Fossils and Geologic Time
Fossils are the remains and traces of ancient organisms. It is an informal rule that such remains and traces must be at least 10,000 years old to qualify as a fossil, but age is seldom a question. Suffice it to say that few people would be tempted to regard last year's road-kill as a fossil, although some might be tempted to refer to a seashell imbedded in a 30 year old beach rock as a fossil (beer cans, clearly traces of life, are also sometimes imbedded in beach rocks, which form quickly through the interaction of sea spray and carbonate sand). Fossils include ancient bones, teeth, shells, tracks, trails, burrows and even dried or frozen corpses (e.g., wooly mammoths from northern Siberia and Alaska). Trace fossils, such as tracks, trails and burrows, are commonly referred to as ichnofossils. Some fossil insects, spiders, and even frogs and lizards are extrordinarily well preserved in amber millions of years old, with eyes, bristles and skin surfaces still visible. More commonly, though, just the hard parts of organisms remain, especially shells, bones and teeth.
Arrowheads, stone tools and even pottery shards are sometimes mistakenly called fossils. Even though they may date from prehistoric times (up to 2 Ma for some stone tools), these works of humans are properly called artifacts, and their study lies in the domain of paleoanthropology and archeology rather than paleontology.
Paleontologists describe, name, classify and analyze fossils to determine when, where and how life evolved, to determine evolutionary rates for particular lineages (clades), and to provide indirect evidence for causes of evolutionary radiations and extinctions, ancient climates, and even ancient continental positions. Over the past century and a half, data from the fossil record have transformed the idea that humans evolved from distant primate ancestors from "just a theory" to an historical fact (although certain details of the process of evolution remain theoretical).
Odd as it may seem from the modern perspective, fossils have not always been regarded as traces of past life. Some American Indians once believed that petrified logs are the scattered weapons of warring gods. About 350 B.C., the Greek philosopher Aristotle wrote that fossils were formed out of moist earth and slime, and that fossils grew within rocks by virtue of a mysterious plastic force, or vis plastica. His evidence was that fossil seashells are commonly seen on and within mountains, far from modern oceans, and they are commonly crumpled as if distorted by growing within the rock. Similar ideas were stated by Theophrastus (372?-287? B.C.) and in modified form by Edward Lhuyd. In 1699, Lhuyd wrote that the seeds which develop into organisms within rocks were planted there by vapors rising from the sea.
Pliny (Gaius Plinius Secundus; 23-79 A.D.), the Roman naturalist who died investigating the eruption of Mt. Vesuvius, wrote that some fossils fell to earth during thunderstorms, while others fell from the sky when the moon was eclipsed. His ideas may reflect the fact that waterspouts are known to deposit aquatic life (fish and frogs) long distances from their home waters.
As early as the 5th and 6th Centuries B.C., Greek philosophers and historians such as Xanthos of Sardis and Herodotus deduced correctly that fossils were once living organisms, and that inland fossil seashells indicated that the seas once covered the land. One of the most complete early statements about the origin of fossils, including the burial of shelled remains in ancient oceans, compaction of the enclosing sediment, and subsequent elevation and erosion of rocks, came from the brilliant Italian scientist, engineer and artist, Leonardo da Vinci (1452-1519 A.D.). He recorded this theory in mirrored hand writing in his personal notes. These notes were not translated from this simple mirror code until the 19th century.
Despite these early accurate views about fossils, as late as 1750 A.D. many Americans and Europeans still believed that fossils were left on mountain tops by the Biblical great flood. This idea is still held by some fundamentalist Christians and "scientific" creationists, despite clear evidence that most fossils predate by tens or hundreds of millions of years all traces of human life.
A feeling for the immensity of time represented by the fossil record comes from the realization that the great pyramids of Egypt, the oldest of which were built about 5,000 years ago, barely show the effects of erosion by rain and wind, whereas the ancient Appalachian Mountains have risen many thousands of feet, become flattened by wind and rain, and have risen once again, all since the appearance of fossil seashells some 543 million years ago.
The earliest monographs of fossils appeared during the 1400's in Italy. One of these works illustrated the fabulous Eocene tropical fishes from Monte Bolca, near Verona (see Monte Bolca countryside, entrance to mine, and view inside the mine, the latter courtesy of the Ente Provinciale Turismo in Verona, 1980). The Monte Bolca mine has yielded magnificent fossil fishes, including giant reef fishes, as well as boa snakes and even entire palm trees. Many of these fossils are displayed in the Museo Civico di Storia Naturale in Verona, and in the Natural History Museum in Milan. Some of the Monte Bolca fishes may have been killed by volcanic gasses venting through the sea floor. These and certain other early paleontological monographs were prepared by monks who used the fossils to illustrate "antediluvian" life, i.e., life before Noah's flood.
