Louisiana State University Museum of Natural Science Collections from late Cretaceous through Early Eocene Microvertebrate Sites, Big Bend National Park, Texas

Judith A. Schiebout1, Julia T. Sankey1, Barbara R. Standhardt2, and Jason Ramcharan1

1Louisiana State University Museum of Natural Science and Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803
22313 W. Alabama, Hobbs, New Mexico 88242
Abstract—Over twenty years of research by Louisiana State University scientists on the Late Cretaceous through early Eocene rocks of Big Bend National Park has focused on the recovery of microvertebrates by bulk screening. This research has produced vertebrate paleontological collections from 95 sites, and a total of 1,822 specimens or lots have been entered into the LSU Museum of Natural Science computerized database summarized in Table 1. Every stratigraphic unit studied has yielded vertebrate microsites, even though the Big Bend section is relatively thin for the time spanned, and although classic bonebeds are scarce. Computerization of sites and of currently identified fossils has been completed, allowing an examination of patterns of productivity. Floodplain sites are the most productive, particularly one, Joe's Bonebed Conglomerate, a conglomerate including carbonate nodules concentrated from soils. Conglomerates rich in pedogenic nodules, formed in channels in a marsh, have also been productive.

Introduction

In Big Bend National Park, deposition of the Javelina Member of the Tornillo Formation spans the Cretaceous-Tertiary boundary, and the Paleocene/Eocene boundary lies between the overlying Black Peaks Member and the Hannold Hill Member of the Tornillo Formation. Lehman (1991) discussed the sedimentological and tectonic significance of these deposits in the Laramide Tornillo Basin. The Cretaceous Aguja Formation and the Tornillo Formation have been the focus of vertebrate paleontological research at Louisiana State University since 1976 (Hartnell, 1980, Rigsby, 1982, Rapp, 1983, Rapp et al., 1983, Sankey, 1995, 1996, 1997, Sankey and Schiebout, 1997, Schiebout, 1979A, 1979B, 1981, 1995, Schiebout et al., 1987, 1988, Standhardt 1986, 1995, Sankey 1998, Schiebout et al., in press). The stratigraphically lowest major sites considered here are from the Late Cretaceous (Campanian), from the base of the upper shale member of the Aguja Formation, deposited during the last regression of the Interior Seaway from the region. Within the overlying Tornillo Formation, the Black Peaks/Hannold Hill Member contact marks the transition from deposition low on a floodplain to a higher elevation floodplain. The contact of the Tornillo Formation and the overlying Chisos Formation marks the initiation of local volcanism (Schiebout et al., 1987).

Where paleomagnetic work has been done and rates of net sedimentation can be calculated, rates in Big Bend are considerably lower than those in classic northern sites of comparable age. From Tiffanian zone Ti3 (late Paleocene) to Wasatchian zone Wa1 (early Eocene), the rate was 21 meters per million years (Schiebout, 1995,p. 43). Sedimentation rates of 90 to 200 meters per million years occurred in the Bighorn Basin in Wyoming at approximately the same time (Sloan, 1987). In general, Big Bend sites were in a coastal plain setting further from uplands and had lower rates of sedimentation than comparable northern sites. Faunal differences
between Big Bend and the northern sites have been an ongoing research focus, with the goal of producing a more geographically complete picture of life for the time. The abundance of carbonate pedogenic nodules in parts of the Big Bend section, such as the late Paleocene, is a major difference from northern sites, attributed to climate differences (Schiebout, 1979B). Nodules have hampered paleontological study by encrusting and breaking bone and covering weathered outcrop surfaces, thereby obscuring fossils.

Much of the LSU work has focused on microvertebrate sites, in part because Big Bend quarry sites had been extensively worked earlier. Late Cretaceous dinosaur and large crocodile quarries in the Aguja Formation in the vicinity of Talley Mountain had been collected by WPA (Works Progress Administration), American Museum of Natural History, and University of Texas at Austin crews since the 1940's. The early Eocene Hannold Hill Member site on Exhibit Ridge, the first in-place exhibit of Tertiary mammal remains in the United States, had been located and the large vertebrates from it studied previously (Wilson, 1967). Most fossils in the LSU collection, including those listed in Table 2, have been recovered by screening. Only Ray's (Wilson, 1967, Schiebout, 1974) and Joe's Bonebeds (Schiebout, 1974) have yielded appreciable larger specimens to the collections, and neither "bonebed" would qualify for the term in comparison to sites where several partial skeletons or hundreds of bones have been recovered. Identifiable fossils from screening in Big Bend are usually individual teeth, the hardest part of the vertebrate skeleton, but can include bones, small jaws or jaw pieces, seeds, invertebrates, and even ostracodes (Standhardt, 1986).

