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Fossil Record

Fossil Record

The fossil record is the set of remains or traces of past organisms identified in fossiliferous rock formations and sedimentary strata (the stratigraphical record) from previous geologic ages to present. Both strati­graphical and fossil records constitute the geological record. The fossil record, or paleontological record, is a primary source of information for evaluating the history of life, including evolutionary processes and extinction events. Paleontologists have studied the fossils of many thousands of species that lived in the past. They have identified evolutionary successions through time, manifesting the morphological transi­tions from some species to others.

The fossil record confirms a basic prediction of evolutionary theory: the past organisms were quite different from present ones. Furthermore, the far­ther back in geological time we look, the simpler and more morphologically similar are the larger taxonomic groups that became fossilized, precisely what to expect if the hypothesis that all life on Earth originated from a universal common ances­tor were true. Other features are observed in the fossil record that provide evidence for evolution­ary theory: transitional fossils between ancient species and more modern ones, consistent hierar­chy in homologies and analogies of past organ­isms, paleobiogeographic data that show that proposed descendants appeared in the same general area as their predecessors, and so on. Exceptional outcrops containing very well- preserved fossils of past fauna and flora, what is called a Konservat-Lagerstatte, have been found worldwide for the different geological time periods and epochs.

Archean Fossil Record

The oldest fossil traces are the stromatolites of 3,500 million years ago (Paleoarchean era, Archean eon). The stromatolites consist of attached, lith­ified growth structures, accretionary away from a point or limited surface of initiation. The result is a laminated rock built of layer upon layer of sedi­ment and precipitants. The stromatolites are formed by the trapping, binding, and cementation of sedimentary grains by microorganisms, espe­cially cyanobacteria. The first stromatolites were probably built by green sulfur-bacteria and purple bacteria, and they were not very abundant. However, the stromatolites became common dur­ing the Paleoproterozoic and Mesoproterozoic eras (early and middle Proterozoic eon, between 2,500 and 1,000 million years ago [mya]), due probably to the great proliferation of colonial cyanobacteria.

The first putative fossil bacteria were found in a stromatolitic formation from the Warrawoona Group (Western Australia) of 3,460 mya. They seem to belong to the domain Eubacteria, but microfossils of extremophilic domain Archaea have also been identified in the Archean eon. Archean life was surely limited to simple nonnucle­ated single-celled microorganisms, or prokaryotes (bacteria). There are no known eukaryotic fossils in Archean rocks, although they might have evolved then and simply not left any fossil. Cyanobacteria and oxygenic photosynthesis first evolved in the Mesoarchean era (3,200-2,800 mya), and were common during the Neoarchean era (2,800-2,500 mya). The combination of oxy­gen released by photosynthetic cyanobacteria and dissolved iron in oceans formed a distinctive Archean (and Proterozoic) type of rocks, called banded iron formations.

Proterozoic Fossil Record

Pre-Ediacaran Time

While it was involved in oxidizing the oceanic iron and precipitating banded iron oxides (magne­tite), biogenic oxygen could not be accumulated in the atmosphere, which continued to be a reducing environment, containing less and less oxygen. This geological process ended in the Proterozoic eon. At that time, the progressive accumulation of oxygen in the atmosphere might have caused the extinc­tion of numerous groups of anaerobic bacteria, what sometimes has been called the oxygen catas­trophe of the Paleoproterozoic era (2,500-1,600 mya). It is considered that the atmosphere changed to oxygen-rich during the Orosirian period (2,050­1,800 mya), since the banded iron formations were abundant in the previous periods, mainly during the Siderian period (2,500-2,300 mya) but also during the Rhyacian period (2,300-2,050 mya).

A major event in the history of life occurred at the beginning of the Statherian period (1,800­1,600 mya): the evolution of the eukaryotic cell. Unlike prokaryotic cells, the genetic material of the eukaryotes is organized in a membrane-bound nucleus. Moreover, eukaryotic cells have a variety of internal membranes and structures, called organ­elles (mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, etc.), and an inner cytoskeleton that includes motile cytoplasmic pro­jections (pseudopods, flagella, and cilia) and other structures (e.g., centrioles). The origin of the eukaryotic cell is disputed. It is believed in general that the nucleus and the internal membranes derived from the invagination of the plasm mem­brane (F. J. R. Taylor’s autogenous model), whereas mitochondria, chloroplasts, centrioles, flagella, and cilia evolved from certain types of eubacteria that a group related to the archaeobacteria engulfed through endophagocytosis (Lynn Margulis’s endo- symbiotic model). Sphaeromorph microfossils (assigned to acritarchs) from the 1,800-million- year-old Changshoigou Formation (Jixian, north China) are interpreted as probably representative of first unicellular eukaryotic protists.

During the Mesoproterozoic era (1,600-1,000 mya) another major event in the history of life occurred: the evolution of sexual reproduction. It has been suggested that cannibalism by primitive protists (protogametes) in times of starvation could have led to a stalemate, with cells becoming fused (protofecundation) but eventually separating when the environmental conditions improved (protomei- osis). The cannibal fusion of cells during conditions of environmental stress was accompanied by the formation of cysts. Cysts are resistant structures, sometimes mineralized, that become preserved in the fossil record. The first fossil group with sexual reproduction is thought to be the acritarchs, known from at least 1,400 mya; that is, approximately at the boundary between the Calymmian and Ectasian periods. The acritarchs were single-celled organ­isms (protists) that included a wide range of differ­ent taxonomic groups. Some acritarchs might be cysts of unicellular algae that were ancestors of the dinoflagellate (a large group of flagellate protists). Microfossils attributed to clear dinoflagellate have been found in 1,100-million-year-old rocks (Stenian period, which lasted from 1,200 to 1,000 mya). Other acritarchs might be related to the prasino- phycean green algae (probable unicellular ancestral stock from which descended all other green algae and later the plants). Clear microfossils of green algae or chlorophytes were identified in 1,000- million-year-old rocks.

Fossil evidence of eukaryotic red algae or rho- dophyte (Bangiomorpha) has been identified in the transition between the Ectasian and Stenian peri­ods (approximately 1,200 mya), in the Hunting Formation (Somerset Island, Canada). This is the earliest known complex multicellular organism. The development of sexual reproduction in unicel­lular protists might have precipitated the evolution of multicellular life. The multiplication of cells in the first multicellular organisms could occur in the interior of a resistant cyst after fecundation and before meiosis.

