Dian Fossey was to gorillas what Jane Goodall is to chimpanzees. Her long-term studies of Mountain Gorillas (Gorilla beringei beringei) in the Virunga Volcanoes of Rwanda, in central Africa, laid the basis for understanding the behavior and social life of gorillas in general, and all subsequent fieldworkers on gorillas have built upon her work.
Like Jane Goodall, Fossey was a protégée of Louis Leakey, the renowned paleoanthropologist. On the grounds that the study of our closest living relatives, the great apes, would teach us a good deal about our own past, Leakey encouraged a number of enthusiastic individuals, almost all of them female, to study these species in the wild. Fossey and most of the others were untrained, but acquired academic training during the course of their fieldwork and in this way were able to put their own observations into perspective and so could go on to make further insights. Dian Fossey (gorillas), Jane Goodall (chimpanzees), and Birute Galdikas (orangutans) were the three who achieved long-term observations and so have contributed most to the field.
Fossey was working as an occupational therapist in California when she met Leakey during a trip to Africa in 1963. In 1963, on his urging, she began studying gorillas at Kabara, in the western part of the Virunga Volcanoes, in the Democratic Republic of Congo (at that time called Zaire). Political unrest made this location unsafe, and she crossed into Rwanda, where she set up a new field station at Karisoke, in the central part of the Volcanoes. (The Virungas span the borders of Democratic Republic of Congo, Rwanda, and Uganda). Here she stayed, on and off, for the rest of her life. She was devoted to Robert Hinde, her academic mentor in Cambridge, but was always anxious to be back in Rwanda.
Her contacts with gorillas become very close and intimate, and adults as well as youngsters would approach her, touch her, and sit by her while she wrote her field notes. She devised a system of recognizing individuals by the pattern of wrinkles on their noses, which she called “noseprints,” on the analogy of fingerprints. She recorded the relationships of individuals within and between troops, and the means by which new troops are formed and established troops are inherited from father to son.
She was engaged in a continuous war with poachers, who would kill adults to sell their heads and hands as souvenirs to tourists or to capture youngsters for sale to zoos. She also constantly battled government officials who wanted to excise land from the Volcanoes National Park to plant crops, such as pyrethrum, or to graze the cattle herds there. On December 26, 1985, she was found murdered in her cabin. Two men, a camp steward who had been discharged and an American researcher, were initially arrested, but her murder was never satisfactorily solved.
Since her death, tourist visits to see gorillas in the National Park have been instituted. Dian Fossey knew about the plans and had been in two minds about this prospect, but the idea has proved a great success, earning Rwanda far more foreign exchange than pyrethrum or the cattle industry, such that during the massacres and civil war in the 1990s, the gorillas were hardly interfered with. Apart from the understanding of gorillas that she promoted, her enduring legacy is that the Mountain Gorilla still survives and flourishes to this day.
The origin and diversity of life on Earth has been one of humankind’s most enduring sources of curiosity. The questions of, “who, how, when, where, and why” are still widely debated. These questions cross the boundaries between science, philosophy, and religion. It is not the intent of this entry to settle this controversy. Rather, it will look at these topics from the standpoint of science based on our current level of knowledge and understanding. This entry is arranged chronologically by era. The time spans discussed are compared to their relative positions from the “recent” end of a 100-meter-long rope to illustrate the quantity of time discussed and its distance from the present. Dates presented herein are primarily from the revised stratigraphic chart of the International Commission on Stratigraphy, 2004. The abbreviation mya means “million years ago.”
One feature of the fossil record is that preserved fossil localities increase in direct proportion to how recently the localities were preserved. In most cases, the preservation is limited to hard body parts, and these parts are often poorly preserved. However, unusually complete and well-preserved assemblages that often include soft-tissue preservation, called lagerstatten, greatly increase the available information for the time when the assemblage was preserved. When interpreting trends in the fossil record, it is easy to infer a gradual increase in diversity through time when, in reality, the increase is merely an artifact of an increased number of fossil localities. Likewise, lagerstatten have nearly always contributed to interpreted “explosions” of diversity. Unfortunately, these glimpses of possibly true representations of biotic diversity are limited to one environment and one instant in time, and these glimpses usually occur tens of millions of years apart. The fossil record thus does not contain enough information to provide any degree of accuracy concerning any but the most basic trends in biotic evolution. For this reason, this report should be considered a tentative representation of our current understanding rather than a definitive statement concerning Earth’s evolutionary events.
Earth’s Origin-Hadean Era: 4,550-3,800 mya (100 to 83.5 meters)
The Hadean is the most poorly known part of Earth’s history. For much of this time, the Earth was in the process of coalescing, cooling, and continent forming. The 4.55-billion-year date for the beginning of the planet was not obtained from Earth’s rocks, but rather from meteorites and moon rocks. These sources seem to place the “solidification” of our solar system, including the Earth, at that date. Then, for a period of time, the Earth likely underwent heating, compression, and eventual landmass solidification. The name “Hadean” was actually given to this period of Earth’s history because of the tremendous heating and volcanic events that are speculated to have happened during that time. It is thought that a collision of Earth and another planetoid at approximately 4.5 billion years ago produced our moon and significantly remodeled the Earth in the process. This and other collisions kept the Earth very hot. However, it should be noted that this picture of Earth’s distant past is highly conjectural. No rock outcrops are known that date to the first half-billion years of Earth’s history. Some scientists have postulated the opposite effect, a “snowball Earth,” for Earth’s very early history, and still others have postulated an environment rich in oxygen since earliest times. Only the most widely accepted views will be presented here.
The oldest radiometric date obtained from any rock of potentially terrestrial origin is 4.4 billion years.
This date was obtained from zircons embedded in younger sedimentary rock in west-central Australia. However, no parent rock has been found, and thus the zircons could theoretically have been of extraterrestrial origin. The oldest rock positively identified as terrestrial in origin is the 4.03-billion-year-old Acasta Gneiss, found near Great Slave Lake, in northwestern Canada. Other ancient sediments include the Porpoise Cove supracrustal sequence of northern Quebec (3.8-3.9 billion years old), the Isua Supracrustal rocks of Greenland (3.7-3.9 billion years old), and Barberton (southern Africa) and Pilbara (Western Australia) greenstone belts (3.3-3.5 billion years old).
