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?
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