Sankey Research

    My research focuses on understanding the effects that climatic and environmental change had on the Late Cretaceous (~75-65 million year old) vertebrate communities, prior to the mass extinction at the end of the Cretaceous.  My main field area is Big Bend National Park, Texas.  Big Bend National Park, Texas is in far southwestern Texas, at the U.S./Mexico border.  It is an incredibly remote and beautiful park.  In the center of the Park are the Chisos Mountains, which are made up of igneous rocks.  However, surrounding these mountains, are deposits of sedimentary rocks, including the Aguja Formation, which is full of dinosaurs and other animals. 

The Aguja Formation is composed of sandstones interbedded with shale and lignite, and was deposited in floodplain environments next to the Western Interior Seaway.  Typical large vertebrates are the giant crocodilian Deinosuchus riograndensis, the horned dinosaur Chasmosaurus mariscalensis, and the hadrosaur Kritosaurus sp. which are known from quarries with associated skeletons. 

Late Cretaceous vertebrate fossils in Big Bend are actually pretty rare, especially in comparison to other deposits further to the north such as in Dinosaur Provincial Park, Alberta.  Part of the reason for this was because Big Bend was relatively distant from mountains in the Late Cretaceous, and had slower rates of sedimentation rates, resulting in infrequent fossil burial. 

Luckily, Big Bend does contain numerous microvertebrate sites.  These are accumulations of small bones and teeth from small vertebrates such as fish, lizards, and mammals.  However, microsites can also contain small teeth from dinosaurs such as juvenile tyrannosaurids and fragments of dinosaur eggshells.  My field work in Big Bend during the past ten years has focused on finding and recovering vertebrate fossils from these sites.  This work has produced abundant and important fossils.  In fact, information on the Late Cretaceous vertebrates in Big Bend would be missing a great deal of information without microsites.  For example, there would no information about Late Cretaceous mammals from Big Bend without microsite collection and especially screenwashing.  Additionally, most of our knowledge of Big Bend theropod dinosaurs, dinosaur eggshells, and juvenile dinosaurs comes from microsites. 

The purpose of my research in Big Bend is to increase our knowledge of the Late Cretaceous vertebrates in this area, especially from microsites and to determine their paleoecological trends during the last ten million years of the Cretaceous, prior to the mass extinction at the K/T boundary, 65.5 million years ago. 

My previous work on microsites in Big Bend focused on the upper Aguja Formation at Talley Mountain Field Area.  In this area, I collected five microsites from stream channel deposits.  Because the rocks were carbonate-cemented sandstones, they had to be dissolved and broken down in a weak solution of acetic acid before they were screened and picked.  However, it was worth the effort as these samples yielded 3,349 identifiable specimens from 38 different kinds of animals.  The fossils are:  fish (68%), sharks and rays (14%), crocodilians (11%), dinosaurs (3%), amphibians (2%), turtles (not counted), lizards (1%), and mammals (1%).

The dinosaur teeth proved to be really interesting, and all were from juvenile or hatchling dinosaurs, including ceratopsian (horned), hadrosaurs (“duckbilled”), pachycephalosaur (dome-head), and various theropod (carnivorous) dinosaurs including a tyrannosaurids and small theropods  such as Saurornitholestes and Richardoestesia isosceles, a new theropod named from Big Bend fossils!

My more recent work has focused on the upper Aguja Formation at Rattlesnake Mountain where my student crews and I have made some important discoveries.  At one particularly rich horizon, we are finding numerous teeth of juvenile hadrosaurs, small theropod teeth, and numerous (~300) dinosaur eggshell fragments.  Student Ed Welsh studied these fragments and identified eight types of eggshells from both theropods and herbivorous dinosaurs, so it is likely that there were at least two nesting sites in this particular area.  All of the pieces are fragments and no complete eggs or nests have been found.  However, this is the first published record of dinosaur eggshells in Big Bend.  Another really interesting discovery during our January 2005 expedition was burned wood, which is an important piece of evidence about Big Bend’s paleoclimate in the Late Cretaceous.

Big Bend in the Late Cretaceous was considerably different from other areas further to the north at this time.  One example is the giant crocodilian, Deinosuchus riograndensis, which is common in Big Bend, but is rare or absent in the north.  Additionally, preliminary results indicate that dinosaurs in Big Bend were less diverse than contemporaneous northern areas. 

