Smithsonian National Museum of Natural History

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Carboniferous

Paleoecology of Mesozoic Terrestrial Ecosystems

 

Like many environments today, the ancient terrestrial communities of Mesozoic ecosystems were populated by a very charismatic megafauna. Like modern mammals, dinosaurs are often the “centerpiece” of reconstructions of their host ecosystem – but were neither the most diverse nor the most abundant organisms in their environments.

Since 1994, I have been studying these Mesozoic ecosystems using fossils recovered from microvertebrate sites. These localities provide rich concentrations of vertebrate fossils that appear to have accumulated over relatively short time intervals (hundreds to thousands of years) and from a geographically small area. Within Mesozoic deposits at least, many of these sites share a similar depositional history that suggests they may be broadly comparable. Thus they may represent an unusual opportunity to study Mesozoic ecosystems across space and time under broadly similar taphonomic conditions.

I am currently working on a long-term project to use these deposits to better understand the ancient environments preserved in the Late Jurassic Morrison and Early Cretaceous Cloverly formations. My recent fieldwork has produced a number of new sites that are being studied in detail.

The work typically involves identifying and bulk-sampling sites in the field, where we collect up to 1 metric ton of sediment.

Most of the sediment is screenwashed initially in the field to reduce its bulk, after which it is shipped to the NMNH for finer processing.

Once washed of sediment, the fossils are removed and identified as specifically as possible, using standard and scanning electron microscopy. This work takes place in my laboratory, often with the help of student interns.

An SEM of an amphibian jaw with teeth from the Cloverly Formation is shown below.

The result has been a significant improvement in understanding the diversity of these formations. For example, study of the Morrison Quarry 9 site demonstrated the importance of accounting for specimens as thoroughly as possible. This site has produced more than 200 specimens of mammals, representing more than 20 species, and thus the site was often interpreted in light of this high mammalian diversity.

However, specimen abundances reveal that mammals represent only a small fraction of the total fossils.

This is similar to many other Mesozoic microvertebrate sites, even from outside the Morrison Formation. Given the potential mode of origin of the site, abundances may be more informative than taxonomic diversity for the purpose of paleoecological interpretation.

A different mode of origin may be evident at a new Morrison site that has produced hundreds of mammal, reptile, and baby dinosaur bones along with thousands of eggshell fragments. The theropod tooth below, still in a fragment of jaw, shows one of the embryonic dinosaur specimens.

Work in the Cloverly has focused on illuminating the poorly understood ecosystem. Large fossils are uncommon in the Cloverly, and so microvertebrate remains have been important in augmenting the known vertebrate diversity. At least 40 different species are present, including fishes, amphibians, crocodilians, lizards, mammals, and dinosaurs. A freshwater shark tooth is seen below.

This presents an opportunity for comparison with the Arundel Clay Facies deposits of Maryland, of similar age, which have produced microvertebrate remains as well. In the longer term, I am interested in exploring these sites in other formations.

Finally, I have begun a new project with Ray and Kristi Curry Rogers (Macalester College) to develop a new model for the origin, interpretation, and utility of microvertebrate sites in general.

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