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130-666: The Lance Formation occurs above the Baculites clinolobatus ammonite marine zone in Wyoming, the top of which has been dated to about 69 million years ago, and extends to the K-Pg boundary, 66 million years ago. However, the characteristic land vertebrate fauna of the Lancian age (which take its name from this formation) is only found in the upper strata of the Lance, roughly corresponding to

260-653: A disaster fauna , a group of survivor animals with low diversity and cosmopolitanism (wide geographic ranges). Temnospondyli recovered first and evolved into large aquatic predators during the Triassic. Other reptiles also diversified rapidly, with aquatic reptiles such as ichthyosaurs and sauropterygians proliferating in the seas. On land, the first true archosaurs appeared, including pseudosuchians (crocodile relatives) and avemetatarsalians (bird/dinosaur relatives). The Middle Triassic spans from 247 million to 237 million years ago. The Middle Triassic featured

390-475: A negative feedback that arises from resource limitation, or both. The hyperbolic model implies a second-order positive feedback. The hyperbolic pattern of the human population growth arises from quadratic positive feedback, caused by the interaction of the population size and the rate of technological growth. The character of biodiversity growth in the Phanerozoic Eon can be similarly accounted for by

520-555: A Berriasian–Barremian warm-dry phase, an Aptian–Santonian warm-wet phase, and a Campanian–Maastrichtian cool-dry phase. As in the Cenozoic, the 400,000 year eccentricity cycle was the dominant orbital cycle governing carbon flux between different reservoirs and influencing global climate. The location of the Intertropical Convergence Zone (ITCZ) was roughly the same as in the present. The cooling trend of

650-541: A body mass over 10 kilograms became extinct, and the Age of Dinosaurs came to an end. The Cenozoic featured the rise of mammals and birds as the dominant class of animals, as the end of the Age of Dinosaurs left significant open niches . There are three divisions of the Cenozoic: Paleogene, Neogene and Quaternary. The Paleogene spans from the extinction of the non-avian dinosaurs, some 66 million years ago, to

780-507: A cooling trend that would continue into the Cenozoic Era. Eventually, the tropical climate was restricted to the equator and areas beyond the tropic lines featured more seasonal climates. Dinosaurs still thrived as new species such as Tyrannosaurus , Ankylosaurus , Triceratops and hadrosaurs dominated the food web. Whether or not pterosaurs went into a decline as birds radiated is debated; however, many families survived until

910-476: A feedback between the diversity and community structure complexity. It has been suggested that the similarity between the curves of biodiversity and human population probably comes from the fact that both are derived from the superposition on the hyperbolic trend of cyclical and random dynamics. Across the Phanerozoic, the dominant driver of long-term climatic change was the concentration of carbon dioxide in

1040-615: A geologic signature associated with the mass extinction that lies between the Mesozoic and Cenozoic Eras . The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822 as the Terrain Crétacé , using strata in the Paris Basin and named for the extensive beds of chalk ( calcium carbonate deposited by the shells of marine invertebrates , principally coccoliths ), found in

1170-594: A lesser extent shelled cephalopods (such as orthocones ). Almost all phyla of marine animals evolved in this period. During this time, the super-continent Pannotia began to break up, most of which later recombined into the super-continent Gondwana . The Ordovician spans from 485 million to 444 million years ago. The Ordovician was a time in Earth's history in which many groups still prevalent today evolved or diversified, such as primitive nautiloids , vertebrates (then only jawless fish ) and corals . This process

1300-426: A result, the world was warm and partially tropical, though possibly with short colder intervals. Plesiosaurs , ichthyosaurs and ammonites dominated the seas, while dinosaurs, pterysaurs and other reptiles dominated the land, with species such as Dilophosaurus at the apex. Crocodylomorphs evolved into aquatic forms, pushing the remaining large amphibians to near extinction. True mammals were present during

1430-973: A severe extinction of sauropods in northern continents, alongside many ichthyosaurs. However, the Jurassic-Cretaceous boundary did not strongly impact most forms of life. The Cretaceous is the Phanerozoic's longest period and the last period of the Mesozoic. It spans from 145 million to 66 million years ago, and is divided into two epochs: Early Cretaceous , and Late Cretaceous . The Early Cretaceous Epoch spans from 145 million to 100 million years ago. Dinosaurs continued to be abundant, with groups such as tyrannosauroids , avialans ( birds ), marginocephalians , and ornithopods seeing early glimpses of later success. Other tetrapods, such as stegosaurs and ichthyosaurs, declined significantly, and sauropods were restricted to southern continents. The Late Cretaceous Epoch spans from 100 million to 66 million years ago. The Late Cretaceous featured

