Fossils are the preserved remains of once-living organisms, formed through the burial of plants or animals in sedimentary rock, which over time transforms into a solid structure. There are four main types of fossils: molds, casts, trace fossils, and whole organisms, each providing insight into past life and environments. Fossils found in the Fort Worth area, including dinosaur footprints and various shell casts, reveal that the region was once covered by a shallow sea during the late Cretaceous period.

1. By Moira Whitehouse PhD

2. 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.

3. Plants and animals buried
here might become fossils.

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

5. 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.
The youngest fossils
would be found in
the
a. top layers of
rock
b. the middle rock
layers
c. the bottom
Natural Resources Canada
layers of rock

6. There are four types of fossils:
Molds
Casts
Trace fossils
Whole or part of an organism that has
been preserved

7. Molds and Casts

8. 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.
Present day scallop Mold of a scallop in clay Mold of scallop in rock

9. 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.
Picture of present day snail Picture of fossilized snail
Bob Hermann//Cochise College.
R.Weller/Cochise College.

10. Ammonite
mold Ammonitec
ast
Molds and casts are often found together.

11. gastropod mold gastropod casts

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

13. Clam mold
Clam cast

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

15. www.oum.ox.ac.uk/thezone/fossils/intro/for
m.htmhttp://www.oum.ox.ac.uk/thezone/fos
sils/intro/form.htm

16. 3. Trace Fossils

17. 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.

18. Leaf
imprints
R.Weller/Cochise College

19. Dinosaur tracks
Wikipedia Creative Commons

20. Coprolite (dinosaur poop)
dmr.nd.gov

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

22. 4. Whole or a part of a body is preserved
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:

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

24. A sabretooth 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

25. Sometimes, a whole organism can be
preserved if it is frozen very quickly.
Photo: Francis Latreille/National Geographic
40,000 year old frozen baby mammoth found in Siberia

26. 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.
• Fossils help scientists understand what
past environments may have been like.

27. In the next few slides you will see fossils
found either in or near Fort Worth. They
tell us:
•what organisms lived here millions of
years ago.
•about the environment of this area at
that time.
•and help us see how things have
changed over time.

28. Gastropod casts

29. Oyster casts

30. Clam cast

31. Scallop cast

32. Sea urchin cast

33. Ammonite cast

34. One arm of an ammonite cast

35. 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.

36. 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.

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

38. 100 million years ago Central Texas was a shallow sea
As the sea receded, Central Texas rose above sea level.

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

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

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

42. 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?

43. A
Protohadrosdin
osaur skull was
found in Flower
Mound, Texas
in 1994.
The Protohadros
lived 98 million
years ago.
A Protohadro dinosaur belongs to the
Hadrosaurs family, the formal name of
duck-billed dinosaurs.

44. Flower Mound is very close to Fort Worth.

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

46. A cast of its
Dinosaur remains of a Tenontosaurusdossiwere
foot
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..

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

48. To conclude, let’s review a few things
we have learned.
Fossils are
a. always made of rock
b. an artist’s painting of animals that
lived long ago
c. the remains of plants and animals
that lived long ago
d. dishes that people used who
lived long ago

49. Fossils give us information about
a. organisms that lived long ago
b. the climate and environments of
the past
c. how the earth has changed over
time
d. all of the above

50. Fossils are usually
found in
a. soil
b. sedimentary
rock
c. igneous rock
from volcanoes
d. the interior of
the Earth

51. The fossils in layer C
are the
A
B a. the youngest
C b. the oldest
c. the same age as
the fossils in
layer A