Eberle in the Denver Basin
Project: Fossil mammals near the
K-T boundary
Dates: August 8 to 29, 2000
Scientist: Jaelyn
Eberle
Setting the Stage
Jaelyn Eberle
Picture this: It's a hot August day in Denver -- our radio tells us it's 98°F, which translates to really, really hot in Celsius (36.7°C). My husband David and I are cruising west on Denver's I-70 freeway. Our nine-month-old son Nicholas coos from the back seat of our Toyota extracab, while our dogs Louie and Fritz bark at passing cars.
While we could be any one of the thousands of North American families that migrate each summer to see Colorado's glorious Rocky Mountains, our mission is different. We've driven more than 4000 km, braving the Chicago traffic and Montana forest fires, but not to camp in Rocky Mountain National Park or hike up Pike's Peak. Quite the opposite, our interest lies in the low relief, rather non-descript, brownish sediments that blanket the flatlands to the east of the Rockies -- the strata on which Denver was built. While their appearance pales in comparison to the rugged peaks towering over Denver to the west, these rocks, or specifically the fossils within them, tell an amazing story of life and death -- extinction and evolution in the vertebrate world -- in a hot and humid Colorado 65.5 million years ago.
As a palaeontologist who specializes in fossil mammals, I am an optimist. I like happy endings. Several years ago, I asked myself "Why study dinosaurs, who, like so many other organisms, met their demise at the Cretaceous-Tertiary (K-T) boundary? Why not study the tiny mammals that kept right on going, across the K-T boundary, seemingly oblivious to the huge meteorite that crashed into Earth, killing off the dinosaurs (or so many believe)"?
In the more-than-four-billion-year history of Earth, the Cretaceous-Tertiary boundary is but a geologic "millisecond". It is defined primarily by the extinction of one great group of organisms -- the dinosaurs -- at the end of the Cretaceous Period, and the subsequent takeover of the Tertiary world by another great group -- the mammals. In terrestrial rocks around the world, the K-T boundary is drawn above the last, or stratigraphically highest, occurrence of dinosaur bones, at the first appearance of Tertiary mammal fossils. The K-T boundary also can be recognized on land by dramatic changes in fossil plants; some 80 per cent of Cretaceous plants went extinct at the K-T boundary. In some places, an unusually high concentration of the element Iridium, which also occurs in meteorites, marks the K-T boundary. Coined the "Iridium Anomaly", it is believed to represent fallout from the impact of a meteorite.
For a century, considerable press (and debate) has centred on dinosaur extinction at the K-T boundary. Yet, just as provocative and unknown is "Why did the mammals survive"? Furthermore, what happened in the mammalian world, right after dinosaur extinction? Answers to such questions must lie in the rock record of western North America, which preserves by far the most and the best-documented K-T boundary sections in the terrestrial realm. One such K-T boundary section is in Colorado, and this is what brings me to Denver.
At 1560 m above sea level, Denver, the Mile High City, sits in a sediment-filled depression -- the Denver Basin, which stretches about 100 km east from the Rocky Mountains, and is about 200 km from north to south. Sedimentary rocks in the centre of the basin (spanning 15 different rock units or formations) are 3600 m thick, and range in age from 300 million years (MY) to 35 MY before present. Included in this thickness is the Denver Formation, which not only spans the K-T boundary, but also preserves a rare window into the vertebrate world in the first million years after dinosaur extinction -- a critical time when most modern orders of mammals were born.
South Table Mountain
Dark-coloured rocks at the top of the ridge
are remnants of a 63-million-year-old lava flow that has protected South
Table Mountain from erosion. Light-coloured rocks below are sandstones
and mudstones near the mammal locality at South Table Mountain.
Larger image: 80Kb jpg
Let's return to the I-70 freeway in Denver. We follow two trucks from the Denver Museum of Nature and Science (DMNS), filled to their brims with scientists and students. From I-70, we take an exit that plunks us into an interesting mix of office buildings, mini-malls, and expensive homes. We wind our way through the neighbourhood, and come to a stop in a church parking lot. While we're still surrounded by city, just off to our west is South Table Mountain, a flat-topped hill (or butte) rising about 1800 m above sea level. South Table Mountain exists because it is capped by a lava flow that has protected it from erosion for some 63 million years. If it were not for the lava, South Table Mountain would not exist, and in its place a continuation of the same flat surface underlying Denver, covered in high-rises and mini-malls.
Our destination (a five-minute stroll from our truck) is an exposure of sandstones and mudstones on a ridge along the eastern flank of South Table Mountain. Historically very important, this is the first site in North America where the K-T boundary was identified. In 1943, geologist Roland Brown documented the occurrence of dinosaur bones at the base of the ridge, and earliest Tertiary plants and mammals at the top of the ridge. Brown surmised that the K-T boundary lies within a 15-or-so-metre interval between the dinosaur-bearing rocks and the mammal-bearing rocks.
In the 1990s, study of fossil pollen (palynology) refined the position of the K-T boundary to within an interval of 20 to 25 cm. About the same time, DMNS recovered the complete skull of an early Tertiary mammal. Our mission for today and the next several days is to find more earlyTertiary mammals at Brown's original locality, explore for new fossil vertebrate sites, and quarry the beautifully preserved fossil leaves found near the mammal site.