The origin of life remains a mystery. Even the search for evidence of ancient life is difficult due to the rarity and metamorphism of preserved rocks of appropriate age and an agreement on what constitutes unequivocal evidence of life. Some researchers are convinced that unusual carbon isotopes found in the Isua rocks are proof of life on Earth nearly 3.9 billion years ago. Widely accepted evidence of life on Earth is fossilized algal mats (stromatolites) dating approximately 3.5 billion years old. However, considering how life must have started, these archaea (formerly known as archeobacteria) were exceedingly complex organisms. They could photosynthesize, if only through the primitive Photosystem I series of reactions, and could form colonies. Several lines of research now point to an origin of life (or possibly several origins) in a relatively hot, anoxic environment such as is found today around hydrothermal vents. Such environments were purportedly common in the Hadean, and much less common since. It should also be noted that virtually all information on early life comes from sedimentary rock, and undisputed evidence of life is present in the some of the earliest sedimentary rocks found on Earth so far.
The Planet Cools, and Life Thrives: Archaean Era: 3,800-2,500 mya (83.5 to 54.95 meters)
This era of geologic time occupies over one quarter of all time since the Earth was formed. It is marked by the presence of life and the absence of free atmospheric oxygen. The types of life present during this portion of Earth’s history are represented today by similar forms living in hydrothermal pools or near volcanic vents and fumaroles on the ocean floor. These life forms did not need oxygen to survive. In fact, they produced free oxygen as a form of organic waste. It is probable that these organisms were largely responsible for the atmosphere we have today.
The beginning of the Archaean is marked by the presence of free water in liquid form. The Earth, by this time, had cooled sufficiently that seas were forming, as evidenced by the presence of sedimentary rocks. It is unknown how long prior to the admittedly arbitrary 3.8 billion years Hadean/Archaean boundary water existed on Earth as a liquid, but it is likely that seas, continents, and atmosphere had combined to create a cycle of erosion and deposition by the beginning of the Archaean. Unfortunately, this early Archaean time is also poorly represented in the rock record.
The first undisputable evidence for life on Earth is found in the early Archaean sediments of the Barberton greenstone sequence of the Kaapvaal Craton found in South Africa and the time-equivalent sediments of the Australian Pilbara Craton. These sediments have been dated at 3.5 billion years old and may have been part of the earliest recognized super-continent, Vaalbara, between 3.45 and 2.87 billion years ago. The African sediments contain carbon-lined tubules in lava rock, and both sediments exhibit fossilized algal mats. It is speculated that the lava tubule discovery is evidence not only for life but also for the trophic behavior of that life. The tubules are purported to show the feeding habit of at least one form of life at that time, burrowing into and eating the rock itself. There is also some evidence that even at this early stage in the history of life, genetic exchange (sexual reproduction) was occurring in some organisms.
Throughout most of the Archaean era, the relatively primitive Archaea consumed carbon dioxide and produced limited quantities of free oxygen. The oxygen was then “trapped” by several oxidation processes. Most researchers agree that the early stromatolites actually represent films of Archaea that exceeded the environment’s carrying capacity, then succumbed to a mass die-off. Once most of these cells died, the environment stabilized, allowing the process to repeat, thus producing the stromatolite laminae. It is unknown how many types of Archaea lived during the 1.3 billion years when they were the most common life forms. Today, we have 3 major types of Archaea, and these show remarkably little genetic similarity to each other. In fact, each group is more closely linked genetically with other forms of life than with each other. It has been suggested that these groups became separate life lineages at the “protolife” level, and thus true life may have arisen more than once.
At approximately 2.8 billion years ago, or approximately 1 billion years after life on Earth first appeared, a new form of life made its appearance. Cyanophyta, commonly known as cyanobacteria, or blue-green algae, were the earliest life forms known to produce oxygen in large quantities. The Cyanophyta accomplished this by utilization of a series of reactions, known as Photosystem II, that break water into its components. Many believe that it was the early success of the Cyanophyta that produced the relatively large amounts of oxygen in our atmosphere. In a relatively short time from the first appearance of Cyanophyta, approximately 300 million years, the atmosphere of the Earth became saturated with oxygen past the point where oxidation processes could “trap” it. Oxygen then became a significant component of the atmosphere itself, setting the stage for a major environmental readjustment.
The Planet’s Oxygen Atmosphere Arrives: Proterozoic Era: 2,500-542 mya (54.95 to 11.91 meters)
Approximately 43% of the history of the Earth is represented by the Proterozoic era. Its beginning is marked by the first appearance of “redbeds” in the stratigraphic record. These redbeds are sedimentary layers colored by oxidized iron. This worldwide oxidation of iron in subaerially exposed sediment required large quantities of oxygen in the atmosphere. The earliest occurrence of these redbeds has been dated at 2.5 billion years ago.
Although Archaea made up a significant portion of the total biomass during the Proterozoic, Cyanophyta were the dominant life forms. It was only during Vendian time, from approximately 600 million years ago until the end of the Proterozoic, that other life forms began to contribute significantly to the biomass. During the Proterozoic, an increase in the oxygen content of the atmosphere, from 1.5% at the beginning to 15% at the end, is attributed to being produced largely by Cyanophyta. It should be noted that heterotrophic eubacteria and eukaryotes are also found in the fossil record of this time period.
The Proterozoic has been broken into three sub-units, the Paleoproterozoic (2.5-1.6 billion years ago), Mesoproterozoic (1.6-0.9 billion years ago), and Neoproterozoic (0.9-0.54 billion years ago). The Paleoproterozoic is marked by the stabilization of several island continents and the significant occurrence of stromatolites. The island continents formed the supercontinent, Kenoraland, and this is the most likely cause of the first recognized major ice age event. The breakup of Kenoraland approximately 2 billion years ago roughly coincides with the end of this ice event. Unfortunately, even though sediments of this age are much more plentiful than those from the earlier eras, there are still large gaps in the preserved record of this time.
Of particular importance is the appearance in the fossil record of eukaryotic organisms (organisms with an encapsulated nucleus) during this time. Current interpretation is that eukaryotic organisms originated as symbiotic multiple prokaryotic cells existing within a common membrane. In the simplest model, an Archaea thermoacidophile and a nonphotosyn-thesizing, heterotrophic eubacterium formed a symbiosis that included a common cell membrane. Eventually, the Archaea portions evolved into mitochondria, and the eubacterium portions became nuclei. It is widely accepted that chloroplasts are evolved from photosynthesizing Archaea in a similar symbiotic relationship.
