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Beachcombing in Austin?
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Background Information (PDF, 21KB)
What did the Austin “beach” look like?
In the Cretaceous period (about 145 million to 65
million years ago), a shallow sea covered much of North America, from the Gulf
of Mexico to the Arctic Ocean. Huge predatory reptiles such as mosasaurs
cruised the waterways, and corals and rudist bivalves flourished
in great mounds on the sea floor. Ammonites and starfish were both the
hunters and the hunted. Imagine Central Texas as a tropical place, with a
warm sea, sandy beaches, and palm groves.
Where Austin is today, there was a sea floor. At times, when the sea was
shallow, broad mudflats formed. At other times, when the sea was deeper, this
mud was deposited on the sea floor. Over millions of years the mud hardened,
forming the limestone beds that we now see in Central Texas. Today, trapped
in that hardened mud, are the remains of both sea creatures and land animals
that roamed along the beach. By looking at these remains, we get clues about
the depth of the sea, and the animals that lived in each environment. For
example we find great dinosaur trackways in what would have been the shoreline
or very shallow sea. We find invertebrates from what would have been the sea
What is a Fossil?
Fossils are traces of past life. They were
typically formed when an animal died, and then was buried in mud. Left
undisturbed for a long time, the original material was replaced by minerals
carried by water in the mud. Occasionally an original shell or bone is found
preserved in the fossil. At other times the only evidence that remains is an
empty space in the shape of the animal, like a shell print. In addition,
fossils can form when mud fills a footprint or covers a feather, and then
eventually dries and turns to stone. The outline of the footprint or feather
is a fossil too.
By studying fossils, scientists learn about the history of life on Earth.
These fossil traces are known as the fossil record. Fossils
allow us to see how plants and animals interacted, how they lived in their
environments, and even how they and their environments changed over time. For
example, the presence of fossils of sea creatures in some rocks in Texas tells
us that these rocks were formed from a sea floor. Paleontologists,
scientists who study fossils, can teach us a lot about the history of our
Many of the rocks around Austin were formed during the
Cretaceous period, from 145-65 million years ago. The most common animals in
these rocks are invertebrates, such as snails, oysters, clams,
scallops, starfish, and sea urchins. Many of these animals had hard shells
that were made of calcite or aragonite, just like
the shells of many animals today. When the animals died, these
shells accumulated on, or within, the sea floor.
These materials, or sediments, first piled up loosely on the sea
floor, the same way shells naturally scatter on a beach. They continued to
accumulate for millions of years. Eventually, the weight of overlying
sediments began to squeeze the water out of the mud, forming distinct layers.
These layers, compacted and held together with natural cements, formed the
rock called limestone.
Today, these limestone rocks remain as evidence of an undersea past. Have you
seen the gray or cream-colored cliffs around Austin? That’s the limestone
that contains these invertebrate fossils.
The marine invertebrates
Today, you can easily find living snails, starfish,
oysters, clams and scallops in their ocean habitat off the coast of Texas.
These animals are related to the invertebrates that lived in the Texas seas
millions of years ago.
Ammonites are cephalopods. They are extinct but considered most
closely related to squid and octopuses. They looked very similar to modern
Nautilus, with a coiled, multi-chambered shell. They were very common in
the Cretaceous sea and have left an abundant fossil record. Snails are
gastropods. They typically have a single spiraled shell, and use a
tooth-like tool to scrape food from rocks, or bore into the shells of other
Oysters, clams and scallops are in a group called bivalves. Their soft
body is protected by two shells. By opening their two shells, the bivalves
filter food through their soft (and edible) body. They are benthic,
which means that they live on the seafloor. Rudists were a special
group of bivalves that lived at the margins of Cretaceous oceans. They often
lived in groups and built up mounds on the sea floor.
Brachiopods, also known as lamp shells, have two shells like a bivalve
but each shell helps the animals in different ways. One shell attaches to a
stalk that lifts the animal off the ocean floor. The other shell houses a
feeding organ that has thousands of sticky hairs that collect and transport
food for digestion.
Sea urchins and starfish are echinoderms. They usually
show five-fold symmetry, for example the starfish has five arms. Their
skeleton is made up of interlocking calcium carbonate plates. The sea urchin’s
plates are very tightly fitted together; the starfish plates are more loosely
How is a fossil formed?
Oceans Over Texas
Modeling Petrified Wood