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A Field Guide to Fossils of the Smoky Hill Chalk

Part 3:

Marine Reptiles



Copyright 2000-2011 by Mike Everhart

Updated 01/25/2011


LEFT: Clidastes and Calcarichelys (a marine turtle) Copyright Dan Varner; used with permission of Dan Varner.

MARINE REPTILES - Several varieties of reptiles were well adapted for living and competing successfully in mid-ocean during the Late Cretaceous. It is likely that some, including mosasaurs, plesiosaurs and pterosaurs, were as highly evolved for survival at sea and in the air as the dinosaurs were on land. All marine reptiles were air breathers, and had to surface periodically in a manner similar to modern whales and porpoises.

Turtles - There were two major groups of marine turtles that lived in the Western Interior Sea. These turtles probably ate seaweed and jellyfish and must have migrated to the east or west to find sandy shorelines where they could lay their eggs. Turtle bone is dense and very finely grained.

For more information on turtles, take a look at Dr. Elena G. Kordikova's excellent Chelonia Website from Stuttgart, Germany.

Toxochelyids - Toxochelys latiremis is the most common species of turtle found in the Smoky Hill Chalk.

Toxochelys was a small to medium sized turtle (up to 2 m (6 ft) in length) that is found throughout the chalk. This turtle was similar in outward appearance to a modern leatherback turtle (i.e. the Green Turtle, Chelone mydas). Common fossils include shell, limb and skull material. (Right: Drawing of the skull of Toxochelys latiremis Cope - Dorsal view, from Williston, 1901).

Williston, S. W., 1901. A new turtle from the Kansas Cretaceous. Kansas Academy Science, Transactions 17:195-199.

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Protostegids - Protostega and Archelon were giant (Volkswagen-sized!) sea turtles that are found in Cretaceous deposits.

BELOW: Left: Pam Everhart works on the partial remains of a Protostega gigas that she discovered in July, 1994 in Logan County, Kansas. Center - Most of the right half of the turtle's lower plastron is exposed here.  Right: Removed from the chalk, the 'spikey' look of the the ;lower shell is very apparent.  We donated this specimen to the Sternberg Museum in 1997 where it is curated as FHSM VP-13448 (Our EPC 1994-42; about 30 inches in maximum length).  
p-gigasa.jpg (13451 bytes) p-gigasb.jpg (12726 bytes)   p-gigasc.jpg (12417 bytes)
sternbra.jpg (8809 bytes) LEFT: A photo by Charles H. Sternberg of a mounted specimen of Protostega gigas from the chalk of western Kansas. The specimen is now in the Carnegie Museum of Natural History in Pittsburg, PA.  A more recent picture is HERE. This is a composite specimen (#1420 and 1421), both discovered by Sternberg about "three miles northwest of Monument Rock" and acquired by the Carnegie in 1904. 

See also: Sternberg, C. H. 1899.  The first great roof.  Popular Science News 33:126-127, 1 fig.

The following information was provided by Dr. Kraig Derstler of the University of New Orleans. Kraig is one of the few people currently studying protostegid turtles.

"Very little is written about Protostegids, other than the descriptive stuff by Wieland and his colleagues in the 1890's and earliest 1900's. Protostega is a huge-headed sea turtle. The species are pretty poorly defined at present. Specimens come from the Niobrara of Kansas, the Pierre of Kansas, Colorado, Wyoming, and South Dakota, the Mooreville, Demopolis, and Eutaw formations of Alabama and Mississippi, and the Campanian - Santonian marls of Texas. Possible scraps also come from other Santonian-lowermost Maastrichtian deposits around the world. Other Protostegids (Calcarichelys, Chelosphargis, and a couple of unnamed things) range from the Albian through the latest Maastrichtian worldwide.

Archelon ischyros has a normal-sized skull, proportionally much smaller than Protostega. However, it has that distinctive hooked snout. It is so far confirmed only from the upper half Pierre in South Dakota. (Reports from Colorado and elsewhere are pretty unbelievable.)

Concerning size, the largest Protostega is a 3.4m beast in the Dallas Museum of Natural History. I was consultant for this exhibit and I'm pretty confident of the identification as well as the size. The Yale Archelon is 3.0-3.1m, but another I've studied is 4.6m long! It is virtually perfect and articulated. As a result, the size and the identification are both solid.

I've never seen any Niobrara Protostega material from an animal that was more than about 2-2.5m long. Niobrara giants may have existed, but I haven't seen any evidence. And, I've never seen any signs of Archelon in the Niobrara. However, there are lots of small to medium-size Chelosphargis specimens and possibly some pieces of the "thorny Protostegid" Calcarichelys".  ( Dr. Kraig Derstler )

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LEFT (Dorsal) AND RIGHT (Ventral): Old pictures of the type specimen of the  protostegid Archelon ischyros (YPM-3000) in the Yale Peabody Museum.

It was collected from the upper Pierre Formation of South Dakota in 1895 by Dr. G. R. Wieland, and described by him in 1896. While very similar to Protostega, it is much larger and lived several million years after the Smoky Hill Chalk species. (Click LEFT photo to enlarge)

Note the missing right rear limb.

ARCHE01A.jpg (15697 bytes) ARCHE02A.jpg (14799 bytes) Drawings of Archelon ischyros, adapted from Wieland, 1909. (CLICK TO ENLARGE)

FAR LEFT: Dorsal view

LEFT: Ventral view

RIGHT: Ventral view, plastron removed.

