What Is the Difference Between a Cast Fossil and a Mold Fossil?

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Fossils are an exciting part of paleontology and archaeology, as they help scientists determine what life was like during prehistoric times. A fossil is an impression of a living thing, and it’s petrified either in a mold or cast. There are several distinct types of fossils: mold fossils, cast fossils, trace fossils and true form fossils. There are slight differences in each. 

What Is a Mold Fossil?

A mold fossil is an impression, or imprint, in substrate. Substrate is another term for sediment or rock, and the fossil is petrified within. Mold fossils are completely hollow. Because of the way they’re imprinted, they offer a backwards look at the organism or creature. The most common mold fossils include teeth, claws, skin and embryos. They can also include organisms such as leaves.

What Is a Cast Fossil?

Cast fossils are very similar to mold fossils. Like mold fossils, they form and are imprinted within a type of substrate. However, instead of being hollow, minerals, rocks or other materials have filled in the gaps for a more “solid” fossil. Like mold fossils, cast fossils are commonly skin, claws, teeth, leaves and embryos.

What Is a Trace Fossil?

A trace fossil is decidedly different from mold and cast fossils and does not offer much information about the organism

— thus the word “trace.” Examples of trace fossils include footprints, nests, tooth marks, excrement and burrows. Trace fossils do offer information about life during prehistoric times, as they give us hints about how an organism lived or hunted.

What Is a True Form Fossil?

As its name suggests, a true form fossil is often a limb or a large part of an organism that has become petrified over time by means of sediment, rocks and minerals. These differ from molds and casts because they are not an impression, but an actual part of a living thing. Common true form fossils include heads, fingers, torsos and limbs.

How Are Fossils Formed?

When an organism comes to the end of its life in the outdoors, it slowly becomes buried in the substrate, or sediment. During this process, the organism decomposes. As it does, it begins to form an impression in the sediment, which leaves a distinct imprint. When it comes to mold and cast fossils, no actual material of the original organism remains, but what is left is a cast or mold of the organism. Over time, water flows through the imprint to form either

a cast or mold fossil. This imprint can help scientists discover new things about life in prehistoric times. 

How Are Fossils Used?

Taking proper care of fossils is imperative. Even though they’ve existed for many years, they are fragile and must be preserved. Typically, when a fossil is found, the paleontologist will make a plaster of Paris or fiberglass mold of the fossil, and this is what is put on display in museums and other exhibits. Sometimes, the actual fossil is put on display, but if it’s too heavy, such as is often the case with true form fossils, a cast must be used. Paleontologists also make

molds for students to study and handle so that they can learn more about prehistoric times.