The Mesoproterozoic is better represented in the rock record than any previous period of time. Stromatolites are abundant, and fossils of eukaryotic organisms become increasingly prevalent throughout this time period. Island continents from the breakup of Kenoraland were again drifting toward each other, and eventually they came together to form Rodinia, the best-known early supercontinent, approximately 1 billion years ago. During this time, life forms capable of genetic exchange and sexual reproduction became common. Broad diversity of eukaryotic organisms, especially phytoplankton, is observed in the Mesoproterozoic of 1.1 billion years ago. Evidence of the first life on land, in the form of fungus (or possibly lichen), also exists from this time. However, possibly coinciding with the beginning of a major ice age, a decrease in observed diversity exists in sediments from that age to those from the end of the Mesoproterozoic and into the Neoproterozoic. The timing of this decrease in biodiversity begins roughly 100 million years earlier than the observed first advance of the glacial ice, so a more likely explanation is that a yet unknown environmental factor caused the decrease in biodiversity, which may also have led to a later glacial event. A third possibility is that these two events are unrelated and the timing is merely coincidental.
During the Neoproterozoic time, life complexity increased. Spongelike animals appear in the fossil record of this time. A major series of glaciations, the Varanger glaciation event, took place between 950 and 570 million years ago. It seems likely that major ice events occurred at approximately 800, 700, and 600 million years ago. At least three separate glaciation events have been documented between 750 and 600 mya. The first firmly documented extinction and evolutionary bottleneck event occurred during this time. This extinction records the loss of nearly 70% of then-living species approximately 650 million years ago. It is unclear to what extent the extinction event was related to the ice age environmental changes. The glaciations included continued cold (to -40 degrees average global temperature) for tens of millions of years. It is also thought that ice covered over 80% of the Earth’s surface at times and that the only reason temperatures were able to moderate was due to an extreme greenhouse effect caused by carbon dioxide buildup. In any case, this extreme event was accompanied by a greatly reduced diversity of life during that time.
This point in geologic time also marks the reassembly of the subcontinents from the breakup of Rodinia into the new supercontinent of Pannotia. Pannotia is difficult to describe in both its extent and duration, since some portions were assembling while other portions were breaking apart. This process continued into the Cambrian. The major assembly of land masses during this process seems to have occurred during the latest Proterozoic.
The later portion of the Neoproterozoic is remarkable for the metazoan life diversity. The last period of the Proterozoic era is the Vendian period. It is rocks of Vendian age that contain the Ediacaran fauna of Australia and similar specimens from elsewhere around the world. The Ediacaran fossils include abundant precursors to corals, worms, sea-pens, and many forms that are difficult to classify. The Ediacaran fauna lived at least 20 million years after the Varanger glaciation, and the diversification of life that occurred during those 20 million years is impressive. Tissue-level organization of life is abundantly represented in the Ediacaran fauna, and organism-level organization of life is also represented. Although many of these animals seem unrelated to known forms, others appear to be primitive arthropods, cnidarians, molluscans, echinoderms, and possibly chordates. There is little, if any, preservation of hard body parts in these animals, and it is thought that these fossils represent only soft tissue preservation. However, the most common rock in which these specimens are preserved is sandstone, usually a very poor preserver of soft tissue.
Life Becomes Complicated: Paleozoic Era: 542-251 mya (11.91 to 5.52 meters)
The Paleozoic era is marked by the appearance of animals with hard body parts and ever-increasing complexity. The boundary of the Paleozoic era is determined by the lowest occurrence of the complex trace fossil, Trichophycus pedum. This trace occurs lower than any other known complex trace fossil, and it is found globally at or below the first occurrence of shelly fossils. This boundary is unusual in that it is not associated with an extinction event. Vendian-type fossils are commonly found above the boundary, together with the remains of animals with hard body parts.
The Paleozoic era, especially the Cambrian period, is noted for its “explosion” of life diversity on Earth. For nearly 90% of Earth’s history, life had remained simple and had barely achieved organism-level complexity. Then, all of a sudden, complex organisms abound in the fossil record.
The Paleozoic era is broken into six periods, each containing a different set of dominant life forms. Increasing complexity of organisms correlates with increasing diversity of organisms, and various groups appear, dominate the environment, and then give way to other types of organisms in comparatively short periods of time. Since most of what is known concerning the events leading to the current biodiversity of Earth occurred during this time, it is appropriate to discuss these biotic events by period rather than era. This pattern will be followed through the Mesozoic and Cenozoic eras as well.
Cambrian Period: 542-488 mya (11.91 to 10.73 meters)
The Cambrian period was a time of warm climates worldwide and diverse flora and fauna in marine environments. Climate was, from all indications, moderate, and there is no evidence of polar ice during this period. Several localities of unusually fine fossil preservation, including the Chengjiang Locality of China, dated at 535 mya, and the Burgess Shale Locality of British Columbia, Canada, dated at 525 mya, provide much of the data from which the current understanding of metazoan origins and early evolution is obtained. Terrestrial flora and fauna, as far as is currently known, consisted only of algae and possibly lichen. The dramatic increase in complex life forms found in marine environments was not initially mirrored in terrestrial environments.
Arthropods, in the form of trilobites, dominated the Cambrian seas. These “water bugs” were equipped with many “legs” for mobility and eyes for sensory input. Other animals, such as the 2-meter-long anomalocarids, also seem to be early arthropods or at least their close relatives. By the end of the Cambrian, arthropods had diversified, and early representatives of the chelicerates, crustaceans, and arachnids appear in the fossil record of this time.
Other animals, including representatives of nearly all current phyla and of several extinct phyla, are also represented in the Cambrian fossil record. Chordates such as segmented worms and conodonts appear and diversify, providing the first major step up the ladder of vertebrate evolution. More important, the appearance of two early jawless fish from a recent discovery in China attests to the presence of true vertebrates in the mid-Cambrian. These agnathans, one resembling the lamprey in body design and the other resembling the hagfish, indicate a relatively broad diversity in Agnatha by that time. This diversity suggests that vertebrate origins had to have occurred at least in the early Cambrian and may have occurred in the Neoproterozoic.
The end of the Cambrian period is marked by a major extinction event. The loss of taxa during this event included many brachiopod, trilobite, and con-odont species. This extinction event seems related to a cooling of the ocean, possibly due to a relatively localized glaciation event. It is speculated that the faunal losses at that time were of warm-water species unable to tolerate a cooler environment. However, most of the evidence available indicates that mean temperatures could have altered only a few degrees, and the cool period was of short duration. These data suggest a minor, short-lived fluctuation that had a great effect on specialized organisms that were not able to adjust to these relatively minor fluctuations.