The eukaryotic cell, sexual reproduction, and multicellular eukaryotic organisms could have had an early evolution. Because the first organisms would not have had hard parts, the first unicellular and pluricellular eukaryotic organisms are proba­bly not preserved in the Archean and Proterozoic fossil record. It is hypothesized that these three significant milestones in the history of life were developed very early. For example, the first eukaryotic cells could have appeared 2,700-2,100 mya, coinciding with the release of atmospheric oxygen; and sexual reproduction could have devel­oped at least 1,800 mya, the age of the first possi­ble acritarchs in China. Moreover, 1-meter-long fossil tubes called Grypania were identified in 2,000-million-year-old rocks of Lake Superior and have been attributed to unicellular eukaryotic algae, although also to giant bacteria.

The Neoproterozoic era (1,000-542 mya) cov­ers one of the most interesting times in the history of life and the fossil record, such as the develop­ment of complex multicellular organisms and the appearance of animals or metazoans. Different lineages of eukaryotic protists also evolved during this era. The first radiation of acritarchs occurred during the Tonian period (1,000-850 mya). The first fossil record of ciliates and testate amoebae occurred more than 750 mya, that is, in the Cryogenian period (850-630 mya). This period is characterized by worldwide glacial deposits, sug­gesting that during this time Earth’s climate suf­fered the most severe ice ages in its history (Sturtian and Marinoan glaciations). It has been hypothe­sized that planetary oceans were frozen over com­pletely (the Snowball Earth hypothesis).

In spite of the fact that the acritarchs suffered a significant mass extinction event during the cold Cryogenian period, the development of multicellular animals may have been favored due to increased evolutionary pressures during the multiple icehouse­hothouse cycles of the Cryogenian period. Animals are generally accepted to have evolved from choano- flagellates, collared flagellate protozoa that have the same structure as certain sponge cells. Using nuclear- encoded protein sequences, geneticists and molecu­lar biologists have suggested that the appearance of animals occurred as early as 1,200 mya (the Mesoproterozoic era). However, a conflict between these molecular models and the traditional percep­tion of the fossil record makes some paleontologists wish to reconsider the molecular data. In spite of this criticism, the Proterozoic fossil record is supply­ing new, unexpectedly complex life forms probably related to multicellular metazoans: enigmatic bed­ding-plane impressions (Horodyskia) of colonial benthic eukaryotic organisms living 1,500 mya (Calymnian period from the Belt Supergroup of Montana); fossils of worm-like segmented organ­isms (Parmia) in 1,000-million-year-old Tonian deposits in Timan (Russia) and interpreted as a probable predecessor of the annelid worms; and macroscopic fossils of worm-like organisms (Protoarenicola and Pararenicola) discovered in 740-million-year-old Cryogenian deposits from Huainan (China) and related probably to annelids and pogonophorans.

Ediacaran Fossils

The end of the global glaciations of the Cryogenian period occurred 630 mya, and after it came one of the most interesting periods in the evolution of life: the Ediacaran period (635-542 mya), also called the Vendian period. This period is characterized by a group of ancient life forms called Ediacaran or Vendian biota. This fauna is older than the oldest shelled fossils of classical paleontology: the faunas. Its first fossil record was found in the Ediacara Hills Konservat-Lagerstatte (South Australia). Dozens of outcrops with related fossils have been found since then, especially important being the deposits found in the White Sea (Russia), in southwestern Africa, in northwestern Canada, and in southeastern Newfoundland. Another sig­nificant Ediacaran Konservat-Lagerstatte deposit was found in the Doushantuo Formation (Guizhou, China). In addition to bacteria, protists, and algae fossils, the Ediacaran fossil record includes the old­est definitive multicellular animals. They are gener­ally segmented or frond-like organisms, and many of them are difficult to interpret. Some have been redescribed as trace fossils, such as Dickinsonia, and often interpreted as ancestral arthropods; forms such as Spriggina can resemble traces. Ediacaran organisms include frond-like forms called rangeomorphs; they have disk-type morphologies with various ornamentations (e.g., Charniodiscus and Charnia). They could belong to cnidaria (related to modern sea pens) or be possible precursors of arthropods and mollusks. Various disk-like forms could be also related to anemones (e.g., Mawsonites) orsponges(e.g., Palaeophragmodictya).Nevertheless, many of the best-known Ediacaran creatures appear to have no obvious relationship to later metazoans, including organisms that resemble sessile bags, annulate disks, and air mattresses. An alternative interpretation is to consider that these extinct organ­isms belonged to a new separate kingdom (kingdom Vendobionta) that comprises nonmetazoan, coeno- cytic groups, or giant multinucleate eukaryotic uni- cellulars. These organisms disappeared across the Ediacaran-Cambrian boundary (approximately 542 mya), perhaps wiped out by a mass extinction event.

Phanerozoic Fossils

The Phanerozoic eon (545 million years-present) is a period of geologic time during which abun­dant hard-shelled animals lived. Its name derives from the Greek and means “visible life,” referring to the abundant fossil record. Significant events in the history of life occurred during this eon: rapid radiation of a great number of animal phyla; emergence of terrestrial plants and ani­mals; evolution of complex plants; and evolution of fishes, amphibians, reptiles, and mammals. All of these evolutionary events allowed the develop­ment of the modern faunas and floras, although this process was also sprinkled with multiple mass extinction events. Two of these extinction events (Permian-Triassic and Cretaceous- Paleogene events) mark the boundaries between the Phanerozoic eras: the (542-251 mya), Mesozoic (251-65 mya), and Cenozoic (65-0 mya) eras.

Paleozoic: From the to the P-T Extinction Event

Cambrian

The Proterozoic-Phanerozoic boundary (or Neoproterozoic-Paleozoic boundary, or Ediacaran- Cambrian boundary) has been classically placed at the first appearance of trilobites and archeocyathids. These extinct life forms are complex compared with the preceding Ediacaran fauna. A large number of animal groups appeared nearly simultaneously at the beginning of the Cambrian period (542-488 mya), such as mollusks, brachiopods, bryozoans, arthropods, echinoderms, and even vertebrates. There is some fossil evidence that simple life (algae, fungi) may have invaded land in this moment, although plants and animals do not take to the land until the or . This rapid episode is known as the Cambrian explosion, and represents the most significant evolutionary radiation to hap­pen in the history of life on Earth. The major groups or phyla of animals emerged suddenly, in most cases without evidence of precursors. The Early Cambrian metazoan radiation was also accompanied by a diversification of organic-walled protists, including prasinophyte and dasyclad green algae, benthic for- aminifers, and acritarchs. Exceptional fossiliferous outcrops (Konservat-Lagerstatten) of the Cambrian have been found at Burgess Shale (British Columbia, Canada), Maotianshan Shales (Chegjiang, China), Sirius Passet (Greenland), Orsten and Alum Shale (Sweden), Murero (Spain), Emu Bay Shale (South Australia), Kaili Formation (Guizhou, China), and the House Range (Utah).