Georges Cuvier is widely regarded as the father of vertebrate paleontology. Balzac said of Cuvier that he "rebuilt, like Cadmus, cities from a tooth." He got this reputation in 1804 when he acquired the fossilized skeleton of a small mammal from limestone quarries at Montmarte. Noting that the jaw had a low condyle (articulating bump), Cuvier knew that it could not be a dog, cat or weasel. Only moles, hedgehogs, bats and opossums were known to have such a jaw bone. He then observed that each molar had three cusps. Moles and bats have seven cusps, hedgehogs
four, and only North and South American opossums, and some of their pouched relatives from Australia, have three. From these features alone, Cuvier successfully predicted the remaining, yet unseen, parts of the opossum-like skeleton, which he demonstrated to witnesses in his laboratory. This was an amazing deduction, considering that opossum-like animals do not live in modern day Europe and, as it turns out, had not done so for millions of years. In addition to starting the science of comparative vertebrate anatomy, Cuvier had discovered the concept of faunal extinction and had
established a new standard for zoological theories. According to Cuvier, "the true seal of a theory is beyond dispute the ability it has to predict phenomena."
Fossil wood is commonly crudely replaced with chert (finely crystalline quartz), leaving only traces of its original internal structure. This coarse replacement by silica is characteristic of the Triassic fossil wood from the famous Petrified Forest in Arizona, which shows only faint traces of internal growth bands (picture at left). At other times, the fine structure of the xylem and phloem is preserved after the hollow spaces and cell walls have been replaced by chert or by opal. The latter process is called permineralization. Permineralization after burial by volcanic ash (the source of silica) has produced some beautiful fossil woods with finely detailed internal growth bands, such as the example on the right from Washington State. Rarely, fossil wood can remain largely intact, except for darkening and the loss of some volatile compounds, for hundreds of millions of years.
Fossil shells and corals generally consist of one or both of the two common crystal forms of calcium carbonate, aragonite and calcite. Calcite is more resistant to dissolution by ground water than aragonite. The calcitic shells of rugose and tabulate corals, articulate brachiopods, echinoids, crinoids, trilobites, oysters, and scallops commonly remain well preserved in rocks which contain void spaces left by dissolved aragonitic shells of scleractinian corals, clams, snails and cephalopods. It is rare to find the original aragonite of corals and shells in rocks over 65 million years old. However, well-preserved calcitic corals and shells are common in rocks as old as 500 million years.
The Late Cretaceous Owl Creek Formation near Ripley, Mississippi, and the Late Cretaceous Ripley Formation at Coon Creek, Tennessee (see Coon Creek exposure) preserve scores of aragonitic-shelled marine clams, snails, and ammonite cephalopods. Over 65 million years old, most of the genera are now extict. The world's best preserved Triassic aragonitic shells and corals are found in the Alpine exposures of the Cassiano Formation near Cortina d'Ampezzo, northeastern Italy. Oriiginal aragonite occurs in the Late Carboniferous Buckhorn Asphalt of Oklahoma, and in the Late Carboniferous Breathitt Formation near Hazard and Ligon, Kentucky. The oldest presently known original shell aragonite occurs in Middle
Devonian strata near Melville Island, Arctic Canada, studied by Paul Johnston of the Royal Tyrrell Museum in Canada.
A few fossil shells (e.g., certain inarticulate brachiopods) and rare corals (the four-sided, cup-like conularids) consisted of calcium phosphate. These fossils share the resistance to dissolution characteristic of bones and teeth, so they are commonly well preserved in ancient rocks.
The Lower Jurassic Holzmaden Shale of southwestern Germany has yielded ichthyosaur skeletons with carbon films of their skin, including dorsal fins and dorsal tail flukes which are not reflected in their internal skeleton.Carbonized compressions of sharks, rays and even squids occur in the Upper Jurassic Solnhofen Limestone in southwestern Germany. The Solnhofen Limestone quarries near Eichstatt have been operating for centuries as a source of limetone blocks for lithography. In their course of operations, these quarries have produced all of the specimens of the famous transitional dinosaur/bird or at least reptile/bird Archaeopteryx. In both the Holzmaden and Solnhofen oceans, the tissues of dead animals were preserved in the muds of the ocean floor
because of their anaerobic chemistry. Low oxygen excluded scavengers and most of the bacteria involved in tissue decay. The dark clayey and silty muds of the Holzmaden Shalewere became anaerobic because abundant rotting organic detritus had removed most of the free oxygen. The light colored carbonate sediments comprising the Solnhofen Limestone were deposited in a much different back-reef environment, but there the temperature may have been so high that the water could not retain sufficient O2 to support scavengers and aerobic bacteria.
Pyrite pseudomorphs of soft tissues can be studied by x-ray photography without removing the fossil from the rock. Such pseudomorphs have revealed the fine structure of Paleozoic trilobite legs and gills, and the tentacles and the single pair of gills in Jurassic belemnoid cephalopods.
Miocene lake sediments at Snawang, eastern China show similar extraordinary preservation of soft tissues, including flowers with original coloration, feathers, hairs, skins, and even the eyes of tadpoles. Fossils there include insects (some even preserving the bright metallic green of scarab beetles), fish, salamanders, frogs, snakes and mammals. At both Messel and Snawang, stomach contents of the animals are commonly also well-preserved.