Ordovician Period: 488-444 mya (10.73 to 9.76 meters)
The early Ordovician is characterized by an increase in faunal diversity similar to that seen in the Cambrian. By the end of the Cambrian period, approximately 200 marine invertebrate families are represented in the known fossil record; by the end of the Ordovician, that number had increased to 500. Animals appear abundantly in the Ordovician that were unknown or of minor importance in the Cambrian biosystem. These animals include corals, crinoids, bryozoans, foraminifera, graptolites, agnathans, eurypterids, bivalve molluscans (the alternative spellings of mollusks and molluscs are commonly used, though probably not correct), and nautiloid cephalopods. Ordovician trilobites were, for the most part, significantly different from their Cambrian counterparts in having developed bizarre spines and other specialized structures.
Significant terrestrial flora and fauna were present for the first time during this period. Very few Ordovician terrestrial fossil remains have been recovered, but those that have been suggest the presence of a developed terrestrial biota. Fungi have been identified from macrofossils of this time, while embryophytes have been identified from spores. Arthropod trackways are also documented from terrestrial sediments from the Ordovician.
Two significant geologic events occurred during this period. Extrusive volcanics of the Appalachians and large volcanic ash deposits suggest that volcanic eruptions of both types occurred that were possibly the largest single event of each type to have happened at any time during the Phanerozoic (Cambrian through present). These events may have been largely responsible for the significant cooling and glaciation of the end of the Ordovician and early Silurian periods.
The end of the Ordovician is marked by paired extinction events, most likely caused by extensive glaciation. The first is a drastic sea level drop, and the second, occurring approximately 500,000 years later, is a significant rise in sea level. Faunal counts across this extinction zone indicate that up to 85% of species on Earth became extinct during that time, making this extinction second only to the Permian/Triassic extinction in terms of numbers of species lost. The most likely explanation for these events is that the ocean levels dropped while ice sheets were building and advancing, then rose when the ice melted. The extinctions are thought to have been caused by marine animals not being able to adapt to the shallower and later, deeper environments to which they were subjected.
Silurian Period: 444-416 mya (9.76 to 9.14 meters)
The beginning of the Silurian period was a time of slowly moderating cold worldwide. However, during the period, the Earth’s climate stabilized, and the runaway hot and cold cycles seen earlier in Earth’s history have not been seen since. This moderation in temperature produced a subtropical climate over much of the Earth, but glaciers did occur above 65 degrees paleolatitude. Areas of arid landscape were common near the equator, and the equatorial seas were generally warm and shallow.
This environmental stabilization provided near-ideal conditions for another increase in diversity and complexity of life. Coral reefs became common for the first time. Brachiopods made a strong comeback after being decimated in the Ordovician-Silurian extinctions, comprising approximately 80% of the total known hard-shelled species by midperiod. Stromatoporoids and bryozoans contributed significantly to reef building, as do calcareous algae. Graptolites, conodonts, molluscans, and crinoids were also common. Trilobites were not as diverse as they were in the Cambrian and Ordovician, possibly because of the increase and diversity of eurypterid species. Jawless fish became widespread and diverse, and the period also marks the first of both freshwater fish and fish with jaws. Eurypterids, xiphosurans, and scorpions are also found in freshwater sediments of this period.
The first good evidence of life on land is seen in Silurian sediments. Discovered remains include relatives of spiders and centipedes and the earliest fossils of vascular plants. Most vascular plant fossils found have branching stems and no leaves. However, a lycophyte, a plant with leaves and a fully developed vascular system, has been reported from Australia.
The end of the Silurian is unusual in that it is not marked by a significant geologic or extinction event. The marker used to separate the Silurian and Devonian is the first occurrence of the graptolite, Monograptus uniformis.
Devonian Period: 416-359 mya (9.14 to 7.89 meters)
The Devonian period saw the continued rapid evolution and diversification of both plants and animals. The Devonian is known as the “age of fishes” because of the virtually simultaneous appearance of precursors of all living groups of fishes and groups now extinct. However, plants experienced a similar radiation, and the first land vertebrates, insect precursors, and terrestrial spiders appeared. In the seas, ammonites made their first appearance, as did siliceous sponges. Brachiopod and coral diversity reached its peak, but trilobite diversity was declining.
The fossil record of the Devonian includes a broad diversification of jawed fish. Placoderms, acanthodians, sharks, and bony fish are all well represented. An extinction event between the middle and late portions of the period seems to have provided an opportunity for shark and bony fish evolution by removing many of the placoderms. The extinction occurred during a period of relatively uniform, warm climatic conditions, and many animal and plant groups were unaffected. Its severity (70% of known species) primarily represents the decimation of benthic, shallow marine species.
The most significant events of the Devonian occurred as both animals and plants began to occupy terrestrial environments. Terrestrial plants including lycopods, ferns, and horsetails appeared, and by the end of the period, woody, treelike lycopods up to 30 meters tall were common. By the end of the Devonian, seed-bearing plants had developed, although their colonization of drier, more inland habitats would come later. Insects were likely common at this time. However, the only fossil insects known from this period are springtails. Lobe-finned fish evolved that were equipped with both gills and lungs. They also had fins that resembled legs. It is thought that these adaptations were of benefit to survival in shallow marine environments. By the end of the Devonian, this lineage included true amphibious forms capable of walking on land for extended periods. This transition placed these vertebrates into an environment where eurypterids, arachnids, and primitive insects already existed.
Ironically, the terrestrial conquest by plants during the last portion of the Devonian may have been the cause for the extinction event that marks the end of the period. The end of the period is marked by slow cooling, increased atmospheric oxygen, and a decrease in atmospheric carbon dioxide. Terrestrial plants accelerated the formation of soils, thus accelerating the weathering of silicate rocks. The calcium and magnesium released, coupled with a significant increase in terrestrial photosynthesis, provided for significant additional removal of carbon dioxide from the atmosphere. The loss of this greenhouse gas concentration, in turn, allowed the Earth to cool by several degrees. Although the cooler environment did not immediately remove species from the system, apparently it hindered evolutionary adaptation and replacement. Thus, the Devonian extinction event was gradual, with species being removed from the system and not being replaced, rather than following the more catastrophic pattern seen in earlier extinctions.
