How to Become a Primate Fossil

Only a small fraction of the primates that have ever lived has been preserved as fossils. After death, most primate bodies are eventually dismantled and devoured by predators, scavengers, and microbes. If not, then their remains undergo chemical decomposition and, ultimately, their molecules are scattered across the ecosystem and recycled. The same is true for all organisms throughout history. Because life as we know it depends on the death and decomposition of organisms, the fossil record is necessarily incomplete. As such, finding fossils involves not only perseverance and luck, but the discovery of any particular fossil also depends on the chance that the specimen preserved in the first place. Although fossilization affects only a small fraction of the organisms that have ever lived, there are well-documented conditions that foster the process. Thanks to Efremov's founding of the science of taphonomy -translated from its Greek roots meaning "laws of burial" - the steps to fossilization are not as mysterious as they once were (see Shipman, 1993). The following taphonomic sequence will increase a primate's chances of becoming a fossil that will one day be studied by paleontologists and marveled at by museum visitors.

The primate dies. This may occur within the primate's habitat or, for example, the carcass might be deposited in a carnivore den.

For the best chances of fossilization, the primate carcass will be largely neglected until it is buried: i.e., not entirely destroyed by predators and scavengers, trampled by other organisms, washed down a turbulent riverbed, and, ultimately, not completely recycled into the ecosystem so that parts of its teeth and bones are left intact (Figure 1). Even when such things occur, fossilization can and does proceed, but the forces that dismantle the carcass diminish the likelihood of fossilization and greatly reduce opportunities for discovery, identification, and analysis of the ancient primate's remains. Nevertheless, the processes that affect the carcass form an important part of the context scientists need to best understand the circumstances of the death - and by extension, life - of the ancient primate.

The primate carcass is buried by geologic processes. The sediment in which it is buried cannot be too acidic. Some silty lake bottoms are hospitable environments, and so are bogs and tar pits which lack many micro-organisms which could further disintegrate the remains. Although some wet environments, especially anaerobic ones, offer fossil-friendly conditions, so do very dry ones. Sedimentation occurs most frequently and most rapidly in places where water and air are moving, or when other events (for example, volcanic eruption) deposit soil, sand, mud, ash, or dust on the landscape. If the primate dies in proximity (space and time) to such sedimentary forces, then its chances for fossilization are improved. Burial by volcanic ash is especially helpful because, depending on the geochemistry of the ejected sediment, not only could the conditions foster fossilization, but the sediment could be dated by geochronologists. Ideally, underground microbes, burrowing animals, and plant roots do not disturb, break, or disperse the remains any further.

Once buried in what is now termed the matrix, the organic materials that comprised the living primate can begin to disintegrate or to chemically change along with the matrix. Over deep, or "geologic," time, and with the right temperature, chemical, and pressure conditions, hard rocks are formed from soils, sands, silts, muds, ashes and other types of loose sediment. This process is called diagenesis and refers to the lithification of both the sediment to the biological remains within the sediment. When once-living organisms are transformed into rocks or leave impressions or imprints within rocks, then those rocks are referred to as fossils, and the process is known as fossilization. Trace fossils are imprints left in sediment by an organism - either during its life (e.g. footprints, insect nests, and tooth pits on the bones of prey) or during its death - that become rock. Many small soft-bodied creatures are preserved this way and it is how a primate's hair, skin, and organs can preserve (e.g. Franzen et al., 2009; Figure 2). However, these are very rare cases. Instead, primates are usually preserved as petrified teeth and bones that can be extracted from the surrounding rock. These kinds of fossils can be formed in two major ways.

Casts occur when the decaying remains dissolve completely and leave a space that becomes filled-in or replaced by sediment which then becomes rock. This is how brains fossilize and sometimes how whole organisms, like invertebrates, can be preserved (Figure 3). This process of replacement can also extend to isolated bones and fossils if none of the original material is preserved.

However, much of what primate paleontologists collect and study are not merely casts but are comprised, in part at least, of the actual fossilized bones and teeth that have transformed over deep time (Figure 4a & 4b). Bone, like wood, is porous and full of canals which, when the bone is buried, are filled-in by the calcite or silica (or other minerals and compounds) that precipitate out of the ground water or that leech out of the surrounding sediment matrix. These new minerals may change the color of the remains. In addition to petrifying or cementing the biological tissues, this process may also leave the delicate cellular structures intact.

Tooth enamel is densely packed with a durable mineral called hydroxyapatite, which makes teeth much better than bones at withstanding the chemical and physical degradation that occurs during fossilization. As a result, teeth are the most abundant elements in the primate fossil record. Fortunately, paleontologists can use tooth morphology to identify species fairly accurately.

