Trackways can provide clues to the social nature of the animal, was it gregarious (social), and did the animal travel in herds? Trace fossils can be combined to provide multiple lines of evidence. Dinosaur nesting sites and trackways support the idea that some herbivorous dinosaurs were gregarious. This same evidence may also point to migrating behavior. Tracks of multiple organisms (footprint assemblage or ichnocoenosis) combined with an analysis of rock formation can help to build an ecological picture of ancient environments. Trackways at Davenport Ranch in Texas record a herd of 23 sauropods apparently being tracked by 5 theropod dinosaurs (Prothero, 1998, p. 414)

The system for naming footprints (ichnotaxonomy) runs somewhat parallel to the taxonomy for body fossils. A track made by Tyrannosaurus would be given the formal name Tyrannosauripus. Footprint names end in -pus ("a foot"), -podus ("foot"), or -ichnus ("track or trace") (Borror, 1988, pp 47, 78, & 82). Determining who made tracks is a prime objective of tracking. Looking at body fossils from the same time period to compare foot anatomy to the track is the key. For example, Pterosaur feet (body fossils) are an excellent match for Pteraichnus (footprints).

In very rare instances the tracks are found with the maker. A specimen of Kouphichnium walchi (a horseshoe crab), found in the Solnhofen strata, is preserved at the end of its track (Boucot, 1990, p. 314). Bivalves with escape trails have been found in siderite nodules from Mazon Creek (Nudds & Selden, 2008, p. 123).

Marine Trace fossils are often classified into behavioral categories. Does the trace fossil represent resting, dwelling, crawling, grazing or some other type of feeding behavior (see Prothero, 1998, p 406 for more details). Perhaps the most practical way to classify trace fossils is by their associations with a particular sedimentary environment or ichnofacies. In marine environments different ichnofacies are associated with different water depths, physical energies (wave & current conditions), or even type of substrate. Ichnofacies have become a standard tool for sedimentary geologists as well as paleontologists (Prothero, 1998, p. 406).

Patterns of tracks through time known as palichnostratigraphy corresponds well with biostratigraphic zones (time intervals defined by fossils). The longevity of the average dinosaur species, defined by appearance and disappearance from the geologic record, is 7 to 8 million years. Ichnologists find essentially the same span of time, 7 to 8 million years for changes in the footprint record (Lockley & Meyer, 2000, p. 10).

Coprolites

The Reverand William Buckland (1784-1856), an English geologist/paleontologist, was the first scientist to recognize the true nature of fossilized feces. Buckland coined the term coprolite or "dung-stone" to describe these trace fossils. The oldest known coprolites come from Silurian deposits and represent fish feces (Eschberger, 2000, Coprolite Article).

Dr. Karen Chin, curator of paleontology at the University of Colorado Museum in Boulder identifies several criteria for coprolite identification. Coprolites often have a high calcium phosphate content (Williams, 2008, p. 47). Phosphate helps mineralize feces. This may help to explain why there is a preservation bias for carnivore coprolites over those of herbivores. Carnivore's coprolites contain their own source of phosphate in the bones and teeth of the consumed prey. Herbivorous coprolites contain the cellulose and lignin of plants and require an outside source of phosphate, usually in the form of marine sediments. Shape can also be an important clue. Sharks and many primitive fish produce spiral-shaped coprolites. The shape of coprolites from larger organisms such as dinosaurs may be affected by splatting issues, trampling, weather and dung consumers. Shape can also be misleading.

Salmon Creek in the state of Washington produces beautifully shaped siderite (iron carbonate) deposits, which resemble feces. These structures are sold as turtle or even ground sloth Miocene-aged coprolites. There is some debate about the nature of these deposits. Adolf Seilacher of Yale University suggests they may represent intestinal casts or cololites (Seilacher, 2001, p. 1). George Mustoe of Western Washington University points out that there is a real lack of evidence as to an animal origin for these deposits. First, they lack calcium phosphate. Second, they lack internal clues such as pollen, scales, seeds, bones or plant fibers. Third, The deposit in which they are found lacks fossils. These siderite deposits most likely do not represent coprolites. The real value in coprolites is what they contain.

Coprolite research is carried out primarily with thin sections. Herbivorous dinosaur coprolites studied by Chin have established a relationship between plant eating dinosaurs and tunneling insects related to dung beetles (Eschberger, 2000, Coprolite Article). Dr. Chin has also established that T-rex pulverized its victims from studying microscopic bone fragments found in a coprolite. Indian and Swedish scientists have found grass phytoliths in dinosaur coprolites. This helps to establish that dinosaurs and grasses coexisted (Williams, 2008, p. 51). Coprolites provide important insight into diet, physiology as well as the geologic and geographic distribution of plants and animals.

Evolutionary Trends

Trace fossils can also help to establish evolutionary trends. Deep burrows in marine sediments first appear in the fossil record during the late Precambrian and indicate the presence of soft-bodied coelomates (Prothero, 1998, p. 227). The earliest evidence for herbivory in insects appears in the Carboniferous. Specimens of Neuropteris and Glossopteris seed fern leaves have been found that show signs of marginal and surface feeding. The first undisputed bee nests and termite borings appear in Cretaceous deposits adding evidence to body fossils that social insects had evolved by this time (Grimaldi & Engel, 2005, pp 50-55).

Bibliography

Garcia, F.A. & Miller, D.S. (1998). Discovering Fossils: How to Find and Identify Remains of the Prehistoric Past. Pennsylvania: Stackpole Books.

Grande, L. (1984). Paleontology of the Green River Formation, with a Review of the Fish Fauna [2nd edition]. The Geological Survey of Wyoming, Bulliten 63.

Grimaldi, D. & Engel, M.S., (2005). Evolution of the Insects. New York: Cambridge University Press.

Lockley, M. & Meyer, C. (2000). Dinosaur Tracks and Other Fossil Footprints of Europe. New York: Columbia University Press.

Nudds, J.R. & Selden P.A. (2008). Fossil Ecosystems of North America: A Guide to the Sites and Their Extraordinary Biotas. Chicago: University of Chicago Press.

Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.

Seilacher, A., Marshall, C., Skinner, H.C.W., Tsuihiji, T. (2001). A fresh look at sideritic "coprolites". Paleobiology, vol 27 No. 1: 7-13.

Thompson, I. (1982). National Audubon Society Field Guide to Fossils. New York: Alfred A. Knopf.

Williams, D. B., (2008) Its a Dirty Job, But Someone's Gotta Do It: Fossilized feces reveal significant details about ancient life. Earth, Sept.