The Oldest Late Cretaceous Dinosaurs in North America~

f' ' THE DEPARTMENT OF NATURAL RESOURCES I( • • • • I miSSISSippi • geology Bureau of Geology 2525 North West Street Volume 3, Number 2 December 1982 • Jackson, Mississippi 39216 _..... \..,. FOSSIL MICROALGAE (COCCOLITHOPHORIDS) IN THE YAZOO CLAY EXPOSURES AT THOMPSON CREEK, YAZOO COUNTY, MISSISSIPPI Will H. Blackwell Department of Botany and Department of Geology Miami University, Oxford, Ohio and Martha J. Powell Department of Botany Miami University, Oxford, Ohio Thompson Creek, long a favorite collecting haunt among "rockhounds," has become an increasingly well known paleontological locality . Recent studies have turned up the exciting discovery of a fossil whale about 50 million years old {Frazier, 1980) as well as an abundance of id enti fiable fossil wood of Pleistocene age {Blackwell and Dukes, 1981}, i.e., not more than one million years B.P. (before present). A third paleontological aspect which has received little notoriety is that Thompson Creek is also a good place to study microscopic fossil algae. The upper Eocene (45-50 million years B.P. ) muds underlying the Pleistocene terrace gravels and loess {fig. 2) are an excellent source of a minute type of fossil algae known as coccolithophorids Figure 1. Line drawing of generalized coccolithophorid. {figs. 1,3,4) as well as possibly related forms, the dis The cell surface is covered with calcite plates coasters {figs. 5-10}. {scales}, the coccoliths. In fossils, these are usu Some attenti on was accorded these interesting fossil ally found separated and scattered. When all algae in one previous study of the Yazoo Clay, exposed together, as in the drawing, the unit is referred within the city limits of Yazoo City. This study by Levin to as a coccosphere. The apical flagella, hapto (1965) provided rather extensive light microscopic descrip nema {between), and other soft cell parts are tions of taxa new to science and some very excellent line not preserved as fossils . drawings as well. Our study is in a sense an addition to this the life cycle may be covered by organic scales or else by in that it looks at a similar exposure ~f the Yazoo Forma calcareous plates composed of calcite (calcium carbonate) tion about 8 miles south of Yazoo City, along the banks crystals (fig. 1) . These tiny and often ornate calcite plates, of Thompson Creek, near Tinsley. In our study there is the coccoliths, are usually formed within the endomem no formal attempt to describe new taxa, but rather the brane system of the cell before being deposited externally aim is to demonstrate the penetrating power of the micro as a coat or "armour." Coccolith formation is light depen scope, especially the scanning electron microscope, in per dent and is known to be related to the photosynthetic ceiving the delicate microstructure of these tiny organisms process (Sieburth, 1979). The principle carbonate source or parts of organisms. There is a growing opinion that is the bicarbonate ion of sea water. The deposition of the certain identification to species may only be possible with individual calcite crystals of each coccolith is very orderly, the use of the electron microscope, and even then it is often producing an imbricated rosette (Black, 1965, and sometimes questionable due to structural variation associ fig. 14). Coccoliths are the only part of fossil coccolitho ated even with a given representative organism (Bold and phorids known to be preserved, and are most often found Wynne, 1978; Sieburth, 1979). The biostratigraphic value separated from one another and scattered. Individual of coccolithophorids and related forms is also a point we coccoliths are often no more than 1 \Jm in diameter. attempt to stress in this discussion, as well as possible When, rarely, all the coccoliths surrounding one cell are paleoenvironmental interpretations. found intact, the unit is referred to as a coccosphere (Bold Methodology is fairly simple. Several samples of the and Wynne, 1978, and fig. 1). The often complex and bluish, semicompacted Eocene clays of the Yazoo Forma· unusual designs (figs. 12-15) of coccoliths make them very tion at Thompson Creek were collected into plastic bags suitable objects for examination under the higher powers and stored for later examination. Literally months later, of the microscope. The viewer is visually rewarded by the it was possible to take a small pinch from a sample bag kaleidoscopic array of forms seen. and make a smear on a glass slide for light or polarizing As in the case of the Yazoo Clay at Thompson Creek, microscopic examination, or else upon a metal stub for when such calcareous ultramicroplankton is found to be examination under the scanning electron microscope. present in large numbers in "rock" or ancient muds, the Light microscope slides are readily made permanent with usual interpretation is that the facies had a marine origin, epoxy. When examining through a microscope, the viewer perhaps deep in the open sea, and formed in a warm or at is quickly