Carboniferous Period (Mississippian + Pennsylvanian): 359-299 mya (7.89 to 6.57 meters)
The Carboniferous period was first described in England and referred to the lowest occurrence of extensive coal deposits. In North America, the same geologic strata were broken into two periods, the Mississippian and the Pennsylvanian. The Mississippian period featured extensive limestone, as the North American continent was mostly underwater during the entire period. The Pennsylvanian saw a lowering of sea levels and the formation of lowland, deltaic forests that were intermittently inundated by marine transgressions. For the geologic timescale to be applicable worldwide, some arbitrary “tweaking” of the units has made the Mississippian and Pennsylvanian of North America match the Lower and Upper Carboniferous of Europe, respectively. This shift has relegated the Mississippian and Pennsylvanian to an unnamed subperiod status.
The forests of the late Devonian gave way to low-lying shrubs in the early Carboniferous. Sea levels became high worldwide, and much of the Devonian forested land was inundated. The first truly terrestrial tetrapod vertebrates show up in the fossil record. Euramerica is tropical and equatorial, and life flourished in terrestrial, aquatic, and marine environments. It was during this time that the earliest known “reptile,” Hylonomus, appears. It should be here noted that the term reptile is a catchall that contains representatives from several lineages and excludes their descendents, the mammals and birds. The appearance ofHylonomus places the origin ofthe amniotic egg in the Early Carboniferous. As the seed allowed plants to become separated from aquatic environments, so the amniotic egg allowed vertebrate animals to become so separated. It is a still unresolved mystery why animals with this obvious advantage did not become diverse and prevalent until more than 50 million years later.
In the oceans, the placoderms, so diverse in the Devonian, are missing from the Carboniferous record. Chondrichthyan have became diverse and were the top predators in the environment. Echinoderms, in the forms of crinoids and blastoids, became extremely common, and their calcareous skeletal structures add significantly to the limestone sediments of this period. Graptolites became extinct, and trilobites were reduced to one type.
By the late Carboniferous, the ocean levels alternately rose and lowered as Gondwana became covered by glaciers. Euramerica remained subtropical, and coal swamps became commonplace. Forests once again covered large areas of the Euramerican land-mass. These forests consisted of diverse plants including 15-meter-tall tree ferns; Catamites, a giant version of the modern “horsetail” plant; Lepidodendron, a 30-meter-tall lycopod; seed ferns; and primitive conifer-like plants that reached 40 meters in height. These extensive forests eventually produced most of the coal on Earth today. Flying insects arrived, including cockroaches resembling those of today and dragonflies with 70-centimeter (2-feet) wingspans. Bony fish inhabited lakes and streams as well as marine environments. Many types of tetrapods were present, including representatives of virtually all amphibian groups and the precursors for the anapsid, synapsid, and diapsid amniotes. By the end of the Carboniferous, the supercontinent, Pangaea (the commonly used spelling, Pangea, is incorrect), had formed, and its formation contributed to a warming and drying trend that carried through into much of the Permian. No major extinction event is associated with the Carboniferous/Permian transition, but the coal beds disappear as the climate became warmer and drier.
Amniote origins and evolution has its roots in the Carboniferous period. However, the order of appearance of specimens in the fossil record does not match the current understanding of amniote evolution. It is thought that amniotes diverged into two groups, those in the synapsid lineage leading to mammals and those in all other lineages, very close to the point of amniote evolution itself. This would put the divergence somewhere in the early Carboniferous, no later than 325 million years ago. The second separation that is postulated to have occurred is that of the anapsids, the lineage leading to turtles, branching off nearly as early as that of the synapsids. Finally, the diapsid lineage leading to birds would branch off. However, in the fossil record, diapsids appear first, in the mid-Carboniferous; the synapsids appear next, in the Upper Carboniferous, and the anapsids appear last, in the mid-Permian. While it is certainly possible that these lineages did exist but have not been discovered in the fossil record, it is unsettling that the postulated evolutionary pattern is reversed in the known record.
Permian Period: 299-251 mya (6.57 to 5.52 meters)
The Earth during the Permian period experienced a varied climate, from glaciers in the south to swampy forests at the equator. As time progressed, the glaciers receded, and the equatorial areas became much warmer and drier. The large landmass of Pangaea likely interfered with the normal climate mitigation from the oceans. As a result, deserts and hypersaline inland seas were common. In fact, most of the Earth’s salt beds were laid down at this time. During this nearly 50-million-year sequence, climatic fluctuations and increased loss of shallow sea environments precipitated the largest extinction event in Earth’s history, the Permo/Triassic extinction. However, throughout the entire 50 million years of the Permian preceding the extinction, marine habitats were dwindling, and species diversity within families of marine animals was lessening. All marine animal groups, with the possible exception of the conodonts and fish, showed a marked decline in diversity throughout the Permian.
During the early portion of the Permian, amphibians such as anthracosaurs and cotylosaurs were competing with early synapsid reptiles for terrestrial dominance. However, amphibian diversity dwindled throughout the Permian. Although the Permo/Triassic extinction event had little effect on the amphibians, only a few forms survived into the Triassic. Pelycosaurs, including the sail-backed Dimetrodon, became the largest terrestrial predators. In a parallel evolution, Edaphosaurus and caseid pelycosaurs became the first large plant-eating amniotes, some reaching over 4 meters in length. In the mid-Permian, pelycosaurs disappeared, to be replaced by very large-headed but much more mammal-like therapsids. These ther-apsids are thought to be closely related to the pely-cosaurs, but more specialized. In turn, the therapsids were out-competed by one of their daughter lineages, the theriodonts, during the late Permian. This new lineage of mammal-like reptiles, including dicynodonts, gorgonopsids, therocephalians, and cynodonts, most likely included the first warm-blooded, furry animals.
These groups, with the exception of the gorgonopsids, survived the Permo/Triassic extinction and were prominent in the early Triassic terrestrial ecosystem as well. Diapsid reptiles also made their appearance in the Permian, and the late Permian diapsid, Coelurosauravus, is the earliest known vertebrate capable of flight.
A series of relatively rapid fluctuations in temperature and moisture seems to have been the driving force behind a significant floral turnover during the Permian. Moist, tropical plants gave way to gymnosperms and seed ferns capable of withstanding a much wider range of climatic conditions. However, at the Permo/Triassic boundary, floral changes occurred rapidly. In some areas, gymnosperm forests were wiped out and lycopods took over, while in others, gymnosperm forests appeared. In most cases, the pattern observed throughout the Permian was disrupted at the Permo/ Triassic boundary. Even where gymnosperms remained the dominant floral component, there was a species turnover, and the preboundary forms are not the same as those preserved after the boundary event.