The most famous Cambrian fossils are the trilo- bites, a group of extinct marine arthropods that form the class Trilobita. Many thousands of trilobite spe­cies are known, and they serve as excellent Paleozoic index fossils because they evolved rapidly and pres­ent a good fossil record. Their ancestors seem to be morphologically similar to taxa like Sprigginia, Parvancorina, and other trilobitomorphs of the Ediacaran. It is reasonable to assume that the trilo- bites share a common ancestor with other arthro­pods that lived prior to the Ediacaran-Cambrian boundary. The most primitive groups appeared dur­ing the Early Cambrian and belong to the orders Agnostida, Redlichiida, Corynexochida, Naraoidia, and Ptychopariida. Although the agnostids have the most primitive characteristics, the redlichid trilobites are the first true arthropods to appear in the fossil record. The earliest known trilobite is the redlichid genus Fallotaspis.

The trilobites could be related to the chelicerata (spiders, scorpions, and related forms), the Cambrian aglaspids being an intermediate group. Crustaceans also first evolved in the earlier Cambrian, as part of the great radiation of Cambrian coelomate animals. Although the crus­taceans have hard exoskeletons reinforced with calcium carbonate, their fossil record is rarer than that of trilobites during the Cambrian.

Arthropods are thought to have branched from an ancestor of the segmented worms, probably prior to the Cambrian, and the onychophorans are a good example. The onychophorans (or velvet worms) are a phylum that today includes seg­mented terrestrial animals somewhat resembling both annelids and arthropods. Traditionally the annelids have been considered the closest relatives of arthropods and onychophorans, due to their common segmentation. Annelids are known at least as early as the Cambrian, and there is some evidence that they appeared during the late Neoproterozoic (Cloudina may be an early serpu- lid worm fossil of the Ediacaran). They comprise the segmented worms, and include the well-known earthworms, leeches, serpulids, and tube worms. However, the similarities between annelids and arthropods have been more recently considered convergent evolution. The arthropods may be more closely related to certain pseudocoelomates such as nematodes (roundworms) or priapulids (penis worms); further, the annelids are more closely related to mollusks, bryozoans, and brachiopods.

The brachiopods (lamp shells) are sessile, two- shelled, marine animals with an external morphol­ogy resembling bivalve mollusks. However, they have a lophophore (a ring of ciliated tentacles sur­rounding the mouth) similar to that of the bryozo- ans. Although possible brachiopods have been found in the Ediacaran period, the earliest unequiv­ocal brachiopods in the fossil record occur in the Early Cambrian, appearing first as inarticulate forms (e.g., Lingula) followed soon by articulate forms (e.g., Obollella). Apart from perhaps the trilobites, brachiopods are one of the most impor­tant Paleozoic index fossils. Unlike bryozoans, they are solitary and never form colonies. Together with bryozoans (ectoproctans) and phoronids (horseshoe worms), brachiopods belong to the informal lophophorates and they almost certainly share a common ancestor.

The briozoans are small colonial animals that build skeletons of calcium carbonate, superficially similar to coral. They were initially subdivided in two subgroups, ectoproctans and entoproctans, based on their similar body plans (with lophohore) and mode of life. Nevertheless, the ectoproctans are coelomate (possessing a body cavity) and the entoproctans are acoelemate. Molecular studies do not support a phylogenetic relationship between the two groups. For this reason, the entoproctans are now considered a phylum of their own and related to mollusks and annelids, whereas the ecto­proctans are considered synonymous with bryozo- ans and related to brachiopods and phoronids. The true bryozoans, the ectoproctans, perhaps evolved from a phoronid-like ancestor in the Cambrian, but their first fossil record occurs in the period. The phoronids are worm­shaped creatures that secrete chitinous tubes in which to live, and some early Cambrian forms (e.g., Iotuba) have been attributed to them.

The earliest mollusk fossils belong to the uni­valved monoplacophorans. They are probably the ultimate common ancestor of all univalved and bivalved mollusks; they first appeared in the fossil record in the earliest Cambrian. By the Late Cambrian, most living classes of mollusks has been found in the fossil record, including Gastropoda (snails and slugs), Cephalopoda (nau­tilus, ammonites, squids, octopuses, cuttlefish), and Pelecypoda or Bivalvia (clams, scallops, mus­sels, cockles). All of them were initially marine. The first true gastropods appeared in the Middle Cambrian (e.g., Protowenella) and Late Cambrian (e.g., Chippewaella and Sterpsodiscus), although some Early Cambrian forms have been related to gastropods (e.g., Scenella). The cephalopods also developed during the Cambrian, with the plec- tronocerids (e.g., Plectronoceras) considered the basal group. A little-known but important Cambrian group of mollusks was the helcionel- lids, such as Helicionella, Yochelcionella, and Latouchella. They could have affinities with other primitive groups like poliplacophorans (chitons). The Lower Cambrian fossil Tommotia, which had squid-like tentacles and a snail-like foot, is considered to be a basal cephalopod.

Many Cambrian taxa do not seem to have any relationship to modern taxonomic categories. Halkieria could be an inarticulate brachiopod or at least a stem group, or even represent the procoelo- mate ancestors of all higher (coelomate body plan) animals, including brachiopods, annelids, and mollusks. The present aplacophorans (small, cylin­drical, and worm-like mollusks) could be relics of the original Ediacaran or Cambrian procoelo- mate-molluskomorph radiation. Wiwaxia and Odontogriphus have been considered protomol­lusks, and related to Ediacaran Kimberella, which could have also mollusk-typeradula. Orthrozanclus, a fossil recently discovered at the Burgess Shale Formation (Canada), could be the evolutionary link among mollusks, annelids, and brachiopods. The genera Halkieria and Wiwaxia have been placed tentatively in a group called Halwaxiida. Other poorly understood Cambrian taxa have been included in an informal Lobopodia family. These include such famous fossils as Anomalocaris, Opabinia, Xenusion, Amiskwia, Odontogriphus, and Hallucigenia. Anomalocaris seem to be closely related to the arthropods. It is included in the col­lective group named anomalocarids, which also includes Laggania, Amplectobelua, and probably Opabinia. Some of the lobopodia taxa, such as Xenusion, Aysheaia, or Microdictyon, are ony- chophore-like forms. The famous Hallucigenia was considered to belong to Lobopodia, but is now considered to be a true onychophoran. Finally, Amiskwia could be a chaetognath (arrow worm) or a nemertean (ribbon worm); its affinities are not clear.