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1560-674: A straight shell, flourished in the seas along with reef-building rudist clams. Inoceramids were also particularly notable among Cretaceous bivalves, and they have been used to identify major biotic turnovers such as at the Turonian-Coniacian boundary. Predatory gastropods with drilling habits were widespread. Globotruncanid foraminifera and echinoderms such as sea urchins and starfish (sea stars) thrived. Ostracods were abundant in Cretaceous marine settings; ostracod species characterised by high male sexual investment had

1690-532: A supervolcano eruption in Sumatra 74,000 years ago caused a global population bottleneck of humans, a second wave of Homo sapiens migration successfully repopulated every continents except Antarctica. As the Pleistocene drew to a close, a major extinction wiped out much of the world's megafauna , including non- Homo sapiens human species such as Homo neanderthalensis and Homo floresiensis . All

1820-490: A trend of desiccation which resulted in the creation of the Sahara, Namib and Kalahari deserts. Mammoths , giant ground sloths , dire wolves , sabre-toothed cats and archaic humans such as Homo erectus were common and widespread during the Pleistocene. A more anatomically modern human , Homo sapiens , began migrating out of East Africa in at least two waves, the first being as early as 270,000 years ago. After

1950-564: A ~0.6 °C increase in temperature. The latter warming interval, occurring at the very end of the Cretaceous, was triggered by the activity of the Deccan Traps. The LKEPCI lasted into the Late Palaeocene , when it gave way to another supergreenhouse interval. The production of large quantities of magma, variously attributed to mantle plumes or to extensional tectonics , further pushed sea levels up, so that large areas of

2080-429: Is a period in which grass spread further across, effectively dominating a large portion of the world, diminishing forests in the process. Kelp forests evolved, leading to the evolution of new species such as sea otters . During this time, perissodactyls thrived, and evolved into many different varieties. Alongside them were the apes , which evolved into 30 species. Overall, arid and mountainous land dominated most of

2210-538: Is debated. The Oligocene Epoch spans from 34 million to 23 million years ago. The Oligocene was an important transitional period between the tropical world of the Eocene and more modern ecosystems. This period featured a global expansion of grass which led to many new species taking advantage, including the first elephants , felines , canines , marsupials and many other species still prevalent today. Many other species of plants evolved during this epoch also, such as

2340-619: Is found in England, northern France, the low countries , northern Germany , Denmark and in the subsurface of the southern part of the North Sea . Chalk is not easily consolidated and the Chalk Group still consists of loose sediments in many places. The group also has other limestones and arenites . Among the fossils it contains are sea urchins , belemnites , ammonites and sea reptiles such as Mosasaurus . In southern Europe,

2470-509: Is known as the Great Ordovician Biodiversification Event or GOBE. Trilobites began to be replaced by articulate brachiopods , and crinoids also became an increasingly important part of the fauna. The first arthropods crept ashore to colonise Gondwana , a continent empty of animal life. A group of freshwater green algae , the streptophytes , also survived being washed ashore and began to colonize

2600-550: Is mostly a transitional recovery period between the desolate aftermath of the Permian Extinction and the lush Jurassic Period. It has three major epochs: Early Triassic , Middle Triassic , and Late Triassic . The Early Triassic lasted between 252 million to 247 million years ago, and was a hot and arid epoch in the aftermath of the Permian Extinction. Many tetrapods during this epoch represented

2730-617: Is the ninth and longest geological period of the entire Phanerozoic . The name is derived from the Latin creta , ' chalk ', which is abundant in the latter half of the period. It is usually abbreviated K , for its German translation Kreide . The Cretaceous was a period with a relatively warm climate , resulting in high eustatic sea levels that created numerous shallow inland seas . These oceans and seas were populated with now- extinct marine reptiles , ammonites , and rudists , while dinosaurs continued to dominate on land. The world

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2860-638: The Earth's surface , beginning with the Cambrian period when animals first developed hard shells that can be clearly preserved in the fossil record . The time before the Phanerozoic, collectively called the Precambrian , is now divided into the Hadean , Archaean and Proterozoic eons. The time span of the Phanerozoic starts with the sudden appearance of fossilised evidence of a number of animal phyla ;

