Niobrara Chalk

The Niobrara Chalk is a geologic formation that was layed down between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. The chalk formed from the accumulation of coccoliths from microorgansms living in what was once the Western Interior Seaway, an inland sea that devided the continent of North America during much of the Cretaceous. It underlies much of the Great Plains and extends up into Canada, but is only exposed in northwestern Kansas. Evidence of vertebrate life is common throughout the formation and includes specimens of plesiosaurs, mosasaurs, and pterosaurs as well as several primitive aquatic birds.

History of Exploration

The Niobrara Chalk was first studied during an expedition led by Othniel Charles Marsh of Yale University in 1870. This and following expeditions to the area in 1871 and 1872 yielded the first of many fossil vertebrate remains commonly attributed with the formation. Excavaions continued through the following years up to 1879 under the direction of professional fossil collectors such as B. F. Mudge and S. W. Williston appointed by Marsh.

The Niobrara Chalk has been continuously explored ever since, with specimens being found by H. T. Martin of the University of Kansas and George F. Sternberg, the son of the famous fossil collector Charles H. Sternberg. Much of the best material from the formation is on desplay at the Sternberg Museum of Natural History in Hays, Kansas.

tratigraphy

The Smokey Hill Member of the Niobrara Chalk contains the majority of the fossils found in the formation, and is subdevided into 23 marker beds. Most vertebrates are present from the upper half of the member. Most of the vertebrate remains were collected and described before the stratigraphy of the Niobrara Chalk was fully understood. Specimens were described as being from layers referred to as being either of gray-blue shale or yellow chalk. This dichotomy is not indicative of different stratigraphic units as was previously thought, but rather is seen as a weathering phenomenon that can be found at varying points in the same outcrop. [Williston, S. W., 1897, The Kansas Niobrara Cretaceous: The University Geological Survey of Kansas, v. 2, p. 237–246.]

Flora and Fauna

During the time of the deposition of the Niobrara Chalk, much life inhabited the seas of the Western Interior Seaway. By this time in the Late Cretaceous many new lifeforms appeared such as mosasaurs, which were to be the some of the last of the aquatic lifeforms to evolve before the end of the Mesozoic. Life of the Niobrara Chalk is comparable to that of the Dakota Formation, although the Dakota Formation, which was deposited during the Cenomanian, predates the chalk by about 10 million years.

Invertebrate Life

Clams, oysters, crinoids, ammonites, and squid are all common in the Niobrara Chalk and must have coprised of the majority of life at the time. Evidence of sponges, annelid worms, and crustaceans are less common and are usually found as trace fossils.

Vertebrate Life

Fish

Fish are by far the most common fossils found from the formation, with remains of pehistoric sharks, ray-finned fishes, and lobe-finned fishes in abundance. As well as smaller fish, many large predatory fish were present in the seas at that time, most noteably "Xiphactinus". Several fish were of close relation to modern day fish including primitive coelocanths, slime heads, lancetfish, gars, swordfish, and salmonids.

Plesiosaurs

Plesiosaurs are present from two different families within Plesiosauroidea in the Niobrara Chalk: the Polycotylidae, or short-necked plesiosaurs, and the Elasmosauridae, or long-necked plesiosaurs. Polycotylids superficially resemble pliosaurs, which are not present within the formation, but are unrelated. They were fast swimmers, unlike the Elasmosaurs that used their long necks to catch fish. [http://www.oceansofkansas.com/FieldGuide3.html] Plesiosaurs are rare in the formation and were therefore likely uncommon in the seas at the time. Specimens become much more numerous in the Pierre Shale situated above the chalk.

Nonavian Dinosaurs

Nonavian dinosaurs have been found in the Niobrara Chalk despite it being located hundreds of miles out to sea at the time. The most reasonable theory is that the carcasses drifted out to sea. It is unlikely that the bodies were carried out by outgoing tides along the shorelines where they died, but rather it is more probable that the dinosaurs were carried offshore by floodwaters during a storm. In the shallow waters the bodies would have began to decompose and bacteria within the carcass would have produced gasses that would have accumulated in the gut, thereby making the body buoyant. Next, the prevailing winds and currents would have carried it out to sea, where it would eventually settle to the bottom and be buried in sediment. [http://www.oceansofkansas.com/Dinosaur.html]

A few caudal vertebrae from a hadrosaur have been found with bite marks and have been eroded, suggesting at one point they were digested. A single tooth belonging to "Squalicorax" was found in situ under the vertebrae. This suggested the shark consumed the posterior end of the tail of a floating hadrosaur carcass and had partially digested it before fossilization. [http://www.oceansofkansas.com/New-dino.html] [Everhart, M. J. and K. Ewell. 2006. Shark-bitten dinosaur (Hadrosauridae) vertebrae from the Niobrara Chalk (Upper Coniacian) of western Kansas. Kansas Academy of Science, Transactions, 109 (1-2):27-35.] Most dinosaurs in the chalk were nodosaurs. No remains of cerapods nor non-avian theropods were found, both being common dinosaurs present at the time.

Birds

Three genre of birds are present in the formation, although rare. Both were unrelated to modern birds, as they still retained teeth. "Baptornis" and "Hesperornis" were large flightless aquatic birds suited for diving. "Ichthyornis" was a seabird that resembled the gulls and petrels of today. Both probably preyed on small fish and were preyed opon by sharks, large bony fish such as "Xiphactinus", and mosasaurs.

References

External Links and References

* [http://www.dinodata.org/index.php?option=com_content&task=view&id=2539&Itemid=75 The Niobrara Chalk Formation] at DinoData.
*Oceans of Kansas [http://www.oceansofkansas.com website] .

Other References

*Bennett, S. C. 2000. Inferring Stratigraphic Position of Fossil Vertebrates from the Niobrara Chalk of Western Kansas. College of Chiropractic, University of Bridgeport, Bridgeport, CT 06601-2449.