These differences are due to paleoclimatic and paleoenvironmental constraints.  Big Bend experienced periodic aridity earlier than more northern areas like Alberta, and this was an important influence on Big Bend’s paleocommunities.  There are several lines of evidence for this aridity.  One, the dinosaur bonebeds in the upper Aguja formed during periodic droughts severe enough to cause marshes to dry up.  Two, the upper Aguja has abundant horizons with paleocaliche nodules; caliche forms in soils during hot and dry conditions.  Three, burned wood in common in the upper Aguja.

The Late Cretaceous had extremely high CO2 levels, high temperatures, and high sea levels.  However, by ~74 million years ago a cooling trend and corresponding drop in sea level and retreat of the Western Interior Seaway caused changes in paleoenvironments and paleocommunities in the Western Interior of North America.  Then at ~69.5 to 68.5 Ma and 65.5 to 65 million years ago two short greenhouse events occurred, before the final K/T mass extinctions. 

My current research in Big Bend will focus on the effects of these climatic changes on the vertebrate paleocommunities.  In particular, climatically sensitive vertebrate groups such as turtles and crocodilians, will be sampled from microsites to track these paleoclimatic changes.  I am collaborating with geologists from Baylor University (add link), who are working paleoclimatic information from the paleosols in Big Bend.

Fossils are collected in Big Bend National Park with permits, and Vidal Davilla and Don Corrick of Big Bend’s Natural Resource Division are thanked for their assistance.  Fossils are curated within the Louisiana State University Museum of Natural Science Vertebrate Paleontology Collections, with permission from curator, Dr. Judith Schiebout.  Students, friends, and family helped the author with lab and fieldwork.  Funding is appreciated from Louisiana State University, Dinosaur Society, US-Canada Fulbright Program, Jurassic Foundation, South Dakota School of Mines and Technology, Earthwatch Institute Student Challenge Awards Program, and California State University, Stanislaus.

References

Colbert, E. H. and Bird, R. T.  1954.  A gigantic crocodile from the Upper Cretaceous beds of Texas.  American Museum Novitates, 1688:  1-22.

Davies, K. and T.M. Lehman.  1989.  The WPA Quarries.  In:  Busbey III, A.B. and Lehman, T.M., eds.  Vertebrate Paleontology, Biostratigraphy, and Depositional Environments, Latest Cretaceous and Tertiary, Big Bend Area, Texas:  pp. 32-42 Guidebook, Field Trips Nos. 1a, b, c.  Society of Vertebrate Paleontology 49th Annual Meeting, Austin, Texas.

Lehman, T. M.  1982.  A ceratopsian bone bed from the Aguja Formation (Upper Cretaceous), Big Bend National Park, Texas.  Unpublished M.A. thesis, University of Texas, Austin, 209 p.

Lehman, T.M.  1985.  Stratigraphy, sedimentology, and paleontology of Upper Cretaceous (Campanian-Maastrichtian) sedimentary rocks in Trans-Pecos, Texas. Ph.D. dissertation, University of Texas, Austin, 299 pp.

Lehman, T.M.  1997.  Late Campanian dinosaur biogeography in the Western Interior of North America.  In: Wolberg, D.A. and Stump, E., eds, Dinofest International, Proceedings of a Symposium Sponsored by Arizona State University:  pp. 223-240, Special Publication, Academy of Natural Sciences, Philadelphia.

Nordt, L; S. Atchley; and S. Dworkin.  2003.  Terrestrial evidence for Two Greenhouse Events in the Latest Cretaceous.  Geological Society of America Today: 4-9.

Rowe, T., R. L. Cifelli, T. M. Lehman, and A. Weil.  1992.  The Campanian Terlingua Local fauna, with a summary of other vertebrates from the Aguja Formation, Trans-Pecos, Texas. Journal of Vertebrate Paleontology 12:  472-493.

Sankey, J.T.  1996.  Vertebrate paleontology and magnetostratigraphy of the Upper Glenns Ferry (latest Pliocene) and Lower Bruneau (Pliocene-Pleistocene) Formations, near Murphy, southwestern Idaho.  Journal of the Idaho Academy of Science, 32(1/2):71-88.

