Paleocene turtles and crocodilians directly above the Cretaceous/Tertiary (K/T) boundary in Pulaski County, Illinois.
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. pulaskensis (6% of the total) represented a non-burrowing element of the fauna that occurred mostly in the form of single valves. A shallow, burrowing pelecypod, Venericardia smithii, represented 2% of the total. A form of Venericardia that could not be identified to species represented 2% of the total.
The predatory gastropod Natica reversa represented 2% of the total and was represented mainly by internal molds. Strepsidura contorea, a gastropod represented mainly by internal molds, represented 2% of the total. Less common Clayton invertebrates include at least eight species of pelecypods, 11 species of gastropods, one annelid worm, and three decapod crustaceans.
The Non-Reptilian Vertebrate Fossils
Non-reptilian vertebrate fossils all consisted of fishes. Cartilaginous fishes were represented as follows. Teeth of the shark Lamna cuspidata composed 2% of the individual fossils. A wide range of maxillary and mandibular teeth were recovered. Less common cartilaginous fishes were represented by teeth of the ray Ptychodus sp., teeth of the ratfish Ischyodus williamsae, and unidentified shark vertebrae. Bony fishes were represented by a ceratohyal element of Pachyrhizodus caninus and bone fragments of Bananogmius cf. B. crieleyi. Propenser hewletti was represented by a frontal bone, hyoid fragments, and shoulder girdle elements. Other fish bones and teeth were unidentified.
This section deals with the identification and interpretation of the two turtles and a crocodilian not previously detailed in the Clayton Fauna. The fossils are housed in the collections of the Illinois State Geological Survey (ISGS). The classification here follows that of King and Burke (1989). All measurements are in millimeters unless indicated otherwise.
Class Reptilia Laurenti, 1768
Order Testudines Batsch, 1778
Suborder Cryptodira Cope, 1869 
Family Cheloniidae Gray, 1825
Genus Chelonia Brongniart, 1800
Chelonia sp. indet.
Material: A complete suprapygal bone, Illinois State Geological Survey (ISGS) No. 90P72, Fig. 4A,B); also see Figure 5, top.
Remarks: The suprapygal bone of Chelonia appears to be very characteristic. The element is roughly hexagonal (Figure 2A,B) with the borders of its two anterior sides directed anteromedially and the borders of its two lateral sides directed posteromedially. The borders of the posterior edges are very weakly directed anteromedially. The posterior side of the suprapygal forms a strong, somewhat swollen articular surface for the pygal. Although the Clayton suprapygal seems to be unquestionably Chelonia, there appear to be some differences in proportion between the fossil and the modern species Chelonia mydas (Table 1).
In a modern Chelonia mydas (Michigan State University uncataloged) carapace (midline length of carapace 48 cm, greatest carapacial width 43.5 cm) the suprapygal is somewhat narrower and more tapered posteriorly than that of the Illinois Paleocene fossil and the posterior articular surface is not as swollen, possibly a sexually dimorphic character. Nevertheless, the characteristic shape of the suprapygal of both species is essentially the same.
As far as I am able to determine, this is the first recorded Paleocene occurrence of the genus, although the genus has a fairly good fossil record. Chelonia is known from Late Cretaceous localities in Europe, the Eocene to the Recent of Australia and North America, and the Miocene of Japan (Ernst et a1. 1994; Limpus 1987; Tachibana 1979). Chelonia fossils that probably belong to the modern species are known from the Miocene of California (Gilmore 1937) and the Pliocene and Pleistocene of Florida (Dodd and Morgan 1992 [Bone Valley Formation]; Weigel 1962 [Vero Locality]). ?Chelonia is known from the Pliocene of the Lee Creek Site in North Carolina (Zug 2001).
At present, three species of living Chelonia occur (King and Burke 1989). Chelonia agassizii Bocourt, 1868 is known from the Eastern Pacific from southern California (USA) to Chile, west to the Galapagos and Papua New Guinea. Chelonia depressa Garman, 1880 is known from the northern, eastern, and western coastal regions of Australia and in the vicinity of New Guinea. Chelonia mydas (Linneaus, 1758) is known in temperate and tropical seas throughout the world. Some consider Chelonia agassizii to be a subspecies of C. mydas (Ernst et al. 1994). Chelonia mydas may migrate as much as 2,000 km across open ocean (Carr and Coleman 1974) but mainly they stay near shore. Most prefer water less than 50 m deep (Fritz et al. 1983) and sea surface temperatures at the siting locations were 24-27 degrees C (mean 25.6). They feed in shallow water where there is abundant vegetation for them to eat. This might be a clue as to the temperature and depth of the embayment where the Clayton Chelonia lived.
