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.
Monday, Dec. 27
This winter’s field season in Ethiopia is my tenth since I began working there, and despite my experience I am filled with anticipation. Our project is a relatively new one — studying rocks and fossils from an important period of history, 22 million years ago — and the location, Mush Valley, is also somewhat new to our team (last year was our first collecting trip here).
Mush Valley is only about 160 kilometers northeast of the modern capital city, Addis Ababa, but it feels as though it could be a thousand miles away. Very little of city life intrudes into the villages of Upper and Lower Mush.
What really takes me away from it all are the rocks and fossils exposed by and alongside the Mush River. They provide us an exciting opportunity to document life, climate, landscape and atmosphere 22 million years ago. As we excavate blocks of fine-grained sediment — primarily shale — looking for clues to the past, the pivotal role played by that ancient time period is always on our mind.
Why is it important to know about the Ethiopian Plateau 22 million years ago? The Mush Valley preserves plants and animals from a time soon after a land connection was established between Afro-Arabia and Eurasia — a land connection that marked the end of Africa’s island status and that was used by animals to migrate between the two previously separated land masses. By looking at the fossil record from that period of time — before the Red Sea was formed — we can gain a clearer view of which species survived this great migration and which did not.
A handful of Africa’s unique island mammal families that were present 26 million years ago are absent from sites younger than Mush, but some originally Eurasian families are found in their place. Circumstantial evidence is pointing toward the influx of invaders as deadly competitors responsible for the demise of the ancient African mammalian families — including Arsinoitheres (an extinct rhinoceroslike mammal), some elephant families and some relatives of living hyraxes.
When and how did this happen? Did it happen all at once, very rapidly, or perhaps more gradually? And what can the historical record tell us about the evolution of Africa’s modern iconic mammals and the potential effects of mixing native and non-native animals today?
We have already found some mammal bones at Mush — as yet unidentifiable — in addition to amazing fossil frogs with soft body parts preserved. We hope to find more! And we wonder: Will any of the extinct forms be among them, or will the fauna have a modern aspect?
We will also look at the plant world and ask the same questions. Already we have studied fossils that are 27 million to 28 million years old and come from a region called Chilga that is to the northwest of Addis Ababa. We know the Ethiopian Plateau was clothed by forests with trees, shrubs and vines whose relatives can be found today in the now separated forests of Central and West Africa on the one hand, and Eastern Africa on the other. We think that an increase in the length of the dry season sometime after 27 million years ago caused the loss of those plant groups on the Ethiopian Plateau. But when did the flora change, and can we find evidence of such a climate change?
Many people think the forests of the East and West became separated with development of the arid and semi-arid East African Rift, a slash in the earth running thousands of kilometers from the Gulf of Aden and Red Sea through Ethiopia, Kenya, Tanzania and Malawi. But the rift did not begin to develop until around 15 million years ago. Will the 22-million-year-old Mush flora be essentially the same as Chilga floras?
We have noticed a big difference between the forests of Chilga and what we have found so far at Mush. At Chilga we found palm fossils in rocks all over the depositional basin — they were ecologically important 27 million to 28 million years ago. Today, palms are rarely important in African forests, and are not diverse. We know from fossil pollen that palms were flourishing when tropical Africa and South America were splitting apart, about 70 million years ago, and they continue to flourish in the Amazon and Southeast Asia.
If palms were still present and important 27 million years ago at Chilga, when was their demise? We are anxious to see if any palms turn up at Mush. If they are present, what kind are they? If they are absent, does it tell us something about climate change -– or was there another reason?
Past climate can be assessed from fossil plants — the shapes and sizes of their leaves, and by comparison with where their living relatives live — and various other measures from the rock record that are independent of the plants. We are able to estimate the mean annual temperature, and the mean annual and seasonal rainfall. The chance to use more than one means of estimating climate and the rest of the physical environment is part of what makes us so excited about our work.
Estimates of past climate are useful for testing the accuracy of computer-generated global climate models (useful for estimating scenarios of future climate), and for understanding how earth’s climate system operates in general. Climate scientists often run models meant to represent a time in the past to see if they can generate a global climate system that is consistent with what is known from estimates that come from fossils.
The record is woefully poor for tropical latitudes, and we expect that estimates from both Mush and Chilga will be useful for these kinds of studies. Perhaps more importantly, there is currently some scientific uncertainty about the climate during the interval between 27 million and 22 million years ago — and more data is needed to settle the issue.
The plant fossils are exquisitely preserved. They occur as leaf and fruit compressions with the organic outermost cuticle layer on both sides of the leaves still intact. Cuticle is a waxy substance laid down on the leaf surface as it develops, so looking at leaf cuticle is essentially the same as looking at a leaf’s outermost cells — the epidermis. Included among these cells are hairs (yup, leaves have hairs), any sort of bump or ornamentation, and most important, stomata. A stoma is an opening through which carbon dioxide goes in and oxygen and water vapor escape. I like to think of them as akin to mouths (botanists, forgive me!). The pattern made by the cells of a leaf’s stomatal openings can help paleobotanists to identify it.
Here’s another cool thing about stomata: It has been shown that when the concentration of carbon dioxide in the atmosphere is high, many plant species reduce the number of stomatal openings on their leaves (relative to the regular cells). Conversely, when atmospheric carbon dioxide is low, stomatal density tends to increase — it’s like getting more mouths to breathe with. We plan to use this relationship — which must first be established or calibrated using modern plants for the genus or species of plant fossil — in order to estimate the concentration of carbon dioxide in the atmosphere 22 million years ago.
Our team in the field this year consists of Mulugeta Feseha, an associate professor of geology at Addis Ababa University; Ellen Currano, an assistant professor at Miami University in Ohio who specializes in paleobotany and insect damage analysis; Aaron Pan, a paleobotanist who is curator of the Fort Worth Museum of Science and History; Louis Jacobs, a professor of earth sciences at Southern Methodist University whose specialty is vertebrate paleontology; and Jordan Noret, a paleobotany graduate student at Southern Methodist University.