The Permo/Triassic extinction event is identified as the most serious extinction of life ever in Earth’s history. Approximately 90% of known marine species and nearly half of terrestrial species were lost. Several factors have been suggested as the cause of this catastrophe, including global trophic collapse, meteor impact, rapid climate change, anoxia, supercontinent formation, glaciation, and both explosive and nonexplosive volcanics. Unfortunately, there is seldom a single cause for any worldwide event. It appears that all the previously listed factors affected the Earth’s biosphere at or near the Permo/Triassic boundary. No single factor has yet been identified that positively correlates with every major extinction event in the Phanerozoic, and the Permo/Triassic is no exception. Perhaps the key to this event is in the universality rather than the exceptions concerning this event. The Permo/Triassic extinction event is concurrent with meteor impacts, a supervolcano explosion, huge volcanic flows, climate fluctuations, a supercontinent, and food chain collapse. It may be, indeed, that the reason for this biggest extinction even in Earth’s history is the concurrent series of events, rather than any single event.
Dinosaurs Rule: Mesozoic Era: 251-65.5 mya (5.52 to 1.44 meters)
The Mesozoic era was a time when the last major groups of animals and plants evolved. It is broken into three periods: the Triassic, Jurassic, and Cretaceous. The Mesozoic was, generally, a time of uniform warm temperatures and subtropical climates. However, the Permo/Triassic extinction radically altered the roughly steady progression of evolution and diversity of life on Earth. Wonderfully complex and interdependent reef communities were reduced to a few simple forms, and 250 million years later, reef communities were still relatively primitive compared with their previous complexity.
Triassic Period: 251-200 mya (5.52 to 4.40 meters)
Nearly total devastation in the marine environments coupled with hot, dry conditions over most of the terrestrial landscape made the early Triassic period relatively inhospitable. Although Pangaea is still intact, major ecological provinces are observed in the fossil record. These provinces seem to coincide with Laurasia versus Gondwana. However, these two subcontinents were joined by the early Permian and did not separate until the beginning of the Cretaceous period, some 150 million years later. Why such eco-logic provincialism existed is unknown.
The two main marine environments were the deep-ocean Panthalassic Sea, which surrounded Pangaea, and the shallow, warm Tethys Sea, which was mostly surrounded by Pangaea (the Mediterranean Sea is a remnant of the Tethys). The seas supported a wide variety of chondrichthyan and osteichthyan fish, cephalopods, molluscans, and brachiopods. Marine reptiles, including ichthyosaurs, also made their appearance for the first time. However, this did not occur immediately after the Permo/Triassic boundary. Both seas exhibited anoxic conditions immediately, and for several million years after, the Permo/Triassic event. In addition, data suggest that the ocean chemistry included high levels of carbon dioxide and hydrogen sulfide during this time as well. Atmospheric oxygen was also depleted to its lowest levels in the Phanerozoic. Life seems to have maintained (and in some cases even flourished) only in isolated habitats during that time.
On land, amphibians made a brief comeback during the early Triassic, but were replaced in most terrestrial habitats by reptiles during the mid-Triassic. Extensive redbeds appear during this time, and these have been interpreted as indicators for hot, dry climate. It has been suggested that the amphibians made a resurgence because they were tied to rivers and streams and the water ameliorated an otherwise harsh environment. Several lineages of synapsid reptiles survived the Permo/Triassic extinction, including Cynodonts, Dicynodonts, Gorgonopsians, and Therocephalians. However, Gorgonopsians vanish from the fossil record shortly after the Permo/Triassic boundary, and Dicynodonts and Therocephalians slowly dwindle and fade out of the record. It should be noted that Lystrosaurus, a Dicynodont, has been found worldwide in Late Permian and Early Triassic sediments. In general, therapsids constitute roughly 90% of early Triassic terrestrial tetrapod fossils.
Diapsid and anapsid reptiles were also present, although early Triassic fossils from these groups are virtually unknown. By mid-Triassic, a diverse fauna again appears in the fossil record. It appears that in the scramble for life during those first few million years after the Permo/Triassic extinction, animals were able to diversify quickly and to survive even if they weren’t very specialized. As time went on, however, those lineages whose ancestors were somewhat specialized were able to out-compete those who weren’t, and the number of different lineages dwindled. By the mid-to-late Triassic, the diapsid fossil record contains rhamphorynchoid pterosaurs, proto- and true dinosaurs, crocodilians, all major swimming-reptile groups except mosasaurs and lizards. Turtles and true mammals appear in the late Triassic fossil record as well. Dinosaurs diversified very soon after their origin. By the end of the Triassic, theropod, sauropod, and ornithischian dinosaur lineages were present.
The end of the Triassic period is marked by one of the “Big 5” extinction events. A regression of the oceans followed by an early Jurassic transgression are partially responsible for the loss of certain species during that time. Also during that time, one of the largest volcanic events, the mid-Atlantic rifting, began. This rifting and volcanic activity greatly increased the amount of atmospheric carbon dioxide, and it is likely that this event also played a role in the Triassic/Jurassic extinction event. The fossil record is, however, quite incomplete from this particular point in time, and it may be that these events precipitated a protracted faunal turnover that should not properly be considered an extinction event.
Jurassic Period: 200-145 mya (4.40 to 3.19 meters)
The beginning of the Jurassic period is marked by the breakup of Pangaea into Laurasia and Gondwana.
The diversity of life seen in the late Triassic is generally seen to continue within the same groups in the Jurassic. A marine transgression covered much of the continental landmass during the early to middle Triassic. This transgression provided greatly expanded shallow marine habitat, and this habitat was soon teeming with marine reptiles, teleost and chondrichthyan fish, plankton, bivalves, arthropods, and other forms. Crocodilians were represented by species similar to the mostly aquatic forms found today.
Terrestrial ecology was dominated by a now diverse dinosaur fauna in both herbivore and carnivore roles. Cynodonts, including primitive mammals, diversified as small, insectivorous forms that did not directly compete with the larger dinosaurs. The tritylodonts survived as herbivorous protomammals well into the Jurassic. Gymnosperms and cycads were common, replacing the last of the seed ferns. Dinosaurs evolved as larger and more specialized forms throughout the Jurassic. The smaller and more generalized prosauropods of the early Jurassic gave way to the larger vulcanosaurids and cetiosaurids, which, in turn gave way to the diplodocids and camarasaurids. By the end of the Jurassic Seismosaurus and Brachiosaurus, the longest and tallest animals, respectively, to ever roam the Earth, wandered in North America. Likewise, the largest predator of the early Jurassic, Dilophosaurus, was replaced by the much larger Allosaurus by the end of the period. Smaller dinosaurs, such as Othnelia and Ornitholestes, were present at the end of the Jurassic. However, they were literally and figuratively overshadowed by the larger forms.