Family Trionychidae Bell, 1828
Subfamily Trionychinae Lydekker, 1889
Material: A bony plate (ISGS 90P73, Fig. 4C); also see Figure 5, middle.
Remarks: This bony plate, which measures 31.4 mm in its greatest length, has a sculpturing pattern that is typical in many genera of the subfamily Trionychinae of the softshell turtle family Trionychidae. The pattern consists of rounded pits with somewhat raised borders, with the pits being sometimes linked and some times isolated, and with its pit borders occasionally open (Hay 1908, plate 86) as in Trionyx (Amyda in Hay 1908). The bone is tentatively referred to the genus Trionyx, for it does not seem wise to assign this imperfect bony plate to the generic level without some reservation. Trionyx goes back in the North American fossil record to the Late Cretaceous and has tentatively been reported from the Hell Creek Formation (Maastrichtian) of North Dakota (Quammen 1992).
Trionyx ("Amyda") has been reported from Cretaceous and Eocene deposits of the Atlantic slope (Hay 1908) where the animals must have existed in an estuarine situation. It has been reported that the Florida Softshell, Trionyx ferox, sometimes occurs in brackish water near the mouths of streams where the tides sometimes carry it out to sea (Ernst et al. 1994). This might indicate the reason for the appearance of the softshell in the Clayton Paleocene fauna. Unlike green turtles (Chelonia) that feed on submerged marine vegetation, trionychids are carnivores.
Order Crocodylia Gmelin, 1788
Material: One general type tooth (see Massare 1987) ISOS 90P79 (Figure 4D) and one newly erupted tooth ISGS 90P80; also see Figure 5, bottom.
Remarks: The general type tooth identified (ISGS 90P79) is slender, somewhat curved, striated, and has a pointed apex. Such teeth function to pierce and hold prey. No crunch type teeth (see Massare 1987) were found, but one very small tooth with a total length of 4.6 mm is striated but relatively blunt, and is interpreted as being a newly erupted tooth. The larger tooth (ISGS 90P79) could be either from a crocodylid or an alligatorid. The larger tooth appears to be identical to crocodyloid teeth reported from the Eocene Jim Woodruff Dam Site of Florida (Holman 2001). Such teeth are well adapted for holding fish that are seized as prey by crocodilians.
The reptile fauna is an especially interesting part of the Clayton Formation fauna from two standpoints. The first is that the green and softshell turtles and crocodilians in this particular region survived whatever forces caused the extinction of the dinosaurs and great sea reptiles at the end of the Cretaceous. The second involves the fact that the reptile fauna puts a finer edge on the previous interpretation of the sedimentary environment indicated by the
Clayton Formation fauna, which was the recognition that nearly marine to brackish water conditions occurred at the head of the Danian-age Mississippi Embayment.
To begin with, in the refinement of the previous interpretation, the occurrence of the green sea turtle and the crocodilian together indicate a tropical or semitropical climate during the time of the deposition of the bones. Moreover, other early Tertiary faunas that appear to be analogs to the Clayton Formation fauna (Table 2) provide other suggestions as to the sedimentary environment of the Clayton Formation fauna. The only possible very late Paleocene brackish to normal marine fauna that I am aware of that contains turtles and crocodilians is Carnegie Museum Locality 517 in the upper part of the Tuscahoma Formation at Meridian in Lauderdale County, Mississippi. This locality is overlain by the fossiliferous Early Eocene Bashi Formation. Carnegie Museum Locality 517 was previously referred to the Early Eocene (Case, 1986; Dockery et al., 1991; Parmley and Case, 1988), but Ingram (1991) has suggested that it is of latest Paleocene age.
Unfortunately, although turtles and crocodilians occur there, they have not been detailed, although four species of fossil sea snakes (see Table 2) from the locality are well known (Parmley and Case 1988; Holman and Case 1992). However, all of these sea snakes have previously been reported only from the Eacene of North America (Holman, 2000) and are considered to be "index fossils" for the Early and/or Middle Eocene. Therefore, I have strong doubts about the Paleocene age of the deposit.
Middle Eocene reptiles from the Point "A" Dam Site Locality in the Tallahatta Formation, northwest of Andalusia in Covington County, Alabama (Holman and Case, 1988) show taxonomic affinities to the Clayton Formation fauna in that a trionychid turtle, probably of the genus Trionyx, is present, and crocodylids are represented by the some of the same kind of piercing tooth types as those in the Clayton Formation fauna. Moreover, a shark of the genus Lamna is present in the Alabama fauna as it is in the Clayton Formation fauna. A large river turtle of the family Emydidae or Bataguridae is, however, not duplicated in the Clayton.
