Texas Memorial Museum: Science Live! @ the Museum: Lessons

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 floor.

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 Earth.

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.

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 organisms.

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 connected.