Echinoderms also evolved in the Early Cambrian from deuterostome animals with bilateral symme­try, but later acquired their typical pentaradial symmetry. Their larvae are organized in a bilater­ally symmetric fashion similar to embryonic chor- dates. An extinct class of bilateral forms, called helicoplacoidea, includes the earliest fossil echino­derms (Helicoplacus), found in the lowest Cambrian fossil record of the White Mountains (California). With scarce exceptions, all extant echinoderms fall into five well-defined classes: crinoidea (sea lilies), asteroidea (sea stars), ophiuroidea (brittle stars), echinoidea (sea urchins), and holothuroidea (sea cucumbers). All of them could have evolved from an extinct Cambrian echinoderm group called homalozoa (Stylophora, Soluta, Cincta, and Ctenocystis), which have been connected to the chordates. Arkarua, a small disk-like taxa of the Ediacaran with fivefold symmetry, has been proposed as a possible precursor of the extinct Cambrian asteroidea (edriosteroidea). Echinoderms are a sister group of hemichordates and chordates.

Hemichordates are worm-shaped marine animals—deuterostomes like echinoderms and chordates—that also evolved in the Lower Cambrian. They include a significant fossil group named grap- tolites. The earliest graptolites are known from the Middle Cambrian, such as Chaunograptus of the Burgess Shale Formation (British Columbia, Canada) or Chepalodiscus of the Wheeler Shale Formation (Utah). The graptolites were colonial hemichor­dates, and their fossils resemble hieroglyphs. The graptolite colony, known as a rhabdosome, has numerous individuals (zooids) with theca distrib­uted in a variable number of branches (stipes). Because many graptolites are good index fossils, they are used in Paleozoic biostratigraphic scales. The most primitive graptolites, the dendroids, had many stipes and were generally benthic. Emmonaspis, an enigmatic worm-like animal from the Lower Cambrian of Vermont, has been associated with graptolites and chordates.

Chordate origins are not fully understood, although Cambrian Burgess Shale outcrops have some small fossil taxa called Piakia with all the ancestral chordate characteristics. It is very similar to the most primitive living chordate, Amphioxus. The chordates are distinguished by a notochord, a semiflexible rod running along the length of the animal. This is replaced and surrounded by the vertebra or backbone in the vertebrates. There are three major groups of chordates: urochordata (tunicates), cephalochordata (lacenlets), and verte- brata. Yunnanozoon, a suspected chordate or hemichordate similar to the most primitive verte­brate Haikouella, was found at the Lower Cambrian Maotianshan shales of Chengjiang (Yunnan, China). Haikouella is surely a chordate related to the first agnathans vertebrate.

Vertebrates also started to evolve during the Cambrian explosion. They are chordates with backbone, vertebral or spinal columns, and include fish, amphibians, reptiles, birds, and mammals. Between the first clear vertebrates are fish-type forms like Haikouichthys and Myllokunmingia, which are probably primitive agnathans related closely to the lampreys. Agnatha are fish-like ver­tebrates characterized by the absence of jaws and paired fins, and they include such extant groups of jawless fish as lampreys (petromyzontids) and hag­fish (myxinids).

Some paleontologists and molecular biologists question the apparent suddenness of the Cambrian radiations, suggesting it is a problem of disconti­nuity in the fossil record. The Cambrian explosion could represent simply a swift increase of animals with hard parts easily fossilizable. Several modern phyla, mainly those with small and soft bodies, have no fossil record in the Cambrian or any another period of the Phanerozoic. Therefore, the first appearance of those phyla could truly occur in the Proterozoic, but their first forms were made up of soft parts that did not fossilize.

Ordovician

The Ordovician period (488-444 mya) is char­acterized by shallow epicontinental seas rich in life, particularly trilobites and brachiopods. Corals flourished in the Ordovician, including rugose and tabulate forms. Mollusks became common during the Ordovician, especially bivalves, gastropods, crinoids, and nautiloids. Nautiloids are a group of shelled cephalopods that flourished during the Ordovician, although they evolved in the Late Cambrian (they are first known from the Fengshan Formation, China). Their shells may be straight, curved, or coiled. The straight-shelled nautiloids (e.g., Orthoceras) are common in Ordovician lime­stones from Scandinavian and Moroccan outcrops, and some of them reached large dimensions, as Endoceras (measuring up to 3.5 m in length) and Cameroceras (probably 11 m in length). Exceptional marine Late Ordovician Konservat-Lagerstatte were discovered in the Soom Shale (South Africa), which includes well-preserved fossils of sea scorpi­ons (eurypterids), conodonts, orthocerid nautiloids, brachipods, trilobites, and chitinozoas. Graptolites also thrived during the Ordovician, evolving the first pelagic forms (graptoloids) at the beginning of this period. They are most commonly found in black shales (deepwater, dysoxic facies). Abereiddy Bay (Wales) is a well-known outcrop for Ordovician graptolite fossils. A significant fossil group of jawless fishes appeared in the Ordovician: the ostracoderms (shell-skinned). They were primitive agnathans covered in an armor of bony plates and usually less than 30 centimeters in length. An important evolutionary event that occurred during the Ordovician was the first appearance of terres­trial plants, evolving from green algae (charo- phyta). Fossil spores from terrestrial plants, probably from bryophites (mosses, hornworts, liverworts), have been found in uppermost Ordovician sediments. The symbiosis between the roots of land plants and fungus (mycorrhiza) occurred surely at the same time, and this mutual­istic association is today essential for the growth of the land plants. The Ordovician finished with a series of extinction events (between 444 and 447 mya) that, taken together, comprise one of the five major mass extinction events. About 85% of all species became extinct, causing the disappearance of one third of all brachiopod and bryozoan fami­lies, as well as numerous groups of conodonts, trilobites, and graptolites. Much of the reef-build­ing fauna was also decimated. The most commonly accepted theory is that these events were triggered by the onset of a glaciation, causing a progressive fall in sea level and a drastic change of the marine habitats. This ice age ended the long, stable green­house conditions typical of the Ordovician.