Birds also appeared in the Jurassic. However, these remains consist of only a few specimens of Archaeopteryx from the 150-million-year-old limestones of Solnhofen, Germany. The next oldest avian remains are those of Confusciusornis and others from 120-million-year-old sediments in China and the approximately 100-million-year-old remains of Ichthyornis from the chalk beds of Kansas. Archaeopteryx is so similar to small theropod dinosaurs that for many years, a featherless specimen was incorrectly identified as Compsognathus. Most researchers now believe that birds evolved from a Compsognathus-like small theropod dinosaur ancestor.
The Jurassic/Cretaceous boundary is the most problematic of any in the Phanerozoic. In many localities, unconformities (areas of missing sediments) are present at this level, and no identifiable marker has been established where the sediments are present. Sea levels had dropped, but it had happened gradually. Small polar ice caps had formed, and temperatures did reach their coolest of any time in the Mesozoic. However, this “cool” period was still quite warm, even when compared with the Earth today.
It seems that the Earth experienced a faunal turnover around the time of the Jurassic/Cretaceous boundary, but it was apparently a gradual event. Animals would die out and be replaced in one habitat, while thriving for several million years longer before succumbing in other habitats. Perhaps the most visible event is the loss of most sauropod and carnosaurid dinosaurs from Laurasia at that time. Sauropods and carnosaurs continued to thrive in Gondwana but were nearly wiped out on the Laurasian continent.
Cretaceous Period: 145-65.5 mya (3.19 to 1.44 meters)
The Cretaceous period is marked by yet another marine transgression. Indeed, the name, Cretaceous, means “chalk,” and refers to the chalk formations of the white cliffs of Dover, England. Early Cretaceous terrestrial deposits are rare. There was an approximately 20-million-year gap between the beginning of the Cretaceous period and the lowest occurrence of well-preserved terrestrial fossil remains. The most likely reason for this is that in most localities, these strata are missing.
By the middle of the Cretaceous period, designated as Aptian-Albian, the Earth’s oceans had once again receded, and a diverse terrestrial flora and fauna are present in the fossil record. Angiosperms, the flowering plants, have made an appearance, although they did not become diverse and widespread until the very end of the Cretaceous. Both marsupial and placental mammals appear in the fossil record. Saurischian dinosaurs, including Acrocanthosaurus, Deinonychus, and Utahraptor, and ornithischians, such as Tenontosaurus, Psittacosaurus, and Sauropelta, represent the wide dinosaur diversity achieved by the Middle Cretaceous.
Another marine transgression separates the middle of the Cretaceous from the late, although terrestrial sediments are preserved in some locations during this time. Late Cretaceous Earth was dominated by a broadly diverse dinosaur fauna. By the middle of the Campanian age, approximately 75 million years ago, dinosaurs had reached the height of their diversity. Mosasaurs had appeared in the oceans, and flowering plants were common, although not yet the dominant flora. Marsupial mammals were common and diverse. From the mid-Campanian until the end of the Cretaceous, dinosaur diversity dwindled. Even though the numbers of specimens in the fossil record remain roughly equivalent, end-of-Cretaceous dinosaur fossil diversity is not nearly that of the Campanian. A few specialized forms seem to have taken the place of many less specialized types.
Undoubtedly the most famous extinction event is the one that marks the end of the Cretaceous period and the end of the nonavian dinosaurs. However, this extinction event was actually a relatively gradual turnover in flora and fauna that began at least 100,000 years before the iridium-enriched clay layer was produced and lasted for at least 300,000 years afterward. Other estimates place the beginning of the turnover at least 500,000 years before the iridium-producing event. In any case, several events occurred that would have caused at least regional biotic crises. First, mid-latitude climate on Earth changed from subtropical to temperate. Second, grass made its appearance as the dominant ground cover. Third, the Deccan Traps Volcanics were extremely active and were producing huge quantities of ash, methane, and carbon dioxide. And finally, an asteroid approximately 10 kilometers (6 miles) in diameter slammed into carbonate rocks of the Yucatan Peninsula, Mexico.
Although any one of these events may have caused the loss of significant life in Earth’s ecosystem, it is likely that it required the combination of these events to actually produce the extensive species turnover seen in both the marine and terrestrial fossil records of the Cretaceous/Tertiary (K/T) boundary. Dinosaurs, and possibly pterosaurs, were likely among the first to disappear. The highest dinosaur or pterosaur remains found to date are still several meters below the iridium layer. However, in certain ocean regions, the turnover observed in fossil plankton is restricted to a dramatic change from one side of the clay layer to the other. This signifies a nearly instantaneous event related to the impact. Taking the evidence in combination, multiple causation of the K/T event is the only viable explanation.
Opportunity Knocks and Mammals Answer: The Cenozoic Era: 65.5-0 mya (1.44 to 0 meters)
The last 65 million years or so of Earth’s history is well represented in the fossil record. It is interesting that this last 1.5% of Earth’s history has received more scientific attention than the other 98.5% combined.
Originally, all rocks and fossils of this age were labeled “Tertiary.” Later, the most recent sediments were broken off into a Quaternary period. The Cenozoic era is currently divided into two periods: the Paleogene and the Neogene. This is a recent division that has replaced the Tertiary and Quaternary divisions at the level of period, and Tertiary and Quaternary have been raised to sub-eon status. This change has created a more equal period division and has placed the division at a point of major global climate change. It should be noted that the spellings, Palaeogene, and Paleogene, are both used in print. However, the International Commission on Stratigraphy uses Paleogene, and that spelling is thus considered proper.
Paleogene Period: 65.5-23 mya (1.44-0.51 meters)
The K/T extinction event removed most life, and many species, from both terrestrial and marine ecosystems. The loss of the top predators from both ecosystems meant that other animals no longer had to hide to survive. The fossil record indicates a very rapid increase in both number of species and in individual size. The climate itself, except for a short cold spell at the K/T boundary, remained a subtropical greenhouse over most of the Earth. Glaciers were unknown until the latest Paleogene, when the Earth began a gradual cooling that continued into the Pleistocene ice age. The breakup of Pangaea continued, and by the middle of the Paleogene, nearly all current landmasses had formed. These island continents provided reproductive isolation for their respective faunas, and life evolved along different paths in Australia, Eurasia, Africa, North America, and South America.
