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:
Figure E-1. Trace fossils (Ichnofossils): Dinosaur tracks, Navajo Canyon, Arizona
Dinosaur tracks, Navajo Canyon, Coconino County, Arizona, Marsh Pass Quadrangle Formation July, 1910. Photo by Gregory, H.E., USGS
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 () where there is an atom-by-atom replacement of the original organisms tissues and/or bones by minerals.
Figure E-2. Petrified wood from Princeton, B.C.
This large sample of petrified wood is displayed at the Pacific Museum of the Earth, Earth and Ocean Sciences Main Building, UBC, and is of Eocene age (54 -33 million years old). Here, the original wood from this tree has been replaced by silica (quartz). Photo by S. Sutherland.
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:
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:
Figure E-3. "Iron Age" Man
Photo of "Lindow Man" a naturally-preserved bog body of an Iron Age man, discovered in a peat bog at Lindow Moss, northwest England in 1984, nearly 2,000 years after his death. Chemicals in the bog preserved the body. Photo courtesy of the BBC.
Figure E-4. Baby Mammoth Preserved in the Permafrost of Northwestern Siberia
A baby mammoth, discovered in the permafrost of northwestern Siberia, has been dated at 10,000 years old, sometime near the end of the last Ice Age. Photo courtesy of the BBC.
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.
Figure E-5: Preservation Potential of Organisms as a Factor of Their Mode of Life
To understand fossil creatures we have to consider the following:
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 (, 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
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:
A fossil example can be drawn from ostracods (). 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.
Figure E-6. Ostracods
Image from H. Armstrong and M. Brasier, 2005, Microfossils, 2nd ed, 304 pp, Wiley-Blackwell.
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 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 ().
Figure E-7. Dinosaur tracks at Dinosaur Ridge, Colorado
Map from M. Lockley and A. Hunt, Fossil Footprints of the Dinosaur Ridge Area, Friends of Dinosaur Ridge and the University of Colorado at Denver Dinosaur Trackers Research Group.
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.
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 ().
Figure E-8. Nurturing young: Buried alive.
This adult Psittacosaur could do nothing to save the 34 hatchlings in its care. Note how erosion has truncated several skeletons, including the adult, on the left and upper sides of the specimen. Scale bar, 10 cm. Photo from Q. Meng, et al., 2004 ("Palaeontology: Parental care in an ornithischian dinosaur", Nature 431:145-146).
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.
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.
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.
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.
There are many examples of predation in the fossil record. Perhaps one of the most famous concerns Mosasaur bite marks on ammonite shells ().
Figure E-9. Mosasaur and Ammonites in the Cretaceous Marine Environment
(Karen Carr. Images ( ) from E.G. Kauffman, 2004 ("Mosasaur Predation on Upper Cretaceous Nautiloids and Ammonites from the United States Pacific Coast", Palaios, 19:96-100).) Reconstruction of a Mosasaur attacking an ammonite. ( ) Close-up views of ammonite fossils showing penetration of adult mosasaur teeth. Illustration ( ) copyright by
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.
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.
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.
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.
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 E-10. Velociraptor vs. Protoceratops
Fossil skeletons of Velociraptor and Protoceratops apparently locked in mortal combat. It is speculated that the two animals in the original scene were fighting when a sand cliff collapsed on top of them, preserving their skeletons in that precise posture until their discovery in 1971 in Southern Mongolia. Photo courtesy of Dinosaur Kingdom Nakasato.
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.
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 ().
Figure E-11. "4-Legged Sprawling" Iguanodon Statues
Reconstruction of Iguanodons by Waterhouse Hawkins were completed in 1854 and still stand today in the Crystal Palace Park, London. In reality, Iguanodons are capable of bipedal movement. The horn placed on the creature's nose actually belongs on Iguanodon's thumb. Photograph provided by Dr. Giles Miller, Natural History Museum, London.
Today, dinosaurs are not regarded as slow moving lizard-like creatures. Reconstructions of many dinosaurs show them as active and dynamic creatures (.
Figure E-12. A Modern Reconstruction of Iguanodon
Note the dynamic pose, bipedal behaviour, and the thumb spike in the correct position. Illustration by Werner Willmann, Wikimedia Commons.
Not all types of interpretations of dinosaurs are welcomed by the scientific community and the general public.
Download and read this article entitled BBC website.and make notes on the information presented. You may also access the article directly from the
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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.
Have you noticed that any landscapes that can observed today are very rarely crowded with creatures? For example, thewas 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.
Figure E-13. The Okanagan Valley, British Columbia, Canada
Now, compare this with the illustration of thebelow. 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.
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.
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.
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.
(movie available to registered students only)
BBC documentary, 2001
The story of an Allosaurus, a large carnivorous dinosaur, which lived in the Late Jurassic about 145 million years ago. "Big Al" is the name given to a real Allosaurus fragilis specimen, whose nearly complete fossilized skeleton was found in Wyoming in 1991. This documentary describes how scientists reached their conclusions about this carnivorous dinosaur's life.
Viewing Time: 30 minutes