Fossils--Text

Earth Science Essentials

by Russ Colson

Introduction

Fossils are not found mixed into rocks willy-nilly as though dumped there by aliens from space. Rather, fossils are found with other fossils and rocks in groupings that make sense in terms of the living communities we see in modern environments.

For example, some types of creatures are suspension feeders, meaning they eat by extracting particles of food suspended in the water. Their filtering apparatus would be quickly overwhelmed and plugged in muddy water, thus, in the modern world, we are more likely to find these creatures in clear, clean water.

Likewise, in ancient rocks, we see communities of creatures and rock types with the suspension feeder fossils that suggest clear, clean water.

Fossils also give us clues to past climate. Polar bear fossils (or their equivalent from the geological past) tell us about cold climates.

Coral often tell us about tropical climates (this is partly because CaCO₃ which coral make their homes out of is more easily dissolved in cold water, thus making cold water less hospitable to them).

One of the most basic questions about an environment is whether it was marine (ocean) or terrestrial (rivers, lakes, etc).

The type of creature can reveal whether water was fresh or marine (think about walleye vs sharks). Fossils can reveal whether there was nearby land.

For example, petrified wood indicates the presence of trees which don't grow out in the middle of the ocean. Also, dinosaurs were terrestrial and are found mostly in terrestrial rocks although sometimes they got washed out to sea.

Rather than simply recap the ideas from the Fossils powerpoint lecture, I'm going to give you a few additional examples of stories told by fossils.

Example Environmental Stories Told by Fossils:

Present is the Key to the Past:

Sometimes it's fun to read the stories written in the shells of a modern beach, as practice for reading stories in fossils.

In doing this, we are looking at the modern world to help us understand how things worked in the past. Geologist call this concept "the Present is the Key to the Past".

Here are a few stories of life and death from a modern beach. I collected these samples along the Texas Gulf Coast at Galveston Island.

Brachipods and turbulence of the environment

As in the example of the suspension feeders from above, the physiological needs of the creature sometimes tells us about the environment of deposition. Thickness of shell of brachiopods can give us clues to an environment.

Which one of the brachiopod species below is least likely to live in a highly energetic, turbulent environment?

Algae and water depth:

You might think that we can never know the depth of an ocean that no longer exists, but we can!   The fossils tell us.   Algae need light to live, and different varieties of algae use different types of light (red, yellow, green).   What's more, the type of light varies with depth.   This gives us a way to figure out the depth of water in specific environments of long-vanished oceans.

First:   different algae use different types of light.   We're going to look at red versus green algae.   If algae look green, what color of light must they be absorbing?   Well, if we see green, then they aren't absorbing much of the green light!   Green algae use red light!   Likewise, the red algae are using green light.

Second:   different types of light dominate at different depths.   Have you ever noticed that in movies and documentaries that the color of water gets bluer as the scuba divers go down into the watery depths?   Red light is absorbed more by the water and so as one goes deeper, there is less red light and more blue and green.  

With that, which type of algae tells the story of greater depth of water?

Green algae (on left) and Red algae (on right) from the modern Florida Keys.   Images are roughly 3.5 inches wide.

Monsters and Desolation--Clues to an ecosystem:

Think about this:   If we find evidence of a large abundance of life (maybe lots of fossils for the amount of sediment deposited), what is one thing do we know for sure about that environment?

Answer:   There was plenty of food!   Abundance of creatures must go along with an abundance of food.

Another concept, perhaps not quite as apparent, is that for a diverse ecosystem to develop, the environment must be stable.   This is comparable to human economies:   under harsh and changing conditions, people tend to generalize tasks, with each individual doing a wider range of activities (such as is typical in a subsistence-level economy).   Under stable conditions, many more specialized lines of work develop (typical of complex economies in developed nations).

Thus we can associate abundance of creatures with abundance of food, and diversity of creatures with stability of environment.   The table below offers one example earth environment for each of the four possibility that we have when considering these two variables.

Suppose that our hero, Czella, is a star squid, a creature born to the world of the magnetic flux and explosive heat of a star.   Below is a moment in a story when Czella first enters a new region of the star that she has never before explored.

She felt the magnetic field pulsing through her, steady, unchanging.   No bursts of energy, no flashes of heat as the twisted strands of the magnetic field lines broke and released their pent-up energy.   The relative shortage of that food made her hungry.   She wondered what kind of monstrous creatures might have evolved in this dark hole in the Sun.   A shiver slid down her magnetic spine, but she forged on toward the Deep where the strange signal had originated.

What kind of stellar wildlife should Our Hero Czella expect in this environment?

Was it catastrophic or attritional?

 Back in our sedimentary rock text, we examined a puzzle from the Hell Creek formation of North Dakota.   Based on very small particle size of the sediment in which the Hadrosaur bones were found, we concluded that it was not likely deposited in a big flood, which argued against a catastrophic deposit of those bones.   But what do the bones themselves have to say about it?

We can generate a conceptual distribution of ages for catastrophic versus attritional deposits by considering that a catastrophic deposit will contain fossils of creatures who lived all at one time—old and young together—whereas an attritional deposit will contain more of those individuals more likely to die, often the older and weak (although sometimes more of the very young as well).   Conceptually, the different distributions might look like that shown in the graph below.  