The Middle Cambrian Burgess Shale of British Columbia, western Canada, is the most spectacular example of carbonized compressions of marine organisms. This fauna, discovered in the early 1900's, was described at length by Stephen Gould (1989) in his book Wonderful Life. This locality is particularly interesting because it shows a wide range of soft-bodied invertebrates near the beginning of the first (or second, counting the late Precambrian Ediacaran fauna) adaptive radiation of large organisms on earth. Shown here are two examples from the Burgess Shale fauna. The worm-like organism Ayshaeia (on the left) is a representative of the Onychophora, which is evolutionarily intermediate between
annelids and arthropods. This was a marine species, whereas all modern onychophorans are terrestrial. The trilobite on the right shows impressions of the mineralized exoskeleton and carbon compressions of its walking legs with attached gills (in lower left part of picture).
Amber. Fossil amber is well-known for its ability to preserve details of insects and also, sometimes, vertebrates such as small lizards and frogs. Terpenes and other fragrant vapors from tree resin penetrate the trapped animal's tissues, replacing the water and quelling bacterial activity. Nevertheless, not all tree resin can become amber. The formation of true amber requires a special kind of terpene-bearing resin, present in various gymnosperms and angiosperms, that can polymerize, like epoxy, into hard, insoluble masses when exposed to air. Among conifers, the Araucariaceae (e.g., Norfolk pines) and Pinaceae (true pines) produce large quantities of such resins. They have a fossil history dating back to the Late Cretaceous, mainly in tropical and subtropical environments. Their resins are used today for adhesives, incense, and varnishes. Among angiosperms, the sweet gum (Liquidambar) and Hymenaea also produced amber-forming resins. Their fossil record extends back to the Eocene. Famous insect-bearing ambers include the following:
Working with Oligocene amber from the Dominican Republic, researchers have traced the ancestry of termites to a cockroach-like genus that is ancestral to modern termites, cockroaches and mantises. Microbiologist Raul Cano of California Polytechnic State University reported in 1996 that he revived bacteria from the stomach of bees preserved in amber 25 and 40 million years old. The bacterium is a symbiotic species much like the Bacillus sphaericus found in bees today, although differing enough in its DNA to suggest a different strain. The bacterium reportedly survived by constructing a tough protein coat called a spore. Cano has reportedly revived many other ancient microbes from flies, ants, wasps, beetles and termites in amber, including Bacillus, Actinomyces, and yeasts. He has even
started a biotech corporation called Ambergene, and he uses ancient yeast to brew a "Jurassic Ale" (a misnomer, because the yeast actually comes from Paleogene insects). Certain microbiologists remain skeptical that Cano has revived fossil microbes. In 1993 Cano sucessfully sequenced a segment of DNA derived from a weevil in Lebanese amber 125 million years old.
The Upper Carboniferous Mazon Creek fossils include soft-bodied animals within siderite (iron carbonate) nodules found in spoil piles from a coal strip and shaft mining operation. Two different biotas are represented: the "Braidwood" assemblage of nonmarine to brackish water animals, and the "Essex" assemblage of marine animals, dominated by jellyfish but also including molluscs, crustaceans, fishes, and previously unknown organisms such as the famous enigmatic Tully monster, Tullimonstrum gregarium. The Tully monster had a soft, 5-10 cm long, boneless body with a fishlike tail, two small, beady eyes supported on transverse bars, and a long, anterior proboscis. It may be an annelid or nemertean worm, or possibly even a mollusk or a conodont (early fishlike animal). The Mazon Creek organisms were buried quickly by silty sediments that accumulated at rates up to 1 meter per year. Their anaerobic decay led to the production of iron carbonate, which formed a protective nodular jacket
around their bodies. Mazon Creek faunas include 14 phyla, more than 33 classes, and nearly 100 orders, including the oldest known fossil lampreys and hagfishes. One salamander-like amphibian fossil, Amphibamus, has partially digested food within its gut. Charles Shabica and Andrew Hay (1997) have recently edited a book with many illustrations and descriptions of these fossils.
Certain early Paleozoic Lagerstatten contain tiny animals that are exquisitely phosphatized. Klaus Mueller and Dieter Walossek described in 1985 a remarkable arthropod fauna from the Upper Cambrian of Sweden in which limestone concretions contain tiny phosphatized arthropods, mostly larvae of crustaceans, in beautiful, three-dimensional detail (see their chapter in a 1997 book in arthropod relationships by R.A. Fortey and R.H. Thomas).
In 1994 Xi-guang Zhang and Brian Pratt found embryonic Middle Cambrian trilobites, and in 1997 Stefan Bengtson and Yue Zhao described Lower Cambrian phosphatized embryos from both Shaanxi Province in China, and the Aldan River region of Lena, Yakutia, Siberia. The accompanying photograph shows Lower Cambrian embryos possibly from a jellyfish (in back) and a worm (in front), each about 1/2 mm in diameter (from Science, vol. 277, Sept. 12, 1997; click on image for a link to a web site about the author, Stefan Bengtson). In 1998, Shuhai Xiao, Yun Zhang, and Andrew Knoll reported phosphatized algae and embryos of unknown triploblastic animals in Ediacaran (late Precambrian) rocks from southern China. These embryos include a remarkable series of stages, from single fertilized eggs to two-cell, four-cell, eight-cell, etc., up to small
balls of many cells. They are identified as triploblastic because the overall size of the embryo does not change much as the cells decrease in size to include more cells. Some researchers believe these may be arthropod embryos, based on their general shape.
