Plain-Language Summary:
When calcium carbonate nodules formed in ancient soils are naturally concentrated by erosion into coarse sedimentary deposits (conglomerates, Figure 20), remains of small animals (Figure 1, 16, 34) are often included. These remains may eventually become informative fossils. In very wet areas soluble mineral matter may be washed out of the soils entirely, but in other areas (e.g. parts of the modern and ancient Gulf Coast) mineral matter from the top soil layer may be concentrated in lower layers to form nodules or crusts (sometimes called caliche). The soil-formed (pedogenic) nodules discussed herein are composed mainly of calcium carbonate.
Three Gulf Coast sites (Figure 2), from the Late Cretaceous and Paleocene of west Texas and the Miocene of east Texas and western Louisiana, illustrate how fossils can be recovered from pedogenic-nodule-rich conglomerates and interpreted. The rocks can be partially dissolved in dilute acetic acid, so that the residues can be screened and picked to retrieve the fossils. Amounts of rock are large in contrast to the amounts used in the investigation of small invertebrate fossils, often involving processing of tons of rock material (Movie1, 2, 3, 4, and 5). The effort required to process so much rock is worth it, because remains of tiny animals can be recovered, many of which would often be overlooked in surface search. These erosion-resistant, conglomeratic rocks form ledges (Figure 29) that can be recognized even in small gullies in heavily vegetated areas, where the possibility of finding fossil vertebrates is otherwise low. Surface finds such as the lower jaw of an approximately fourteen-million-year-old camel relative (Figure 32) are very rare in areas like western Louisiana. Nevertheless, such remains have been recovered by processing pedogenic nodules.
Taphonomy, the study of how fossils form, including all the processes undergone between death and study of the fossil, sheds light on the specimens from the nodule-rich conglomerates. They form in areas where soil-formed nodules and vertebrate remains are a substantial part of the coarse fraction of sediments available; i.e., they are likely to be prominent where most of the deposition is clay (Figure 21), where calcareous soils tend to form, and where rates of sediment deposition are comparatively slow, allowing good development of soils.
Concentration of nodules and bones can take place in rivers and between them during floods and in periods of widespread erosion, which can leave extensive blankets of material winnowed from soils to be buried and preserved later. Geochemical studies of the nodules can give information on the chemistry of vegetation and rainfall at the time the nodules were forming, and thus give information on ancient ecology.
Acetic acid - The chief acid in vinegar
Cretaceous - The end of the time of the dinosaurs
Miocene - The middle part of the Age of Mammals Paleocene - The beginning of the Age of Mammals
Free lay booklet on research on the Miocene fossils of Fort
Polk. 1996.
Schiebout, J. A. and B. S. Dooley, Fort Polk Fossils, June, 1996 edition, Popular Report
Submitted to Fort Polk Environmental Learning Center, 23 p. This short booklet is aimed at
elementary school children, and teachers and others are welcome to reproduce it. For a
free copy, send a stamped, self-addressed envelope to:
Department of the Army
HQ, JRTC and Fort Polk
AFZX-PW-EC
(Attn. J. Grafton, re Miocene Fossil Booklet) Fort Polk, Louisiana, 71459-7100
Other popular articles on sites discussed in the article
include:
1986. Schiebout, J. A. Big Bend National Park, A crossroads at the beginning of the Age of
mammals. In Geology of the Big Bend Area and Solitario Dome, Texas. West Texas Geological
Society Guidebook, Publ. 86-82:129-134.
1998. Schiebout, J. A. When Elephants and Rhinos Roamed Louisiana: Hunting Miocene Fossils
on Fort Polk. Phi Kappa Phi Kappa Phi National Forum Winter Edition: 26-29.
Groups within Louisiana may request a speech by Dr. Schiebout on the Fort Polk Miocene research entitled "When Rhinos and Elephants Roamed Louisiana" from the Speaking of Science Program sponsored by NSF/EPSCoR-SI and the Board of Regents by calling 504-342-4253 or E-mailing ofp@regents.state.la.us
Web pages may be contacted for more information on the LSU Museum of Natural Science (http://www.museum.lsu.edu/), the LSU Department of Geology and Geophysics (http://www.geol.lsu.edu/), and the Louisiana Geological Survey (http://www.lgs.lsu.edu/).
J.A.
Schiebout and Suyin Ting, Museum of
Natural Science, Louisiana State University, Baton Rouge, LA 70803, USA. Correspondence
should be directed to Schiebout.
J.T. Sankey, Department of Geology and
Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA