So many people have asked me what could possibly be on a paleontology final... Well this is the first part of the final. The second part will be correctly identifying fossils down to family/genus level. It's not perfect. But it's what I got. Not gonna post the questions either... gotta retain some semblance of mystery. This is the first draft, I'll clean it up laters.
And I'm actually doing this for my own benefit. I'm hoping some paleontologist will stumble on this and correct my ass. I'm an undergrad, but I deserve a scholastic spanking sometimes... In any case, this is the science I intend to do my graduate work in. Enjoy!
1.
Preservation or fossilization of animals can occur in several ways. Hard parts can be preserved in the following ways: Complete preservation by way of immediate burial (which is highly rare), decay of soft parts and/or transport of the hard parts which can lead to the hard parts being preserved unaltered or recrystallized (calcite changing to aragonite, silica, etc); if no recrystallization occurs, material can be removed through many processes, resulting in internal molds and casts, of either partial or complete preservation; if material is added, by pore intrusion by minerals (permineralization), sediment or mineral infill, or molecular replacement, an internal cast may happen. Naturally, most fossils are a combination of any of the above processes. Also there is petrification, which is a chemical process normally used to refer to fossilized plant material. These do not take into account footprints, ripple marks, trace fossils, or burrowing animal evidence, as they do not contain any soft or hard parts of an animal.
Both hard and soft parts of an organism can be fossilized, although obviously hard parts are much more likely. For soft parts, the most complete method (indeed, the most complete of any form of preservation) would be freezing of the body, in whole or part. Such examples as wooly mammoths in Siberia, and even the waste products of certain arctic and antarctic animals, lead to an extremely accurate account of the lives of extinct animals. Of course, preservation like this is entirely climate dependent. Other examples of soft body preservation or fossilization are environment dependent, but much more likely to occur, namely desiccation and burial,. Rapid burial under anaerobic conditions with the concurrent decreased possibility for scavenging and decay (due to the low oxygen) is possible, however rare. These lagerstatten (those Germans and their wonderful words) occur all over the world, with the most famous being the Burgess shale of Canada. Near perfect soft body preservation of organisms led to discoveries that are described in Stephen Jay Gould's Wonderful Life. Also very good preservation systems can be found in Mazon creek formation near Chicago. These, of course, would give the paleontologist a very good amount of information about past ecologies and the biology of the organisms therein. Barring that, a life assemblage (fossil assemblages that are buried in situ) are the second best, while not as perfect as a lagerstatten like the Burgess shale. Life assemblages give us a great glimpse into the ecology of an area and the biology of the critters preserved there (as long as one accepts the concept that modern analogues are accurate when compared with similar animals from a long extinct time).
Modern areas of prime fossilization are rare, as human behavior and habits can disturb many potential fossil-forming areas. One of the prime areas that could be used is a very deep lake with inlets of sediment flowing in rapidly. The lack of oxygen in the deep water, the cold nature of water past the sunlight penetrating zones, an absence of scavengers due to the lack of oxygen, and a rapid burial could preserve and fossilize hard parts quite well. The outflow of large rivers also has the potential to bury animals quickly, although there may be oxygen and scavengers present.
2.
Temperature.
Temperature factors are obtainable from fossil assemblages because of several reasons. First, certain animals will not survive in certain temperatures (too hot or too cold will kill an animal). This is particularly important when thinking about cold blooded animals like lizards or amphibians. If the modern analogues of these animals cannot survive below a certain temperature, it is likely that their ancestors weren't able to tolerate cold temperatures either. On the reverse side, polar bears do not tolerate hot climates very well; their bodies are evolved to tolerate frigid temperatures. If one sees several lizard fossils in an assemblage, then the paleoclimate must have been one that had warm temperatures. Also, the growth rate and reproduction of animals is dictated to a large part by the metabolism of the creature. A cold environment creates a slow metabolism, which in turn decreases the growth rate of the animal. This makes breeding a very late stage process as opposed to warmer climates, in which pedomorphosis is more common rather than peramorphosis, as occurs in colder climates. However, each animal has a particular zone of temperature that it abides best in, and according to Van Hoft's rule, lower the temperature deviates from ideal (every ten degrees Celsius), the slower the biological reaction rates are (by a factor of 1 to 6). Also, morphology can be affected by temperature in many ways. Warm water has more dissolved calcium carbonate for marine critter shells to form, and less dissolved gasses which make cold water animals intolerant to the temperature by a roundabout way. Even the handedness of certain gastropods denotes temperature, with certain species coiling right handedly during warm periods, and left handedly during glacial periods. This is all, of course, based on our assumption that certain species behaved in ways that their modern analogues do. There may be arctic lizards we don't know about, but when diagnosing a fossil assemblage, taking into account all the fossils makes us relatively certain. It is a safe assumption that an arctic lizard, if there ever was one, would not inhabit the same ecological area as a set of other warm weather species.
