Introduction

Fossils are the physical remains of Earth's past biosphere. In this Lesson, we will examine how fossils form and how they can help us reconstruct the Earth's ancient history.

In its literal sense fossil means "obtained by digging". For paleontologists a fossil is any trace of past life. Two broad categories of fossils are:

1. physical fossils, which include bones, shells, gastroliths (stomach stones), skin impressions, coprolites (fossil faces), and eggs; and
2. trace fossil or ichnofossil, which are the tracks and trails left by organisms and preserved in the geological record (see figure below).

How do Fossils Form?

There are many ways a fossil can form. For example, fossils form when insects are trapped in amber, creatures are frozen in glaciers or permafrost, or dehydration forms natural mummies. A common mode of fossilization is mineralization, the replacement of organic material by inorganic minerals.

The most common dinosaur fossils are the mineralized remains of bones and teeth. Bones are composed of calcium phosphate and organic material. Common mineralizing media are calcite, iron minerals, and silica. The finest mineralization is called permineralization (see figure below) where there is an atom-by-atom replacement of the original organisms tissues and/or bones by minerals.

Finding and Understanding a Fossil

Only a very small percentage of creatures that have lived become fossils. Not all creatures have the same potential to become fossils. Factors that come into play that determine the preservability or fossilization potential of a creature include:

• composition of the creature
• the environment where the organism lived and died
• processes affecting the dead organism

All processes that occur to a creature prior to it become a fossil are called taphonomy. These processes include decay (necrolysis) and movement of the corpse. Necrolysis starts a few minutes after death and will continue until the destruction of the corpse or until fossilization. The factors that will determine the degree of necrolysis include:

1. the supply of oxygen. The less oxygen available, then fewer scavengers and microbes can exist. Examples of low oxygen environments are swamps and at the bottom of some large bodies of water such as the Black Sea.
2. pH extremes. A good example of areas that experience extremes in pH are peat bogs. These areas can be extremely acidic and effectively transform flesh into a kind of leather (see figure below).

3. temperature. The lower the temperature, the less microbial activity occurs and decay is prevented (see figure below).

4. nature of the organic carbon. Organic material such as fats and muscle will decay relatively rapidly. Others types of organic carbon, for example cellulose in plants, will decay more slowly.
5. movement of the corpse (also called biostratinomy). Corpses may be moved by scavengers or by physical process such as wind and water transport.

In addition to necrolysis and biostratinomy, the environment in which the creature inhabited (the paleoenvironment) will have a large bearing on fossilization potential. In general creatures that live in water have a much better chance of being preserved as fossils compared to land creatures. This is mostly due to the fact that creatures that live in bodies of water have a much greater chance of being covered by sediment and/or transported to low oxygen environments, which increase the chances of it becoming fossilized. A land-based corpse is much more likely to lie exposed on the surface and therefore be more at risk from scavengers, certain physical processes, and the destructive activity of the Earth's oxygen-rich atmosphere. An attempt to describe the preservation potential of organisms as a factor of their mode of life is presented in the figure below.

Paleobiology: Understanding Fossils as Former Living Creatures

To understand fossil creatures we have to consider the following:

• functional morphology: How did the organisms move?
• developmental morphology: How did the organisms grow?
• behavioural paleontology: What did the organisms do?
• interaction between organisms: How did the organisms interact with others in their environment?

Functional morphology

Some fossils are quite obvious in terms of their function. For example, the bones of a leg of a dinosaur are quite obviously designed to let the creature walk or run. However problems can arise when there are no known creatures that are similar to the fossil under study. This makes consideration of any analogous behaviour quite difficult.

An example of such a strange and troublesome creature is Hallucigenia (see figures below, re-printed from Module B Lesson 5). It lived in the Earth's equatorial oceans during the middle Cambrian. Because nothing like Hallucigenia is found living today, it was considered a very strange creature and first attempts at reconstructing it had the creature presented upside down!

Figure B-29. Hallucigenia

(left) Reconstruction of Hallucigenia. (right) Simon Conway Morris' original reconstruction of Hallucigenia showing it upside down. Illustration (left) copyright by Karen Carr. Image (right) from the Geological Survey of Canada.

Developmental morphology

Sometimes it can be difficult for the paleontologist to determine if a population of fossils represents one species at different stages of development or a group of different species.