Techniques

Prior to 1970, major bulk screening had not been done on the Late Cretaceous to early Eocene rocks of Big Bend National Park. The first bulk screening done in the area was on Joe's Bonebed Conglomerate, a small lens of pedogenic nodules and sand, winnowed out of floodplain mudstones in the late Paleocene (Schiebout, 1974). A bulk sample was transported to the University of Texas Vertebrate Paleontology Laboratory, dried and warmed, and soaked in varsol, a dry-cleaning solvent. The varsol was decanted and water added, displacing the lighter varsol and breaking down the clays, allowing the rock to be screened.

Table 1—Late Cretaceous through early Eocene stratigraphy, Big Bend National Park, Texas, and related vertebrate collections in the Louisiana State University Museum of Natural Science. NALMA is an abbreviation for North American Land Mammal Age.

Age NALMA Formation/ Vertebrate Locs. over Total #

or Epoch Member Localities 10 specimens specimens

Eocene Wasatchian Tornillo/

Hannold Hill 2 1 91

Paleocene Tiffanian Tornillo/

Black Peaks 2 2 255

Paleocene Torrejonian Tornillo/Black Peaks 5 2 56

Paleocene Puercan Tornillo/Javelina 15 7 563

Late K or Pal. ? Tornillo /Javelina 4 0 10

Late K Maastrichtian Tornillo/ Javelina 49 11 504

Late K Late Campanian Aguja 18 8 343


Vertebrate paleontological research at LSU began with work at late Paleocene Joe's and Ray's Bonebeds and early Eocene TT-Jack's site, all three of which had previously been worked by University of Texas at Austin researchers (Wilson, 1967, Schiebout, 1974). In the 1970s, rock was treated with varsol or broken down by ultrasonic vibration in a Bransonic-brand ultrasonic. Next, LSU work focused on the Cretaceous-Tertiary boundary rocks, and bulk wet screening of mudstone, without chemical treatment, began from many sites (Standhardt, 1986). Recently, LSU work has focused on Late Campanian microvertebrate sites in the upper Aguja Formation near Talley Mountain. Five horizons of extremely hard fossiliferous conglomerate containing pedogenic nodules have been bulk sampled, disaggregated with dilute acetic acid, and wet screened at LSU (Sankey, 1995,1996,1997; Sankey and Schiebout, 1997, Sankey, 1998).

Discussion

Several questions can be asked of the results shown in Table 1 and 2. What lithologies are highest in productivity ? How are sites distributed with respect to the ages and stratigraphic units? In the summaries in Tables 1 and 2, the decision to include sites yielding 10 and 50 catalogue numbers, respectively, was arbitrary.

The pattern of productivity reflects both the discovery of fossil concentrations like Joe's Bonebed Conglomerate, which resulted in a concentration of work at its level, and research emphasis on segments of the section for scientific reasons, even if rich concentrations were not available. The Big Bend sites Joe's Bonebed and Ray's Bonebed remain the southernmost North American major sites for the late Paleocene Tiffanian Land Mammal Age, so screening at both continued even when the rich carbonate nodule concentration at Joe's Bonebed was expended. This explains the relatively small numbers recovered at LSU from Joe's Bonebed (Table 2) versus the productivity from Joe's collected prior to 1974. Joe's
Bonebed's original lens of conglomerate yielded 450 catalogued specimens from approximately a ton of rock disaggregated (Schiebout, 1974). Joe's Bonebed conglomerate was a small lens in floodplain mudstone, mainly composed of nodules from local soils with a high component of vertebrate remains from the same source. It is more productive per ton washed than the Talley Mountain conglomerate sites, which have yielded a total of 267 catalogued specimens from 1.9 tons. Rock collected from these sites for processing was pedogenic-nodule-bearing conglomerate, deposited in channels, some showing marine influence (Sankey, 1998), on a marshy coastal plain. Considerable coarse material, such as limestone pebbles, was also being carried by the flow in addition to pedogenic nodules and vertebrate remains.

No early Eocene screening sites were known before 1976, which resulted in an emphasis on screening at TT Jack's Site, which had yielded medium and large sized mammals of early Eocene age (Wilson, 1967; Hartnell, 1980). The fine-grained floodplain sites of Table 2, like TT-Jacks, differ in depositional microenvironment from one to another and are not totally comparable. For example, Ray's Bonebed has been interpreted as a near-river backswamp and contains a comparatively high level of freshwater lower vertebrates, compared to TT-Jack's. TT-Jack's is not as closely associated with any major fluvial sandstone and is much poorer in freshwater lower vertebrates.

