Finding Fossils Help

Fossils Found Together

If paleontologists discover a sedimentary rock cliff full of a variety of fossils, what can they predict about their interaction?

A fossil community is a group of fossils or organisms that lived in the same environment when they were alive.

In a particular area, rocks formed very specifically. Their specific characteristics are called sedimentary facies . This happens today as it has for millions of years. Sedimentary rock in different places can have completely different compositions even though they may have accumulated during the same time period. The same thing happens with their associated fossils.

The fossils in some sites are often very different from fossils at another site because of the environment in which the sediment was deposited. Just as ocean fish, shellfish, and mammals are found at different sea depths, the fossils discovered from sedimentary deposits of former seas are characteristic of their marine environments. The sedimentary rock record often shows a species and sediment gradient. Since everything eventually sinks to the bottom and is covered by sediment, even species that lived far apart (in depth) may end up close together in the sedimentary rock layer.

Different sedimentary facies may have different groups of fossils. When different fossils are found together in one particular sediment type, those species are known as the facies fauna for that sediment.

Fossils in an assemblage can be from more than one fossil community. Individuals may have lived at the same time, but are located in broader strata that can range over millions of years.

A fossil assemblage is a group of naturally associated fossils found in an area of sedimentary rock strata.

Depending on sediment thickness, texture, color, orientation, and other factors, paleontologists can figure out if different individuals of the same genus and species lived at the same time through radiometric dating. They can also compare individuals from different classifications (kingdom, phylum, family, or whatever).

Paleontologists use fossils to date the sedimentary rock layers in which they are found. They look for similarities between fossil specimens in different layers and then apply this information to their knowledge of the climate and history of the region. This is how they were able to figure out which dinosaurs lived during the Jurassic, Cretaceous, and other periods.

A correlation happens when two or more rock units are dated on the basis of having similar types of fossils. Those with like fossil assemblages are thought to have formed around the same time.

Paleontologists have found that dinosaurs were not alone on the Earth. During the Mesozoic era, they lived with a wide variety of other animals. These animals were a lot like groups of animals alive today. The first mammals appeared in the Late Triassic along with frogs, turtles, lizards, and crocodiles. By the end of the Cretaceous period, birds, snakes, and primitive marsupials and mammals were around. They all lived together in a region sharing water holes and food (and being food) like birds and animals, large and small, do today.

One problem paleontologists face when creating a total picture of a fossil assembly is that many marine tube worms and other species are soft bodied. Marine plankton, tube worms, and the like have no hard bones. Their remains decompose quickly. The only clue to their existence is the fossilized tunnels left behind by burrowing creatures. So some species have to be assumed by their remaining habitats.

Fossil Tracks

In 1802, a Massachusetts farm boy named Pliny Moody found fossil footprints in his family’s field while plowing. Local farmers nicknamed the tracks, the footprints of “Noah’s Raven.” In 1835, James Deane, a Connecticut doctor and amateur naturalist, saw two side-by-side sets of footprints in rock slabs cut for a roadbed near his home. He thought they had been made by turkeys. Deane was wrong about the maker of the tracks, but was probably one of the first people to suggest that living animals with very similar tracks to the fossil imprints might be very similar to extinct creatures in other ways.

From the Paluxy River region in Texas, tracks were found by R.T. Bird. In 1937, when Bird visited the area, he found that fossil tracks were common on local ranchland. Among these were the Davenport Ranch Trackway and Paluxy River trackways.

At the Paluxy Trackway, Bird discovered the tracks of more than 12 sauropods ( Brachiosaurus ), followed sometime later by at least three predators (theropods like Allosaurus ). Although Bird thought this was evidence of a hunt in progress, there is no way to tell how much later the predators followed. Like most dinosaur detective work, it’s impossible to know. The track could have been a favorite sauropod path that the theropods took as a shortcut.

Dating Rock And Fossils

As we learned geologic time is calculated by measuring the decay rates of the atoms of radioactive isotopes. The ages of the most ancient rocks can be found by measuring the decay of specific isotopes that are not stable, but break down to other element forms. A rock sample is dated using testing techniques known as radiometric dating, which takes into account the various melting and environmental influences that affected the sample.

Unstable isotopes of elements like uranium, rubidium, potassium, and carbon go through natural decay into stable isotopes of other elements. This is the case when atoms of an unstable isotope of uranium (parent isotope) decay into atoms of a stable isotope of lead (daughter isotope). The ratio of the uranium isotope to stable lead isotope in a sample is a function of time. A longer time period results in a higher percentage of daughter isotopes compared to the smaller percentage of parent isotopes. The decay rates of radioactive breakdown products are constant and can be accurately determined by the laboratory testing of a sample.

The discovery of radioactivity in 1896, by Antoine Becquerel, led to the study of radioactive decay of elements in fossils. By measuring decay rates of various unstable isotopes geologists were able to add numbers to the traditional geologic timescale. Today’s measurement techniques and extremely accurate instrumentation have allowed for precise fossil and sediment dating. The geological timescale is constantly updated with new finds producing time adjustments and increased accuracy.

Practice problems of this concept can be found at: Fossils Practice Test