By BONNIE F. JACOBS AND ELLEN CURRANO, NYT
None of us has ever experienced a site like this. Not only are the shales full of leaf fossils, but we have now also found beautiful and important fossil bones, including the tooth of a small mammal and the scapula of an artiodactyl. (This is an order of hoofed animals that are also known as even-toed ungulates — picture a mammal that walks on its tippy-toes, like a gazelle.)
These discoveries mean there is great potential for finding other mammals here, including primates. This site will fill a gap in the record of African vertebrate evolution — there are no others of this age known.
Treats for the paleobotanists include a fossil flower, many seeds, a bean pod with seeds in it and part of a lobed leaf that, when whole, must have been as big as your head (even if you are big-headed). The leaves are quite abundant, so in two days’ work we have already collected more than 300 specimens.
Ellen Currano A fossil of a legume fruit with seeds.
In order to get a good sample of leaves for ecological and climate analyses, we collect each locality from a single horizontal level, sub-sampling from at least three places across several lateral meters of outcrop. We repeat this method in several spots in order to understand the ecology of this place across space and time. The thickness of a sampled stratum is five centimeters, which represents a short time interval — probably less than a century. So our samples allow us to “see” the forest, with all its variation, across at least half a kilometer and through perhaps a couple of centuries. Once we understand how many species we have and what they were, we will gain some understanding of the ecology of these communities.
Leaves can also be used to estimate past rainfall — and rainfall seasonality — based on their size. As you probably know, the leaves of plants found in a tropical rain forest are moderately large, while the leaves of plants in desert species tend to be small or even tiny. These correlations can be used to draw inferences about fossil assemblages and past rainfall amounts. The luxury of having a large sample size that includes many species will make for a more reliable estimate.
Among other things, we are also looking at the role that insects played in our ancient plant communities. Ellen Currano, an assistant professor at the University of Miami in Ohio, is an expert in the analysis of insect damage on leaf fossils, and here she is to explain what she’s noticing in Mush Valley:
Because our fossil leaves are so well preserved, I can analyze the insect feeding damage on them. Some of the leaves have holes in them, and I can tell that these holes were made while the plant was alive because there is a rim of thickened tissue surrounding the hole, essentially a scab that formed to protect the wounded area. Because insects are the most common culprits today, I assume that these 22 million-year-old holes were also made by insects.
Another type of feeding damage I have seen here are galls. Galls form when an insect lays an egg within a leaf. This stimulates the plant to create a tumorlike protuberance of new tissue around the egg. The insect hatches, lives within the gall, eats the gall tissue and eventually emerges to continue its life cycle.
Ellen Currano Fossil of a leaf with galls.
I study insect feeding damage for three reasons. First, the insect fossil record is patchy. Although there are amazing insect fossils, particularly the ones preserved in amber, they are few and far between. Studying the different types of feeding damage from a fossil leaf site allows me to infer what types of insects lived during times when we have no insect fossils, and how many species there were then.
Second, I am interested in how plant and insect communities respond to climate change. In the middle latitudes, I have found that insect feeding damage increases as temperature increases, most likely because of tropical insects migrating north. But what happens in the tropics, the warmest part of the earth?
Finally, I am looking for clues to solve Darwin’s mystery of why there are so many species of insects. There is particular diversity in the tropics, probably because tropical insects are very specialized feeders, able to eat only a single species of plant. This means that many species of insect tend to be crammed into a region. By studying the types of insect damage and what proportion of the damage was made by specialized feeders, I can determine whether tropical insects were as diverse and specialized 22 million years ago as they are today.
Bonnie F. Jacobs, a paleobotanist at Southern Methodist University, writes from Ethiopia, where she is studying fossils of ancient plant and animal life. The current field season in the Mush Valley of Ethiopia is financed by a grant to Ellen Currano of Miami University, Ohio, from the National Geographic Society Committee on Research and Exploration.