A famous insect lagerstatten occurs in an Early Permian limestone at Elmo, Dickinson County, Kansas. This locality, called "Insect Hill" has yielded thousands of insect impressions. Many of these fossils were collected by the late Carl Dunbar for the Yale University Peadody Museum of Natural History. The following examples were illustrated by J.R. Barbour and A. H. Rehn for the Yale Peabody Museum Invertebrate Paleontology web site: paleodictyopteran Dunbaria, protodonatan
Typus, odonatans Kennedya and Megatypus.
Absolute ages for carbon-bearing fossils less than about 55,000 years old can be calculated with a fair degree of accuracy using the carbon-14 method. However, 55,000 years does not extend beyond the late Pleistocene, so only relatively recent fossils can be dated by this method. Furthermore, carbon-14 dating is limited to fossils containing abundant carbon, such as bones, teeth, shells, and wood. The radioactive isotope C-14 is produced in the atmosphere by cosmic radiation, and it is incorporated into shells, bones and teeth through respiration and metabolism. C-14 undergoes a spontaneous change in its nucleus through radioactive decay. The radioactive decay rate, which is well known, is used to provide an absolute age based on the following facts: C-14 eventually decays back to nitrogen-14, the atom from which it was originally formed. The decay rate is one-half of the number of C-14 atoms every 5730 years. However,C-14 is only one of the isotopes of carbon in the atmosphere. By far the most abundant carbon isotope is C-12, which, unlike C-14, is stable and non-radioactive. There is a more or less constant ratio of C-14 toC-12 in the atmosphere because the production and decay of C-14 is in a state of dynamic equilibrium. When animals and plants incorporate atmospheric carbon into their tissues, the carbon is removed from this atmospheric equilibrium, and the ratio of C-14 to C-12 begins to decrease in the fossil. Measuring this ratio is the key to determining when the fossil lived. For example, if a shell has a ratio decreased by one-half compared to modern values, it would be about 5730 years old. If the ratio is decreased by halfing this ratio twice, i.e., to 1/4 the original ratio, then the shell is 2 x 5730 or 11,460 years old.
One limitation of C-14 dating is that it cannot be applied to fossils much older than about 55,000 years because most of the C-14 has decayed by then, reverting back to N-14. Another limitation is that the ratio of C-14 to C-12 has varied slightly in the past due to changes in the flux of solar radiation, which is responsible for converting N-14 into C-14. Nevertheless, this problem can be corrected using a calibration curve based on C-14 dating of samples from well-known tree ring growth series.
Several other radioactive isotopes can be used to provide absolute ages for fossils, but some, such as uranium-235, uranium-238, and potassium-40, are mainly found in minerals in rocks which crystallize at high temperatures. These isotopes also decay very slowly, so their parent mineral must be many millions of years old to yield detectable amounts of daughter product. However, when these atoms occur in minerals that are part of an ancient lava flow, volcanic ash fall, or vein of magma, they may be used to provide a minimum age for a fossil-bearing bed which they overlie or cut through.
Uranium series dating, which utilizes minute differences in the intermediate daughter products of radioactive uranium, partially fills in the chronological gap between C-14 and uranium and potassium dates. However, this technique depends on having sufficient uranium and daughter products in the fossils analyzed, and this is commonly not the case.
Perhaps most importantly for paleontology, absolute ages permit geologists to place numbers on boundary horizons in the relative geologic time scale. Important boundary horizons separate the Precambrian and Phanerozoic Eons (543 MA, where Ma = millions of years ago), the Paleozoic and Mesozoic Eras (251 Ma), the Mesozoic and Cenozoic Eras (65 Ma), the Tertiary (= alternatively the Paleogene + Neogene periods) and Quaternary periods (1.8 Ma), and the Pleistocene and Holocene epochs (10,000 years ago).
The relative geological time scale is divided into a hierarchy of units, called eons, eras, periods and epochs. The boundaries between the eras and periods are commonly based on marked changes in the fossil record. For example, the Paleozoic, Mesozoic, and Cenozoic eras are separated by two great mass extinctions. The most recent division between eras occurs roughly 65 million years ago between the Mesozoic and Cenozoic. Below this division, the Mesozoic terrestrial vertebrate world is dominated by dinosaurs. Above this boundary, during the Cenozoic, dinosaurs are gone and terrestrial vertebrate life is dominated by birds and mammals. The Paleozoic and Mesozoic Eras are separated by an even greater extinction that affected many invertebrate and vertebrate families and orders (e.g., rugose and tabulate corals, trilobites, many bryozoans, most echinoids, and many reptile families went extinct at this boundary).
