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Taphonomic, Avian, and Small-Vertebrate Indicators of Ardipithecus ramidus Habitat

Antoine Louchart et al.

The stratigraphic unit containing Ardipithicus ramidus was probably deposited rapidly, thus providing a transect through a 4.4-million-year-old landscape. To help reconstruct and understand its biological setting as thoroughly as possible, we recovered an assemblage of >150,000 plant and animal fossils. More than 6000 vertebrate specimens were identified at the family level or below. These specimens represent animals ranging in size from shrews to elephants and include abundant birds and small mammals that are usually rare in hominid-bearing assemblages. Many of these birds and small mammals are highly sensitive to environmental conditions and thus are particularly helpful in reconstructing the environment.

Accurate interpretation of fossil assemblages can be challenging. Even fossils from one layer can represent artificial amalgamations that might have originated thousands of years apart. Moreover, the remains of animals living in different habitats can be artificially mixed by flowing water or by shifting lake and river margins. Ecological fidelity can be further biased by unsystematic recovery if, for example, only the more complete, identifiable, or rare specimens are collected. Thus, interpreting the Ardipithecus-bearing sediments requires that we deduce the physical and biological conditions under which the fossils accumulated and the degree to which these biases operated at the time of deposition—a practice called “taphonomy.”

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Abundance of birds (left) associated with Ar. ramidus. These distributions are consistent with a mostly woodland habitat. (above) An example of the many small mammal and bird bones.

Both the large- and small-mammal assemblages at Aramis lack the damage that would result from transport and sorting by water, a finding consistent with the fine-grained sediments in which the bones were originally embedded. Many of the limb bone fragments of large mammals show traces of rodent gnawing and carnivore chewing at a time when the bones were still fresh. These bones were most probably damaged by hyenas, which in modern times are known to destroy most of the limb bones and consume their marrow. The actions of hyenas and other carnivores that actively competed for these remains largely explain why the fossil assemblage at Aramis contains an overrepresentation of teeth, jaws, and limb bone shaft splinters (versus skulls or limb bone ends).

As a result of this bone destruction, whole skeletons are extremely rare at Aramis, with one fortunate exception: the partial skeleton of Ar. ramidus excavated at ARA-VP-6/500. The relative abundance and damage patterns of the fossils representing small mammals and birds suggest that they are derived from undigested material regurgitated by owls (owl pellets). Because of their fragility and size, bird bones have been rare or absent at most other eastern African fossil assemblages that included early hominids. However, we cataloged 370 avian fossils; these represent 29 species, several new to science. Most of the birds are terrestrial rather than aquatic, and small species such as doves, lovebirds, mousebirds, passerines, and swifts are abundant. Open-country species are rare. Eagles and hawks/kites are present, but the assemblage is dominated by parrots and the peafowl Pavo, an ecological indicator of wooded conditions.

The small-mammal assemblage includes up to 20 new species, including shrews, bats, rodents, hares, and carnivores. Extant counterparts live in a variety of habitats, but their relative abundance in the fossil assemblage indicates that Ardipithecus lived in a wooded area. Avian predators most probably procured the much rarer squirrels and gerbils from drier scrub or arid settings at a distance. Most of the bat, shrew, porcupine, and other rodent specimens are compatible with a relatively moist environmental setting, as are the abundant fossils of monkeys and spiral-horned antelopes.

The combination of geological and taphonomic evidence, the assemblage of small-mammal and avian fossils, and the taxonomic and isotopic compositions of remains from larger mammals indicate that Aramis was predominantly a woodland habitat during Ar. ramidus times. The anatomical and isotopic evidence of Ar. ramidus itself also suggests that the species was adapted to such a habitat.

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