Salinity.
Mean salinity of the ocean is around thirty five percent NaCl. This does not mean, however, that all ocean water is the same percentage saline. Waters near the shore (especially near freshwater outflow) are subject to widely varying salinity (so-called brackish water), which leads to particularly hearty forms of life living there. Massive outflows of freshwater can give way to periods of evaporation that concentrates the salt in the water. Particularly, organisms that have evolved the ability to regulate the amount of salt in their bodies (otherwise known as euryhaline). These are usually relatively simple life forms like certain bivalves, crustaceans, and forams, and can be a definitive argument for the paleoecology of the area being one of fluctuating salinity. Most marine critters are stenohaline, which means they have no ability to regulate the amount of salt in their bodies. Some critters, usually exclusively in fresh water, have very little tolerance to salt, and some, namely certain types of algae, shrimp, and bacteria, can tolerate hypersaline conditions. All this, of course, is exclusively dealing in marine realms, as there are few euryhaline terrestrial animals (some species of reptile excluded) and even fewer inland salt water bodies. In this example, our assumptions are based off of not only the modern analogues, but the positions of animals with each other in life assemblage fossils. Bivalves and molluscs that exist near each other must have had similar salinity tolerances, which leads to better evaluation of paleoecology.
Light.
Most organisms need light to survive, either by primary production or consumption of the primary producers (some deep sea animals live off of the sulfuric compounds that emanate from hydrothermal vents, with other animals living off them: yet more biostratigraphic guide fossils, if they ever fossilize). Plants are the obvious potential fossils, but in marine environments, it gets a bit more tricky. Certain organisms that do not photosynthesize yet are first or second level consumers or are symbiotic with photosynthetic organisms can't exist beyond the photic zone. Hermatypic corals, large forams, and giant clams are dependent not only on photosynthetic organisms for food, but even for their symbiotic relationships with them. As a rule of thumb, any organism that is light dependent or it's primary consumers cannot exist outside of a narrow margin in and directly below the photic zone. Ahermatipic scavengers and the predators that eat them exist further down the ocean column. In an extreme example of our dealing with the inherent problems of defining the ecology of an extinct system, both of the above assumptions (that dealt with temperature and salinity, respectively) can be used. The organisms that exist near each other coupled with the behavior of extant species are accurate estimations of the paleoecological areas and the paleobiological realms of extinct species.
3.
Ordovician.
After the Cambrian extinction, some small evolution of vertebrates was evident in the Ordovician period, along with the extinction of some deep-water trilobite species, but the biggest expansion was in the shelled critters. Among the most dominant were the pentamerid and inarticulate brachiopods, the tabulate corals (which were the primary reef builders after the rudists) like halysitdae, which spread widely during the late Ordovician, the cryptostome and treptostome bryozoans, archaeogastropods like macluitdae, shallow water trilobites like asaphinids, and most abundantly, the crinoids, which went a huge expansion during this time, and are the most dominant fossil presence.
Jurassic.
Scleractinian corals expanded, with types like montastrea being the most dominant, along with saccocoma crinoids, but the gastropods like turritella, molluscs like the oyster-type graphaea, and ammonites like stephanoceras expanded greatly. Trilobites are now all but extinct. What is particularly interesting is the expansion of the vertebrates like liopleurodons being abundant, although rarely fossilized. The shallow marine environment of the Jurassic was abundant with many varied types of life, including pseudodiadema echinoids (sea urchins) that evolved from more primitive echinoids earlier in the Carboniferous period. Overall, the bioecology of the ocean branched out and expanded in several ways, with lots of major diversification from the Permian and Triassic periods, and with the shelled critters overtaking crinoids as the most dominant fossil presence.
Eocene.