2990-528: The Ediacaran period ( Avalon explosion ) of the preceding Proterozoic eon have since been discovered, and the modern scientific consensus now agrees that complex life (in the form of placozoans and primitive sponges such as Otavia ) has existed at least since the Tonian period and the earliest known life forms (in the form of simple prokaryotic microbial mats ) started in the ocean floor during

3120-531: The Hell Creek Formation . Torotix T. clemensi A possible pelecaniform Cretaceous The Cretaceous ( IPA : / k r ɪ ˈ t eɪ ʃ ə s / krih- TAY -shəss ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era , as well as the longest. At around 79   million years, it

3250-713: The Mancos Shale of western North America. These shales are an important source rock for oil and gas , for example in the subsurface of the North Sea. In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group , which forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast. The group

3380-520: The Messinian salinity crisis . Along with these major geological events, Africa saw the appearance of Australopithecus , the ancestor of Homo . The Isthmus of Panama formed, and animals migrated between North and South America, wreaking havoc on the local ecology. Climatic changes brought savannas that are still continuing to spread across the world, Indian monsoons , deserts in East Asia, and

3510-711: The North American Cordillera , as the Nevadan orogeny was followed by the Sevier and Laramide orogenies . Gondwana had begun to break up during the Jurassic Period, but its fragmentation accelerated during the Cretaceous and was largely complete by the end of the period. South America , Antarctica , and Australia rifted away from Africa (though India and Madagascar remained attached to each other until around 80 million years ago); thus,

3640-632: The Ordovician–Silurian extinction , during which 60% of marine invertebrates and 25% of families became extinct. Though one of the deadliest mass extinctions in earth's history, the O–S extinction did not cause profound ecological changes between the periods. The Silurian spans from 444 million to 419 million years ago, which saw a warming from an icehouse Earth. This period saw the mass diversification of fish , as jawless fish became more numerous, and early jawed fish and freshwater species appeared in

3770-553: The Selli Event . Early Aptian tropical sea surface temperatures (SSTs) were 27–32 °C, based on TEX 86 measurements from the equatorial Pacific. During the Aptian, Milankovitch cycles governed the occurrence of anoxic events by modulating the intensity of the hydrological cycle and terrestrial runoff. The early Aptian was also notable for its millennial scale hyperarid events in the mid-latitudes of Asia. The BAWI itself

3900-812: The Tethys Sea , and the Americas were separated by the Panamanic Seaway (as the Isthmus of Panama had not yet formed). This epoch featured a general warming trend that peaked at the Paleocene-Eocene Thermal Maximum , and the earliest modern jungles expanded, eventually reaching the poles. The oceans were dominated by sharks, as the large reptiles that had once ruled had become extinct. Mammals diversified rapidly, but most remained small. The largest tetrapod carnivores during

4030-728: The Turonian Age, based on isotopic evidence. However, this has subsequently been suggested to be the result of inconsistent isotopic proxies, with evidence of polar rainforests during this time interval at 82° S. Rafting by ice of stones into marine environments occurred during much of the Cretaceous, but evidence of deposition directly from glaciers is limited to the Early Cretaceous of the Eromanga Basin in southern Australia . Flowering plants (angiosperms) make up around 90% of living plant species today. Prior to

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4160-495: The evolution of those phyla into diverse forms; the evolution of plants ; the evolution of fish , arthropods and molluscs ; the terrestrial colonization and evolution of insects , chelicerates , myriapods and tetrapods ; and the development of modern flora dominated by vascular plants . During this time span, tectonic forces which move the continents had collected them into a single landmass known as Pangaea (the most recent supercontinent ), which then separated into

4290-597: The flood plains and riparian zones , giving rise to primitive land plants . By the end of the Ordovician, Gondwana had moved from the equator to the South Pole , and Laurentia had collided with Baltica , closing the Iapetus Ocean . The glaciation of Gondwana resulted in a major drop in sea level, killing off all life that had established along its coast. Glaciation caused an icehouse Earth , leading to

4420-483: The tuatara ) disappeared from North America and Europe after the Early Cretaceous , and were absent from North Africa and northern South America by the early Late Cretaceous . The cause of the decline of Rhynchocephalia remains unclear, but has often been suggested to be due to competition with advanced lizards and mammals. They appear to have remained diverse in high-latitude southern South America during

4550-512: The "Age of the Fish", the Devonian features a huge diversification in fish such as the jawless conodonts and ostracoderms , as well as jawed fish such as the armored placoderms (e.g. Dunkleosteus ), the spiny acanthodians and early bony fish . The Devonian also saw the primitive appearance of modern fish groups such as chondricthyans ( cartilaginous fish ) and osteichthyans (bony fish),

