Elasmosaurus: Beast of the Week

 This week we will be checking out a sea beast who's proportions were so outrageous, it confused even scientists! Check out Elasmosaurus platyurus.

Elasmosaurus was a large reptile that lived in he sea that covered what is now central part of the United States during the late Cretaceous period, between 80 and 77 million years ago.  From snout to tail it would have measured about 34 feet (7.1m) and would have eaten meat when alive.  Its genus name translates to "plate reptile" in reference to how wide and flat some of its hip and chest bones were.  

Watercolor life reconstruction of Elasmosaurus by Christopher DiPiazza.

The first thing you notice about Elasmosaurus is its extremely long neck.  Plesiosaurs, the group of marine reptiles it belonged to, are famous for having long necks, but Elasmosaurus stands out even among them.  Proportionally one of the longest necks of any animal that ever lived, it accounts for more than half of its total body length.  Unlike many other kinds of long-necked animals, like giraffes or the sauropod dinosaurs, which evolved their necks by elongating the individual vertebrae, Elasmosaurus' ancestors appeared to have simply increased the number of vertebra instead, resulting in a whopping seventy two vertebra in just the neck! Because of this, Elasmosaurus' neck would have been relatively flexible, at least when it moved its head side to side.  (I like to compare it to one of those wooden snake toys that you can hold by the tail to make it move around.)  Because the neural arches on the tops of the neck vertebra were pretty tall, Elasmosaurus' vertical range of motion would have been much more limited.  This means that the iconic swan-like neck pose many older depictions of plesiosaurs, as well as most images of the Loch-Ness Monster and other lake cryptids based on plesiosaurs, are anatomically wrong.  

Cheap and surprisingly lifelike, this snake toy has limited movement in an up and down motion but pretty flexible side to side, similar to how Elasmosaurus' neck likely was.

The neck of Elasmosaurus is so unusually long, it even tricked scientists at the time of its discovery.  The first skeletal mount of Elasmosaurus accidentally has the head placed on the end of the tail, instead of the neck, since a reptile with a super long tail seemed more logical at first. (at least to the scientist who drew it)

Drawing of the initial interpretation of Elasmosaurus with the head incorrectly placed at the tip of the tail and has what was really the neck as a long tail.

So why such a long neck?  This is something paleontologists are still contemplating and we may never truly know the answer.  One strong hypothesis is the long neck would have allowed Elasmosaurus to get its head closer to fish, which it would have hunted, without scaring them away with its huge body, essentially tricking them into thinking it was smaller and less threatening, than it really was.  Another idea is the neck may have enabled Elasmosaurus to pursue prey that may have hidden between rocks or other difficult to reach places.  A third idea is that Elasmosaurus evolved such long neck due to sexual selection, being a sign of genetic fitness and therefore attractive as a mate.  Maybe rival Elasmosaurus settled disputes by showing off who had a longer neck?  We may never know for sure.

Elasmosaurus skeletal mount (with the head on the right end) on display at the Academy of Natural Sciences in Philadelphia, USA.

Elasmosaurus had a proportionally tiny head which was flattened top to bottom. Like most long-necked plesiosaurs, its eyes were positioned on the top of its head and its jaws were lined with long cone-shaped teeth that interlocked when its jaws were closed.  The longest teeth were in the front of the mouth, likely an adaptation to hold onto slippery fish prey. 

The body of Elasmosaurus is wide and almost turtle-like in shape and function, with virtually no flexibility, since some parts of its pelvis and shoulder blades were fused together at the midline of its torso, forming wide plate-shaped structures.  This would have made its body stable when swimming at higher speeds.  Like all plesiosaurs, its limbs were modified into long rigid flippers, with upper limb bones and joints suggesting they would have been able to move them like the oars of a boat as they swam for sustained periods of time. Similar to modern sea turtles, Elasmosaurus' front flippers appear to have been where most of its power for swimming came from while the hind flippers would have been more for steering.  It's tail was proportionally short and flattened from side to side like a rudder. 

Elasmosaurus' habitat was a shallow warm sea that existed in the middle of what is now the United States during the late Cretaceous, dividing the continent into two landmasses.  Many marine fossils have been unearthed and studied from there.  When alive Elasmosaurus would have shared its habitat with many kinds of fish, ammonites, turtles, seagoing birds, the pterosaur, Pteranodon, and of course, mosasaurs(marine lizards), including Tylosaurus, Globidens, and Prognathodon.

References

Cope, E. D. (1869). "Synopsis of the extinct Batrachia, Reptilia and Aves of North America, Part I". Transactions of the American Philosophical Society. 14: 44–55. 

Houssaye, A. (January 1, 2013). "Bone histology of aquatic reptiles: what does it tell us about secondary adaptation to an aquatic life?". Biological Journal of the Linnean Society. 108 (1): 3–21.

O'Gorman, J. P. (2016). "A Small Body Sized Non-Aristonectine Elasmosaurid (Sauropterygia, Plesiosauria) from the Late Cretaceous of Patagonia with Comments on the Relationships of the Patagonian and Antarctic Elasmosaurids". Ameghiniana. 53 (3): 245–268.

Sachs, S. (2005). "Redescription of Elasmosaurus platyurus Cope, 1868 (Plesiosauria: Elasmosauridae) from the Upper Cretaceous (lower Campanian) of Kansas, U.S.A". Paludicola. 5 (3): 92–106.