What We Can Learn From Fossils Besides Just "What The Organism Looked Like"
Generally, when we go to museums and look at fossils, we assume that the primary thing we can learn from them is what the plant or animal looked like. We see the fossil, and we look at the artists' drawing of what the plant or animal may have looked like in life. Usually, the plant or animal is also put into some type of life-like setting, such as a swamp or a forest or a desert. Dinosaurs are usually depicted with volcanoes erupting in the background. Are all of these additions just the artists' fancy, or are there valid reasons to depict the plants and animals this way?
Crinoids from Allen's Creek: an example that can be used in the classroom.
Perhaps, we can get at this issue in the following way. Below are photographs of two sets of fossil crinoids. One (labeled A) is the surface of a single rock. The other (labeled B) is a number of crinoid stem segments.
[If you are unfamiliar with crinoids, a photograph of one of the few surviving species is shown here, and some beautiful fossils from Crawfordsville, IN, are shown here.]
These two sets of fossils are from the same geographic area, within ¼ mile of each other on the shore of a reservoir that was formed by damming a creek. As shown on the satellite image below (from Google Earth), the composite rock was collected at site A, and the stem segments were collected from site B. Site A is composed of large blocks of limestone, with crinoid pieces embedded in it. Site B is loose shale, which weathers to a fine mud from which the crinoid segments can easily be removed.
What do we see when we study these two fossil assemblages? We see that:
1. Site A has a coarse-grained matrix holding the fossils, while site B has fine-grained shale that weathers to mud.
2. At site A, the crinoid segments are largely broken into individual discs from the crinoid stems, while at site B, the stems are more intact.
3. At site A, some segments are broken, while at site B, segments are largely still attached to one another, and there are often small protuberances still evident on the segments.
4. Fossils form site A show relatively few details; fossils from site B are exquisitely preserved.
What do these differences suggest? It is helpful to distinguish three types of processes that have occurred in the 250 million years since these animals were alive.
A. What processes may have occurred between the death of the animal, and its being covered in sediment? How would these processes affect the fossils as we see them today?
B. What may have occurred in the 250 million years or so that the animals lay buried? Replacement of the organic matter with minerals is certainly one process, since it is the basis of fossilization. Could other things have occurred?
C. What has happened in recent years, months, or days, as erosion releases the fossils from the rock? Because these are on the shore of a lake that is subject to strong storms in the summer, and numerous motor boats, there is frequently considerable wave action. The lake is far enough north that freezing and thawing occur during the winter.
These are important considerations, especially if one examines these data in the classroom. It is essential to separate current conditions from the conditions the animals faced when alive. Very frequently, students make the logical (but unwarranted) leap that because these sites are currently on the edge of a lake, they have always been on the edge of a lake. In building an explanation for the observations, we will need to consider these questions.
It is also helpful to put the information about the fossils in the photographs (or in one's hand, if one collects and examines fossils in this way) into the larger geological context.
i. The surrounding area is generally composed of limestone beds, whose strata are horizontal.
ii. Fossils may be found in much of this area, but in only a few locations (such as the A/B location shown above) are they in such dense accumulations. Indeed, fossils are much less common less than a mile in any direction from this location (except, perhaps, for inland--to the right in the figure--where these particular strata lie under many meters of rock and soil, and have therefore not been examined).
iii. Although not visible in these photographs, it is possible to find very long sections of stems (several feet) at site B, provided one looks relatively far from the waves of the current lake. These long sections are broken into smaller segments by cracks in the shale. Thus, the segments in the photograph are only fragments of the actual fossils that may be found at this site.
Implications of the Data
1. The horizontal strata of this region (for hundreds of miles in all directions) indicate that this general region has seen little geological disturbance. (point i above)
2. The surrounding limestone indicates that this general area was deposited under water. (point i above)
This address the conception that sites A and B were on the edge of a lake when the animals were alive. The geological context tells us that this was part of a large sea. Recent erosion created the valley that was filled by the downstream dam, thereby creating the reservoir.
3. The surrounding limestone, the shale of site B, and what is known of the two surviving species indicate that these organisms are ocean-dwellers. They typically attach to surfaces--and therefore were attached to surfaces in the A/B location. The scarcity of fossils only a short distance away indicates that there were no such surfaces available in the surrounding ocean. It was probably relatively deep water. (point ii above)
4. From the grain size of the surrounding matrix, and the broken-up nature of the fossils, it seems likely that site A experienced fairly rough treatment. Since crinoids are ocean-dwelling animals, it seems likely that the rough treatment was wave action. Rough waves would break up dead crinoids, as well as wash away fine silt. (points 1, 2, and 3 above)
This inference addresses point A above--the conditions between death and burial of the animals. Because the two sites are so close to each other, they probably experienced fairly similar conditions for the millions of years when the fossils lay buried (point B above). The overall horizontal nature of the strata (point i above) suggests that these conditions were fairly uneventful. The strata were deposited horizontally, and remain horizontal. The current conditions are also similar (point C above); the differences in erosion arise solely from the fact that one site is shale and the other limestone. We cannot, therefore, attribute the differences in appearance of the fossils to current conditions. Current conditions can account for general abrasion such as one finds for river rocks or sea shells that have been worn down by waves and sand--but such abrasion is evident at both sites.
5. The fine silt and relatively-larger segments of fossils indicate that site B is likely to have been a much calmer environment. Dead crinoids appear to have settled gently to the bottom, where they were buried in silt, with little agitation. Significant agitation would have washed away the fine silt in which they were buried, and which became the shale in which they are embedded.
