1. Body fossils and trace fossils

Broadly speaking, paleontologists divide fossils into two main groups:

1. Body fossils.
2. Trace fossils.

Body fossil example 1

A wall of large dinosaur bones (permineralized remains) from the Jurassic period on display at Dinosaur National Monument, Colorado. The fossil bones of large vertebrates like dinosaurs are only very rarely found connected, or articulated. Typically, they are not found attached to each other (i.e., they are disarticulated), as is the case here. This requires the scientists who study the bones to make interpretations about how the bones might have fit together when the animal was alive. As new scientific discoveries are made, reconstructions of individual dinosaur species often change, requiring museums to rebuild their specimens on display and artists to create new illustrations.

Body fossil example 2

A fossilized leaf from the Eocene-aged Green River Formation (preserved as a carbonization). Because of the excellent quality of preservation, little imagination is required to reconstruct the appearance of the once-living leaf. This specimen is on display at the Utah Field House of Natural History in Vernal, Utah.

Body fossil example 3:

An assemblage of well-preserved fossil Turritella snail shells from the Miocene-aged Gatun Formation of Panama, showing unaltered preservation, despite being over 10 million years old.

Trace fossils

Trace fossils (sometimes also called ichnofossils) provide evidence about the movements and/or activities of ancient organisms, but not necessarily about their appearance.There are three major types:

1. Movement traces.
2. Predation traces.
3. Digestive traces.

1. Movement traces

As their name implies, movement trace fossils tell us about how ancient animals moved within their habitats. It is easy to grasp why such fossils are so useful for understanding ancient life.

Consider the footprints you might leave behind as you walk on a sandy beach or through the snow. While they don’t reveal much about your appearance (besides the general size and shape of the bottom of your foot or shoe), they reveal the direction you were moving, whether you were traveling alone or in a group, and also provide clues about whether you were walking (footprints close together) or running (footprints further apart).

Dinosaur footprints provide the best examples of movement trace fossils and provide paleontologists with the same kinds of clues that human footprints do.

Discoveries of dinosaur footprints and trackways–left behind in soft sediment that later turned to rock–are not altogether rare and new discoveries are made regularly. For example, a massive footprint left behind by a titanosaur was recently discovered in the Gobi Desert.

Not all movement trace fossils are footprints, however. Other types of movement traces include the trails and burrows left behind by animals in the sediment before it hardened. Sediments that were heavily mixed by burrowing organisms prior to turning to rock are said to be bioturbated. Such bioturbated rocks usually lack finely preserved layers; the absence of such layers is sometimes an indication for geologists that the sediments were well oxygenated, thus allowing animals to crawl through them.

The specimen shown below is a common type of movement trace fossil called Zoophycos. The animal (likely a worm) that made the trace probably lived in a burrow near the position indicated by the arrow. The clockwise swirling lines may indicate the sweeping motions of a feeding structure that gathered food from the sediment.

2. Feeding traces

Feeding trace fossils are usually predatory in nature. They are important because they provide a record of ancient ecological interactions between species, as well as indicate how these interactions may have changed over time.

Some modern snails, especially naticids and muricids, attack their prey (usually bivalves or other snails) by first drilling holes into their shells. Paleontologists commonly find similar holes in fossil shells, allowing them to assume that the same kinds of predators made the holes in the past. This assumption is bolstered by the fact that the shells of the predators themselves are often found in the same deposits.

Crabs are also important predators of snails and bivalves in marine ecosystems. They use their powerful claws to break the shells of their prey. Their attacks are not always successful in killing their prey, however. In some cases, the shell of the prey is damaged, but is later repaired as it recovers from the attack. Evidence of such attacks are commonly preserved as jagged, high-relief scars that cut across normal lines of growth.

3. Digestive traces

Digestive trace fossils consist of fossilized excrement.  These fossils are scientifically known as coprolites. 

Coprolites are scientifically useful because they sometimes tell paleontologists what types of food ancient organisms were eating. For example, some large Cretaceous coprolites contain fragments of dinosaur bone, suggesting that the excrement was produced by a large meat eating dinosaur.