No sites were recognized as definitely early Paleocene (earlier than late Torrejonian) for Big Bend before Standhardt's (1986) research, which resulted in emphasis on the appropriate stratigraphic level, and culminated in bulk sampling and intense study of Dogie Site (Table 2) and discovery and processing at other early Paleocene, as well as Cretaceous sites. Of the microsites in floodplain fine-grained deposits, the one most heavily worked by researchers from LSU was the Dogie Site, from which 3.3 metric tons were processed (Standhardt, 1986). The high amount of material screened is reflected in the high number of Dogie Site specimens (Table 2).

The most productive (per weight processed) of the screening microsites in this part of the Big Bend section remains the first one worked, classic Joe's Bonebed Conglomerate, studied before the work at LSU began. Further search has yielded sites in all relevant formations and members, but not faunas belonging to all NALMA'S. No definitely Clarkforkian animals are known from outcrops stratigraphically between Joe's Bonebed (clearly Tiffanian) and the first Wasatchian fossil sites (Schiebout, 1995). This contrasts with the more complete record for this interval in Wyoming.

Continued LSU field work has not located a single new major quarrying site for medium to large animals. TT-Jack's and Ray's were previously known quarry sites of moderate productivity of medium- and large-sized animals which also became useful microsites, but nothing new equally productive for quarrying has been found, suggesting that initial surveys were very successful in locating concentrations of larger vertebrate remains, and that these sites are rarer than they are in more rapidly deposited rocks, such as the northern sites of Wyoming and Montana. Slower deposition in Big Bend resulted in more weathering and reworking of vertebrate remains and less frequent development and preservation of the type of situations yielding quarryable bonebeds, which explains the preeminence of microsites in the Big Bend Late Cretaceous through early Eocene interval.

Conclusions

The Big Bend area, less thoroughly studied twenty years ago than northern areas exposing Late Cretaceous to early Eocene rocks, in part because of lower richness and nodule-rich ancient soils in Big Bend, was further from sediment sources and more stratigraphically condensed. Although relatively low in real "bonebed" style quarry sites, the Big Bend section has yielded many useful microsites. Although not all North American Land Mammal Ages (NALMA's) are represented, every Big Bend stratigraphic unit for the time span under consideration has microsites of consequence. Vertebrates have been recovered whenever concentrations of soil-formed nodules, like Joe's Bonebed Conglomerate and the Talley Mountain conglomerates, have been collected. Locating more such concentrations will continue to be a goal of work in the Big Bend section.

Table 2—Big Bend National Park Late Cretaceous through early Eocene microvertebrate sites which have produced over 50 catalogued specimens based on material from Louisiana State University Museum of Natural Science (Geoscience section, LSUMG). * Values for "total number of vertebrates" and for the following categories include fossils from surface search, screening, and quarrying. Detailed locality information is on file in the Vertebrate Paleontology collections at LSUMNS.

Age NALMA Formation/ LSUMNS Lithology and Total Total Total Total Total

or Zone Member Vertebrate Depositional verts.* fish, non dino. dinosaurs mammals

Locality Environment sharks, reptiles &

Name,(#) rays amphibians

Eocene Wasatchian Tornillo- floodplain

Hannold Hill TT Jack's (1) mudstone 76 2 7 0 66

Paleocene Tiffanian Tornillo/ conglomerate lens and

Black Peaks Joe's Bonebed(3) floodplain mudstone 76 3 6 0 67

Tiffanian Tornillo/

Black Peaks Ray's Bonebed(9) floodplain mudstone 77 12 32 0 33

Puercan Tornillo/

Javelina Dogie (108) floodplain mudstone

w. limonitic concretions 279 75 51 0 153 Tom's Top(111) floodplain silts and fine

ss w/ limonitic concretions 96 11 31 0 54 Late

Cretaceous Early- Tornillo/ Running Lizard(113) mudstone, fine ss, 93 34 38 7 14

Maastrichtian Javelina marls, prob. lacustrine

Late

Campanian Aguja Judy'sConglomerate carbonate-cemented 120 64 29 7 20

(140, 489) conglomerate; lag of

distributary channel


Acknowledgments

Support was provided by the National Science Foundation under grant EAR 8216488, the Dinosaur Society, the LSU Museum of Geoscience Associates, the LSU Museum of Natural Science, the LSU Department of Geology and Geophysics, and Joe Schiebout. Work was conducted under Antiquities Act Permits granted to the LSU Museum of Geoscience and the LSU Museum of Natural Science. Conversations with colleagues, including Jill Hartnell, Thomas Lehman, Earl Manning, Steven Rapp, Katherine Rigsby, Tim Rowe, Robert Sloan, Suyin Ting, Anne Weil, and John A. Wilson, have been important. Ruth Hubert and three anonymous reviewers are thanked for reading the manuscript and providing helpful comments. Cooperation of U. S. National Park Service employees is much appreciated. Joe's Bonebed was named after Joe Schiebout, Ray's Bonebed was named after Ray Alf, and TT Jack's was named after John A. (Jack) Wilson.

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