Ardipithecus ramidus: The Geological, Environmental, and Taphonomic Background

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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.

20 Comments

“I would like to look at the geological,environmental, and taphonomic background”

The only background needed is that the universe was created less than 10,000 years ago by Mr. Jeebus H. Cripes.

JF

Tcha! No need to go out of your way to pick fights, guy. Somebody will show up to pick one for you quickly enough.

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

It’s a dirty job, but somebody’s gotta do it?

That’s what grad students are for.

fnxtr said:

That’s what grad students are for.

Precisely.

You know what the difference is between grad students and galley slaves? They feed galley slaves every now and then.

… the universe was created less than 10,000 years ago by Mr. Jeebus H. Cripes.

Gee, let’s see, now who else do we know that’s a troll fond of the word “Jeebus”?

The Discovery Channel has a special about Ardi this Sunday at 9 pm.

Karen S. said:

The Discovery Channel has a special about Ardi this Sunday at 9 pm.

Yes, you can also get a sneak preview here - there should be about 7 clips on Ardipithecus.

Science is just so cool!

40% tree cover in the east, 65% tree cover in the west, and no primates were found in the east.

Isn’t this one of the puzzles the finders are trying to solve: That is, if a thinning forrest forced us to walk, why are there no fossils in the east? Is 65% tree cover sufficient to cause bi-pedalism, or were there other factors? I believe the researchers mentioned something about the size of the male’s canines as evidence for bipedalism. It was suggested that life pair bonding reduced male competition for mates, but increased the need to insure your DNA passed on, How does that increase bi-pedalism? Cheers.

Certainly, an amazing find, one of the key ones of the last century.

What has always struck me: the amount of effort that goes into finding such an old fossil skeleton. They’ve been searching in that area for decades to come up with one partially complete and very fragile Ardipithecus.

And how little effort is actually expended relative to the task. From the science articles, it looks like there is only one or a few teams searching that strata every year.

Given our curiosity about our evolutionary trajectory, if it was up to me as (hypothetical) grant distributor, a lot more effort would be expended. I suppose the next step is to go back another 1-3 million years and find out what gave rise to A. ramidus.

Mounting field work presents significant challenges. Most people having teaching obligations, limiting time for field work. Crews can be large and need to be fed and guarded, especially when working in an area that is not entirely safe. The weather often dictates when one can and cannot work in a given area. Local politics can present significant challenges. These fossils are housed at the Ethiopian National Museum. The Ethiopian government is building a wonderful new facility there, but resources for cleaning and preparing specimens are limited. Even electricity can be a problem – last time I was there I had trouble sending email because the power outages kept cutting me off. Trained and skilled staff for handling all the material are limited.

Finding fossils at any given time period requires exposed strata. There are few available. This has been a major issue in searching for hominins. And remember that in most areas, the hominin fossils are a small component of the vertebrate fossil assemblage. This means that it can take a long time to find hominins, and of course that one needs to handle all the vertebrate and botanical specimens, all of which need the same care and attention as the hominins. That’s a LOT of work.

Considerably more material was found than just the skeleton. Complete specimens of anything are rare. Compared to what is known for other fossil hominins, the ramidus material is abundant.

An enormous amount of work went into these things. A lot of us have disagreements and a lot of questions about the fossils and the interpretations. There is a lot of behind the scenes discussion about how White and colleagues have handled things. But everyone concedes that these are excellent and important fossils, and that Tim and colleagues have done a tremendous amount of work.

raven said:

Certainly, an amazing find, one of the key ones of the last century.

What has always struck me: the amount of effort that goes into finding such an old fossil skeleton. They’ve been searching in that area for decades to come up with one partially complete and very fragile Ardipithecus.

And how little effort is actually expended relative to the task. From the science articles, it looks like there is only one or a few teams searching that strata every year.

Given our curiosity about our evolutionary trajectory, if it was up to me as (hypothetical) grant distributor, a lot more effort would be expended. I suppose the next step is to go back another 1-3 million years and find out what gave rise to A. ramidus.

Mounting field work presents significant challenges.

Ann Gibbons, Science mag.:

The 1st week, White and a paleontologist were sick, and White is still fighting a harsh cough that keeps him awake at night. The 2nd week, some aggressive Alisera tribesmen who live near the Ar. ramidus site threatened to kill White and Asfaw, making it difficult to return there. (That’s one reason the team travels with six Afar policemen armed with AK-47s and Obama caps, dubbed “The Obama Police.”) The day before, a student had awakened with a high fever and abdominal pain and had to be driven 4 hours to the nearest clinic, where he was diagnosed with a urinary tract infection, probably from drinking too little water in the 35°C heat. “The best laid plans change every day,” says White, who has dealt with poisonous snakes, scorpions, malarial mosquitoes, lions, hyenas, flash floods, dust tornadoes, warring tribesmen, and contaminated food and water over the years. “Nothing in the field comes easy.”

That is what the science articles said. They had armed guards and had to dig their own well for water. Digging their own well in an arid environment sounds tricky. Who knows how deep the water table is? And how does one dig a well in the African desert in the middle of nowhere. With shovels?

Money solves a lot of problems. We spent who knows how many trillions of USD bailing out Wall Street after they helped wreck our economy.

The hostile tribespeople sound like more than a nuisance. I’m tempted to say they were creationists whose idea of doing human origens science is threatening researchers like they do in the USA. But really, they probably aren’t that sophisticated,…yet.

I doubt they were creationists as we define it. Ethiopia has had several military coups and, if memory serves, one province declared independence. Consequently, the Alisera are somewhat suspicious of outsiders.

afarensis,

Great post, which illustrates why it is important to collect as much data as possible pertaining to the fossil bones, not merely the bones themselves. Was wondering why Ardi was well preserved, and am pleased that you’ve discussed at length its taphonomy.

Watched the Discovery Channel program on the finds – they did a pretty good job, including a follow-up roundtable discussion of the significance of the materials.

Unfortunately I missed it, but look what Nova has coming up November 3, 10, and 17: a 3-part series on human evolution called Becoming Human

Karen S. said:

Unfortunately I missed it, but look what Nova has coming up November 3, 10, and 17: a 3-part series on human evolution called Becoming Human

That looks really interesting. I should mention that I hope to have the next post - covering the anatomy of Ardipithecus up on Thursday…hopefully…

Scott said:

Science is just so cool!

stop !!!!!!!!!!!!!

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This page contains a single entry by Afarensis published on October 10, 2009 1:15 PM.

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