Paleocene Turtles and Crocodilians Directly above the Cretaceous/Tertiary (K/T) Boundary in Pulaski County, Illinois
Holman, J. Alan, Michigan Academician
During the earliest part of the Paleocene (Danian Age), the Mississippi Embayment extended into southern Illinois. Recently, fossils from this interval were found immediately above the K/T(Cretaceous/Tertiary) boundary in the Clayton Formation near Olmsted, Pulaski County, Illinois. Bryozoans, gastropods, and pelecypods dominate the fauna, and sharks are common. Rare fossils include annelid worms, crabs, a spiny lobster, a ratfish, a ray, a few bony fishes, and the reptiles reported here Reptiles have not been previously reported from the Paleocene or Eocene of any state or province bordering the Great Lakes. Moreover, all reptiles reported here represent survivors of the massive extinction of the dinosaurs and great sea reptiles at the end of the Cretaceous. Turtles are represented by remains of a large sea turtle of the modem genus Chelonia and a softshell turtle assigned to cf. Trionyx, a modern genus of the family Trionychidae; a small crocodilian is also represented. Previously, the Clayton fauna was sai d to represent a near shore brackish to nearly marine environment. Based on the Clayton reptiles, one can further refine this interpretation by suggesting a riverine, coastal environment in a semitropical or tropical climate.
Reptilian fossils directly above the boundary between the Cretaceous and Tertiary Periods (K/T boundary) are important because they represent survivors of the massive reptilian extinction episode at the end of the Cretaceous, which not only resulted in the complete extermination of the dinosaurs, but also the demise of the last of the great sea reptiles. Therefore, the discovery of three fossil reptiles in the Clayton Formation of Pulaski County, in extreme southern Illinois (Figure 1) directly above the Cretaceous/Tertiary (KIT) boundary is of more than local importance, as Paleocene reptiles have not previously been reported from any state or province bordering the Great Lakes. These fossils were found in the near shore marine sediments of the Mississippi Embayment (Figure 2), an inland sea that penetrated northward to extreme southern Missouri and Illinois in the Early Paleocene. The fossils were collected from spoils of the Golden Cat Company Clay Pit.
The Clayton Formation (Earliest Paleocene, Danian Age) is immediately above the K/T boundary. The sedimentary material that underlies the Clayton Formation sediments forms the Owl Creek Formation which represents the Maastrichtian Age of the Late Creraceous (Figure 3).
The Clayton Formation, itself, is overlain by the later Paleocene Porters Creek Formation (not figured), which contains the material that is mined to manufacture the product "Kitty Litter."
The lithology of the Clayton Formation is quite distinct from that of either the overlying Porters Creek or underlying Owl Creek Formations, and contains a clay pit (Golden Cat Company Clay Pit) which consists of 5 to 6 meters of bioturbated, dark green, glauconitic, micaceous, fine to medium sands alternating with layers of dark gray clay. Fossils were collected and processed from spoil piles from the clay pit under the direction of Dr. John E. Utgaard of the Department of Geology of Southern Illinois University, Carbondale Illinois. The collectors retrieved 8750 fossil specimens that are thought to represent a fair representation of the fossil assemblage.
THE NON-REPTILIAN FAUNA
The Invertebrate Fossils
The preponderance of the fossils collected (91%) represented eight species of invertebrates. These included the gastropod Turritella alabamiensis (39% of the 8750 fossils), which was mainly represented by phosphatic internal molds. The nodular bryozoan Conopeum damicornis (30% of the total fossils) encrusted most of the mollusks at the site and even the base of some shark's teeth. Pitar? ripleyanus (8% of the total), apelecypod, was a shallower burrower. A pelecypod of the oyster genus Ostrea, O. …