by Bonnie F. Jacobs, Monday, Jan. 3
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.
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.
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