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Mosasaurs (moze-uh-sores) are extinct aquatic reptiles resembling giant lizards with paddles.  They were plentiful in the Niobrara Sea and were dangerous predators, rivaled only by the large ferocious fish, Xiphactinus.  Mosasaurs were air breathers and it is assumed they spent all of their time in water, filling a niche in the late Cretaceous sea like that occupied by killer whales, dolphins and sperm whales in the ocean today.  There is also mounting evidence that mosasaurs gave live birth, probably at sea.

Mosasaur skullMosasaurs were probably better suited for hide-and-attack methods of hunting rather than prolonged pursuit of prey.  They were excellently equipped predators who hunted fish, ammonites and even other mosasaurs.  Their hinged jaws were much like those of snakes and varanid lizards, allowing them to swallow their prey whole.  This was aided by inner-gullet jaws (pterygoids)  that allowed them to pull their catch into their stomachs.  Mosasaurs could also dive very deep and some fossilized mosasaur bones show oxygen deprivation traits associated with "the bends."

There are three main groups of mosasaurs from the Kansas Cretaceous. Tylosaurus (tie-loe-sore-us) is the largest, sometimes reaching a length of over forty feet. It is long and slender with small paddles, and its skull is large with an elongated snout. Tylosaurus was undoubtedly the most dangerous predator in the sea because of size alone.


Platecarpus ictericus

Nicknamed “Ole Yeller” for the yellow colored chalk matrix, it is an unusually large mosasaur for its type. Displayed in Keystone Gallery and twenty-one feet long.

Platecarpus-MuralThe most common mosasaur in the Kansas chalk is Platecarpus, (plat-e-car-pus) the mid-sized type, averaging fifteen feet in length.  It had proportionately larger paddles than the other mosasaurs and a blunter, more robust skull. In 1967, Marion Bonner found and collected the most complete Platecarpus to date. The specimen has external scales, internal organ shapes, all four paddles, and a fish still in its belly. It was found with the entire spinal column in place, establishing that Platecarpus have one hundred twenty-seven vertebrae. It is a wonderful study specimen and now resides in the collection of the Los Angeles County Museum of Natural History in Los Angeles, California.

Clidastes-MuralClidastes (clyde-ass-tees) is the smaller, more delicate variety of mosasaur. It reaches an average length of ten to twelve feet and probably inhabited shallower areas of the sea. Clidastes was the fastest of the mosasaurs, with small paddles and a flanged tail section that helped generate more speed.

The first mosasaur specimen described was found on the Meuse River in 1778. More detail and images can be found here.


Dolichorhynchops-MuralPlesiosaurs from the Kansas chalk are mainly represented by a single genus—Dolichorhynchops (doe-lick-o-rine-cops). It is a short-necked variety that reaches lengths of fifteen feet. Plesiosaurs have oar-like paddles composed of small structural phalange bones called podials. Since these appendages were large and solid, it is thought that plesiosaurs either skulked slowly in the water or moved their limbs up and down in a flying motion through the water.

Marion Bonner's Dolichorhynchops
One of the best specimens of Dolichorhynchops was found and collected in 1955 by Marion Bonner and his son, Orville. Complete except for one paddle, it is now on display at the Sternberg Museum of Natural History in Hays, Kansas.

Because of its small skull, Dolichorhynchops spent most of its time seeking out schools of small fish to feed on. It was not a strong competitor with mosasaurs and partially digested plesiosaur bones have been found among stomach contents of some mosasaur specimens.

Chuck Bonner found a plesiosaur in 1987 in the Pierre shale just above the Smoky Hill chalk. The head and neck were missing but inside its body were the remains of fetal material, proving that plesiosaurs gave live birth. More information about it can be found on the Bonner Family fossil hunting history page.

Elasmosaurus Though seldom found in the Smoky Hill chalk, the plesiosaur Elasmosaurus (eh-laz-moe-sore-us) is a famous fossil find from Kansas. In 1867, Dr. Theophilus Turner, post surgeon at Fort Wallace, Kansas, found the remains of a large, fossilized animal in the Pierre shale deposit. Parts of the specimen were sent to Edward D. Cope in Philadelphia. Cope described it as a long-necked plesiosaur, which he named Elasmosaurus. In his hurry to describe the specimen, he placed the head on the wrong end of the vertebral column and was publicly ridiculed by Professor O.C. Marsh of Yale. This started an intense feud between the two men that lasted throughout their lives. The rivalry caused an ongoing competition to name new species of fossils from the American West, including many dinosaurs, fish and mammals. Good for science, but not for friendships.


Turtle-GalleryFossil turtles from the Late Cretaceous Niobrara formation are similar to those in the ocean today. Specimens are not very plentiful, as the turtle groups may have stayed closer to the shorelines.

The most common turtle from the Kansas chalk is Toxochelys (tox-o-key-leez). Similar in anatomical structure to present day green turtles, Toxochelys probably also had the same habits and traits. Turtles today lay their eggs on the shore and it is assumed the Kansas Cretaceous turtles did the same.

Protostega-MuralProtostega (pro-to-stay-guh) is another type of turtle from the Kansas Niobrara Sea. It was a large turtle reaching a length of up to eight feet. Protostega could have traveled great distances, much like a modern leatherback turtle.

©2015 Keystone Gallery / Photos © Barbara Shelton unless otherwise noted

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