Death and Fossilization

Este ensayo fue escrito por W. Scott Persons, Victoria Arbour, Matthew Vavrek, Philip Currie, y Eva Koppelhus para el MOOC Dino 101: La Paleobiología de los Dinosaurios y es publicado en este blog para ser traducido al español con Duolingo.

Learning objective for lesson 2: Students will be able to describe how fossils form and interpret the taphonomy of skeletons and bonebeds.

Learning objective 2.1: Classify fossil occurrences as articulated skeletons, associated skeletons, isolated elements, or bonebeds.

Learning objective 2.2: List environments in which fossils can become preserved through burial.

Learning objective 2.3: Understand which environments are best for preserving fossils.

Learning objective 2.4: Define the three main categories of rocks.

Learning objective 2.5: Identify which kinds of rocks preserve dinosaur fossils.

Learning objective 2.6: Classify types of fossil preservation.

Learning objective 2.7: Describe the techniques used to collect, prepare, and curate dinosaur fossils.

Learning objective 2.8: Identify taphonomic features common to dinosaur bones.

Learning objective 2.9: Arrange the sequence of events in the fossilization process.

Learning objective 2.10: Evaluate the taphonomic history of a dinosaur fossil.

After Death

We have discussed how fossils can be used to learn about a dinosaur’s life. Often, there is more to a fossil’s story. The moments immediately after a dinosaur’s death may have been an eventful period, and a great deal could happen in the more than 65 million year interval between a dinosaur’s death and the discovery of its fossils. Taphonomy is the study of all natural processes that involve an organism after it dies -- this includes how it decays, is scavenged by other organisms, becomes fossilized, and erodes.

Although you might think that a dinosaur would naturally stay put after it dies, it is not uncommon for dinosaur carcasses to have been moved considerable distances from the original site of their deaths. Predators, and later scavengers, may carry a carcass to their dens or some other more secure feeding area. Shortly after death, decay may cause a body to swell with putrid gasses, and this may cause the carcasses of even large animals to float easily and to be transported by shallow and weakly flowing water. This phenomenon is known as bloat-and-float.

Finding complete dinosaur skeletons is rare. More commonly, only a single bone or a few isolated bones are found. There are many taphonomic factors that can contribute to the disarticulation of a skeleton. Partial consumption by carnivores is one such factor. Carcasses that have rotted for some time may be easily broken apart if swept away by rivers or flood waters. Water currents may also carry different portions of a skeleton to different locations, based on the weight and shape of the different bones. Prolonged exposure to sunlight gradually weakens and disintegrates bone. Skeletons that become only partially buried will eventually lose their exposed portions. Portions of skeletons may also be trampled by animals or have their mineral content leached away by the roots of plants. These are only a few examples, and there are a large number of other taphonomic factors that can contribute to both the transportation and the disarticulation of a skeleton.

Even while buried, taphonomic factors may modify a skeleton. The weight of layers of rock and sediment above a bone may flatten it, and even bone that has already fossilized may be subjected to plastic deformation. Plastic deformation occurs when pressure causes the shape of a buried fossil to be changed such that, even when the pressure is later removed, the fossil does not return to its original shape. Plastic deformation is an important process to understand and to be mindful of. Otherwise, plastically deformed fossils may be incorrectly assumed to display their true original shapes.

Fossilization Environments

Fossils may form in a variety of ways. The different ways that fossils form are called preservation styles. Most dinosaur bone fossils form through either permineralization or replacement. Permineralization occurs when the empty internal spaces of a bone are filled with minerals. These minerals are first dissolved in water and are then deposited in the empty bone spaces as water soaks through the bone. Replacement occurs when the original bone gradually decays and minerals fill the space that the bone once occupied.

To become fossilized, a bone needs to be buried. Burial can occur if an animal dies in its own burrow, if it falls into a sinkhole, or if it, or one of its bones, is buried by a predator. But most often, burial occurs when water washes sand or mud over a carcass. Fossilization is, therefore, more common in wet environments than in dry environments, where there is no water to help bury carcasses. Fossilization is also more common at low elevations, where sand and mud carried in by water are able to build up, than at high elevations, where sand and mud are often carried away by erosion before they can build up and ‘permanently’ bury and protect a carcass. For this reason, we most often find dinosaur fossils in ancient rocks whose sediments were deposited by rivers, streams, and lakes.

River and stream deposits are called fluvial deposits, and lake deposits are called lacustrine deposits. Lacustrine deposits have the best chance of preserving soft tissues like hair or feathers. This is because there is relatively little water movement at the bottom of a lake to disrupt the skeleton, and the sediments laid down in lakes are very fine-grained mud – which naturally preserve impressions of soft structures better than course sand. Even though there were no marine dinosaurs, dinosaur skeletons are sometimes found in ancient coastal deposits and even in deep-water marine deposits. Many dinosaurs lived along ancient sea shores, and occasionally dinosaurs were washed out to sea by storms and tidal waves.

Aeolian deposits form from sediments that are accumulated not by water, but by wind. Aeolian deposits are characteristic of dry deserts. Usually, wind deposits sediments much more slowly than water, making deserts poor places for fossils of large animals to be preserved. For this reason, few desert dwelling dinosaurs are known. However, one amazing exception is the the ancient environment represented by the fossil-rich rocks of the Gobi Desert in Mongolia. During the Cretaceous much of what is now Mongolia was a sand swept dessert, but it was not all dry. A river also coursed through the dessert and, like the modern Okavango River system of modern Africa, the river formed a large deltaic plain that created a huge oasis. In this deltaic plain, many dessert animals, including large dinosaurs, had a chance to be buried by the sediments that were deposited by the river. Dinosaurs in the desserts of Mongolia could also be buried in another way: by sand dunes that suddenly collapsed onto the still living animal. Sand dune collapses happen when the stability of a dune is compromised by saturating rainstorm. The skeletons of dinosaurs that were buried in this way are often preserved in crouching positions, with their necks bent upwards reaching for air.