6. In what kind of environment did these animals live? What can account for the differences between sites A and B? The data indicate that the environment included deep water nearby, strong wave action at site A (meaning a location near enough to the surface to experience waves), and calm water at site B (but just as near the surface as site A). What current environments provide these conditions? Today, the types of environments with these characteristics are coral reefs. A typical reef usually is exposed to waves on one side, but the reef itself forms a breakwater, so on the landward side, there is a relatively calm lagoon.
7. THEREFORE: It is a reasonable inference that site A was the oceanward side of a crinoid reef, and site B was the lagoon side.
Below are two photographs of the same ammonite fossil, from Seboyeta, NM. The photo labeled "top" is the side of the fossil that was facing up when it was found. The photo labeled "bottom" is the side that was facing down.
Why might the downward-facing side show much better preservation than the upward-facing side? Again, we need to consider conditions between life of the organism and its being covered in sediment. Again, we need to consider subsequent events, particularly weathering and erosion. Did the upper surface weather more severely than the lower surface? This is entirely plausible.
But look in the center of the fossil. Where a complete ammonite fossil would have a central coil, this one has none. Instead, there are fossils of other organisms (arrow). These are pelecypods -- oysters. The clearest example is shown in the enlargement on the right. Now this changes things! The ammonite was apparently broken before it settled out of the sea, and was colonized by oysters.
What would account for the breakage of an ammonite? Probably waves. What kind of environment would allow growth of oysters? Probably a near-shore environment in a shallow sea. It's relatively easy to picture the ammonite dying, being battered by waves, and this fragment settling out where things were a bit calmer. This kind of thing happens today, just not with ammonites.
On maps of North America during the Cretaceous, paleontologists have drawn an outline of the shore of an inland sea. How do they know where to draw the shoreline? This type of fossil tells us.
The Significance of This Exercise:
From these fossils, we learn not only what crinoids were like as animals, but we also can gain some insights into the kind of environments in which they lived. The additional information is gleaned from the characteristics of the fossils, the nature of their preservation, the nature of the matrix in which they are found, the geological context of the surrounding area, and the types of other fossils found in the same assemblage. From such clues, it is possible to make inferences about the ecological and environmental conditions in which the species lived.
It is this "other" information that is used by paleontologists and artists and museums to reconstruct images (or even 3-dimensional displays) of long-extinct animals in their natural habitats. For example, dinosaurs are often depicted with erupting volcanoes in the background. Why? The Morrison formation, from which a great many dinosaur fossils have been recovered, was formed by sedimentation of volcanic ash. The presence of extensive beds of volcanic ash is a pretty good clue that there were volcanoes nearby. An excellent example has been preserved as The Painted Desert and Petrified Forest National Park, in Arizona. Both trees and animals were buried, enabling an excellent opportunity to reconstruct the ecosystem that was destroyed by the ashfall.
Using this kind of analysis to address an Important Evolutionary Transition--Fish to Amphibians
A particularly helpful use to which this kind of ecological information has been put is in reconstructing plausible scenarios for the kinds of selection pressures that could have existed at different times and in different places. Consider, for example, the transition of fish to land animals. This transition is not easy to picture, and has spawned a great many misconceptions. It has also become a symbol of the Creation/Evolution debate, showing up as automobile stickers of a "truth" fish eating a "Darwin" fish--it just seems so wildly implausible that many people simply declare it wrong.
Set aside, for the moment, the issues of genetics, genetic and molecular control of development, the mechanism of evolutionary change, etc. These are discussed elsewhere in these pages. Here, we consider what the fossils tell us.
First, recall the older idea--the traditional scenario, perhaps--that fish "had to" evolve feet "in order to survive" as their lakes and ponds dried up. The idea was that they scrabbled overland from pool to pool, much as mudpuppies and Snakehead fish do now, and the scrabbling selected for stronger fins. Well, the fossils make this scenario unlikely.
In association with Devonian fossils of lobe-finned fish, are other fossils. They tell us little about what the fish were like, and are thus overlooked in the typical diagram of the fossil progression. What they do tell us, however, is that these fish did not live in drying pools. Rather, they lived in coastal swamps reminiscent of current mangrove forests. Within such swamps, we would find smaller fish reminiscent of today's "typical" fish (the ray-finned fish that have many very thin bones in their fins) and much larger fish reminiscent of today's coelacanth (with more robust bones supporting the fins). Large, carnivorous fish living in a mass of submerged roots would not swim quickly; smaller prey would negotiate the tangled vegetation more easily. However, fins work not just for pushing against water, but also for pushing against things like submerged roots. In this particular environment, there would be selection pressure that favors stronger fins that would enable harder pushing. Based on this information, and a great many fossils, it seems likely that the fins-to-fingers transition occurred fully underwater, with no scrabbling on land involved. Of course, once an animal is well-adapted for pushing against things, what better thing to push against than the ground? We now have a quite plausible, multi-step scenario in which lobe-fins underwent selection for stronger, more robust skeletal elements, leading eventually from fins to paddling feet to walking feet.
In short, we should not look at fossils in isolation. What is particularly important is the fossil assemblage, and the geological context, from which we learn about ancient environments and ecosystems. This information is as important as the fossils themselves in evaluating evolutionary progressions.
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Last updated: 21 August 2009
Comments: Jose Bonner, OSO
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