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Bringing the past to life
 Enlarged Fossil View |
Imagine you have just collected the rock sample shown in the photo at left from an outcrop in northwestern Maine. The rock surface has an interesting shape -- could it be a fossil? You want to figure out what this object is, so you might ask yourself the following questions:
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Was it alive?
To begin our investigation, we'll start with a general question -- what kind of rock is the sample? First, let's review the three basic rock types. Igneous rocks crystallized from molten rock that cooled and became solid. Sedimentary rocks formed from layered deposits of sediment particles which were then compacted or cemented into rock. Metamorphic rocks formed when heat or pressure altered sedimentary or igneous rocks. The occurrence of fossils is very dependent on rock type. Fossils usually occur in sedimentary rocks. The molten material that forms igneous rocks would destroy the remains of living organisms. It is common to find fossils in low-grade metamorphosed sedimentary rocks, although the fossils are usually somewhat deformed or distorted. However, fossils become increasingly rare to nonexistent in sedimentary rocks that have been subjected to intense metamorphism. The great pressures and temperatures to which these rocks are subjected obliterates any visible organic remains.
So rock type could quickly indicate if the object is not a fossil. Examination of our rock specimen shows layering and a very fine-grained texture. To test for composition, we put a drop of dilute acid on the rock surface. Strong fizzing results, indicating that the rock is made up of calcium carbonate (i.e. the mineral calcite). Acid also fizzes on the surface of the object. Based on this examination, we decide that the rock is limestone -- a sedimentary rock composed largely of calcite. Fossils are a possibility -- the object might represent a once living organism, but we need to search for more evidence.
 Annotated Fossil Photo |
Now let's examine the shape and form of the object. Note the following characteristics and refer to the photo at left:
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Let's return to our original question -- was it alive? What does our list of shape characteristics tell us? Could the object be just a cluster of calcite crystals? Such a calcite growth would likely have the same composition all the way through the cluster of crystals, but instead we see a thin outer lining, like a wall, around each hexagon. The hexagons also weather to form hollow tubes - demonstrating that the wall of each hexagon is more resistant than the filling. Examination of the inside of the tubes through a microscope reveals a particle "filling" of rounded grains inside each thin hexagonal wall. This filling looks like the material that makes up the surrounding rock. It appears that both the surrounding rock and the filling of the hexagonal tubes are more easily eroded than the tube walls. These factors would lead us to believe that the object is not just a growth of calcite crystals. Besides, inorganic calcite generally does not crystallize in this pattern. If these resistant tubes are not the natural form of calcite crystals, could they be the skeleton of some type of living organism? It could be a fossil, but we're still not sure.
Where can we get more information? Let's expand our investigation and look at a larger portion of the limestone sample that contains this object. Look closely at the shape below right of the object. Looks like a shell! Now let's look at the entire rock specimen that contains our object. Once again, look at the shapes contained in the rock. They definitely do not look like mineral crystals. In some places, they look like a pile of organic debris. Putting our object in a larger context provides us with more definitive clues that our object was alive. Based on the accumulated evidence, we conclude that the object is a fossil of a living organism.
What was it?
Now comes the hard part. What was it? The best way to answer this question is to compare the shape of this object to living organisms. If the organism is extinct, there may be only a few features which compare to something in the modern world. Also, remember that a fossil usually preserves only the "hard" parts of an organism. Soft parts like skin, tentacles, or muscles are rarely preserved. The shape of this object might be outside of our experience and unrecognizable. We might have to consult museum reference collections or reference books to find something comparable. This can be the exciting or frustrating part of paleontology.
When looking at an unknown object, the easiest way to figure out its identity is to compare it to objects or shapes that are familiar. Let's start with an easy example. Look at the photo at left. What do you see? By examining the shape of the objects in the photo and comparing them to objects that you're familiar with, you'd probably say it's a scattering of seashells. It's a good guess. The photo shows the imprints of shells on the surface of a rock from southeastern Maine. This process of comparing the shape and structure of living organisms to fossils is actually how paleontologists have identified and classified fossils of extinct organisms.
Looking at our object, you might guess that some type of animal lived in the tubes. Since the tubes are clustered, the organism probably lived in a "colony." Could it be a fossil beehive? The honeycomb shape seems to match. It's possible, but a beehive is rather fragile, with no real "hard" structure. It would probably be destroyed long before it could be preserved as a fossil. So if it's not a beehive, what other "colonial" organisms exist today?
Look at the object again. What other clues could help us discover its identity? First, remember that the surrounding rock is limestone. A little research into rock types tells us that most limestones form in clear, warm, marine waters. Additional reinforcement for this conclusion is the shell imprint found near our fossil. Add up all of our conclusions thus far and we have a colonial organism with a hard skeletal structure that lived in the ocean.