Diagenesis is not a uniform process. Some remains experience fossilization quickly, and some slowly, depending on the conditions. Specimens that are nly partially fossilized are dubbed "subfossils." These include the skeletal remains of animals that have recently gone extinct, like many species of giant lemurs on Madagascar, some of which were lost within the last 2,000 years (Godfrey et al., 2006). These recent fossils may also preserve DNA, which has been successfully extracted from the remains of subfossil lemurs (Karanth et al., 2005) and Neanderthals (Green et al., 2006).

Deep, geologic time is absolutely crucial to fossilization (and also to the evolutionary processes revealed by the fossil record). There are tricks for attempting to comprehend deep time like unrolling a long strip of paper and ticking off time intervals over millions of years (e.g. Mr. Croll's strip of paper (Darwin, 1872)); calibrating distances on the globe to match depth of time (Parker, 2011); or animating a few branches of the primate fossil record into a timeline (Dunsworth, 2011). Demonstrations like these reveal the fact that there are more gaps in the fossil record than there are fossils. Earth's evolutionary history includes so many organisms and covers such deep time that it is taking generations upon generations of paleontologists to go out into the field, collect new specimens, and fill in the gaps in the fossil record, a subset of which is the primate fossil record.

Before undertaking an expedition to collect fossil primates, paleontologists first look to geologic maps of a region to determine if rocks of the right time period are actually present and exposed on the landscape. If they are interested in finding the most ancient fossil apes, like Proconsul, then they look for rocks that have been dated to the early Miocene epoch. If the rocks have not been dated either absolutely or relatively with stratigraphic methods, then prior collections from the region may demonstrate that early Miocene species are present, indicating that Proconsul may be present as well.

Once rocks of the right age are pinpointed on a map, then paleontologists may use Google Earth and aerial photographs or they might visit by airplane, automobile or foot to determine if those rocks are exposed and accessible and free of thick vegetation, agricultural crops, or water cover.

The word “fossil” derives from the Latin verb fodere which means “to dig.” However, one need not always dig in order to find fossils. There are localities such as Koobi Fora in northern Kenya (Figure 5) and Rusinga Island in western Kenya (Figure 6a & 6b) where many fossil primates have been found and where vertebrate fossils litter the ground.

Primate fossils are often hard to find, not because they are hard to see, but because of the slim chance that a dead primate will be preserved, transformed into rock, and then found thousands or millions of years later. If a primate is going to become a fossil, then it needs to get buried quickly before other organisms devour and scatter its bones. It should die near a river, pond or lake, for example, where water is constantly building up sediment. The sediment cannot be too acidic or contain too many micro-organisms since these promote chemical degradation. If the conditions are sufficient, the organic materials in the primate's bones and teeth lose their organic properties and take on the mineral and chemical properties of the surrounding matrix. This transformation is fossilization. Over time, as the bones and teeth and the surrounding sediment metamorphose, soft tissues are rarely preserved. However, crania may leave brain casts behind and skin may leave impressions as trace fossils. Paleontologists strategically survey rocks with geologic ages that correspond to the chapter in Earth's evolutionary history that they're interested in learning about. For primate evolution, that chapter begins roughly 65 million years ago, continues through to today, and will end at an unknown time in the future.

Fossil remains reveal the paleobiology of the organism, but none of that information has any scientific value without context, which includes data like the location, the sedimentary matrix, the taphonomy, and the age of the specimen. Without context, scientists cannot relate a fossil to other living or fossil organisms. Understanding change over time - adaptation, variation, relatedness of organisms (a.k.a. phylogeny), and so on - are the goals of paleontology, and knowing the context of fossils is absolutely crucial for making or breaking scientific hypotheses. If a fossil has no context, then it cannot be placed in the Tree of Life, it cannot be evolutionarily linked to living organisms, and, therefore, it can do nothing to further our understanding of how evolutionary processes work and how present-day species arose.

Many governmental agencies require that people obtain permits before collecting fossils, and this is done, in part, to protect a fossil's context. In the United States it is illegal to remove fossils from public lands like National Parks. (Learn about Utah's fossil collecting rules and regulations here: http://geology.utah.gov/online/pdf/pi-23.pdf. See Kenya's requirements on the website for the National Museums of Kenya: http://www.museums.or.ke/.) When someone picks up fossils, hikes them out to the nearest town, and sells them at a shop, then those fossils are rendered scientifically meaningless. Without context, fossils are just funny looking rocks.