Long after the first mammals inhabited the land, they returned to the ocean, with the first species of whales evident in the Eocene. The neogastropods were in full flourish, with several genera evident in the fossil record, like nassarius and conus. Nautiloids were taking the niches of ammonites,which were dying out. Sea urchins like Lutetiaster were abundant in the warm, brackish waters, as well as several hundred species of fish (the dominant vertebrates in marine environments), like diplomystus, which is now extinct. The Eocene contains several modern analogues to current genera, if not those exact genera that have changed little in thirty four million years, adding or dropping species occasionally. The most dominant form of fossil are still the shelled critters, with neogastropods and molluscs overtaking the more primitive shelled animals.
4.
It would be safe to assume that since limestone is made primarily of calcium carbonate, that the production of the rock would be predicated on the production of animals that produce calcium carbonate for some reason, be it shell production, stalk production, or even as a byproduct of some other process (though this is far from likely). It therefore stands to reason that the best way to discover what a particular limestone formation is made of is dependent on the forms of life that make the calcium carbonate. Sure, geological reworking has a definite factor to the overall texture of the limestone, but the basic parts, not to mention the larger fossils, would remain unchanged. So, in essence, the only real way to distinguish on sight a particular type of limestone would be to analyze its skeletal components and what type of fossils lie in it.
The most predominant forms of life that contributed to the past petrogenesis of limestone were usually confined to shallow tropical ocean water environments. Of course, in order for limestone to be made, things have to die. When they die, they collect and are lithified. Limestones like the enormous chalk beds that formed during the cretaceous (creta, in Latin, means chalk) are made up of microscopic organisms called coccolithophores, the poorly cemented coquinas made of fragmented shells (and because of this have to be at least as old as the evolution of CaCO3 shells), and oolitic limestone, made out of oolites, onion layer-like deposits of calcite around a grain of sediment, can and have formed over vast swaths of time. But here, condensed, is an approximate overview of some of the early eras.
Archean:
Stromatolites dominated the life form arena, up until the Proterozoic, and produced what CaCO3 we can find from this time period today.
Proterozoic:
Still mostly stromatolites. Evolution occurring among some groups of corals and critters, but not much is in the fossil record.
Cambrian:
The Cambrian explosion led to a vast diversification of organisms; corals, bivalves, trilobites, a few molluscs, etc. All of which are carbonate producers. From here on out, with a few variations, the major players in limestone production are here.
Ordovician:
Anthozoan corals abound, and more and more critters take shape and contribute to the fossil record in limestone. The development and expansion of bryozoans as well as the radiation of gastropods and articulate bivalves occurs, along with uncoiled nautiloids.
Silurian-Devonian:
Tabulate corals and forams are the primary limestone contributors here.
This trend occurs up until the Cretaceous, when some of the main distribution of limestone takes the form of the coccolithophores in chalk. All throughout this time, shelled organisms are dying and being lithified or recycled and turned back into CaCO3 for the other critters to use. It's difficult to keep tabs, but as a general rule, the animals that were able to use CaCO3 created and were fossilized in limestone, and generally they were of increasing complexity, from simple bivalves and solitary corals to complex, colonial corals and other bivalves, gastro/cephalopods, and complex arthropods.
5.
Forams/ Diatoms
Heterotrophic feeding/ Autotrophic feeding
Shell composed of:/ Shell composed of:
Tectin, agglutinated/ Silicious “frustule”
material, or secreted/
calcite/
Precambrian-Holocene/ Jurassic-Holocene
The foraminefera are more likely to be in marine environments, although some can live in brackish waters. Diatoms dominate the freshwater environments. Fusilinidae forams are great for correlating upper Paleozoic rocks, as they went extinct at the end of the Permian.
6.
Interval zone: Biostratigraphic zone which correlates the beginning and/or end of one or more fossil taxa
Assemblage zone: Biostratigraphic zone which includes multiple taxa
Abundance zone: Where a particular fossil taxon reaches a higher level of abundance
(Created with Microsoft paint program. Technically, I drew it. I'm kosher here.)
Planktonic organisms are useful for naming zones and time stratigraphic relationships because the appearance and extinctions of certain types of organisms clearly mark certain time boundaries. For instance, diatom biostratigraphy, which is based on time-constrained evolutionary apperances and extinctions of unique diatoms, is well developed and widely applicable in marine systems.
Phew!