Silurian

The Silurian (443-416 mya) was a period char­acterized by high sea levels and warm shallow seas that provided a hospitable environment for marine life. Wenlock Series Konservat-Lagerstatte (UK) contains well-preserved fossils of worms, sponges, graptolites, chelicerates, and mollusks, as well as abundant radiolarians, a protist group that pro­duced intricate siliceous microskeletons. The first appearance of coral reefs occurred in the Silurian, built by extinct rugose and tabulate corals. Trilobites, mollusks, brachiopods, and briozoans were very common and quite diverse. Besides the trilobites, other arthropods began to be diversified. Giant sea scorpions (Eurypterus, Pterygotus), some of them more than 2 meters in length, lived during the Silurian, and were formidable predators. Although they are not true scorpions, they are related to the arachnids. The arthropods colonized the terrestrial environment for the first time in the Late Silurian. Among them, the myriapods (millipedes) were surely the first terrestrial animals. The first appearance of terrestrial arachnids occurred in the Late Silurian, and the trigonotar- bids (spider-like arachnids) are one of the oldest known land arthropods. Myriapod and arachnid cuticles have been found in Upper Silurian rocks in Shropshire (England). The first true fishes were the acanthodians (spiny sharks), which appeared in the Early Silurian. This extinct group had features transitional between the two main groups of extant fishes, the bony fish (osteichthians) and cartilagi­nous fish (chondrichthians). Placoderms, armored fishes, are also known from Upper Silurian rocks of China (e.g., Wangolepis). The first fossil record of vascular plants or tracheophytes (with special­ized tissues for conducting water, minerals, and photosynthetic products) occurs in Silurian rocks, and it includes primitive vascular plants (rhynio- phytes) like Cooksonia (early Silurian of the Northern Hemisphere). Most evolutionary rhynio- phytes (as Rhynia) and primitive lycopods (as Baragwanathia from the Upper Silurian rocks of Australia) were other vascular plants that appeared in this period.

Devonian

The Devonian period (416-359 mya) continued to be dominated by brachiopods, bryozoans, cri- noids, and corals in the marine environment. Great barrier reefs extended a thousand kilometers around the Devonian continents, built by sponges (stromatoporids), corals (tabulate and rugose), and calcareous algae. Trilobites were still very common. Armored otracoderms and placoderms were the most abundant fishes of the Devonian, coexisting with the first sharks and ray-finned fish. The first shark, Cladoselache, which appeared in the Devonian, was 1.2 meters in length, and exhib­ited a strange combination of ancestral and derived characteristics. Placoderms went on to inhabit and dominate almost all known aquatic ecosystems, including freshwaters. Canowindra (New South Wales, Australia) is a Konservat-Lagerstatte that contains excellent fossils of fishes, including placo- derms and lobe-finned crossopterygians. Originating from within the nautiloid, the ammo­nites first appeared in the early Devonian. The ammonoids are excellent index fossils for the Upper Paleozoic and Mesozoic. Their shells usually take the form of planispirals, although some of them were trochospiral (helicoid) or nonspiraled. The most primitive group of ammonoids is the goniatitids, which were superficially similar to the nautiloids. Excellent lagerstatte-type outcrops with well-preserved marine fossils (trilobites, corals, brachiopods, sponges, arthropods, trace fossils, etc.) come from the Hunsrük Slates (Germany). In the terrestrial environment, primitive plants cre­ated the first soils, which sheltered such arthro­pods as myriapods, scorpions, and mites. The early Devonian plants still did not have true roots, but the rapid appearance of plant groups with roots occurred during the Late Devonian: lycophytes, sphenophytes, ferns, and progymnosperms. Rhynie chert Konservat-Lagerstatte (Scotland) includes significant well-preserved fossils of Devonian vas­cular plants. This rapid evolutionary radiation of plants has been called the Devonian explosion, and includes the first appearance of Archaeopteris. This is the first tree-like fern, and it can be found in worldwide outcrops. It was a genus of the extinct progymnosperms, plants with gymnosperm­like wood but that produce spores rather than seeds. The development of soils probably acted as a carbon dioxide sink, dropping the levels of greenhouse gases. This may have cooled the cli­mate and caused the mass extinction event toward the end of the Devonian (the Frasnian-Famennian boundary event, 374 mya). The Late Devonian extinction was one of five major extinction events and affected about 82% of the species, including trilobites, ammonites, brachiopods, conodonts, and acritarchs. Ostracoderms and placoderms were extinguished. Meteorite impacts at the Frasnian-Famennian boundary have been sug­gested as possible agents for the Late Devonian mass extinction.