Sedimentology

With only a few rare exceptions, all fossils are found in sedimentary rocks. Sedimentary rocks are rocks that form when mineral and organic particles accumulate and become either cemented or compacted together. The two other basic rock types are igneous rocks, which form when magma or lava cools, and metamorphic rocks, which form deep underground when sedimentary or igneous rocks are changed by extreme heat and pressure.

Sedimentology is the science of how sedimentary rocks form. Different kinds of sedimentary rocks form in different environments. Understanding the environmental conditions that led to the formation of the particular sedimentary rocks that contain a fossil can give important clues about the habitat of the fossil organism. Sedimentary rocks that form from mud and silt are called mudstone and shale. Lakes are places where large amounts of mud and silt accumulate, and large deposits of mudstone and shale often indicate a former lake bottom environment. Sedimentary rocks that form from sand are called sandstone, and sandstone can indicate a former beach, river channel, or ocean floor environment. Coal is a special kind of sedimentary rock that forms from the compressed remains of plants, and coal indicates a former swampy environment. Limestone is formed from the accumulation of shells and exoskeletons of small marine invertebrates, and limestone always indicates a former shallow marine environment.

Where to Dig

Simply because a dinosaur bone managed to beat the odds and become buried and fossilized, does not mean that the fossil will ever have a chance to be discovered by a paleontologist. Most of the dinosaur fossils that ever formed have either been destroyed (they have been melted or metamorphosed by geologic processes deep within the earth or have eroded away to dust on the earth’s surface) or they remain buried too deep for current excavation technology to detect or to reach. Just as becoming a fossil requires a special set of circumstances, so does becoming a fossil that is discoverable.

To prevent a fossil from eroding away, it must remain buried. However, the burial process must be at least partially reversed in order for the fossil to be near enough to the surface to be found. Dinosaur fossils are, therefore, most commonly found in modern environments where there is considerable recent erosion. Modern environments that are covered with vegetation are bad places to hope to find fossils. Vegetation covers and holds together an environment’s topsoil and prevents erosion. Badlands, such as those throughout the Canadian and American west, are arid environments where vegetation is sparse, where erosion rates are high, and where large expanses of ancient sedimentary rocks are exposed. Badlands are among the best places to hunt for fossils.

Excavation

Using geologic maps, paleontologists can identify locations where there are exposures of sedimentary rocks that are the right age to contain the fossils of dinosaurs. Often, a paleontologist that is hunting for dinosaurs returns to a particular location where fossils have been found before. Whether hunting in a new location or returning to an old location that has previously yielded good specimens, a paleontologist does not simply grab a shovel and immediately commence to digging. First, a paleontologist, and usually an entire paleontological field crew, prospects for promising specimens. The ideal dinosaur skeleton is one that is freshly, and only just barely, exposed above ground. Fossils that are not exposed at all are simply not detectable, and fossils that are completely exposed, and have been for a long time, may be badly weathered.

Once found, the first step in the excavation of a large fossil specimen is overburden removal. Overburden is the rock and earth that covers a fossil specimen and that must be removed before the full extent of the specimen can be judged. Overburden removal usually involves large indelicate tools like shovels, pickaxes, and occasionally even jackhammers and bulldozers. However, such tools are not used in close proximity to fossils. At close distance, the work of the final excavation switches to hand picks and brushes.

Large dinosaur skeletons or bonebeds (accumulations of the bones of many dinosaurs) cannot usually be excavated and removed all at one time. Instead, they must be excavated in parts and usually over the course of many field expeditions. Before any one bone is removed, it is important to map its location relative to the other bones. Mapping the relative positions of bones may help in putting a skeleton back together and may also give important taphonomic clues. For instance, if, in a bone bed, all the long limb bones share a similar orientation -- are observed to lie roughly in parallel with each other, this may indicate that the bones were carried and deposited by a strong river and that this river oriented the limb bones in line with its current.

Once a bone has been mapped, it is ready to be dug up. Although fossil bones are mineralized, they are usually brittle and unable to support their own weight. This makes them delicate to transport. To protect a fossil bone on its trip from the field to the laboratory, the bone is wrapped in a layer of protective material (this can be cloth, paper towel, or aluminum foil) and is then covered by strips of burlap that have been soaked in plaster. Once the plaster hardens, it forms a strong and rigid jacket around the fossil. These plaster jackets are not opened until they have reached the laboratory. Then, special glues are applied to the fossils to strengthen them. The final work on removing the rock that surrounds a fossil takes place in the laboratory, and this process often takes more time than the field excavation.

There are many clues available to help a palaeontologist understand what happened to a dinosaur after it died, and sometimes even evidence for how the dinosaur might have died. Disarticulation of a skeleton may occur as carnivores eat the carcass, or because the specimen was transported by water. In a bonebed, the orientation of the fossils is important: long bones (like the femur or humerus) that are aligned in the same direction indicate that the bones were transported by water, and tell us the direction the water was flowing. The amount of abrasion on the bones can give a relative sense for how far the bones may have been transported by flowing water. Scratches on the bones can be tooth marks, which indicate that carnivores fed on the carcasses (but do not necessarily indicate that the dinosaur was killed by that carnivore)