Imagine the problems extraterrestrial paleontologists may have if they were to come across human fossils if no living humans could be found on planet Earth. Here are just some of the differences we can find in our own species:

• Humans that live in hot arid climates tend to be tall and long limbed (for example the Maasai people of Africa) whereas humans adapted to cold conditions tend to be shorter and squatter (for example the Inuit peoples in arctic areas).
• There are also obvious differences between the sexes with men tending to be larger and women having adaptations in the pelvis for childbirth.
• Fossils of children would also look quite different from adults.
• Aside from the obvious size differences, babies have a different proportion of body parts with the head being much larger when compared to the rest of the body than adults.

A fossil example can be drawn from ostracods (see figure below). Ostracods are small crustaceans that secrete a bivalved shell around them. Like all crustaceans, these creatures (and their fossil relatives) grow by shedding their old external skeleton and growing into new larger one. As a result each ostracod can produce a whole series of different sized shells through its life cycle from juvenile to adult. In addition, female ostracods often posses brood pouches where eggs are stored. These can make the external shell of a female ostracod look very different to that of the male.

Developmental morphology is an important factor that paleontologists have to consider if they are to accurately determine the biodiversity and biological composition of ancient environments.

Behavioral paleontology

Given the restrictions that fossils place on paleontologists in interpreting and reconstructing ancient creatures, you could be forgiven for thinking that determining the ancient behavior of these creatures could be almost impossible. This is not necessarily correct. In fact, there are many clues that can be taken from the fossil record that can shed light on the way these ancient creatures lived. Some examples of these clues from dinosaurs are discussed below.

Trace fossils (tracks and trails)

Trace fossils are one way that we can examine the way creatures moved and interacted with their environment. For example, tracks preserved at Dinosaur Ridge in Colorado have been used to demonstrate that Allosaurs were actively tracking other dinosaurs like lions track game today (see figure below).

Concentrations of dinosaur bones

These "bone beds" are sometimes composed of a single species of dinosaur. These beds, when present in sediments deposited by rivers, have been interpreted as herds of migrating dinosaurs attempting to cross rapidly-moving rivers. This is very much like the way that large numbers of Wildebeest are swept away by rivers that they have to cross during there annual migration.

Nurturing dinosaurs

Exceptional fossils of dinosaurs protecting their young in nests have been discovered. These clearly demonstrate that (at least) some dinosaurs cared for their young after they hatched from the nest (see figure below).

Interactions between organisms

To develop a more complete picture of how fossil organisms were a part of the environment they lived in, we need to try and understand how they interacted with other individuals. Once again, although the evidence may be a little spare, there is still a surprising amount of information that can be gathered concerning such behavior. Four general types of interaction between organisms are defined by biologists. Each of which may also be recognized in some exceptional fossil samples.

Mutualism

In mutualism both species benefit from the interaction. An example in today's environment are bees and flowers. An example of mutualism in the fossil record occurs between a sea snail called Platyceras and a creature related to sea stars and sea urchins called a crinoid. Platyceras is often found attached to a crinoid over its anus, which in crinoids is positioned close to its mouth. It is thought that Platyceras benefits from feeding on the waste material from the crinoid and the crinoid benefits by having the waste material removed effectively and kept isolated from repeat ingestion.

Commensalism

In commensalism, two species live in close proximity with little or no benefit or harm to each other. For example, different species of herbivorous dinosaurs sharing the same geographical location.

Parasitism

In parasitism, one organism (the parasite) benefits from the relationship while the other (the host) may suffer severe detrimental effects. A modern example is malaria, where a parasitic protozoan, Plasmodium, inhabits the bloodstream and liver of animals, including humans. Parasitism is difficult to recognize in the fossil record but certain fossil species of crinoid do demonstrate unusual growths that suggest a reaction to a parasite that has entered their external skeleton. This is similar to the way that certain modern trees react to a parasitic invasion of their trunks.

Predation

There are many examples of predation in the fossil record. Perhaps one of the most famous concerns Mosasaur bite marks on ammonite shells (see figures below).

Dinosaur Fossils

Dinosaurs are difficult fossils to find as they were exclusively land-based creatures. As already discussed, land-based (terrestrial) fossils do not fossilize as well as those that live in bodies of water. Despite this, a variety of dinosaur fossils have been discovered including bones and teeth (the most common), skin impressions, eggs, trace fossil, coprolites (fossil dung), and internal brain casts.