Sankey, J.T.  1998.  Vertebrate Paleontology and Magnetostratigraphy of the Upper Aguja Formation (Late Campanian), Talley Mountain Area, Big Bend National Park, Texas.  Ph.D. dissertation.  Louisiana State University, Baton Rouge, LA.  263 pp.

Sankey, J.T.  2002.  Vertebrate paleontology and magnetostratigraphy of the Glenns Ferry and Bruneau Formations (Plio-Pleistocene), near Murphy, southwestern Idaho; pp. 1-49 In:  W.A. Akersten, H.G. McDonald, D.J. Meldrum, and M.E.T. Flint (eds), And Whereas... Papers on the Vertebrate Paleontology of Idaho Honoring John A. White, Volume 2.  Idaho Museum of Natural History Occasional Paper 37.

Sankey, J.T.  2001.  Late Campanian Southern Dinosaurs, Aguja Formation, Big Bend, Texas. Journal of Paleontology 75(1):208-215.

Sankey, J.T.  2005.  Drought, Fires, Dinosaurs, Eggs, and Babies in the Late Cretaceous of Big Bend National Park, Texas.  North American Paleontology Conference, Halifax, Nova Scotia, Canada. June 20-25, 2005.  PaleoBios Museum of Paleontology University of California, Berkeley:  25(2):101. 

Sankey, J.T.  2005.  Late Cretaceous vertebrate paleoecology, Big Bend National Park, Texas; pp. 98-106 In:  Dinosaur Park Symposium, Short Papers, Abstracts, and Program, D.R. Braman, F. Therrien, E.B. Koppelhus, and W. Taylor  (eds).  Special Publication of the Royal Tyrrell Museum, September 24-25, 2005.  Drumheller, Alberta, Canada. 

Sankey, J.T.  2006.  Turtles of the Upper Aguja Formation (Late Campanian), Big Bend National Park, Texas; pp. 235-243.  In: Late Cretaceous Vertebrates from the Western Interior.  Bulletin 35, S. Lucas and R. Sullivan (eds).  New Mexico Museum of Natural History and Science (Albuquerque).

Sankey, J.T.  In Press.  Vertebrate paleoecology from microsites, upper Aguja Formation (Late Cretaceous), Big Bend National Park, Texas (26 pp, 4 figs, 1 table).  In: The Unique Role of Vertebrate Microfossil Assemblages in Paleoecology and Paleobiology, J.T. Sankey, and S. Baszio (eds).  Indiana University Press (Bloomington).

Sankey, J.T. and W.A. Gose.  2001.  Late Cretaceous mammals and magnetostratigraphy, Big Bend, Texas. Occasional Papers of the Museum of Natural Science, Louisiana State University No. 77:1-16.

Sankey, J.T., B.R. Standhardt, and J.A. Schiebout.  2005.  Theropod teeth from the Upper Cretaceous (Campanian-Maastrichtian), Big Bend National Park, Texas (27 pp, 6 figs, 5 tables) in: Carnivorous Dinosaurs, Ken Carpenter (ed), Indiana University Press (Bloomington).

Schiebout, J.A., S. Ting, and J.T. Sankey.  1997.  Microvertebrate Concentrations in Pedogenic Nodule Conglomerates: Recognizing the Rocks and Recovering and Interpreting the Fossils.  Palaeontologia Electronica.  1(2): 1-54. http://wwwodp.tamu.edu/paleo/1998_2/schiebt/issue2.htm

Welsh, E. and Sankey, J.T.  In Press.  First dinosaur eggshells from Texas: Aguja Formation (late Campanian), Big Bend National Park (20 pp, 3 figs) In: The Unique Role of Vertebrate Microfossil Assemblages in Paleoecology and Paleobiology, J.T. Sankey and S. Baszio (eds).  Indiana University Press (Bloomington).

Ziegler, A.M., and Rowley, D.B. 1998.  The vanishing record of epeiric seas, with emphasis on the Late Cretaceous “Hudson Seaway”.  In Tectonic Boundary Conditions for Climatic Reconstructions.  Edited by T.J. Crowley and K. Burke, K.  Oxford University Press, Oxford.