A search for ocean-dwelling colonial organisms might lead you to bryozoans, hydrozoans, and corals. Comparing the shape and structure of our fossil would eliminate the jellyfish-like hydrozoans and the skeletal structure is different from that of modern bryozoans.
This leaves coral as a possibility. Modern corals come in a variety of shapes and sizes. Examine the photo of the modern coral specimen shown at left. Note the cavities in the upper surface of the specimens and the tube-like structures seen on the side. Individual coral animals called "polyps" live in these cavities. Polyps resemble inverted jellyfish (in fact, jellyfish are closely related to corals) and feed using their tentacles. When feeding, the polyp extends outside the tube, but when it contracts for protection, it withdraws almost completely into the tube. The polyps secrete an external skeletal "cup" made of limestone (calcium carbonate). The skeletons of adjacent polyps are cemented together, and succeeding generations of polyps attach to the surface of dead skeletons, eventually forming the structure seen in the photo.
Now compare our list of fossil characteristics to the form and structure of this modern coral. You can see a similar cluster of cavities and hollow tubes, the domed shape, and a honeycomb appearance on the top. Add this to our knowledge of an ocean environment, and the comparison of these two skeletons looks like a reasonable match. So, based on this comparison, we'll call our specimen a fossil colonial coral.
What was its habitat?
Now that we've decided that our object is a fossil coral, can we describe the habitat in which it lived? Stop a moment and think about where corals live in today's world. What comes to mind -- coral reefs around warm tropical islands with crystal clear waters? Good guess. Modern corals have several habitat requirements. These requirements include clear shallow water that allows sunlight to penetrate and warm ocean temperatures (68 to 82 degrees F). These habitat requirements limit the range of modern coral reefs to a zone of warm water near the equator, generally between 30 degrees N and 30 degrees S latitude. So where did our fossil coral live? By referring to the habitat of its living relatives, we will assume that the habitat of the fossil coral was similar to its modern day counterpart. Therefore, since today's coral lives in shallow, clear, warm ocean water, our fossil coral probably did also. But wait a second -- this fossil came from 45 degree latitude in northern Maine. It's cold up there and nowhere near the ocean! We obviously still have some investigating to do.
| More Information: Coral reefs: A range of information on modern corals and reefs from the Sea World Education Department. |
How did it become embedded in solid rock?
When our fossil coral was living, it must have been similar to modern corals, its colonies growing on the surface of the sea floor. The tubes in the fossil skeleton would have been filled with living polyps, feeding themselves with soft tentacles. So how did the fossil coral in our specimen become embedded in solid rock? First let's look at the area surrounding our coral. The rock contains a haphazard jumble of the remains of corals, shells, and other organisms. Think of the piles of shells and debris that you often see when walking on a beach at the seashore. These coral skeletons probably were moved by waves or ocean currents and deposited in quiet water. Over time, this jumble of dead corals on the sea floor was buried in an ooze of limy mud. Remember when we looked at the "honeycomb" pattern of the fossil through the microscope? The coral tubes were filled with rounded particles that looked the same as the particles that made up the surrounding rock. These are particles of the original mud that buried the coral skeleton and seeped into and filled all the cavities in the skeleton.
Burial in the mud protected the coral skeletons from being destroyed. Think of shells you've seen at the shore and how they are often in the process of being buried by either sand or mud. Those that aren't buried can be destroyed by decomposition, waves, or other sea creatures. So now the coral is buried in a protective layer of mud, but the mud is still soft and hasn't yet hardened into rock. Over time, the mud containing the coral will be buried more and more deeply by sediments deposited on top. The weight of the overlying material becomes greater and greater, squeezing the mud together. At the same time, temperature may be gradually increasing and water is squeezed out. In the limestone, the lime (calcium carbonate) acts as a cement, literally gluing the mud together to become a rock. The coral is now totally encased within the limestone.
| More Information: Fossils: Window to the Past: An introduction for the general reader to types of fossils, conditions leading to fossilization, and the information contained in fossils (University of California Museum of Paleontology). |
How old is it?
But just when was this coral living? Ten years ago? A thousand? A million? If we think about the amount of time necessary to bury the coral skeleton in mud and turn that mud into rock, it must have been a VERY long time. To determine the age of this fossil, we have to investigate the rocks surrounding the area where it was found. Fossils are most often found in sedimentary rocks which are deposited in beds like a layered cake. The oldest layers are at the bottom of the "cake," with successively younger layers deposited on top. By mapping the "stack" of rock layers, geologists can come up with a "relative" age of the coral by seeing where its rock layer is in the stack, as well as by noting its relative position in other rock sequences that have been dated by other means or by comparing it to the position of other fossils within the overall regional setting. For example, if this coral had been previously described in a group of rocks in England and dated as Silurian, we could then assign a date of Silurian to our sequence based on that information. Alternatively, if this particular coral had never been described before, we could still look for fossils with known ranges in the beds below and above the coral bed, in order to obtain a bracketed age for the unit. For example, let's say that the coral bed was underlain by rocks that contained fossils of organisms which only lived during the Cambrian, and the beds above the coral contained fossil brachiopods which were only known from the Devonian. Based on this information, we could say that the coral was no older than the Cambrian and no younger than the Devonian.