4680-472: The Age of Reptiles, Age of Dinosaurs or Age of Conifers, the Mesozoic featured the first time the sauropsids ascended to ecological dominance over the synapsids, as well as the diversification of many modern ray-finned fish , insects , molluscs (particularly the coleoids ), tetrapods and plants. The Mesozoic is subdivided into three periods: the Triassic, Jurassic and Cretaceous. The Triassic ranges from 252 million to 201 million years ago. The Triassic

4810-999: The Albian regularly expanded northward in tandem with expansions of subtropical high pressure belts. The Cedar Mountain Formation's Soap Wash flora indicates a mean annual temperature of between 19 and 26 °C in Utah at the Albian-Cenomanian boundary. Tropical SSTs during the Cenomanian-Turonian Thermal Maximum were at least 30 °C, though one study estimated them as high as between 33 and 42 °C. An intermediate estimate of ~33-34 °C has also been given. Meanwhile, deep ocean temperatures were as much as 15 to 20 °C (27 to 36 °F) warmer than today's; one study estimated that deep ocean temperatures were between 12 and 20 °C during

4940-500: The Carboniferous as semiaquatic amphibians such as the temnospondyls , and one lineage developed extraembryonic membranes that allowed their eggs to survive outside of the water. These tetrapods, the amniotes , included the first sauropsids (which evolved the reptiles , dinosaurs and birds ) and synapsids (the ancestors of mammal ). Throughout the Carboniferous, there was a cooling pattern, which eventually led to

5070-486: The Cenomanian between the Albian and Turonian. The Cretaceous is divided into Early and Late Cretaceous epochs , or Lower and Upper Cretaceous series . In older literature, the Cretaceous is sometimes divided into three series: Neocomian (lower/early), Gallic (middle) and Senonian (upper/late). A subdivision into 12 stages , all originating from European stratigraphy, is now used worldwide. In many parts of

5200-571: The Cenozoic Era   —   the ichthyosaurs , last remaining temnospondyls ( Koolasuchus ), and nonmammalian cynodonts ( Tritylodontidae )   —   were already extinct millions of years before the event occurred. Coccolithophorids and molluscs , including ammonites , rudists , freshwater snails , and mussels , as well as organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, ammonites are thought to have been

5330-645: The Coniacian through the Maastrichtian. During the Cretaceous, the late- Paleozoic -to-early-Mesozoic supercontinent of Pangaea completed its tectonic breakup into the present-day continents , although their positions were substantially different at the time. As the Atlantic Ocean widened, the convergent-margin mountain building ( orogenies ) that had begun during the Jurassic continued in

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5460-574: The Cretaceous is sharply defined, being placed at an iridium -rich layer found worldwide that is believed to be associated with the Chicxulub impact crater , with its boundaries circumscribing parts of the Yucatán Peninsula and extending into the Gulf of Mexico . This layer has been dated at 66.043 Mya. At the end of the Cretaceous, the impact of a large body with the Earth may have been

5590-570: The Cretaceous is usually a marine system consisting of competent limestone beds or incompetent marls . Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European continental shelf , at the margin of the Tethys Ocean . During the Cretaceous, the present North American continent was isolated from the other continents. In

5720-537: The Cretaceous seas. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half of the world's petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic shales were formed during this interval, such as

5850-514: The Earth, and the large amounts of trees sequestered much of the carbon that became coal deposits (hence the name Carboniferous and the term " coal forest "). About 90% of all coal beds were deposited in the Carboniferous and Permian periods, which represent just 2% of the Earth's geologic history. The high oxygen levels caused by these wetland rainforests allowed arthropods, normally limited in size by their respiratory systems , to proliferate and increase in size. Tetrapods also diversified during

5980-540: The Jurassic but remained small, with average body masses of less than 10 kilograms (22 lb) until the end of the Cretaceous. The Middle and Late Jurassic Epochs span from 174 million to 145 million years ago. Conifer savannahs made up a large portion of the world's forests. In the oceans, plesiosaurs were quite common, and ichthyosaurs were flourishing. The Late Jurassic Epoch spans from 163 million to 145 million years ago. The Late Jurassic featured

6110-616: The Jurassic, the North Atlantic already opened, leaving a proto-ocean between Europe and North America. From north to south across the continent, the Western Interior Seaway started forming. This inland sea separated the elevated areas of Laramidia in the west and Appalachia in the east. Three dinosaur clades found in Laramidia (troodontids, therizinosaurids and oviraptorosaurs) are absent from Appalachia from