The names of the eons, eras, periods and epochs may appear formidable, but most of them have straightforward origins. For example, Phanerozoic means "visible life", i.e., the age of abundant life forms that leave readily visible fossils. The Precambrian simply means before the Cambrian. The names Paleozoic, Mesozoic and Cenozoic translate to ancient, middle and modern life, respectively. The period and epoch names come from geographic or descriptive terms. The Carboniferous contains vast carbon reserves in the form of coal. The Triassic comes from the three-fold division of rocks of this age in Europe. The Jurassic is named for the Jura mountains of Europe, and the Cretaceous is named for chalky strata of southern England (the white cliffs of Dover), northern France (Normandy), and Belgium ("Cretaceous" comes from the Latin for "chalky").
A famous 19th century English geologist, Sir Charles Lyell, named some of the Cenozoic epochs (e.g., Eocene and Miocene) on the basis of their percentage of living marine species.
Some of the geological periods have alternative subdivisions. For example, Europeans tend to use the name Carboniferous whereas many Americans still use the terms Mississippian for lower (older) Carboniferous, and Pennsylvanian for upper (younger) Carboniferous. In additon, the older part of the Tertiary is now commonly called the Paleogene, and consists of the Paleocene, Eocene, and Oligocene epochs. The medial Tertiary is now called the Neogene. The Neogene contains the Miocene and Pliocene epochs (formerly in the Tertiary), and the Quaternary includes the Pleistocene and Holocene epochs.
Major subdivisions of the relative geologic time scale and the absolute ages of their major boundaries are listed below. Epochs are listed only for the Paleogene and Neogene periods, although they hve been defined for the other periods as well. This chronology is shown in "stratigraphic order", i.e., with the oldest beds on the bottom. The numbers on the left refer to boundary dates, not to durations. A few major evolutionary events are also indicated.
II. PHANEROZOIC EON:
C. Cenozoic Era:
3. Quaternary Period:
b. Holocene Epoch
0.01 Ma ---------------------------------------------------
a. Pleistocene Epoch (Homo sapiens appears)
1.8 Ma ----------------------------------------------------
2. Neogene Period:
b. Pliocene Epoch (early humans appear)
5.3 Ma ------------------------------------------------------
a. Miocene Epoch (apes diversify in Africa)
23.8 Ma -----------------------------------------------------
c. Oligocene Epoch (apes originate)
33.7 Ma -----------------------------------------------------
b. Eocene Epoch
54.8 Ma -----------------------------------------------------
a. Paleocene Epoch (major primate radiation)
65 Ma -----------------------------------------------------
B. Mesozoic Era:
3. CretaceousPeriod (primates appear)
144 Ma ----------------------------------------------------
2. JurassicPeriod (birds appear)
206 Ma ----------------------------------------------------
1. TriassicPeriod (dinosaurs and mammals appear)
248 Ma ----------------------------------------------------
A. Paleozoic Era:
290 Ma ----------------------------------------------------
5. CarboniferousPeriod (reptiles appear)
354 Ma ----------------------------------------------------
4. DevonianPeriod (amphibians appear)
417 Ma ----------------------------------------------------
443 Ma ----------------------------------------------------
490 Ma ----------------------------------------------------
1. CambrianPeriod (fishes appear)
540 Ma ----------------------------------------------------
I. PRECAMBRIAN EON:
B. Proterozoic Era:
2. EocambrianPeriod (large invertebrates appear)
605 Ma ----------------------------------------------------
1. Unnamed periods (eukaryotes appear)
2500 Ma ---------------------------------------------------
A. Archean Era:
1. Unnamed periods (prokaryotes appear)
4600 Ma: earth begins to cool from molten state.
Relative ages of fossils are determined by correlating the rocks that contain them with a reference section, using any of a wide variety of possible time markers. The latter include temporally restricted associated fossils (i.e., "index" fossils), ash falls blanketing an entire region in a short period of time (e.g., at the end of the Cretaceous, when an asteroid left an ash layer over much of the earth's surface), or even physically tracing out rock units that are believed to have been deposited synchronously (e.g., thin black shales believed to represent relatively short, widespread episodes of anaerobiosis in an ocean basin). Microscopic oceanic fossils like planktonic foraminifera, diatom frustules, radiolarians, and coccoliths are commonly used as index fossils for the latter Mesozoic and Cenozoic. Some larger fossils, such as trilobites for the Cambrian; trilobites, echinoderms and corals for the later Paleozoic, and
ammonite cephalopods for the Mesozoic, are also useful as local time markers, although they generally lack the wide geographic distribution of marine microfossils such as planktonic forams and coccoliths.
Biostratigraphy is the science of combining stratigraphic and paleontologic data to establish relative ages of sedimentary deposits, and to interpret their history of deposition and erosion. Several different kinds of relative time zones can be defined using individual index fossils, pairs of fossils, or fossil assemblages. A taxon range zone is the time interval between the origin and extinction of a single genus, species or subspecies. Boundaries of range zones are difficult to define precisely. Many of them are actually acme zones, which represent only the time of a species' greatest abundance, the assumption being that the earliest and latest occurrences are generally too rare to be generally known. Concurrent range zones are defined by the overlap of two taxon range zones or two acme zones.
