How ancient plants responded to geographic and climatic events has implications for predicting the effects of current global change on modern ecosystems. The tropical regions are of special concern because that is where Earth’s greatest percentage of biodiversity resides. Fossils hold the key to past community ecology, and can be used to document past climates.
This three-year project will greatly improve documentation of paleoecology, paleoclimate, and floral communities in eastern Africa between 28 and 27 million years ago. To do this, we plan to focus on a wealth of plant fossils in an area near Chilga, in northwestern Ethiopia, by involving students and specialists in paleobotany, paleosols (fossil soils), isotopes, dating, and the rock record.
This collaboration will allow us to obtain independent estimates of landscape characteristics, ecology, and climate from paleosols and isotopes that can be compared and integrated with information from plant and vertebrate fossils. Fossil leaves provide estimates of precipitation and vegetation on the landscape. Paleosols and isotopes provide estimates of climate and the physical landscape.
Plant fossils will help document aspects of floristics and evolution not currently available for tropical Africa. The plant fossils occur as leaf litter assemblages with cellular detail, wood, and flowers, fruit and seed casts co-occurring with vertebrate fossils; and silicified (petrified) wood assemblages representing standing forests.
Researchers are testing three hypotheses:
- Fossil plants and the geochemistry of ancient soils from Chilga provide estimates of past climate that are statistically equivalent to each other.
- Separate leaf assemblages that are the same age and from the same depositional settings provide estimates of precipitation that are statistically equivalent to each other.
- Plants now found separately in the forests of West and East Africa (and not in NW Ethiopia), were present at Chilga 28 million years ago.
Fossil plants can be used to reconstruct past climate. Documenting past climates helps us understand better how Earth’s global climate system works – but all regions of the earth should be well studied to reconstruct the past. Until recently, very little was known from Africa during the interval between 65 and about 20 million years ago. The work in Ethiopia helps to fill that gap. Fossil soils and isotopes are another indicator of past climate – and will provide independently derived climate information that can be compared with that based upon plant fossils.
The project has documented forests both different from and similar to forests of today – for example palms were much more prevalent 28 million years ago than they are now, but close relatives of other kinds of plant fossils can still be found in living West African forests. Of interest will be the response of these ancient communities to climate and environmental stresses – such as volcanic eruptions, known to have impacted the landscape.
Lots of large mammal fossils have been found in the field area and document an odd and interesting mix of extinct unique families and others that are ancestral to the living African fauna. This year, SMU’s Louis Jacobs joined the team to search for the remains of small mammals, which have so far been elusive.
Ethiopian students are participating in a field school learning from paleontologists, geologists, and graduate students. This will advance plans for a paleotoursim heritage site. Collectively, our efforts will improve infrastructure and positively impact the local economy.
Project members have participated in three Ethiopian conferences about Ethiopian paleontology, paleotourism, and tropical African botany. The geological map created by this project will be available for publication by the Ethiopian Mapping Authority.
All new geochronologic, stratigraphic and paleontological data will be entered into the Paleostrat Paleobiology databases.
This project, funded by a $300,000, three-year grant from the National Science Foundation, began as an exploration of the potential of the region for fossil vertebrates – particularly human ancestors – in 1999.
At that time, John Kappelman (UT Austin Department of Anthropology) and Tab Rasmussen (Washington University Department of Anthropology), funded by The National Geographic Society, discovered vertebrate fossils indicating the age of Chilga deposits was much greater than had been thought by previous researchers (28 rather than 8 million years old). They also found an abundance of plant fossils.
Since that time, and with further funding from The National Geographic Society andThe National Science Foundation, these colleagues and several others who specialize in geology (Mulugeta Feseha, Peter Copeland, Jeff Crabaugh, Neil Tabor [SMU]), vertebrate paleontology (Alisa Winkler [SMU], Sevket Sen, Bill Sanders, Louis Jacobs [SMU]) and paleobotany, (Bonnie Jacobs [SMU]) and her students have collaborated to bring the landscape and life of Chilga to light.
How Does The December 2006 Work Fit In With The Other Work Done So Far?
Previous paleobotanical work at Chilga includes the work of Aaron Pan (PhD student at SMU), and has focused on leaf assemblages that tell us about the composition (kinds of plants) of forest communities living in this place about 27.5 million years ago, and some things about their ecology. These assemblages have also been used to estimate past rainfall amounts. Estimates of rainfall are based upon the size of the leaves of the species found in the assemblages studied – it’s known from studies of plants and climate across Africa and other regions today, that wetter climates support plant species with larger leaves. By quantifying this relationship, one can estimate the amount of rainfall in the past from a well-sampled assemblage of leaf fossils.
Another focus of our work is a study of the response of vegetation to massive ash falls. Ash beds are common among the Chilga deposits, but one in particular is prominent across the basin. Juan Garcia Massini (SMU PhD student) is studying the ecological and landscape effects of the sequence of ash falls that created this prominent ash bed.
This diagram shows a representation of all the sedimentary beds at Chilga in an area of the basin where our work has been concentrated. Leaf sites are shown in green, fruit and seed localities are in brown, and the red star shows the location of CH 3 toward the top of the column.
Also shown in this figure are the dates we have on the deposits – which tell us the age of the fossils. A radiometric (isotope) date on the ash being studied by Juan is 27.36 million years old, and a basalt (solidified lava flow) underlying the sediments is dated at 32.4 million years old.
Another technique is called paleomagnetic dating, which uses the precisely-dated pattern of changes in the Earth’s geomagnetic field during geologic history to better constrain the age of sediments. The part of the geomagnetic time scale relevant to Chilga sediments is shown in this diagram – our sediments were most likely laid down during the part of the time scale labeled “C9n”. For more information on dating techniques used here, you can visit http://en.wikipedia.org/wiki/Potassium-argon_dating ,http://en.wikipedia.org/wiki/Argon-argon_dating and http://en.wikipedia.org/wiki/Geomagnetic_reversal .
What Is The Bigger Purpose Of Working In Ethiopia?
We need to understand the evolution of Africa’s biomes (forests, woodlands, and savannas), the plant species that comprise these, and the climate history of the continent. Why? Climate has no geographic boundaries – atmospheric circulation in tropical Africa, interaction between these air masses and others and with oceanic circulation off the African coasts, impact climate elsewhere in the world. This holds true now and for times in the past.
To understand our current changing climate, as well as that of the past, we must know what was going on at low latitudes, including in Africa. As for the vegetation – the early origins of Africa’s flora is largely a mystery. What we know comes primarily from hypotheses generated by the modern distributions of plants, rather than from the fossil record. The African angiosperm record (angiosperms, or “flowering plants”, make up nearly all the plants living in today’s tropical, subtropical, and temperate regions) is especially poorly known for the time interval between their evolutionary origin about 130 million years ago and about 20 million years ago.
The floral record, ecological information about the vegetation, and the climatic information from Chilga (28 million years ago) provides us with a unique and important view of the past – which has implications for the origins of Africa’s current flora and for better understanding global climate change.
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