Dinosaur coprolite. Photo by courtesy of MNH CU Boulder
Theropod coprolites from Hell Creek Formation
In 2011 I worked as a research assistant on a paleontology research project with Professor Karen Chin, Curator of Paleontology at the University of Colorado Boulder’s Museum of Natural History and one of the leading experts on dinosaurs and other vertebrates coprolites (fossilized feces).
I developed my Bachelor’s degree dissertation: Hypothetical theropod coprolites from the Late Maastrichtian Hell Creek Formation of North America: analysis, paleobiological significance and interpretation on the identity of the animal source.
Paleoichnology is the study of trace fossils, called ichnofossils, produced by the biological activity of organisms. It is within this group that we find coprolites which are studied today by many scientists who spend their time analyzing these fossils and who have so far been able to discover remarkable biological characteristics belonging, for instance, to the dinosaurs.
Coprolites are biological fossil remains with a significant role in the study of past organisms. They unveil important clues on feeding habits, social interactions, the metabolism and physiology of animals, and on ancient ecosystems of millions of years ago.
My research project focused on the analysis of some probable unique theropod coprolites from the Late Maastrichtian of North America with the objective of formulating a hypothesis on the most probable producers, based on shape, and the biotic inclusions in the specimens.
Food remains found as external or internal biological features can be preserved thanks to particular diagenetic conditions which save the biotic matter from being altered.
Powerful jaws were the strength of carnivorous dinosaurs, that had very short forelimbs. Very popular is the theory that suggests that big theropods were both predators who killed their prey as well as scavengers who looked for dead animals.
The coprolites analyzed by this study were found in three different sites within the Hell Creek Formation of North America, known for its abundance of Tyrannosaurus-rex body fossils. The Formation dates to 66.8 Ma to the event which apparently caused non-avian dinosaur extinction at approximately 65.5 Ma; the K/T boundary is in fact the upper limit of Hell Creek. Spread from Montana, North and South Dakota, to Wyoming, the Hell Creek Formation consists of non-marine sediments representing fluvial and swamp deposits.
The first step in analyzing coprolites is to produce scaled photographic documentation of the specimens in order to get high quality pictures of their shapes and dimensions to use as a good source of information. The experimental procedure upon which we worked on can be summarized as follows:
• Non destructive analyses. The overall structure of the specimen is preserved.
- The analysis of external and internal surfaces (if visible) of the coprolites to detect the occurrence of important impressions or inclusions was carried out by direct observation and by using a stereomicroscope.
- Volume measurement by water displacement. This technique provides information about the probable producer with respect to fecal volume/animal size ratios.
• Destructive analyses.
- X-ray technique of two specimens previously done provided chemical composition data to help determine if the coprolites were produced by a carnivore or an herbivore. This step required the pulverization of some of coprolite material.
- Thin sections of two specimens (one of which was subjected to XRF) were performed. Analysis of the internal mass provided information on the inclusions. We used a diamond saw to cut the specimens, a polisher/grinder to polish and grind down the sections, an ultrasonic cleaner to remove particles, and an optical microscope to look at the completed thin sections.
Stereomicroscope observations of the surfaces have been fundamental in detecting biotic and non-biotic features still preserved such as bone fragments, muscle tissue, and plant remnants. Chemical analyses showed that phosphorous and calcium are the most abundant in the coprolites, and thin sections analyses revealed what the actual contents inside the coprolite masses are. Many examples of probable fossil muscle fibers were discovered, but no apparent pieces of bone were detected.
The abundance of P and Ca as in the coprolites, helps us assign these coprolites to carnivorous animals. These elements are autochtonous components of carnivore diets due to bone and flesh digestion. Moreover, they can also come from the decomposition of biological remains led by bacteria and fungi, and from occasional ingestion of insects. The abundance of likely muscle tissue detected in thin sections analysis can suggest a fast gut-residence time caused by easy access to food resources and gorging behavior.
The apparent absence of bone tissue is more likely due to the fact that coprolite producers avoided ingesting large quantities of bone.
All the specimens have an overall globular shape, a maximum length of 13 cm, and a volume of less than 360 ml. Therefore it is unlikely that an adult Tyrannosaurus rex or a theropod of the same bulk would have produced such fossilized feces. The identification of a coprolite producer is always difficult to determine, though an analysis of the associated fossils can offer clues about the potential animal sources. T.-rex body fossils along with dromaeosaurid, crocodilian and turtle remains were collected from the sampling locations and suggests that one of these types of carnivorous animals may have fed on some herbivorous carcasses and left fecal traces. On the other hand this association can just be a random event. Thus it is likely that either young T.-rex, small theropods or other meat-eaters from Hell Creek Formation were responsible for the coprolites.