By Moira Whitehouse PhD

What is a fossil?• A fossil is the preserved remains of

a once-living organism.

How are fossils formed?• Fossils are formed when animals or

plants are buried in sedimentary rock.

• You should remember how sedimentary rock is formed and how to identify which layers are the oldest and which are youngest.

Plants and animals buried here might become fossils.

If sediment covers a dead organism before the it has time to rot, the organism’s body may become fossilized.

As many, many years pass, new types of sediment are deposited on top of the original sediment containing the fossil and the layers of sediment change to rock. As a result, the fossils are finally buried under layers of rock.

fossils

The youngest fossils would be found in the a. top layers of

rockb. the middle rock

layersc. the bottom

layers of rockNatural Resources Canada

Based on what you already know about how sedimentary is formed, you should be able to tell which fossils in this picture are older or younger.

There are four types of fossils:

Molds

Casts

Trace fossils

Whole or part of an organism that has been preserved

Molds and Casts

1. A mold forms when: a. an organism dies and is buried in sediment such as sand, silt or clay b. the sediment changes to rock and the organism’s body decomposes leaving an imprint or mold in the rock. Molds can be seen if the rock is broken open.

Mold of a scallop in clayPresent day scallop Mold of scallop in rock

2. A cast forms when an organism dies and is buried in sediment. Its body rots leaving a “hole” in the shape of its body. Water with minerals fills up the hole. When the water evaporates, a copy of the original structure of organism is formed as a rock.

Bob Hermann//Cochise College.

Picture of present day snail

R.Weller/Cochise College.

Picture of fossilized snail

Ammonite mold Ammonite

cast

Permission for use in educationViney, M. (2008). The Virtual Petrified Wood Museum, http: petrifiedwoodmuseum.org

Molds and casts are often found together.

gastropod castsgastropod mold

Which is the cast of the clam and which is the mold?

Clam cast

Clam mold

Dinosaur bones are not really bones. They are the fossilized cast of a dinosaur bone that has changed to a rock.

www.oum.ox.ac.uk/thezone/fossils/intro/form.htmhttp://www.oum.ox.ac.uk/thezone/fossils/intro/form.htm

3. Trace Fossils

Trace fossils include leaf prints, burrows, coprolites (feces or poop), trails, footprints.

Trace fossils of an animal tell something about its movement and behavior.

Trace fossils are not an imprint of the hard part of an animal’s body.

Leaf imprints

R.Weller/Cochise College

Wikipedia Creative Commons

Dinosaur tracks

dmr.nd.gov

Coprolite (dinosaur poop)

Worm tubes made by ancient worms as they moved through soil.

In rare instances an entire organism or its skeleton is preserved because the organism gets trapped in a substance that protects its body from decaying. Examples are:

4. Whole or a part of a body is preserved

A spider trapped in tree sap. The tree sap hardened into amber, preserving the whole spider.

Wikimedia Commons

A sabre tooth cat skeleton found in La Brea Tar Pits in California. The tiger fell into the tar and got “stuck”. His skeleton was preserved by the tar.

Wikimedia Commons

40,000 year old frozen baby mammoth found in Siberia

Sometimes, a whole organism can be preserved if it is frozen very quickly.

Photo: Francis Latreille/National Geographic

• Fossils help scientists understand what past environments may have been like.

What do fossils tell us?• Fossils give clues about organisms

that lived long ago.

• They also provide evidence about how Earth’s surface has changed over time.

In the next few slides you will see fossils found either in or near Fort Worth. They tell us:

•about the environment of this area at that time.

•and help us see how things have changed over time.

•what organisms lived here millions of years ago.

Gastropod casts

Oyster casts

Clam cast

Scallop cast

Sea urchin cast

Ammonite cast

One arm of an ammonite cast

Looking at these organisms, you can conclude they once lived:a. in a forest c. in a

grassland b. In an ocean d. in a desert

Yes, you could conclude that they once lived in an ocean.

So what must have been here in Fort Worth at that time (100 million years ago)?

Yes, at one time, very long ago, this area was covered by a shallow sea.

The North American continent during late Cretaceous time.

The Western Interior Sea covered most of the Midwest from the present Gulf of Mexico to the Arctic.

Western Interior Seaway during the mid-Cretaceous, about 100 million years ago.

Wikipedia Commons

100 million years ago Central Texas was a shallow sea

As the sea receded, Central Texas rose above sea level.

The next page shows dinosaur footprints found in the Paluxy River bed in Dinosaur State Park, Glen Rose, TX very close to Fort Worth.

Dinosaur tracks made 113 million years ago in the Paluxy River bed near Glen Rose, TX.

Paleontologists who examined the three-toed prints identified Acrocanthosaurus as the likely trackmaker.

Paluxysaurus,

Other dinosaurs had also left their footprints here. Sauropod tracks likely made by Paluxysaurus, a herbivore, appear as huge potholes—sometimes three feet long—pit the rocky bottom of the riverbed.

Acrocanthosaurus

Based on what scientists know about dinosaurs, what can we conclude about the climate at Glen Rose 113 million years ago and the kinds of plants that grew during this time?

A Protohadros dinosaur skull was found in Flower Mound, Texas in 1994.

A Protohadro dinosaur belongs to the Hadrosaurs family, the formal name of duck-billed dinosaurs.

The Protohadros lived 98 million years ago.

Flower Mound is very close to Fort Worth.

Above is a artist’s painting of what a Protohadros might have looked like and the habitat that scientists believe it lived in.

Dinosaur remains of a Tenontosaurus dossi were found in 1998 by a nine year old boy on a ranch just west of Fort Worth near Weatherford. This dinosaur lived 110 million years ago.

A cast of skull and teeth in rock. A cast of a foot.

An artist’s picture of what a Tenontosaurus dossi might have looked like..

A cast of its foot

From these finding what can we conclude about the environment and climate of the region around Fort Worth 98--110 million years ago?

Fossils are a. always made of rockb. an artist’s painting of animals that lived long agoc. the remains of plants and animals that lived long agod. dishes that people used who lived long ago

To conclude, let’s review a few things we have learned.

Fossils give us information about

a. organisms that lived long agob. the climate and environments of

the pastc. how the earth has changed over

timed. all of the above

Fossils are usually found in a. soilb. sedimentary

rockc. igneous rock

from volcanoesd. the interior of

the Earth

The fossils in layer C are the

a. the youngestb. the oldestc. the same age as

the fossils in layer A

AB

C