A large suite of fossils may be used to define an assemblage zone. For example, the Paleozoic assemblage zone includes trilobites, rugose and tabulate corals, primitive fishes, primitive amphibians, and primitive reptiles, but not scleractinian corals, dinosaurs, birds or mammals. The Mesozoic assemblage zone includes scleractinian corals, dinosaurs, early birds, and early mammals. The Cenozoic assemblage zone includes scleractinian corals, modern birds, and modern mammals but not dinosaurs. Note that some elements of adjacent assemblage zones may overlap. It is the total assemblage that is distinctive of an assemblage zone, not necessarily the individial elements.
Calculating relative ages can be difficult when a geological section is missing strata representing significant amounts of geologic time. When the missing time interval is short (only a few thousands or hundreds of thousands of years), the missing time interval is referred to as a diastem.If much more time is not represented (e.g., millions of years) the missing time interval is labeled as an unconformity. Unlike diastems, unconformities areoften marked by a lag deposit of rounded pebbles, commonly phosphatic, which represent an ancient erosion surface. Sometimes, the older rocks are tilted by mountain-building (tectonic) forces before sediment deposition was resumed. This special kind of unconformity surface is called an angular unconformity, and it usually represents tens or hundreds of millions of years.
Fossils can provide important clues about the environment of deposition of a stratigraphic unit. Fossils themselves can also be studied for their life habits, trophic interactions, and community relationships. These subjects comprise the field of paleoecology. Paleoecologists generally specialize in terrestrial or marine ecosystems. Dale Russell, for example, has written extensively on the ecology of dinosaurs, including their life habits, trophic interactions, and inflluence on terrestrial habitats. He has hypothesized that a herd of hundreds or thousands
of duckbill dinosaurs would drastically alter the landscape during seasonal migrations along the shores of the Rocky Mountain Seaway in western North America. Bill Showers and Reese Barrick have analyzed oxygen isotopes in dinosaur bones to determine whether these animals were cold or warm blooded. On the other hand, marine paleoecologists have reconstructed the climatology and biogeography of ancient oceans for several periods during the Phanerozoic Eon. They have produced global paleogeographic maps in which biotic and climatic provinces are reconstructed
relative to ancient continental configurations. For example, the accompanying Upper Cretaceous map shows the Rocky Mountain Seaway cutting north-south across western North America. At that time, there was a relatively wide subtropical climatic belt, and no extremely cold Arctic and Antarctic climatic provinces (green represents cold temperate, as in southeastern Canada today).
The marine paleoecologist James Valentine has studied the life habits, community relationships, paleoenvironments, and paleobiogeography of a great many marine invertebrates to determine the relationship of these factors and of long term climatic change to variations in marine faunal diversity.
One of the present authors (JGC) has studied the paleoecology of marine invertebrates from the Middle Devonian Hamilton Group in a portion of central New York state. The Hamilton Group is famous for its diversity of marine brachiopods, bivalves, and gastropods. These animals lived in marginal marine to open marine environments to the west of the ancient Catskill Delta, on the western flank of the ancestral Appalachian mountains. Marine Hamilton Group rocks are largely dark brown to black in color, varying from shales to siltstones and sandstones, with relatively few limestones, the latter more abundant in the western part of the state (away from the delta sands and silts). Typical exposures in central New York are represented by the Delphi Station Member of the Skaneateles Formation and the Solsville Member of the Marcellus Formation north of Morrisville, New York. A roadside exposure of the Solsville Member in the Peterborough North geologic quadrangle shows a change from silty, sandy shale at the base to muddy sandstone at the top. Bulk faunal samples taken from several horizons along the outcrop reveal a correlated faunal change. The base of the outcrop contains mostly the brachiopods Cupularostrum, Leiorhynchus, and Lingula, the gastropod Palaeozygopleura, and deposit feeding bivalves such as Nuculoidea. The upper part of the outcrop contains mostly the brachiopods Spinocyrtia, Devonochonetes, and Rhipidomella, and the suspension feeding bivalves Modiomorpha, Gosseletia, Leptodesma, and Ptychopteria. Statistical analysis of the faunal samples reveals two well-defined marine benthic communities, one characterized by positive correlation with deposit-feeding nuculoid bivalves, the other by positive correlation with Spinocyrtia. The brachiopod Ambocoelia correlates with neither community, and instead is only locally abundant throughout the outcrop. The ecological separation of deposit feeding and suspension feeding bivalves, the latter largely restricted to the Spinocyrtia community, reflects the fact that finer sediments commonly have abundant adherent organic matter, but they tend to foul the gills of suspension feeders. Coarser sediments are better suited for suspension feeding bivalves, and they have relatively little organic detritus for deposit feeders. Brachiopods, which as impingement feeders were more tolerant of mud than were suspension feeding bivalves, may have specialized for either finer or coarser sediments by virtue of the structure of their lophophore feeding device, their relationship to the substratum, or other anatomical or behavioral factors. Ambocoelia apparently preferred ecologically disrupted environments. Its local and temporary domination of the sea floor is characteristic of modern "opportunistic" species with these tendencies.