The Paleogene is divided into Paleocene, Eocene, and Oligocene Epochs. During the Paleocene, condylarths and creodonts dominated most terrestrial environments. Although ungulates descended from condylarth ancestors, the Paleocene terrestrial herbivore ecospace belonged to the nonungulate condylarths. Likewise, ancestors of modern carnivores were present in the Paleocene, but most terrestrial mammalian predators were creodonts. A flightless bird, Gastornis, was also one of the top predators of the Paleocene, at least in what is now Europe.
Many of the terrestrial mammals that thrived during the Paleocene became extinct about 10 million years after the K/T event. A major climatic event, the Paleocene/Eocene Thermal Maximum, is thought to have been responsible for this loss and also the loss of many types of benthic foraminifera. A very short-term event, measured at less than 20,000 years, saw a 15-degree rise in ocean temperatures, which affected even the deep ocean. It then took several hundred-thousand years for the temperatures to cool to equilibrium values. It has been postulated that methane “burp” caused the greenhouse conditions that raised the temperatures. Methane was, and is currently, trapped in huge reservoirs of frozen clathrate on the ocean floors. The melting of this material is thought to be a self-perpetuating reaction once started, and the reaction would end only when no more clathrate was left to melt. Some scientists believe that this is a recurring event that was the cause of the Permo/ Triassic event and is the most imminent and significant threat to the current ecosystem.
By the Eocene, ungulates and carnivores were vying with the condylarths and creodonts, respectively, for their positions in the ecosystem. Also of note is the appearance in the fossil record of primitive anthropoids by mid-Eocene. Early ancestors of whales were beginning to inhabit aquatic environments, and most modern bird group origins can be traced to Eocene ancestors. A meteor impact formed Chesapeake Bay as a nearly 100 kilometer crater, and another, even larger, struck in Russia near the middle of the Eocene, but these impacts seem to have caused little harm to existing species.
By the Oligocene, the earlier forms had given way entirely to the ungulates and carnivores as the major terrestrial herbivores and carnivores. The first monkeys and apes also appear in the Oligocene. By the end of the Oligocene, Antarctica had taken up its position at the South Pole, and glaciers had formed on its surface. This positioning of Antarctica likely had much to do with the Earth’s cooling. A rapid marine regression at the Paleogene/Neogene boundary was likely caused by the formation of Antarctic glaciers. The position of Antarctica is likely a major factor in the cooler-than-normal temperatures of the Earth today.
Neogene Period: 23-0 mya (0.51-0.00 meters)
The fossil record of the Neogene period generally depicts repeated adaptation to gradual, increasing cold. The four epochs of the Neogene—Miocene, Pliocene, Pleistocene, and Recent—are characterized by floral and faunal adaptations to geologically rapid climatic fluctuations. The sediments of the Oligocene/ Miocene boundary provide the first record of significant glaciation occurring above those of the early Mesozoic.
Cool temperatures and Antarctic glaciation of the early Miocene gave way to global temperatures similar to the late Mesozoic greenhouse by the middle of the epoch. Global temperatures reached an average maximum of approximately 6 degrees Celsius more than is experienced today. Although the temperatures rebounded from the early Miocene lows, temperature-sensitive plants gave way to grasses as the predominant ground covering. Toward the end of the Miocene, kelp forests appeared in the oceans for the first time, and global temperatures were gradually decreasing.
Mammals achieved their greatest diversity during the Miocene. Semi-isolation of continental landmasses allowed reproductive isolation and preservation of some less specialized groups in certain areas. Quality grazing and browsing habitats for herbivores also contributed to Miocene mammalian diversity. This diversity includes the first appearance of hominids in the fossil record.
The Pliocene was a time of cooler temperatures and drier climates. No major extinction event is associated with the Miocene/Pliocene boundary, and the flora and fauna in both terrestrial and marine environments remain largely unchanged throughout this time. The same may be said for the Pliocene/ Pleistocene boundary. However, the global environment changed drastically approximately one third of the way through the Pleistocene.
The mid-Pleistocene glacial event marks the first glacial incursion into the middle latitudes since the Paleozoic. This unusually cold cycle is thought to be related to the continued location of a continent in the South Polar Region, coupled with global temperature and circulation patterns that largely block heat transport to the Polar Regions. It is thought that glaciation is somewhat self-perpetuating. As more of the Earth’s surface is covered by ice, more of the sun’s energy is reflected rather than absorbed. And, the less energy that is absorbed, the cooler the Earth gets and the more ice is produced.
Animals began to appear in the Pleistocene fossil record that were well adapted to cold environments. These animals include the wooly mammoth, wooly rhinoceros, elk, moose, and bison. However, the major glaciation events of the Pleistocene were relatively short-lived, lasting only a few tens of thousands of years on average. In between these events, the Earth experienced interglacial periods when the global climate was at least as warm as that of today.
Technically, the fossil record ends with the beginning of the ice age, because more recent remains have commonly not been “fossilized.” However, the deposi-tional processes that have provided an interesting and occasionally detailed picture of Earth’s history are still at work, and they continue to produce a record of Earth’s life and geologic events.
- Altermann, L. W., & Corcoran, P. (Eds.). (2002). Precambrian sedimentary environments. Oxford: Blackwell.
- Behrensmeyer, A. K., Damuth, J. D., DiMichele, W. A., Potts, R., Sues, H.-D., & Wing Scott, L. (1992) . Terrestrial ecosystems through time: Evolutionary paleoecology of terrestrial plants and animals. Chicago: University of Chicago Press.
- Farlow, J. O., & Brett-Surman, M. K. (Eds.). (1997). The complete dinosaur. Bloomington: Indiana University Press.
- Koeberl, C., Martinez-Ruiz, F. C., & Ruiz, F. M. (2003). Impact markers in the stratigraphic record (impact studies). New York: Springer-Verlag.
- Stiassny, M. L. J., Parenti, L. R., & Johnson, G. D. (Eds.). (1996). Interrelationships of fishes. San Diego, CA: Academic Press.
- Szalay, F. S., Novacek, M. J., & McKenna, M. C. (Eds.). (1993) . Mammalphylogeny: Mesozoic differentiation, multituberculates, monotremes, early therians, and marsupials. New York: Springer-Verlag.
- Walker, G. (2003). Snowball earth: The story of a maverick scientist and his theory ofthe global catastrophe that spawned life as we know it. New York: Crown.