But this still doesn't tell us the exact age of the coral (in millions of years). In order to determine the "absolute" age, geologists use a method called "radiometric dating" which measures the amount of decay in radioactive minerals contained in igneous rocks. Based on worldwide mapping of sedimentary rocks and radiometric dating of igneous rocks, geologists have developed a "geologic time scale."
Let's return to our fossil coral specimen. It was collected in northwestern Maine from a group of rocks called the Hardwood Mountain Formation. Based on mapping of the surrounding rock layers and dating of igneous rocks in the area, geologists have assigned the Hardwood Mountain Formation to the Silurian Period (417 - 440 million years ago).
| More Information: Fossils, Rocks, and Time: General interest publication from the U.S. Geological Survey. Geologic Time: General interest publication from the U.S. Geological Survey. |
Where on EARTH did it live?
Let's return to the climate of northwestern Maine. How could coral possibly have lived in this cold region? Obviously, the climate must have been different during the Silurian Period, over 400 million years ago. But how do we explain this climate change? Was the whole world warmer? Was "Maine" in a warmer location on the planet? For an explanation, we need to investigate the theory of "plate tectonics". Simply put, this theory states that the surface of the Earth is made up of a series of "plates" that are constantly in motion. Granted, it's "slow" motion, but over hundreds of millions of years, the Earth's continents have moved great distances. Land masses that are in northern climates today may have been in tropical regions millions of years ago.
By mapping out the worldwide distribution of particular fossils, ancient oceans and continents can be delineated. For example, fossils of the Mesozoic reptile Mesosaurus occur only in western Africa and in eastern South America, which shows that those continents were next to each other in Mesozoic time. On the other hand, fossils of particular marine shellfish (brachiopods) of Silurian age in coastal Maine and New Brunswick are different from brachiopods of the same age in western New England and northern Maine. This indicates that land that is now connected was not connected in the Silurian. Using this kind of evidence along with a wide variety of other clues, geologists have reconstructed the movement of the Earth's continents over hundreds of millions of years. An animated map will help you visualize how the Earth's continents have moved over time.
Based on these reconstructions, we see that northern "Maine" was actually underwater and in a different location on the Earth's surface when the coral was alive. Since "Maine" was much nearer the equator during the Silurian Period, the climate was also different. "Maine" would have had a tropical to subtropical climate with warm ocean waters. Once plate tectonic motion is taken into account, notice how the location of Silurian reefs was similar to the location of modern-day coral reefs. The distribution of corals has and still does straddle the equatorial regions, restricted to warmer waters.
| More Information: The Paleomap Project: Investigate paleogeographic reconstructions of the Earth during various geologic time periods. Plate Tectonics: Information from the University of California Museum of Paleontology. |
How did it get to the land surface?
Think about the fossil coral being buried in mud during the Silurian Period. The mud became rock when it was deeply buried by overlying layers of sediment. Over 400 million years, a great thickness of sediments that eventually became rock must have been deposited on top of our coral layer. How did it get back to the land surface? If we refer back to plate tectonics, we will see that the Earth's continents are constantly in motion -- new mountains are thrust upward and old mountains are eroded. We call this sequence of mountain-building, erosion, sedimentation, and burial and rock formation the "rock cycle." If rocks that are buried deep within the earth are thrust upward into a new mountain range, they are vulnerable to erosion, which gradually wears the rocks down, exposing older rocks. Northwestern Maine underwent a period of mountain building when the Appalachian Mountains were created. Erosion of these mountains and the scouring effects of the last glaciation have worn away the younger rocks, exposing the Silurian Hardwood Mountain Formation at the surface.
Putting it all together
Let's put all the evidence together:
- Based on the object's shape and other characteristics, we decided it was once a living organism.
- Then we compared it to known organisms and decided it was a colonial coral.
- We correlated its habitat to modern corals and decided it lived in a warm, shallow ocean.
- We investigated the surrounding rock and decided it was buried by soft sediments and cemented into rock.
- We researched its age and assigned it to the Silurian Period, over 400 million years ago.
- Using plate tectonic reconstructions, we located ancient "Maine" much nearer the equator and in the Southern Hemisphere.
- We determined that erosion of the overlying rocks revealed the fossil in an outcrop at the land surface.
Quite a story from what started out as an unknown object in a rock!
| More Information: The Virtual Silurian Reef: Explore a virtual Silurian reef at the Milwaukee Public Museum. |
What the paleontologists tell us.
So did we get the story straight? You bet. Our fossil coral is an extinct tabulate coral belonging to the genus Favosites.
Last updated on April 25, 2012