Carboniferous

The Carboniferous period (359-299 mya), sub­divided into two subperiods (Mississippian and Pennsylvanian) in North America, is characterized by large expanses of soils, which remained succes­sively buried, building up important coal deposits. Atmospheric oxygen levels probably increased, causing gigantism among Carboniferous insects and amphibians. The protodonata (griffin flies) are an extinct group related to dragonflies that reached a wingspan of more than 75 centimeters. The Mazon Creek Fossils (Illinois) Konservat-Lagerstatte con­tains well-preserved fossils of arthropods (insects, millipedes, spiders, scorpions, etc.) and of the other paleontological groups. Some labyrinthodont amphibians were longer than 6 meters, including the largest known amphibian, the 9-meter-long Prinosuchus of Brazil. The labyrinthodonts also include well-known amphibians like temnospondyl Eryops and anthracosaurian Seymouria, and they were accompanied by smaller lepospondyls. There is a controversy over the origin of lissamphibias, which include modern anuras (frogs and toads) and urodeles (newts and salamanders), although it is now considered that they evolved from lepospon- dyls. One of the greatest evolutionary innovations of the Carboniferous was amniote eggs among the earliest sauropsid reptiles. The earliest known reptiles are the theropsid Archaeothyris and the sauropsid Hylonomus from Middle Carboniferous sediments of Nova Scotia (Canada). Carboniferous terrestrial plants include an extensive diversity of equisetales (horsetails), sphenophyllales (vine-like plants), lycopodiales (club mosses), lepidodendrales (scale trees), filicales (ferns), pteridospermales (seed ferns), cordaitales, “pre”-cycaDalíes, and volziales. Lepidodendron and Sigillaria evolved among the lepidodendrales, which are closely related to cycaDalíes. Calamites, tree-like horsetails, is other significant genus of the Carboniferous period. An evolutionary event comparable with the appearance of eggs in the tetrapods was the evolution of seeds among the terrestrial plants (gymnosperms). This innovation occurred toward the end of the Devonian, but it was developed during the Carboniferous. It occurred in a primitive plant group called pteri- dosperms or seed ferns (e.g., Lyginopteris), which are considered to be the common ancestor of all gymnosperms (cycads, ginkgos, and conifers). Cordaites (a tree about 30 m high) is the most sig­nificant genus of cordaitales, a primitive and extinct group related to conifers. True conifers appear in the latest Carboniferous and belong to the volt- ziales, which are believed to be ancestral to living conifers (cedars, cypresses, junipers, pines, firs, red­woods or sequoias, spruces, and yews). In the oceans, ammonoids, brachiopods, foraminifera, corals, bryozoans, echinoderms (mainly crinoids), and chondrichtyes (sharks) flourished. The Bear Gulch Limestone (Montana) is an excellent outcrop (Konservat-Lagerstatte) with well-preserved fossils of marine groups (sponges, starfish, conulariids, worms, brachipods, bryozoans, mollusks, as well as fishes such as lampreys, acanthodians, sharks, coelacanths, etc.). The fusulinids are very abundant in the Carboniferous fossil record. They are excel­lent index fossils for the Pennsylvanian and Permian shallow marine rocks, such as Fusulina. Brachiopods also were abundant and are excellent index fossils, including the well-known productids, spirifierids, rhynochonellids, and terebratulids. Bivalves and gastropods, echinoderms—mainly the crinoids like Cyathocrinus—were also very numerous. Among the cephalopods, the straight and curved-shelled nautiloids and the goniatitid ammonoids were com­mon. Finally, fishes increased their diversity during the Carboniferous, becoming abundant: the elasmo­branch chondrichtyes, that is, sharks such as Psammodus or Symmorium. From among the oste- ichtyans, palaeonisciformes and rhizodonts evolved. The first ones include the most primitive or earliest known actinopterygians (ray-finned fishes). The second ones are an extinct group of sarcopterygians (lobe-finned fishes). Freshwater fishes were com­mon, such as the actinopterygian Cheirodus and the acanthodian Acanthodes. Hamilton Quarry (Kansas,) is an excellent outcrop (Konservat- Lagerstatte) with a diverse assemblage of Carbon­iferous marine, freshwater, and terrestrial fossils, including reptiles, amphibians, fishes, insects, conifers, echinoderms, and brachiopods.

Permian

The Permian period (299-251 mya) began with fauna and flora similar to the Carboniferous, but about the Middle Permian there was a major paleo­geographic, paleoclimatic, and palaeobiologic turn­over. All of the earth’s major land masses were collected into a single super continent called Pangea, changing the ocean currents and causing deserts to increase. The dry conditions favored the gymno­sperms, since the protective cover of the seeds rep­resented a selective advantage with regard to the ferns and other pteridophytes that dispersed spores. The first modern gymnosperms—cycadophytes (cycads), ginkgophytes (ginkgos), and pinophytes (conifers)—appeared during the Permian. In the oceans, ammonoid and nautiloid cephalopods, echi­noderms, brachiopods, and foraminiferal fusulinids were common. Permian marine sediments are rich in fossils of these groups, with ammonoid, brachio- pod, and fusulinid being important Permian index fossils. Among the terrestrial fauna, insects and tetrapods (amphibians and reptiles) dominated. A number of important new insect groups appeared during the Permian, including the first coleoptera (beetles) and diptera (flies). Permian amphibians consisted of temnospondyls, lepospondyls, and batrachosaurs. They dominated the Early Permian terrestrial fauna together with reptilian pelycosaurs, being replaced by the therapsids (mammal-like rep­tiles) during the Middle and Late Permian. The therapsids evolved from a group of pelycosaurs, the sphenacodontids, and included the first large herbi­vores (anomodonts as Eodicynodon) and carnivores (theriodonts like gorgoniopsians and therocepha- lians). They also included the ancestors of mam­mals, the cynodonts, which probably appeared toward the end of the Permian. The other main group of reptiles, the sauropsids, was represented by anapsids such as millerettids, nyctiphurets, pare- iasaurs, and procolophonids. The latter survived into the Triassic and probably include the ancestor of the testudines (turtles). In the marine environ­ment, gastropods, bivalves, echinoderms, brachio- pods, ammonites, and fusulinid foraminifers were very abundant. These two last groups contain the most significant Permian index fossils. The Permian ended with the most extensive mass extinction event recorded in the earth’s history: the Permian-Triassic (P-T) boundary event. It has been estimated that more than 95% of marine species and 70% of land vertebrate species became extinct. Although the cause of the Permian mass extinction is under debate, a multiple scenario including glaciations, volcanic eruptions, and meteoritic impacts is broadly accepted.

Mesozoic Fossil Record: From the P-T Extinction Event to the K-T Extinction Event

Triassic

The P-T mass extinction event marks the Paleozoic-Mesozoic boundary, with the Triassic (251-199 mya) being the first period of the Mesozoic. The Triassic climate was generally hot and dry and therefore suitable for gymnosperm trees and reptiles. Triassic land plants include forms that survived the P-T event, including vascu­lar plants both without seeds (lycophytes, cycads) and with seeds (glossopterid pteridospermato- phytes, ginkgophytes, and conifers). The conifers flourished during the Triassic period, mainly in the northern hemisphere (the Petrified Forest in Sonoma County, California, is the most significant fossil record of Triassic conifers, consisting mostly of extinct Araucarioxylon). Although the archo- saur reptiles first appeared toward the very end of the Permian, they evolved into more advanced types during the Triassic, including crurotarsans, phytosaurs, aetosaurs, and rausichids. In addition, the first crocodylians (Saltoposuchus), pterosaurs (Scleromochlus), and dinosaurs (Marasuchus, Lagerpeton) appeared in the Middle and Late Triassic. Amphibians such as temnospondyls and lissamphibians, the earliest turtles (Proganochelys, Proterochersis), and lepidosauromorphs (similar to tuataras) complete the land fauna. In marine environments, the ammonites diversified from a few survivors of the P-T event. The previous goni- atitids and ceratitids were replaced by true ammonitids, the evolved Phylloceratina. In addi­tion to ammonites and fishes, the marine fauna were characterized by such marine reptiles as pac- hypleurosaurs, nothosaurs, placodonts, plesio­saurs, and ichthyosaurs. Finally, new modern groups of corals, the scleractinians, appeared in the Early Triassic. One of the best Triassic outcrops is the 237-million-year-old Monte San Giorgio Konservat-Lagerstatte (Lake Lugano region, north­ern Italy and Switzerland), where abundant well- preserved fossils of fishes and marine and terrestrial reptiles are recorded. Other Triassic Konservat- Lagerstatten are Ghost Ranch (New Mexico) and Karatau (Kazakhstan). The Triassic ended with one of the five major mass extinctions of the Phanerozoic: the Late Triassic extinction event, which affected 70% of species, including sponges, ammonites, conodonts, brachiopods, insects, and many vertebrate groups (archosaurs, dicynodonts, and cynodonts). The latest Triassic extinction event was probably a cluster of smaller events, with at least two of them standing out: the 208- million-year-old Norian event and the 199-million- year-old Triassic-Jurassic boundary event. The origin of these finitriassic extinctions is poorly known, but such extinctions allowed the dinosaurs to expand into many niches that had become vacant.