6240-519: The LKEPCI. During this period of relatively cool temperatures, the ITCZ became narrower, while the strength of both summer and winter monsoons in East Asia was directly correlated to atmospheric CO 2 concentrations. Laramidia likewise had a seasonal, monsoonal climate. The Maastrichtian was a time of chaotic, highly variable climate. Two upticks in global temperatures are known to have occurred during

6370-532: The Late Cretaceous, where lizards remained rare, with their remains outnumbering terrestrial lizards 200:1. Choristoderes , a group of freshwater aquatic reptiles that first appeared during the preceding Jurassic, underwent a major evolutionary radiation in Asia during the Early Cretaceous, which represents the high point of choristoderan diversity, including long necked forms such as Hyphalosaurus and

6500-555: The Late Cretaceous-Early Palaeogene Cool Interval (LKEPCI). Tropical SSTs declined from around 35 °C in the early Campanian to around 28 °C in the Maastrichtian. Deep ocean temperatures declined to 9 to 12 °C, though the shallow temperature gradient between tropical and polar seas remained. Regional conditions in the Western Interior Seaway changed little between the MKH and

6630-422: The Late Triassic, and pterosaurs evolved a bit later. Other large reptilian competitors to the dinosaurs were wiped out by the Triassic–Jurassic extinction event , in which most archosaurs (excluding crocodylomorphs , pterosaurs and dinosaurs), most therapsids (except cynodonts ) and almost all large amphibians became extinct, as well as 34% of marine life in the fourth mass extinction event. The cause of

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6760-486: The MKH. Mean annual temperatures at the poles during the MKH exceeded 14 °C. Such hot temperatures during the MKH resulted in a very gentle temperature gradient from the equator to the poles; the latitudinal temperature gradient during the Cenomanian-Turonian Thermal Maximum was 0.54 °C per ° latitude for the Southern Hemisphere and 0.49 °C per ° latitude for the Northern Hemisphere, in contrast to present day values of 1.07 and 0.69 °C per ° latitude for

6890-412: The MKH. The poles were so warm that ectothermic reptiles were able to inhabit them. Beginning in the Santonian, near the end of the MKH, the global climate began to cool, with this cooling trend continuing across the Campanian. This period of cooling, driven by falling levels of atmospheric carbon dioxide, caused the end of the MKH and the transition into a cooler climatic interval, known formally as

7020-404: The Maastrichtian, bucking the trend of overall cooler temperatures during the LKEPCI. Between 70 and 69 Ma and 66–65 Ma, isotopic ratios indicate elevated atmospheric CO 2 pressures with levels of 1000–1400 ppmV and mean annual temperatures in west Texas between 21 and 23 °C (70 and 73 °F). Atmospheric CO 2 and temperature relations indicate a doubling of pCO 2 was accompanied by

7150-428: The Mesozoic) ended with the Cretaceous–Paleogene extinction event , a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs , and large marine reptiles , died out, widely thought to have been caused by the impact of a large asteroid that formed the Chicxulub crater in the Gulf of Mexico. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary (K–Pg boundary),

7280-473: The Middle Devonian, shrub-like forests existed: lycophytes , horsetails and progymnosperm . This greening event also allowed the diversification of arthropods as they took advantage of the new habitat. Near the end of the Devonian, 70% of all species became extinct in a sequence of mass extinction events, collectively known as the Late Devonian extinction . The Carboniferous spans from 359 million to 299 million years ago. Tropical swamps dominated

7410-464: The Paleocene were reptiles, including crocodyliforms , choristoderans and snakes . Titanoboa , the largest known snake, lived in South America during the Paleocene. The Eocene Epoch ranged from 56 million to 34 million years ago. In the early Eocene, most land mammals were small and living in cramped jungles, much like the Paleocene. Among them were early primates , whales and horses along with many other early forms of mammals. The climate

7540-423: The Paleozoic Era and ran from 539 million to 485 million years ago. The Cambrian sparked a rapid expansion in the diversity of animals, in an event known as the Cambrian explosion , during which the greatest number of animal body plans evolved in a single period in the history of Earth. Complex algae evolved, and the fauna was dominated by armoured arthropods (such as trilobites and radiodontids ) and to