Selected fossil collecting localities
Paleozoic of Kentucky and Indiana:
The Paleozoic strata exposed in Kentucky, Indiana, and Ohio are well known for their wealth of fossiliferous strata representing the Ordovician, Devonian, and Carboniferous periods. Silurian fossils are also locally present, but they are seldom as well preserved as in the other three periods mentioned. Just a few of these richly fossiliferous Paleozoic localities include:
ORDOVICIAN of KENTUCKY
Kentucky: The Lower Ordovician Lexington Limestone is exposed along the southeastern exit ramp at exit 126 on Interstate 75, south of Lexington, KY (see stratigraphic chart from D.C. Haney and M.C. Noger, 1992, Roadside Geology along Interstate Highway 75 in Kentucky, KY Geol. Survey Spec. Publ. 16). Abundant rhynchonellids and a few spiriferid brachiopods have weathered from the limestone along the upper surface of this outcrop. See geologic section and road map.
The Upper Ordovician Southgate Member of the Kope Formation is exposed on the westward-facing hillside on route 227, immediately southeast of exit 44 on Interstate 71, southeast of Carrollton, Kentucky (see stratigraphic chart from W. Sweet, 1979, U.S. Geol. Survey Professional Paper 1066-G). This limestone and shale sequence contains well preserved brachiopods (Rafinesquina, Onniella, Sowerbyella), crinoids (mostly pieces of stems), bryozoans (Batostoma and Heterotrypa, the latter with small mound-like elevations), trilobites (e.g., Flexicalymene, Isotelus, Cryptolithus), gastropods (Sinuites, Holopea, Loxoplocus), bivalves (Modiolopsis, here replaced by black calcite; Ambonychia), and cephalopods (?Orthonybyoceras and Endoceras). Nearly complete branching colonies of bryozoans can be extracted from the shale here. Many species from this locality are illustrated in the booklet Cincinnati Fossils edited by R.A. Davis (1998) for the Cincinnati Museum Center, Cincinnati, Ohio (513 287-7000).
DEVONIAN of INDIANA
Indiana: Falls of the Ohio State Park: The Falls of the Ohio on the Ohio River between Louisville, Kentucky, and Clarksville, Indiana, is a world-famous Middle Devonian fossil locality. On the Indiana side of the river (just west of Exit 0 on Interstate 65) fossils in the Jeffersonville Limestone can be seen year-round, with easy access by a short trail leading from a Visitors Center to the river's edge. This trail is open year-round, but the Visitors Center has an entrance fee. Fossil collecting is prohibited at this and other Indiana state parks. Here the Middle Devonian Jeffersonville Limestone is overlain by the less fossiliferous, massive to thin bedded, locally dolomitic (with a sugary texture) North Vernon Limestone, also of Middle Devonian age. The North Vernon is in turn overlain by the dark, finely laminated, Upper Devonian New Albany Shale, which is virtually devoid of fossils. The Jeffersonville Limestone is underlain by he Upper Silurian Louisville Limestone, which is often dolomitic and poorly fossiliferous. Fossils from the Falls of the Ohio were described as early as 1820 by the famous botanist and paleontologist C.S. Rafinesque. Since then, over 600 species of Devonian invertebrates have been described from here, with over 30% unique to this site. The uppermost limestone ledge along the visitors trail at Falls of the Ohio State Park is rich in sheet-like bryozoans and brachiopods, and is directly underlain by a bed of the brachiopod Brevispirifer gregarius. The underlying ledges comprise the Amphipora ramosa zone, so named for this branching, tubular, spaghetti-like stromatoporoid which encrusts rugose (horn) corals. The major part of the exposure, comprising the limestone flats at the river's edge, is the underlying coral zone, which contains a wealth of tabulate corals, rugose corals, stromatoporoids, trilobites, bryozoans, and brachiopods. Some of the originally calcitic fossils in the Jeffersonville Limestone have been replaced by quartz, thereby making them more resistant to chemical weathering, and causing them to stand "proud" of the surrounding limestone surface. Common Middle Devonian fossils at this locality include:
Amphipora ramosa: spaghetti-like, tubular encruster..
Solitary rugose corals:
Zaphrenthis: septa are knobby or spiked.
Heterophrentis: septa are smooth.
Aulacophyllum: the calyx (cup) is on the side of a short "horn".
Siphonophrentis: reaches 1.5 meters in length.
Scenophyllum: like Siphonophrentis, but narrower and with a central protrusion) inside the calyx (cup).
Colonial rugose corals:
Eridophyllum: indivudual calices are not connected and commonly spread outward from a common center.
Prismatophyllum: closely packed calices.
Hexagonaria: Similar to Prismatophyllum but less common. See Ida Thompson's (1982) Audobon Society Field Guide to North American Fossils for differences between the two.
