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Bringing the past to life
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.
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.
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.
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.
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).
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.
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:
Quite a story from what started out as an unknown object in a rock!
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
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