Ardipithecus ramidus: The Geological, Environmental, and Taphonomic Background
As Matt Young pointed out recently, the fifteen year wait for the complete publication of the Ardipithecus ramidus skeletal material discovered by Tim White, and his research group, is over. The material was finally published in Science and is open access. I will discuss the morphology of Ardipithecus ramidus and its implications in a second post. In this post I would like to look at the geological, environmental, and taphonomic background to the discoveries. I examine these first because they provide strong evidence to back up some of the behavioral interpretations of Ardipithecus ramidus.
Geology
The Middle Awash area of Ethiopia contains strata ranging in age from the Late Miocene to the Pleistocene. These strata have yielded a wide variety of botanical, invertebrate, and vertebrate fossils. Of particular importance is the Lower Aramis Member of the Sagantole Formation. The Lower Aramis Member sits atop the Gaala Vitric Tuff Complex (GATC) and below the Daam Aatu Basaltic Tuff (DABT) and crops out along a ~9 km arc. The Lower Aramis Member ranges in thickness from 3-6 m and is composed of silt, sand , and clay deposited on a low relief floodplain at some distance from the river channel. The two tuffs that sandwich the Lower Aramis member have been dated via the 39Ar/40Ar laser fusion method. The Gaala Vitric Tuff Complex dates to 4.419 ± 0.068 Ma and the Daam Aatu Basaltic Tuff dates to 4.416 ± 0.031 Ma. These results are statistically indistinguishable and raises the question of how much time elapsed between the two. WoldeGabriel et al, in their paper on the geology of the area argue that this time span may be somewhere between 100-10,000 years - although it could be as high as 100,000. This is an important point, the group argues that most hominid bearing sites elsewhere represent significant periods of time and because of that do not say much about the environment of the hominids.
Environment
There are a number of ways one can reconstruct an ancient environment. White’s group used quite a few of them. First, they looked at stable isotopes, particularly carbon and oxygen. Results suggest the the environment ranged from woodland to grassy woodland savanna. One can also look at plant and animal fossils. One of the footnotes to the paleobiology papers tells how this material was collected and I can’t resist quoting it in full:
In 1994, the Middle Awash project instituted “crawls” of sedimentary outcrop between the GATC and DABT to collect all available fossil material. Crawls were generally upslope in direction, done by teams of 5 to 15 collectors who crawled the surface on hands and knees, shoulder to shoulder, collecting all fossilized biological materials between a prescribed pair of taut nylon cords. Surfaces were repeatedly collected with this technique, invariably resulting in successively depressed specimen recovery numbers in subsequent field seasons.
The result of this procedure was the collection of a large quantity of botanical, invertebrate, and vertebrate fossils (including a number of new species). The botanical material included phytoliths, endocarps and fossilized wood. Analysis of the material indicated an open grassland with ~40% tree cover in the east and increasing to ~65% in the western sites. Interestingly enough no primates were found at the eastern sites. The invertebrate fossils include insect larvae, dung beetle broodballs, gastropods, millipedes, pupal cases, and solitary bee brood cells. Most of these are rather fragile and do not survive transport well so we can conclude that they are locally derived. What these fossils indicate is a lowland forest in a semi-arid region with a high water table. The vertebrate fossils include catfish, cichlids, giant terrestrial tortoise, various species of turtle, frogs, snakes, lizards, crocodiles, 32 genera of small mammals (including 20 new species), 29 different taxa of birds (mainly terrestrial - small species being abundant and derive from owl pellets), bovids, primates, and a wide variety of carnivores. The fauna present support the picture revealed by the geomorphological, isotopic, and botanical evidence.
Taphonomy
Fossil assemblages can arise from a wide variety of different factors. They can, for example, be collected and gnawed on by porcupines, pass through the digestives system of carnivores, be trampled by other wildlife, be transported by fluvial processes, suffer exfoliation and cracking due to exposure to the elements, and a wide variety of other processes. A key to understanding them is in understanding the history of the bones themselves. One example of this is the fish fossils mentioned above. The environment is an open canopied woodland on a floodplain. The fish fossils arose via overbank flooding, yet without understanding the geomorphology of the region one might be lead to think that the environment was much wetter than it actually was.
There were no signs of fluvial transport such as rounding or abrasion of the bones, nor was there indication of sorting. Most showed signs of subaerial weathering and exfoliation. Evidence of trampling was rare. This indicates the bones were exposed for a brief period before burial. The lack of evidence for transport of the larger bones is consistent with the evidence for local derivation and burial of the small mammals and invertebrate fossils. In connection with this, I should mention that the precise location, orientation and dip of the bones was recorded. I mention this because mudcracking does occur in this type of environment. Inevitably, rodent limb bones get tipped into them and become vertically aligned - which I think is really cool. At any rate, most of the large mammal bones showed signs of toothmarks and digestive etching by stomach acids. Complete destruction of the articular surfaces of long bones was also common - something hyenas do to extract bone marrow. This is all summarized in one paragraph:
The overall Ardipithecus-bearing locality and sublocality assemblages indicate that the competition for large mammal carcasses must have been intense. Abundant shaft fragments, rare epiphyseal portions, and the extremely low representation of axial postcrania as compared to those of the appendicular and craniodental skeletons, combined with the high tooth-marking rates, suggest that the Aramis ecosystem may have matched highly competitive modern settings such as Ngorongoro Crater (10). The rarity of late-stage weathering damage characterized by deep cracking and exfoliation (<3% of total specimens at stages 4 and 5) suggests that exposure to subaerial conditions before burial was brief and/or buffered by tree cover and/or leaf litter.
Because of the short time depth of the Lower Aramis Member (mentioned above) White and his group argue that the earliest of hominids did not evolve in a mixed mosaic environment, rather they evolved in more closed habitats until the origin of the Australopithecines.
Literature Cited
Louchart et al. (2009) Taphonomic, Avian, and Small-Vertebrate Indicators of Ardipithecus ramidus Habitat, Science 326, 66 DOI: 10.1126/science.1175823
White, et al. (2009) Habitat of Ardipithecus ramidus Macrovertebrate Paleontology and the Pliocene Habitat of Ardipithecus ramidus, Science 326, 67 DOI: 10.1126/science.1175822
WoldeGabriel et al. (2009) The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of Ardipithecus ramidus, Science 326, 65 DOI: 10.1126/science.1175817
Edited to correct some typos.