Jurassic

The Jurassic period (199-145 mya), together with the subsequent Cretaceous period, is known as the Age of Dinosaurs. It was the golden age of the great herbivorous sauropods, for example, Diplodocus, Brachiosaurus, Apatosaurus, or the giant Supersaurus (40-45 m in length, 50-60 tons), and carnivorous theropods like Ceratosaurus, Megalosaurus, and Allosaurus. The first birds evolved from small coelurosaur theropods, with the famous Jurassic Archaeopteryx being the earliest and most primitive known bird. Solnhofen Limestone (Germany) is a Jurassic Konservat-Lagerstatte that includes excellent preserved fossils of Archaeopteryx. Both sauropods and theropods are saurischian dino­saurs and were more abundant than the other major group of dinosaurs, the ornithischians. Nevertheless, this latter dinosaur group also played an important role in the land environment as small herbivorous, the stegosaurids as Stegosaurus, and nodosaurids like Panoplosaurus. Other significant Jurassic rep­tiles were the pterosaurs. The warm, humid climate that characterized the Jurassic period allowed much of the landscape to be covered by lush jungles dominated by conifers (e.g., extinct Cheirolepidiaceae or extant Araucariaceae and Pinaceae). Bennettitales, cycads, ginkgos, and tree ferns were also common. The bennettitales are curious extinct cycad-like

plants that had hermaphrodite strobili with ovulate and staminate sporangia in a flower-like arrange­ment. This plant group has been connected to angiosperms (true flowering plants), although the relationship is under debate. In the sea, ammonites, belemnites, fishes, and marine reptiles, including ichthyosaurs, plesiosaurs, and crocodiles, were the predominant fauna. Holzmaden Konservat- Lagerstatte (Germany) includes well-preserved Jurassic fossils of ichthyosaurs and plesiosaurs as well as ammonites. The rudists, a group of bivalves, formed reefs in the shallow seas of the Cretaceous. Well-preserved fossils of coeloid cephalopod (octo­pus and vampire squids) are found in the Konservat- Lagerstatte of La Voulte-sur-Rhone (France). In the protista world, new groups appeared, like the evolved planktonic foraminifera and calpionelids. Both are planktonic groups and are good Jurassic index fossils.

Cretaceous

The Cretaceous period (145-65 mya) was very warm and humid worldwide, allowing fossils of warmth-adapted plants and dinosaurs to be found at outcrops as far north as Alaska and Greenland and as far south as 15 degrees from the South Pole. Conifers continued being predominant in the flora, but angiosperms (flowering plants) spread during this period (e.g., magnolias) whereas bennettitales died out before the end of the Cretaceous. Archaefructus, found in the Yixian Formations (China), has been proposed as one of the earliest known angiosperms. The land fauna was dominated by dinosaurs, which were at their most diverse. Some of them are popularly known such as Tyrannosaurus, Apatosaurus (= Brontosaurus), Triceratops, Iguanodon, and Ankylosaurus or Velociraptor. Some reached giant size, like the sau- ropod Argentinosaurus (30-35 m in length, 90 to 110 tons) and Bruhathkayosaurus (40-45 m in length, 175 to 220 tons). Maniraptora include tran­sitional forms between dinosaurs and birds, whereas mammals were small and still a minor component of the land fauna. The best fossil record of dinosaurs are found in well-preserved outcrops (Konservat- Lagerstatte) of Auca Mahuevo (Patagonia, Argentina), the Rio Limay Formation (Neuquen, Argentina), the Santana Formation (Brazil), and the Yixian and Chaomidianzi formations (Liaoning, China). The Xiagou Formation (Gansu, China) is a Konservat-Lagerstatte with well-preserved fossils of Gansus, the earliest true modern bird. Insects began to diversify in a coevolutionary process with flower­ing plants, with new insect groups like Hymenoptera (ants), Isoptera (termites), Lepidoptera (butterflies), and Orthoptera (grasshoppers) now appearing. The Konservat-Lagerstatte of the Zaza Formation (Baissa, Siberia) contains numerous exceptionally well-preserved insects. Ammonites, globotruncanid planktic foraminifera, ichthyosaurs, plesiosaurs, mosasaurs, rays, sharks, and modern teleost fishes dominated the oceans. A radiation of diatoms, a significant phytoplanktonic group of protist algae with siliceous shells, occurred during the Cretaceous. Planktic foraminifers and ammonites are the best index fossils of the Cretaceous. The Cretaceous period ended with one of the major mass extinction events, caused by the impact of a meteorite 10 kilo­meters in diameter. Dinosaurs, pterosaurs, ammo­nites, belemnites, marine reptiles, rudist bivalves, and globotruncanid foraminifers become completely extinct. More than 75% of species were affected by the K-T extinction event.

