Stratigraphy
One of the important issues that will surface again and again in this book is the lack of stratigraphic information on the occurrence of most of the fossils that have been collected from the Smoky Hill Chalk. In this usage, "stratigraphic occurrence" refers to what approximate chronological level the fossil was found within the 2 00-m (600-ft.) chalk unit. As noted earlier, this chalk was deposited in the Western Interior Sea over a period of about five million years. Fossils found near the bottom of the chalk are millions of years older than those found near the top. Knowing the ages of specimens is useful in understanding when species appeared and when they became extinct, and what ecological relationships may have existed. Relatively few of the fossil remains collected since 1870 have even good locality data, let alone stratigraphic information. While this may not seem too important at first, it means that we cannot establish when the animal lived in relation to other remains that we find in the chalk. This certainly limits the usefulness of any
fossil in the study of the ecosystem of the Western Interior Sea. Even though an accurate frame of reference for locating fossils within the five-million-year depositional period of the chalk has been in place since Hattin (1982), few people are aware of it and fewer use it in the field.
The geology of western Kansas was not well understood when most of the early collecting was done. According to Zakrzewski (1 996), geological studies of the western interior of the United States had begun as early as the 1850s, but had proceeded slowly and sporadically. Meek and Hayden (1861) first referred to the chalk and limestone strata as the Niobrara Division in their description of exposures along the Missouri River near the mouth of the Niobrara River in Nebraska. In Kansas, these Upper Cretaceous strata were referred to simply as the "Niobrara" by the geologists and paleontologists of the day (Hattin, 1 982). E. D. Cope (1872a) wrote the earliest substantial account of vertebrate fossils from the "Niobrara Beds." Though he described the geology of the chalk and each of the various species in some detail, he made no attempt to delineate their stratigraphic occurrence within the formation.
In 1889, the University Geological Survey of Kansas, which eventually became the Kansas Geological Survey, was established by the state legislature. Samuel W. Williston was appointed to the faculty of the University of Kansas the following year. These two events provided the basis for much of the early progress that was to be made in the study of geology and paleontology in Kansas. At the time, even reaching an agreement on what to name the formations that cropped out in the western part of the state was no easy matter. Most of the early terminology used to describe the Smoky Hill Chalk was based on the occurrence of the predominant fossils and added little stratigraphic information to individual specimens. The "Niobrara Division" of Meek and Hayden (1861) was made up of two distinct units, a lower limestone member and an upper chalk member. Logan (1897) was the first to call the upper chalk member "The Pteranodon Beds" in apparent recognition of the abundance of well-preserved Pteranodon material that had been discovered there (Chapter 10). That same year, demonstrating his support for Logan's descriptive terminology, Williston (1897) further divided the Pteranodon Beds into the lower Rudistes Beds and the upper Hesperornis Beds, providing essentially the first biostratigraphic subdivisions of what was to become the Smoky Hill Chalk.
Williston (1897) briefly discussed the stratigraphic occurrence of mosasaurs in the Pteranodon Beds for the first time. He also made the observation that Clidastes does not occur in the lower Rudistes Beds, indicating that other genera (Platecarpus and Ty-losaurus) probably occurred within 100 feet of the contact of the chalk with the underlying Fort Hays Limestone. Williston (ibid., p. 2 4 5) was certainly aware of the lack of good stratigraphic data for Niobrara vertebrate fossils when he wrote, "I need not call the attention of future collectors to the importance of locating the horizon of specimens more accurately than has been done heretofore."
Williston (1898b) also published the first comprehensive description of the systematics and comparative anatomy of mosasaurs from the Smoky Hill Chalk, and he discussed their range and distribution in comparison with specimens discovered earlier in New Zealand and Europe. He commented that Ty-losaums, "so far as was known, begins near the lower part of the Niobrara [Smoky Hill Chalk] and terminates at its close or in the beginning of the Fort Pierre [Pierre Shale]." Of Platecarpus, he stated that the species on which the genus is based are "known nowhere outside of Kansas and Colorado, and are here restricted exclusively to the Niobrara." He again concluded that the lowest horizon of Clidastes "is the upper part of the Niobrara in Kansas."
It was not until after the turn of the twentieth century that the exploration for oil and gas in western Kansas enabled rapid advances in understanding the geology of the entire Niobrara Formation. According to Hattin (1982), Moore and Hays (1917) were the first to regard the Kansas Niobrara as a formation and the first to give member status to the currently recognized divisions, the Fort Hays Limestone and the Smoky Hill Chalk.
Russell (1967) reviewed mosasaur specimens in the Yale Pea-body collection and suggested that the Smoky Hill Chalk could be divided into a lower, Clidastes liodontus-Platecarpus coryphaeus-Tylosaurus nepaeolicus zone and an upper, Clidastes propython-Platecarpus ictericus-Tylosaurus proriger zone. He also suggested that the increased abundance of Clidastes specimens in the upper portion of the chalk was an indication of a gradual change from a mid-ocean to a near-shore environment. Russell (1970) noted significant differences between the distribution of mosasaur species in the Smoky Hill Chalk compared to the Gulf Coast species occurring in the Selma Formation of Alabama. In his initial paper concerning the biostratigraphy of the Smoky Hill Chalk, Stewart (1988) stated that he was aware of several exceptions to Russell's stratigraphic distribution of mosasaurs in the Smoky Hill Chalk that caused him to regard it with "a degree of skepticism."
It was only after Hattin (1982) published his composite measured section of the Smoky Hill Chalk that significant progress could be made in understanding the vertebrate biostratigraphy of this formation. Hattin used bentonites and other geological features to delineate his twenty-three lithologic marker units, and he divided the chalk into five biostratigraphic zones based on the occurrence of invertebrate species. In doing so, he provided field workers with the first dependable method of determining their stratigraphic location in the section.
Stewart (1988) used the distribution of the fish genus Proto-sphyraena to further delineate biostratigraphic zones in the Smoky Hill Chalk. Stewart (1990) then incorporated Hattin's marker units as upper and lower boundaries for his six proposed biostratigraphic zones (Table 2.1). This report provided the first comprehensive description of the distribution of known invertebrate and vertebrate species in the Niobrara Formation and was the first at-
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