Favosites: mounds reaching 1 meter in diameter, with clearly developed tabulae (horizontal plates).
Emmonsia: similar to Favosites but with poorly developed tabulae. Emmonsia ramosa may assume the shape of a large rugose coral, such as Siphonophrentis.
Alveolites: delicate branching coral with pin-hole size calices.
Thamnopora: sturdier branching coral with toothpick size calices and thin calyx walls.
Trachypora: like Thanmopora but with larger calices separated by thick, wavy walls.
Pleurodictyum: looks like a mound shaped wasp nest, with rounded calices.
Syringopora: narrow tubes (less than a straw width) connected in fan-shaped or organ-pipe arrangements.
Brevispirifer gregarius: lateral folds are strong; medium to small shell.
Paraspiriferacuminatus: lateral folds are weak, with a strong medial fold and sulcus; large shell up to 2 inches across..
Platyrachella: with sharp wings and long, straight hinge.
Athyris: rounded with a pointed beak; nearly smooth.
Atrypa: similar to Athyris but finely radially ribbed.
Paracyclas: rather smooth, oval shells.
Turbonopsis: fist-sized, commonly covered by stromatoporoids..
Platyceras: calcitic shell.
Hippocardia: anterior end pointed, posterior end flattened with a tube.
Additional fossils: see S.F. Greb, R.T. Hendricks, and D. R. Chesnut, Jr., 1993, Fossil Beds of the Falls of the Ohio, Kentucky Geological Survey Special Publication 19, 42 pp.
Indiana: Lexington/Scottsburg Hanson Quarry: The Silurian and Devonian strata at Falls of the Ohio are also exposed at the Hanson quarry on Indiana Highway 203 between Scottsburg and Lexington, SE Indiana. This quarry is just south of the intersection of Indiana Highway 56 and 203 on the east side of 203. The quarry contains a basal exposure of the Upper Silurian Louisville Limestone, and continues upward with the Middle Devonian Jeffersonville Limestone, the Middle Devonian North Vernon Limestone, and the Upper Devonian New Albany Shale. As at Falls of the Ohio State Park, abundant Favosites and Emmonsia occur in the Jeffersonville Limestone. In August of 2001, large favositid coral heads were well exposed in situ in life position on the vertical quarry walls. The large, solitary rugosan Siphonophrentis also occurs here, as well as a variety of smaller rugose corals. Brachiopods are locally abundant here as well, such as flat shelled Stropheodonta. Brachiopods are also present in the overlying North Vernon Limestone.
Cretaceous and Cenozoic in the Southeastern United States
The biostratigraphic framework for the Cretaceous and Cenozoic deposits of the southeastern United States is relatively well established, and serves as an excellent example of a biostratigraphic synthesis. In the accompanying chart, absolute and relative geologic time are indicated in a column near the left, along with global foraminiferal and nannofossil zones. Cenozoic foram zones are numbered with a P or N, for Paleogene and Neogene, respectively (e.g., P21, N21). Cenozoic nannofossil zones are numbered NP and NN, for Nannofossil Neogene and Nannofossil Paleogene, respectively (e.g., NP21, NN21). Cretaceous foram and nannofossil zones are indicated by C21, etc. Fossil zones are also indicated for Miocene-Recent molluscs (M1-M14 of L.W. Ward), and for Upper Cretaceous pollen (III-V and CA1-CA6, MA1) in the column for Maryland and New Jersey. The foram, nannofossil, and molluscan zones apply to marine deposits, whereas the pollen
zones apply largely to terrestrial deposits. Where foram, nannofossil, molluscan, and/or pollen zones are known, this information is indicated by the name of the formation or member on the main body of the chart.
Outcrop exposures of several of the formations on this stratigraphic chart are illustrated below:
EOCENE FOSSIL LOCALITIES
Bashi Marl Member of Hatchetigbee Formation, exposed in landscaping boulders near Meridian, Mississippi, with abundant shells of Venericardia. Also other exposures of the Hatchetigbee Formation near Meridian. Gordon Creek Shale Member of Cook Mountain Formation, central Mississippi. Cockfield Formation, central Mississippi. Tallahatta Formation in central Mississippi.
Florida: Suwannee Limestone, exposed along the Chipola River, northwestern Florida.
North Carolina: Duplin Formation (= southern aspect of Zone 2 Yorktown Formation) at Murfreesboro, Greenville, Lumberton, and Tarheel, North Carolina.
Virginia: Yorktown Formation, Zone 1 (lower, cooler paleoclimate zone) at Avery borrow pit, Virginia; and Zone 2 (upper, more tropical paleoclimate zone) at Southall pit, Virginia.
PLEISTOCENE FOSSIL LOCALITIES
Florida: Fort Thompson Formation, along the Caloosahatchee River, near La Belle, southwestern Florida.
North Carolina: Chowan River Formation at Colerain Beach, on the Chowan River, North Carolina.
Virginia: Norfolk Formation near Norfolk, Virginia, with fossil trees.