Packing for the trip home

Updated: August 2, 2000

Today we spent the day packing the fossils and preparing the paperwork for transporting the specimens we collected back to the USA. In this report we will say a few words about some of the fossil plants we found.

Above: Packing fossils for the trip to the United States.

The fossil flora at Mahenge is dominated by plants in the legume family (Leguminosae). This is the family that food plants such as beans, peas, and soybeans come from, as well as common trees such as redbud, black locust, honey locust, mimosa, mesquite and many others. Legume trees are very diverse and important in the tropics, especially in South America and Africa. Thus it is not unexpected that the fossil assemblage from Mahenge contains many legumes. However, the diversity and abundance of legumes at Mahenge is greater than we first thought based on the relatively few plant specimens collected in 1996. We now know that there are at least three, possibly four, types of caesalpinioid legumes, two or three mimosoids, and one papilionoid legume (each of these refers to one of three subfamilies).

Above: Legume fruit collected from the site near Mahenge.

We have found fossil leaves and fossil fruits belonging to this family. One of the fossil fruit types is probably a mimosoid legume (like Acacia or Mimosa). These fruits are especially exciting because all but one of them shows evidence of insect damage. Bruchid beetles are a major predator of legume seeds in the tropics today. The fossil fruits that we found show evidence of bruchid predation- a circular exit hole in the center of the fruit valve above each seed. Only one fruit of this type that we found was not damaged by these insects. This is similar to the pattern observed today in which these insects destroy most of the seeds in a population.

Above: Cynometra leaflets from Mahenge.

One of the legume leaf types that we found is similar to the tropical genus Cynometra. The leaflets are strongly asymmetrical and occur in pairs. This leaf type was known from one specimen in the 1996 collections, but it is even more incomplete than the specimen we show here (we have a better one but it is already packed!). The new specimens that we found, although none are perfect, provide much more information about leaf structure in this plant. Leaf structure is very variable among the living members of this group and thus the new specimens we found are very important in helping us to investigate the relationships of this fossil. Even less than perfect fossils can provide important character information.

Among the other fossil plants that we collected is a very interesting and unusual leaf with a strongly serrate margin (photo at left). The majority of our specimens are smaller, and non-toothed, so this represents something new and relatively rare. In fact, the modern vegetation around the site consists of many plants that are related at the family level to those in the fossil collection - it consists of a woodland dominated by trees in the legume family. The implication is that the environment around the lake 46 million years ago was similar in overall appearance to what it is today, even if the species composition was not exactly the same.

This report was prepared by Pat Herendeen