engrossed by the beauty, variety, and fine details least temperate climate (Tappan, 1980). This bottom of these tiny organisms. Sample slides can be readily accumulation of planktonic organisms is the result of assayed for the presence of coccolithophorids because of their postmortem downward drift or settling. There is their characteristic, swastika-like interference figures (fig. thus an attendant general paleoenvironmental meaning 11) under polarized light (Hay, 1965). to the discovery of the presence of these tiny creatures Some background information is perhaps in order. in a particular sediment. The content of such a formation Coccolithophorids are small, one-celled algae (2-20 \.lm in as the clays at Thompson Creek compares well in general size) which even at present form a significant component terms with that of certain modern deep sea muds and of the calcareous marine nannoplankton (small plankton), oozes. The favorable comparison of fossil coccolith muds particularly in tropical seas. Some 200-300 living species with deep sea sediment5 has been known for many years. are known (Sieburth, 1979; Tappan, 1980), and are most It was actually first noted by Sorby (1861) after com abundant to a depth of about 50 meters. Living specimens parison of coccoliths in the English Chalk with coccoliths have one or two golden-brown chromatophores and store of modern deep sea oozes. There is thus relatively little reserves of leucosin and lipid. Coccolithophorids are offi doubt as to the general paleoecological implication of such cially "haptophytes" (Class Haptophyceae), in reference a formation as the Yazoo Clay. to the fact that their motile stages, in addition to pos Though small and inconspicuous, coccolithophorids sessing two flagella per cell, sometimes have a third flagel have nonetheless been the subject of dynamic evolutionary lum-like appendage, the haptonema. At least one stage in processes through geologic time. They are sketchily known Figure 2. Locality photograph of Thompson Creek. Fossil coccoliths were extracted from lowest layer visible (arrow), the Yazoo Formation . Terrace material and loess, respectively, lie above the Yazoo Clay. Figures 3-4. Light micrographs of two different coccoliths. Figure 3 corresponds in general to the form identified as Cocco lithus eopelagicus by Levin, 1965, from a similar exposure of the Yazoo Clay. X 830. Figures 5·1 0. Light micrographs of various fossil discoasters (perhaps another type of coccolith) found in the Yazoo Clay. Figure 5 is probably Discoaster barbadiensis, a characteristic marker organism of the upper Eocene. X 830. Figure 11 . Polarized light micrograph of a coccolith showing the crossed or swastika-like interference figure caused by the orientation of calcite crystals. X 400. MISSISSIPPI GEOLOGY 2 3 DECEMBER 1982 from the upper Paleozoic Era. However, well-preserved unlike coccolithophorids, the discoasters arc considered coccoliths show up in the late Triassic record and become a basically extinct group. They arc, in all probability, abundant in the Jurassic and Cretaceous (Tappan, 1980). si mply another type of coccolith . However, the absence These organisms apparently fe ll particul ar prey to the of li ving, intact re pre se n tat ives leaves their true re Ia ti on · worldwide extinctions or ncar ex tinctions occurring in the ship a bit un ce rtain. Cretaceous/Tertiary hiatus some 70 million years ago The various marine muds and rocks in Mississippi (Bram lette, 1965). This ncar extinction of the entire group are undoubtedly a rich paleontological treasure, and most is thought to have been primarily related to low nutrien t certainly hold an intriguing variety of types of fossil algae. cycles at that particular time as well as to fluc tuations in Continuation of studies of this type would very likely the carbonate compcnsa tion depth. Re gardless, in the be well worth the time of the investigator. For example, earl y Tertiar y (Paleocene) these organ isms began a "rc· some fossi l diatoms we re seen in several of the coccoli tho· newal '' or reevolution of types whi ch reached a peak, phorid samples examined. These could we ll provide the subsequently , in the Eocene. Coccolithophorid dive rsi ty subject of another investigation. declined drastically during the Oligocene, increased per haps only slightly in the Mi oce ne, and, with the exception of sporadic variation, has remained rather cons tant since Acknowlcdgmen ts (Tappan, 1980). As compared to the Cretaceous, forms fou nd in the Eocene sediments are diffe re nt taxonomically, We thank Dr. Thomas N. Taylor and associates of Ohio State Un iversity fo r the usc of their scanning electron and larger in size. The div ers ity during the Eocene (age of Yazoo Clay) was perhaps as great as at any time in microscope facili ty during this study. geologic history . As indica ted, there are numerous living representatives of coccolithophorids, occurring mainly References Cited in tropical waters.

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