Body Fossils

Bones and teeth are common as body fossils. The iconic Dinosaur National Monument Quarry Exhibit Hall Camarasaurus DINO 2580 specimen.

NPS photo.

Introduction

Body fossils are what most people think of when they see or hear the word “fossil.” Body fossils are the remains of plants, animals, and microorganisms preserved in a geologic context.

Body fossils are in contrast to trace fossils (ichnofossils). Trace fossils are the evidence of biological activity of an organism in the geologic record, but without some part of the animal or plant itself actually preserved. Typical trace fossils are footprints or trails, compared to body fossils such as bones or petrified wood.

The vast majority of body fossils are not of the whole organism, but only its most durable parts. Most frequently, body fossils consist of hard parts such as:

• Bone

• Teeth

• Shells, exoskeletons, plates, and other mineralized parts

• Wood

More rarely are the following soft body parts preserved:

• Skin

• Soft tissue and organs

• Leaves, flowers, fruit, or seeds

Body fossils have been recorded in at least 275 units of the National Park System.

Hard Parts Versus Soft Parts

Body fossils are preserved when they are rapidly buried so that they are protected from scavenging, decomposition, and decay.

Body fossils that consist of the hard parts of organisms are much more common in the geologic record than those of soft parts. The simple reason that hard parts are much more likely to be preserved as fossils is because they are more able to survive the various events that may affect them after an organism dies. Bones, teeth, wood, shells, and exoskeletons are more resistant to weathering and decay than soft tissues, and are less likely to be eaten by scavengers. Hard parts can also withstand transportation by water currents, whether in a river system or in the surf zone along a shoreline. Organisms that are entirely composed of nonresistant or delicate material, such as insects, jellyfish, or various worms, have poor fossil records.

Because soft body parts are so delicate, they only persist in the fossil record under particularly rare geologic conditions. For example, the remains of insects may be preserved in fine-grained lake bed sediments when they are rapidly buried, such as what happened at Florissant Fossil Beds National Monument in Colorado.

Lacewing. Florissant Fossil Beds National Monument.

NPS photo.

Freezing and drying, such as may occur in permafrost or dry caves, can also preserve soft body parts, as can entrapment in asphalt or amber. These conditions are rare in the National Park System. The most common in the parks are cave fossils.

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Body fossils may belong to any of four major categories of organisms:

• 

  Plant Fossils

  Plant fossils include petrified wood, pollen, and, more rarely, leaf impressions. Fruits, seeds, and nuts are rarely fossilized.

• 

  Vertebrate Fossils

  Bones and teeth are commonly fossilized. Skin and organs are rare in the fossil record.

• 

  Invertebrate Fossils

  The majority of all invertebrate fossils are shells, exoskeletons, or other hard parts. Fossils of insects are very rare.

• 

  Other Fossils

  Microorganisms that have mineralized cell walls are commonly fossilized, as are the microbial mounds known as stromatolites.

How Body Fossils Form

Plants, vertebrates, invertebrates, and microorganisms become fossilized through a variety of processes:

Permineralization and Replacement

Permineralization and replacement frequently occur during the fossilization of wood, bone, and shell materials. Permineralization is the infilling of natural pores in original organic material by minerals. Replacement is the substitution of inorganic minerals for the original hard parts of organisms that have been dissolved away.

Recrystallization

Recrystallization occurs when one mineral replaces another of the same chemical composition. This processes most often occurs during the fossilization of the hard parts (shells) of invertebrates.

Molds, Casts, and Steinkerns

The formation of natural molds and casts of organisms, especially of the hard parts of invertebrates, is an important fossil preservation process. Molds and casts may also form of hard body parts of plants or vertebrates, including rare natural molds of trees in lava flows.

Compressions and Impressions

Fossil impressions and compressions can preserve fine details of all types of organisms. These processes are especially important for leaf, flower, and insect fossils.

Preserved Remains (Drying, Freezing, Amber, Natural Asphalt)

Drying, freezing, or entrapment in amber or natural asphalt may preserve body fossils of plants, vertebrates, invertebrates, plants, and microorganisms. These processes often include hard and soft body parts.

Taphonomy

Taphonomy is the study of what happens to an organism’s remains between the time that it dies and when it becomes fossilized. Events that happen during this time interval control what parts of an organism, if any, become fossilized. Some fossils are only fragments of an organism, while others are more complete.

Events that happen before remains are buried in sediment can also impact whether a fossil is articulated (e.g., with its body parts in the position that they were when the organism was alive) or disarticulated (jumbled, separated, or scattered).

Learn More

• NPS—Taphonomy—Death & Decay