The postulated habitat for the Middle Eocene Alabama fauna is a tidal riverine system along a tropical or semitropical coastline. In 1988, Holman and Case stated the following:
Softshell and river turtles (Trionyx ferox and Pseudemys floridana), which are usually considered to be freshwater species, commonly occur in tidal riverine situations in gulf coastal rivers in Florida (Suwanee, Waccassasa, Withlacoochie) today, where sharks, rays, weakfishes, sheepshead, croakers, and drum are also found in abundance, especially at high tides (J.A.H., personal observation). This would explain the softshell and river turtle remains in association with marine (littoral) fishes and the estuarine Palaeophis virginianus in the Alabama fossil site. Crocodilians are also abundant in such places in several places in the tropical world today.
A newly reported Eocene (probably Middle Eocene) herpetofauna from the Jim Woodruff Dam Site near Chattahoochee, in Gadsdon County in northwest Florida represents the first reptile fauna recorded from the Eocene of the state (Holman 2001). This fauna also contains reptiles generally similar to those of the Clayton reptiles. This reptile assemblage includes a large softshell turtle with a seemingly identical sculpturing pattern to that of the Clayton softshell turtle (?Trionyx), a large river turtle representing either the family Emydidae or Bataguridae, and crocodilian teeth, including piercing teeth that appear identical to the larger crocodilian tooth of the Clayton fauna. It was suggested (Holman 2001) that the Florida Eocene reptile fauna was analogous to the Point "A" Dam Site reptile fauna in Alabama, and also represented a tidal riverine system along a tropical or semitropical coastline.
The one herpetological element in the Clayton Formation fauna that does not fit the Alabama-Florida-Mississippi early Tertiary analogs is the green sea turtle, Chelonia.
No sea turtles have been reported from these localities. This situation makes the interpretation of the sedimentary environment of the Clayton Fauna somewhat different. I have previously reported in the Chelonia section of this paper that green turtles stay near shore in water less than 50 m deep and that they feed in shallow water where rooted vegetation is abundant. To that effect, I would report that in the 1950ss I have seen green turtles (now exceedingly rare if not extirpated from that area) feeding in grass flats only a few miles at sea from the mouths of three Florida rivers cited above, but never in the rivers themselves.
This, however, would not negate the suggestion that the Mississippi Embayment in the Paleocene of extreme southern Illinois was a riverine coastline situation in a tropical or semitropical climate; but would cause me to speculate that the Clayton fauna probably was deposited in the shallow, nearshore part of the marine system represented, rather than in or near river mouth itself.
TABLE 1 Measurements and ratios of the suprapygal of the Clayton Formation Paleocene fossil Chelonia and a modern Chelonia mydas compared. Measurements are in mm. Measurements and Ratios Fossil Chelonia Modern Chelonia mydas Width of anterior end (WAE) 53.5 45.0 Width of posterior end (WPE) 29.5 17.0 Greatest total length (GTL) 50.5 49.0 GTL/WAE 94.39 100.09 WPR/GTL 58.84 34.69 TABLE 2 Early Tertiary reptile faunas from marine, coastline environments. Clayton Fauna, Carnegie Museum Point A Dam Site, IL. Early Paleocene Locality 517, MS. AL. Middle Eocene at K/T Boundary ?Paleocene Chelonia sp. Turtles (unstudied) Emydidae/Batuaguridae (Green Sea Turtle) (River Turtle Species) cf. Trionyx Trionyx (Softshell Turtle) Crocodylia indet. Crocodylia indet. Crocodylia indet. (Crocodilian) (Crocodilian) (Crocodilian) Tuscahomaophis Tallahattaophis (1 strange sea snake) (1 strange sea snake) Palaeophis Palaeophis (3 sea snake species) (1 sea snake species) Clayton Fauna, Jim Woodruff. IL. Early Paleocene Dam Site, FL. at K/T Boundary Early or Middle Eocene Chelonia sp. Emydidae/Bataguridae (Green Sea Turtle) (River Turtle Species) cf. (Softshell Turtle) (Softshell Turtle) Crocodylia indet. Crocodylia indet. (Crocodilian) (Crocodilian) Palaeophis (1 sea snake species) FIGURE 3 Stratigraphic location of the Clayton Formation fauna. Formation Epoch Age Significance Clayton Paleocene Danian Surviving K/T (Earliest) Cretaceous Boundary Reptiles Present Owl Creek Cretaceous Maastrichtian Age of (Latest) Last Occurrence of Dinosaurs
I am very grateful to Kerri H. Cope, John M. Masters, and John E. Utgaard who allowed me to study the reptile fossils they collected by them. Dr. Utgaard, who supervised the collection of the Clayton Formation fossils, also provided me with written accounts of the site and a digital map of the geological structure of the Mississippi Embayment.
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|Author:||Holman, J. Alan|
|Date:||Jun 22, 2002|
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