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Life Goes On for the Mammals after Dinosaur Extinction
by Jaelyn J. Eberle

While considerable press has been given to dinosaur extinction at the Cretaceous-Tertiary (K-T) boundary, the evolutionary events after the K-T event (which are critical to the history of our own beloved Class Mammalia) are only beginning to be understood. During the first million years of the Tertiary Period (from 64 to 65 million years ago), in a world devoid of the giant reptiles, mammals emerged from the understory and began their greatest diversification ever. During earliest Tertiary time, mammals evolved larger body sizes and experimented with different diets (as shown by differences in their teeth). During this time, most modern orders of mammals were born. Rather than "recovering" from whatever killed the dinosaurs, mammals were "discovering" a new world left to them by the dinosaurs. North America is the only continent on which this critical time in mammalian history is being documented in detail. The Denver Basin is one of the few places in North America (and the world) that preserves a window into the mammalian world in the first million years after dinosaur extinction.

Extensive prospecting for earliest Tertiary mammals in the Denver Basin dates back to the 1930s and '40s, when pioneer discoveries were made by Roland W. Brown and colleagues at South Table Mountain near Golden, and in earliest Tertiary rocks near Colorado Springs. However, the most extensive collecting was done in the 1970s by Dr. Peter Robinson and his students at the museum at the University of Colorado, Boulder (UCM). Renewed collecting efforts by the Denver Museum of Nature and Science and the Colorado Department of Transportation in the 1990s has led to the discovery of new localities and specimens. Despite all of the prospecting and collecting by numerous parties, the actual number of specimens known from the Denver Basin remains small. This is partly because of their size—earliest Tertiary mammals are most often recognizable by teeth, which are small, rare, and consequently difficult to spot in the field.

As a vertebrate paleontologist specializing in mammalian evolution across the K-T boundary, I joined the Denver Basin project in 1999. I have several years of experience working on a K-T boundary section in the Hanna Basin of south-central Wyoming—among the thickest and most complete terrestrial K-T boundary sections. By collecting and studying fossil mammals from the Denver Basin, and comparing them with those from the Hanna Basin and elsewhere in the Western Interior, I hope to provide a more complete picture of what happened among North American mammals in the first million years after dinosaur extinction—the first few "minutes" of the Tertiary Period. Additionally, my research on the fossil mammals, combined with what other members of the Denver Basin project discover about the fossil plants, sedimentology, and stratigraphy, should provide a more complete picture of the ancient ecosystems operating in and around Denver directly after dinosaur extinction.

Their rapid evolution, abundance, and good preservation make Tertiary mammals, or more specifically their teeth, excellent fossils for determining the relative ages of rocks (i.e., older versus younger). The combination of Tertiary mammals, pollen, and paleomagnetism—all valid dating techniques in their own right—should allow us to calibrate the sediments in the Denver Basin, and determine age equivalency between different parts of the basin as well as different basins (known as correlation). For instance, by comparing earliest Tertiary mammals of the Denver Basin with those of the Hanna Basin, I can determine if the same interval of time is represented in both basins, and if temporal gaps exist (as evidenced by the presence of a particular mammalian fauna in one basin and its absence in the other). A mammalian fauna includes all of the mammals in an ancient ecosystem at one particular time, not just one species.

Earliest Tertiary mammalian faunas from the Denver Basin, like those of the Hanna Basin and elsewhere, reflect critical steps in the evolution toward modern orders of mammals. Prior to the K-T boundary and in the first few hundred thousand years of the Tertiary, mammals were small (most were mouse- to rat-sized), and probably had similar diets to one another, as reflected by similarity of teeth. However, a few hundred thousand years after the K-T boundary, larger-bodied mammals appeared on the landscape, and mammals were experimenting with different and more specialized diets, as reflected by differences in their teeth. The primary players in this early Tertiary mammalian expansion were the condylarths, which appeared in North America near the K-T boundary. Condylarths are a group of extinct placental mammals from which modern hoofed mammals (such as horses, deer, and cattle) as well as whales are descended. While condylarths are known throughout the Western Interior, relatively few species are known from the Denver Basin. My fieldwork related to the Denver Basin Project focuses on recovering more of these mammals, because they yield valuable information on the age of the rocks, are the most diverse group of mammals in earliest Tertiary time, and give us clues into the history of today's hoofed mammals. While not nearly as diverse as condylarths, other earliest Tertiary mammals in the Western Interior include opossums and multituberculates. An unusual group with no living descendents, multituberculates are so named for the many bumps (or tubercles) on their teeth. Additionally, a medium dog-sized mammal called a taeniodont also lived in the Denver Basin during earliest Tertiary time. Based upon more complete remains recovered from earliest Tertiary rocks in New Mexico, taeniodonts such as the one known from the Denver Basin probably were good burrowers, much like today's aardvarks, and may have fed on insects and tough plant roots. I hope to recover more fossils of this relatively rare group of mammals.

Much of my time in the field in August 2000 was spent at the South Table Mountain locality discovered by Brown in the 1930s. Although this site has been visited over the decades, a mere handful of mammalian specimens, belonging to only four species, have been recovered. The fossils likely were found on the surface, as there is no evidence for quarrying at the site. Interestingly, most of the specimens belong to a medium dog-sized condylarth known as Baioconodon. Intensive quarrying and screening of sediments, with the help of DMNS volunteers, turned up more fossils. Additional fossil mammal finds can help correlate this locality with other fossil mammal localities in the Denver Basin and elsewhere. I also explored for Tertiary mammals in rocks near Kiowa. In the summer of 1999, important discoveries of fossil mammals were made in the area, one of which was a partial skull of a taeniodont found by amateur fossil collectors.

The Denver Basin holds many clues into a critical time in the evolution of mammals—the first great diversification of Tertiary mammals, and the birth of modern mammals as we know them. The collaborative efforts of the many researchers involved in the Denver Basin project is answering many of our questions about what life on land, particularly among mammals, was like after dinosaur extinction, and what ecosystems operated in the Denver area in early Tertiary time. Just as early Tertiary mammals discovered a whole new world left to them by the dinosaurs, many new fossil mammal discoveries await me in the Denver Basin.

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