Where to find dinosaur fossils

In each of the environments below, a paleontologist will be looking for a place where sediment has been deposited, which might have covered up a dinosaur corpse prior to it being destroyed by taphonomic processes.

Fluvial (river) environments

This is the most common sedimentary environment to find a dinosaur fossil in, probably because open sources of water attract large animals and a flowing river would be able to rapidly cover a dinosaur corpse with sediment (such as silt, sand, and conglomerates). Dinosaur remains found in these environments are often composed of disarticulated bones, the result of the corpse being transported and knocked around by the riverine flow. The most complete dinosaur fossils in this environment are found on the flood plains next to rivers where the water flows a little less quickly and where there is a large supply of finer-grained sediment to cover the dinosaur corpse.

Lacustrine (lake) environments

Dinosaur fossils have been found in the fine sediments around lake margins or on river deltas entering a lake. No dinosaur fossils have been found in deeper lake sediments suggesting that few (if any at all) dinosaurs were aquatic.

Aeolian (desert) environments

In general, fossils are very rarely found in any desert sediments because the wind blown silt and sand are very abrasive and erosive. Even so, some exceptional fossils have been found in Mongolian desert sandstones, including the famous fighting dinosaurs specimen (figure below).

Deltaic environments

A delta forms when a river enters a body of water. The material that has been carried by the river is dropped in a fan shaped deposit. The delta top consists of many branching rivers and is often swampy. Thus delta systems will have similar preservability potential as standard river systems have for dinosaurs.

Interpreting dinosaurs

We learned how taphonomy and paleobiology must be consider when interpreting fossils. However, it is a fact that are own cultural and historical bias are significant factors in interpreting these extinct organisms. For example, one of the first dinosaur reconstructions was of Iguanodon. To Victorian (mid-1800s) scientists, reptiles were slow sluggish creatures that moved around on four legs. As a result the reconstruction of Iguanodon by Waterhouse Hawkins in 1854 reflect that bias (see figure below).

Today, dinosaurs are not regarded as slow moving lizard-like creatures. Reconstructions of many dinosaurs show them as active and dynamic creatures (see figure below).

T. Rex Goes on Trial

Not all types of interpretations of dinosaurs are welcomed by the scientific community and the general public.

Dinosaurs in media and art

Our impression of the fossil world is very much shaped by what we see in books, magazines and increasingly in movies, and on TV. As a result, our perception of ancient environments may be biased by these media. Consider the following three conventions in paleontological art.

Numbers of individuals/crowding

Have you noticed that any landscapes that can observed today are very rarely crowded with creatures? For example, the photograph below was taken in the southern Okanagan of British Columbia, Canada. We know that this environment is rich in creatures: bears, cougars, humming birds, insects, rattlesnakes, scorpions, cattle, and many, many other species. However, none of these are visible in the photo.

Now, compare this with the illustration of the Cretaceous Coastal Environment below. The impression given is that the Cretaceous is brimming with creatures, a very crowded landscape compared to our modern landscape of the Okanagan. In reality, the Cretaceous landscape was probably no more crowded with animals than today!

Figure C-1. Cretaceous Coastal Environment

A mural displayed at the Sam Noble Oklahoma Museum of Natural History. Illustration copyright by Karen Carr.

Activity

Another feature of many dinosaur recreations is extreme activity. Dinosaurs are often shown to be in a perpetual state of hunting or being hunted. In reality, life at that time can be described as "long periods of boredom interspersed with brief moments of terror." In other words, for the majority of individuals, much of their time was spent pursuing mundane activities.

Emphasis

We humans are vertebrates and as a result have a certain bias towards vertebrates in our reconstructions of life in past geological time. This gives the impression that the history of life on Earth has been dominated by vertebrates. In fact, the opposite is true! Of the 1 million species so far described, only 40, 000 or 4% of the total are vertebrates.

Allosaurus: Big Al Uncovered

View the following movie. The study/review questions that follow should be used as a guide/self-assessment tool to help you gauge your understanding of the material presented in the video.

Video Study/Review Questions

1. Who is "Big Al" and in what geological period did he live?
2. In what type of sedimentary environment was he preserved?
3. Who are Big Al’s closest living relatives?
4. How long did it take for an Allosaur to grow to full size?
5. Were Allosaur brains more like birds or alligators?
6. Other than brain shape, what also suggests that Allosaur feeding behavior may have been similar to that of alligators?
7. What do paleontologists think caused Big Al’s death?