  2. Most of the available prey in this ecosystem was a lot smaller than other locations. One of the notable absences in this ecosystem was that of Polycotylids, small Plesiosaurs that made up a good portion of the diets of Mosasaurs. Most remaining prey consisted of large Turtles and fish, and so Kourisodon, lacking the robust teeth that the other Mosasaurs of this area had to bite into the former, adapted to the latter. This also helps to explain why fewer predator genera are known from this area(1).
  3. Kourisodon was not as closely related to Prognathodon as recently thought, and was instead more related to Clidastes, a much smaller Mosasaurine that was also somewhat closely related to Prognathodon. While the tooth shape differed substantially between the two, some authors have suggested that laterally compressed teeth shared between Mosasaurines may be analogous, and not homologous, just examples of different genera evolving similar structures due to similar selective pressures(5). This casts doubt on the ability of flatter teeth to determine the supposedly close relationship between Kourisodon and Prognathodon. Indeed, many recent papers are beginning to classify it as a close relative of Clidastes, with one remarking that the postcranial skeletons of Clidastes and Kourisodon were almost identical. I’m not sure if it has been officially reassigned (a few papers have moved it around to many different branches of the Mosasaur tree), I think that this new classification will likely come forth should any new material be discovered.

The Pender Formation

Kourisodon was, as stated above, not alone in it’s environment. Also recovered from the Pender Formation are the remains of two Elasmosaurs and what is likely a Destmatochelys lowi, a giant turtle closely related to Archelon and (more distantly) the modern Leatherback Sea Turtle(8). Remains from D. lowi have also been found in the neighbouring Haslam Formation. Other Mosasaurs have also been found from this formation and its neighbor, the Haslam Formation. These Mosasaurs have yet to be described and are currently being prepares, although it is almost certain that one is a much larger Mosasaurine, while the other is more likely a Russellosaurine, although its size is unknown(1).

map-of-ice

During the Santonian, the Pender Formation was located at a slightly higher latitude than the previously discussed Puskwaskau Formation, lying at about 65-70 degrees palaeo-latitude(9). While this particular location did not likely see snow, the wide geographic range of Kourisodon specimens seems to indicate that this particular Mosasaur genus was not just limited to a single location and latitude. Could Kourisodon have ventured farther north into the arctic circle as did other marine reptiles(10)? Only time will tell.

 

Final Painting

kouriso

Title: Santonian Pacific Palaeoenvironment

Temporal range: Late Cretaceous (Santonian)

Species shown: Kourisodon puntledgensis, Pachydiscus ootacodensis

Geologic setting: Pender and Lambert Formations

 

That’s all for now. See you soon,

Henry Sharpe

 


Works cited:

1 Nicholls, Elizabeth L., and Dirk Meckert. “Marine Reptiles from the Nanaimo Group (Upper Cretaceous) of Vancouver Island.” Canadian Journal of Earth Sciences 39.11 (2002): 1591-603. Web.

2 Tanimoto, M. “Mosasaur Remains from the Upper Cretaceous Izumi Group of Southwest Japan.” Netherlands Journal of Geosciences 84.3 (2005): 373-78. Web.

3 Schulp, Anne S., Michael J. Polcyn, Octavio Mateus, Louis L. Jacobs, and Maria Luisa Morais. “A New Species of Prognathodon (Squamata, Mosasauridae) from the Maastrichtian of Angola, and the Affinities of the Mosasaur Genus Liodon.” Proceedings of the Second Mosasaur Meeting (2008): 1-12. Web.

4 Konishi, Takuya, and Hans Sues. “The Northernmost Occurrence of Prognathodon (Squamata: Mosasauridae) from the Western Interior Seaway of North America.”Canadian Journal of Earth Sciences 49.9 (2012): 1111-115. Web.

5 Leblanc, Aaron R. H., Michael W. Caldwell, and Nathalie Bardet. “A New Mosasaurine from the Maastrichtian (Upper Cretaceous) Phosphates of Morocco and Its Implications for Mosasaurine Systematics.” Journal of Vertebrate Paleontology 32.1 (2012): 82-104. Web.

6 Konishi, Takuya, Donald Brinkman, Judy A. Massare, and Michael W. Caldwell. “New Exceptional Specimens of Prognathodon Overtoni (Squamata, Mosasauridae) from the Upper Campanian of Alberta, Canada, and the Systematics and Ecology of the Genus.”Journal of Vertebrate Paleontology 31.5 (2011): 1026-046. Web.

7 “Prognathodon.” Fossilworks. N.p., n.d. Web. 11 Feb. 2017.

8 “Desmatochelys.” Fossilworks. N.p., n.d. Web. 11 Feb. 2017.

9 Bell, Phil R., Federico Fanti, Mark T. Mitchell, and Philip J. Currie. “Marine Reptiles (Plesiosauria and Mosasauridae) from the Puskwaskau Formation (Santonian-€“Campanian), West-Central Alberta.” Journal of Paleontology 88.1 (2014): 187-94. Web.

10 “Navigator.” PBDB. N.p., n.d. Web. 11 Feb. 2017.

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