 The red dashed line shows a catastrophic deposit (distribution of creatures at one point in time) for a steady state situation in which creatures have some finite chance of death throughout life, so that there are more young than old individuals.

The blue dashed line shows the attritional deposit for the case where creatures have a finite chance of death throughout life, but the chance of death increases sharply as the creature ages.

The solid green line shows a catastrophic deposit for the steady state situation in which creatures do not die until they have reached a 'death age', and thus there are the same numbers of young as old.

The solid yellow line shows the same case as the green but for an attritional deposit in which no young individuals occur (because none die), with all individuals found in the deposit having reached the 'death age'.

Of course, real populations are not always steady state, meaning that the population itself might be growing or declining, but the graph gives a sense of the kinds of distributions that might occur for different populations and for catastrophic versus attritional deposits.

The Hadrosaur bone bed in the Hell Creek Formation is comprised mostly of scattered bones, not whole skeletons (there's a clue in that too...)   The distribution of bone sizes found at the site as well as a nearby sister site is shown in the graph below (From Colson et al. (2004) Rocky Mountain Geology v. 39, no. 2, pp. 93-111.)   What do you think this tells us about whether the deposit is a catastrophic or attritional deposit?

Identifying the evidence in real research

Identifying evidence is a crucial part of doing science, yet most people struggle to tell the differences among evidence, intermediate reasoning, and conclusion.   Often in public reports of science the evidence is left out entirely.   This might explain why people sometimes refer to scientific conclusions as "just theories" as though they were opinions unsupported by mountains of evidence.   This common difficulty in associating scientific conclusions with evidence undermines the public understanding of science and leads to all manner of problems for science in the public arena.

We are going to examine evidence and conclusion in the report "Secondary Evolutionary Escalation between Brachiopods and Enemies of Other Prey" by Michal Kowalewski, Alan Hoffmeister,Tomasz Baumiller, and Richard Bambach, published in the journal Science, vol 308, (2005), pp 1775-1777.   Here is a graph from this paper reporting percentage of fossil brachiopods with drill holes from predators.   Graphs in scientific reports often illustrate the key evidence on which conclusions are based.

From Kowalewski et al 2005.

Below is the abstract for their report.

The fossil record of predation indicates that attacks on Paleozoic brachiopods were very rare, especially compared to those on post-Paleozoic mollusks, yet stratigraphically and geographically widespread. Drilling frequencies were very low in the early Paleozoic («1%) and went up slightly in the mid-to-late Paleozoic. Present-day brachiopods revealed frequencies only slightly higher. The persistent rarity of drilling suggests that brachiopods were the secondary casualties of mistaken or opportunistic attacks by the enemies of other taxa. Such sporadic attacks became slightly more frequent as trophic systems escalated and predators diversified. Some evolutionarily persistent biotic interactions may be incidental rather than coevolutionary or escalatory in nature.

Consider the following paraphrased statements of their results.   One of these statements is the authors' primary conclusion.   One statement is their primary evidence and reasoning.   One statement is an intermediate logical step connecting the primary evidence to the main conclusion.   One statement is not a conclusion of their report but which their report addresses indirectly.   Can you identify which is which?

(A*)

A few important fossil groups:

In preparation for the lab that some of you will do, I'm going to list a few of the fossil groups.   Although vertebrate fossils get the news headlines, they are actually relatively rare in the geological record.   Therefore, most of the information geologists glean from fossils comes from invertebrate fossils.   Invertebrate phyla important in the rock record (that is, phyla that are commonly fossilized) include the following:

Arthropods :   Perhaps Earth's most successful phylum, includes the insects, spiders, crustaceans, and the long-extinct trilobites.

Mollusks :   Also a very successful group, including the gastropods (snails and the like), the bivalves (making up most of modern sea shells), and the cephalopods (squids, nautilus, and the extinct ammonites).

Brachopods :   The dominant seashells for much of the paleozoic, the lampshells got squeezed off the stage by the molluscs and are not nearly as abundant as they once were.

Echinoderms :   Including the starfish, echinoids, and sand dollars.   Important groups of the past include the crinoids (modern sea lilies) and the extinct blastoids.

Cnidaria :   Including the jellyfish and modern corals, as well as two important extinct groups of corals, the rugose and tabulate corals.

Bryozoa :   This phylum often produces branching or encrusting carbonate structure that superficially resemble coral.   Sometimes you can see bryozoa on the inside of sea shells along modern oceans, resembling a tiny net.

Porifera :   The sponges.   Often they are not well preserved in rock and are identified by the spicules, structures that substitute for a skeleton.

Sarcodina :   This group includes the modern amoeba (which of course doesn't get preserved much in the fossil record) as well as groups that produced a hard test, a shell-like structure that the one-celled creatures lived in.   The fusilinids made a test that resembled a grain of wheat or rice.

Important plant phyla in the fossil record include the Algae and the Tracheophyta (vascular land plants).

Last updated 3/15/2017.   All text and pictures are the property of Russ Colson except as noted.