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ku-1301a.jpg (19091 bytes) Plesiosaurs- There were at least three major varieties of plesiosaurs in the Western Interior Sea during the Cretaceous. The short-necked pliosaurs, short-necked polycotylids, or long necked elasmosaurs were never common, however, and plesiosaur remains are rare in the Smoky Hill Chalk. Most finds of plesiosaur material consist of pieces of paddles or a series of vertebrae that were the remains of shark attacks on dead or dying plesiosaurs. It is likely that plesiosaurs gave birth to their young at sea and may have lived in family pods similar to dolphins and whales. Plesiosaur bone is dense, with a fine grain, and the vertebrae are concave on both sides. The remains of elasmosaurs (Elasmosaurus and Styxosaurus) and polycotylids (Dolichorhynchops) remains become much more common above the chalk in the Pierre Shale formation.
Polycotylids (Short Necked Plesiosaurs) - Short necked plesiosaurs like Polycotylus and Dolichorhynchops are found occasionally in the upper chalk. These animals were apparently strong, fast swimmers and fed on fish. New evidence indicates that polycotylids were also present in the lower chalk.

LEFT:The skeleton of Dolichorhynchops osborni (lateral view, about 10 feet - Adapted from Buchanan, 1984)

Elasmosaurids (Long-necked plesiosaurs) - These plesiosaurs probably lived in the inland sea in small numbers but little data exists on their occurrence.  It's possible that they avoided the deeper water in the center of the seaway, preferring to remain closer to shore. Elasmosaurus apparently swam slowly and used it's long neck to catch small fish from below.

RIGHT: The skeleton of Styxosaurus snowii (Lateral view, 30-45 feet - Adapted from Buchanan, 1984)

Long necked plesiosaurs swallowed smooth stones (gastroliths or gizzard stones) that were used to help grind up their food.  Read the story of Elasmosaurus platyurus here.

Mosasaurs - Last of the great marine reptiles.

1997-10b.jpg (23776 bytes) Mosasaurs were relatively late-comers compared to plesiosaurs, and are only known from the second half of the Late Cretaceous. They were so successful, however, that they became the dominant large carnivore in most of the oceans of the world within a relatively brief (geologically speaking) period of time. Only a few, rare species of plesiosaur were larger in size, although some marine crocodiles (i.e. Deinosuchus) were certainly as formidable. The recent find (1995) of a Plioplatecarpine mosasaur in South Dakota with four or more babies inside appears to be conclusive evidence that mosasaurs bore their young alive, much like porpoises and ichthyosaurs.
The relative abundance of small, immature mosasaurs living in a hostile mid-ocean environment, hundreds of miles from land or sheltered areas seems to indicate that mosasaurs lived in groups and protected their young. Mosasaurs fed on all types of prey, including ammonites, squid, fish, plesiosaurs, turtles, birds, pteranodons and other mosasaurs. A number of genera are present in the chalk but Tylosaurus, Platecarpus and Clidastes were the most common. Mosasaur bone is denser than fish bone, but not as dense as plesiosaur or turtle bone, and has a distinct surface texture. Mosasaur vertebrae are concave on the anterior end and convex on the posterior end.

The remains of cartilage, particularly from the rib cage and the tympanic membrane covering the ear, are fairly commonly fossilized in mosasaur specimens. Imprints of the scales covering the skin have been found in several sets of remains. Although mosasaurs had large paddles, it is more likely that they used their strong, sinuous tails to propel themselves rapidly through the water.  Recent (1995, 1997 and 1998) evidence has shown that even large Tylosaurs were attacked or scavenged by Cretoxyrhina sharks.


LEFT: Dorsal, lateral and ventral views of the skull of Clidastes sp. (about 18 inches).


Tylosaurus - Tylosaurs were the largest of the mosasaurs in the chalk, reaching forty feet or more feet in length. The earliest species, Tylosaurus nepaeolicus, occurs in the lower 1/3 of the Smoky Hill chalk while Tylosaurus proriger, the largest of the Tylosaurs, is found in the middle and upper chalk. A third, as yet undescribed species appears to be intermediate between the other two and may be the most common species found in the lower 1/3 of the chalk.

1tyloa.jpg (7542 bytes) In all Tylosaurs, the teeth in the premaxillary (anterior end of the upper jaw) do not reach the end of the bone. Some researchers believed that the premaxillary (nose) was used like ram in attacking prey or as a defensive weapon against other mosasaurs. However, the current interpretation shows this was not likely. 

Platecarpus - This genus is represented by medium sized animals that are found throughout the chalk. Russell (1967) counted specimens of mosasaurs in various museum collections and found that Platecarpus was the most commonly represented. Currently two species are known: Platecarpus tympaniticus and P. planifrons.

1platea.jpg (8409 bytes) In Platecarpus the teeth in the premaxillary extend to the end of the bone.  While Tylosaurus appears to dominate the lower Smoky Hill Chalk, Platecarpus becomes the dominant genus of mosasaurs in terms of numbers in the upper chalk (about 24 feet).

Clidastes - This genera is the smallest of the three types of mosasaurs found commonly in the chalk. Until recently, it was documented only in the upper chalk. A new specimen found in 1995 in the low chalk may indicate that Clidastes liodontus occurred there in small numbers. Clidastes may have been the most primitive of all the mosasaurs living in the Western Interior Sea (about 12-15 feet). Mosasaur remains found in the Fairport Chalk (Turonian) of Kansas are the earliest mosasaurs found anywhere, and are probably closely related to Clidastes. (See Martin and Stewart, 1977)

1clidasa.jpg (10794 bytes) The skeleton of Clidastes propython (lateral view, about 12 feet)

Continued on next page.................................

A Field Guide to Fossils of the Smoky Hill Chalk - Part 4; Pteranodons and Birds