Cenozoic Fossil Record: From the K-T Extinction Event to the Present

Paleogene

The Paleogene period (65-23 mya) is subdivided into three epochs: Paleocene (65-56 mya), Eocene (56-34 mya), and Oligocene (34-23 mya), and is notable for being the time during which mammals evolved from small insectivores that survived the Cretaceous-Tertiary event and diversified into large forms that dominated the land. Paleocene mammals included monotremes, marsupials, mul- titiberculates, and primitive placentals (mesony- chids). Birds also began to diversify during the Paleocene, including large carnivorous birds like Gastornis. Flowering plants continued to develop and proliferate, coevolving with the insects. In the oceans, sharks became the top predators after the K-T extinction of the marine reptiles. The Eocene epochs started with one of the most rapid and extreme global warming events recorded in geo­logic history (the Paleocene-Eocene boundary event), causing an extinction event in the deep ocean environment (benthic foraminifera) but significant radiation in the shallow seas and land fauna and flora. Fossils of subtropical-tropical trees have been found even in Alaska and Greenland. Modern mammals, like artiodactyls, perissodac- tyls, proboscidians, bats, and primates, appeared during the Eocene. Significant Eocene Konservat- Lagerstatte come from London Clay (England), Monte Bolca (Italy), Messel Oli Shale (Germany), and the Green River Formation (Colorado, Utah, and Wyoming). A gradual extinction event trig­gered by severe climatic and vegetational changes occurred across the Eocene-Oligocene transition. At this time, the world experienced a global cool­ing that caused the formation of the Antarctic ice cap and rearranged many of the existing biomes. Tropical areas, such as jungles and rainforests, were replaced by more temperate savannahs and grasslands. The Late Eocene event drastically affected land mammals (the Grande Coupure), including the extinction of mesonychids. Oligocene mammal assemblages include the well-known perissodactyl Brontotherium, rhinocerotid Indricotherium, creodont Hyaenodon or equid Mesohippus, among others. In the seas, the ceta­ceans (whales) had just evolved from their ances­tors, the archaeocetids, which had first appeared during the Eocene, evolving from artiodactyls (per­haps from hippopotamids). Riverleigh (Queensland, Australia) is a significant Oligocene-Miocene Konservat-Lagerstatte containing ancient marsupi­als (Thylacinus, a Tasmanian tiger; Silvabestius, a giant wombat; Ekaldeta, a carnivorous rat kanga­roo; Nimiokoala, an ancient koala, etc.).

Neogene

The Neogene period (23-0 mya) is subdivided in four epochs: Miocene (23-5.3 mya), Pliocene (5.3-1.8 mya), Pleistocene (1.8-0.01 mya), and Holocene (the past 10,000 years). It is a unit of geologic time during which birds and mammal evolved considerably, culminating with the appear­ance of the genus Homo. Due to a generally cooler and drier climate during the Miocene, grasslands underwent a major expansion at the expense of forests. Miocene mammals and birds, as well as other continental and marine fauna were fairly modern. Significant Konservat-Lagerstatte-type outcrops of the Miocene epoch come from 15- to 20-million-year-old Clarkia Fossil Beds (Idaho) and 10-million-year-old Ashfall Fossil Beds (Nebraska). The Pliocene climate became still cooler and drier than that of the Miocene, forming the Arctic ice cap; reducing tropical plants world­wide; spreading deciduous forests, coniferous for­ests, grasslands, and tundra; creating deserts in Asia and Africa; and evolving essentially modern fauna (mastodonts, elephants, rhinos, giraffes, tapirs, saber-toothed tigers or smilodons, dogs, hyenas, horses, cows, antelopes, etc.). The pri­mates continued their evolution, appearing as Australopithecus in the late Pliocene (more than 4 mya). The cooling of the climate culminated in the Pleistocene, an epoch characterized by repeated glacial cycles. A significant Pleistocene Konservat- Lagerstatte is found in the 20,000-year-old La Brea Tar Pits (Los Angeles, California), a tar pit that trapped numerous animals and plants, includ­ing ancient bison, American camels, coyotes, jag­uars, mammoths, American mastodons, and even humans. The genus Homo first appeared during the Pliocene-Pleistocene transition, with H. habilis and H. rudolfensis being their first representatives. Other Homo species evolved progressively across the Pleistocene, such as H. ergaster, H. erectus, H. antecessor, H. heildenbergensis, H. neanderthalen- sis, H. rhodesiensis, and finally H. sapiens. The best outcrops of Australopithecus and Homo come from Hadar (Ethiopia), Olduvai Gorge (Tanzania), Lake Turkana (Kenya), Kimberley (South Africa), Solo River (Java), Zhoukoudian (China), Atapuerca (Spain), Mauer (Germany), La Chapelle-aux-Saint (France), and Shanidar (Iraq).

The beginning of the Holocene corresponds with a period of warming (Holocene climatic opti­mum) after of the last ice age, and with the devel­opment of human cultures. Humans have evolved into a significant agent of extinction. Deforestation, agricultural practices, pollution, overhunting, and numerous other human activities are causing the so-called Holocene extinction event, including numerous species of plants, mammals, birds, amphibians, reptiles, and arthropods. It is also beginning to be called the sixth extinction, since it may become as important as the five major extinc­tion events of the Phanerozoic: the Late Ordovician, Late Devonian, P-T boundary, Late Triassic, and K-T boundary mass extinction events.

Ignacio Arenillas

See also Archaeopteryx; Chronostratigraphy; Coelacanths; Dinosaurs; Evolution, Organic; Extinction; Extinction and Evolution; Extinctions, Mass; Fossils, Interpretations of; Geological Column; Geologic Timescale; ; Ginkgo Trees; Glaciers; Hominid-Pongid Split; Paleontology; Phylogeny; Stromatolites; Trilobites

Further Readings

Benton, M. J. (Ed.). (1993). The fossil record 2. London: Chapman & Hall.

Harland, W. B., Holland, C. H., House, M. R., Hughes, N. F., Reynolds, A. B., Rudwick, M. J. S., et al. (1967). The fossil record. London: Geological Society of London.

Knoll, A. H. (2003). Life on a young planet: The first three billion years of evolution on Earth. Princeton, NJ: Princeton University Press.

Margulis, L. (1970). Origin of eukaryotic cells. New Haven, CT: Yale University Press.

Paul, C. R. C., & Donovan, S. K. (1998). An overview of the completeness of the fossil record. In S. K. Donovan & C. R. C. Paul (Eds.), The adequacy of the fossil record. Chichester, UK: Wiley.

Schoph, J. W. (1999). Cradle of life: The discovery of the earth’s earliest fossils. Princeton, NJ: Princeton University Press.

Taylor, F. J. R. (1979). Symbionticism revisited: A discussion of the evolutionary impact of intracellular symbioses. In Proceedings of the Royal Society of London, Series B, Biological Sciences, 204(1155), 267-286.

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Fossil Fuels

Fossil Fuels

Interpretations of Fossils

Interpretations of Fossils