Rain Forest Primeval? Colorado fossils show unexpected diversity

The size, shape, and riotous variety of fossil leaves unearthed at a site in central Colorado suggest that the region may have been covered, some 64 million years ago, with one of the world’s first tropical rain forests.

Excavations at a site about 50 meters off Highway I-25 near the town of Castle Rock have yielded the remains of two species of conifers, three types of ferns, six types of fruits, and 90 species of broad-leaved flowering trees. The diverse assemblage of fossils consists primarily of fallen leaves that were suddenly buried by a 25-centimeter-thick flood of mud about 1.4 million years after the demise of the dinosaurs, says Kirk R. Johnson of the Denver Museum of Nature & Science. He and museum volunteer Beth Ellis describe the find in the June 28 Science.

Mathematical relationships between the climate in which a modern forest grows and the size, shape, and variety of leaves on its trees enabled Johnson and Ellis to infer conditions at the site when the forest thrived. The leaves among the ancient forest’s species were large–about 67 square centimeters on average when mature. This leaf size suggests the site received about 225 cm of rain annually, enough to qualify as a rain forest, Johnson says.

About 69 percent of the plant species there had smooth-edged leaves, which hints that the forest had an average temperature of just over 22C, or about the same as modern Miami. Of the 48 species in which the ends of the leaves were preserved in fossil remains, 19 showed abruptly tapered tips, yet another clue that rain was abundant. Leaves with smooth edges and these so-called drip tips shed raindrops particularly well. Most species that prospered in earlier forest ecosystems didn’t have these features.

Johnson says that most of the precipitation at the site came from humid winds that shed their moisture as they cooled while ascending the eastern slopes of the early Rockies. At the time, those peaks may have towered around 3,000 m over sea levels of the Gulf of Mexico, which then stretched into the central United States, and the Cannonball Sea, a long-gone arm of the Arctic Ocean that extended south to where North Dakota is today.

The multitude of tree species found at the Colorado site more closely resembles the diversity found in modern rain forests than the relative monotony found in deciduous woodlands of temperate regions. Also, the variety is surprising because hundreds of other sites dated up to 9 million years later typically preserve only a few species at each locale, says Johnson. Only later, during a worldwide warm spell about 52 million years ago, did widespread reblossoming of ecosystems take place, he notes.

Johnson and Ellis’ survey of plants at the Colorado site is “an excellent and sizable census,” says Leo J. Hickey, a paleobotanist at Yale University. They’ve found an unusual and unexpected set of plants for that time and place, he notes. The unique ecosystem may document a short spike in global temperature lasting less than 100,000 years, says Hickey.