7670-401: The Phanerozoic correlate much better with the hyperbolic model (widely used in demography and macrosociology ) than with exponential and logistic models (traditionally used in population biology and extensively applied to fossil biodiversity as well). The latter models imply that changes in diversity are guided by a first-order positive feedback (more ancestors, more descendants) or

7800-419: The South Atlantic and Indian Oceans were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising eustatic sea levels worldwide. To the north of Africa the Tethys Sea continued to narrow. During most of the Late Cretaceous, North America would be divided in two by the Western Interior Seaway , a large interior sea, separating Laramidia to the west and Appalachia to

7930-514: The Southern and Northern hemispheres, respectively. This meant weaker global winds, which drive the ocean currents, and resulted in less upwelling and more stagnant oceans than today. This is evidenced by widespread black shale deposition and frequent anoxic events . Tropical SSTs during the late Albian most likely averaged around 30 °C. Despite this high SST, seawater was not hypersaline at this time, as this would have required significantly higher temperatures still. On land, arid zones in

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8060-403: The Tethys to the Arctic Ocean and enabling biotic exchange between the two oceans. At the peak of the Cretaceous transgression , one-third of Earth's present land area was submerged. The Cretaceous is justly famous for its chalk ; indeed, more chalk formed in the Cretaceous than in any other period in the Phanerozoic . Mid-ocean ridge activity—or rather, the circulation of seawater through

8190-521: The ancestors of modern mammals. The first conifers evolved during this period, then dominated the terrestrial landscape. The Permian ended with at least one mass extinction , an event sometimes known as " the Great Dying ", caused by large floods of lava (the Siberian Traps in Russia and the Emeishan Traps in China). This extinction was the largest in Earth's history and led to the loss of 95% of all species of life. The Mesozoic ranges from 252 million to 66 million years ago. Also referred to as

8320-425: The ancestors of modern-day birds also diversified. They inhabited every continent, and were even found in cold polar latitudes. Pterosaurs were common in the early and middle Cretaceous, but as the Cretaceous proceeded they declined for poorly understood reasons (once thought to be due to competition with early birds , but now it is understood avian adaptive radiation is not consistent with pterosaur decline ). By

8450-458: The atmosphere, though some studies have suggested a decoupling of carbon dioxide and palaeotemperature, particularly during cold intervals of the Phanerozoic. Phanerozoic carbon dioxide concentrations have been governed partially by a 26 million year oceanic crustal cycle. Since the Devonian, large swings in carbon dioxide of 2,000 ppm or more were uncommon over short timescales. Variations in global temperature were limited by negative feedbacks in

8580-454: The beginnings of the Sahara Desert . The Earth's continents and seas moved into their present shapes. The world map has not changed much since, save for changes brought about by the Quaternary glaciation such as Lake Agassiz (precursor of the Great Lakes ). The Quaternary spans from 2.58 million years ago to present day, and is the shortest geological period in the Phanerozoic Eon . It features modern animals, and dramatic changes in

8710-513: The beginnings of the break-up of Pangaea as rifting commenced in north Pangaea. The northern part of the Tethys Ocean , the Paleotethys Ocean, had become a passive basin, but a spreading center was active in the southern part of the Tethys Ocean, the Neotethys Ocean. Phytoplankton , coral, crustaceans and many other marine invertebrates recovered from the Permian extinction by the end of the Middle Triassic. Meanwhile, on land, reptiles continued to diversify, conifer forests flourished, as well as

8840-499: The boundaries of the Holocene epoch. Human activity is blamed for an ongoing mass extinction that began roughly 10,000 years ago, though the species becoming extinct have only been recorded since the Industrial Revolution . This is sometimes referred to as the " Sixth Extinction " with hundreds of species gone extinct due to human activities such as overhunting , habitat destruction and introduction of invasive species . It has been demonstrated that changes in biodiversity through

8970-450: The climate. It is divided into two epochs: the Pleistocene and the Holocene . The Pleistocene lasted from 2.58 million to 11,700 years ago. This epoch was marked by a series of glacial periods ( ice ages ) as a result of the cooling trend that started in the mid-Eocene. There were numerous separate glaciation periods marked by the advance of ice caps as far south as 40 degrees N latitude in mountainous areas. Meanwhile, Africa experienced

9100-408: The continental crust were covered with shallow seas. The Tethys Sea connecting the tropical oceans east to west also helped to warm the global climate. Warm-adapted plant fossils are known from localities as far north as Alaska and Greenland , while dinosaur fossils have been found within 15 degrees of the Cretaceous south pole . It was suggested that there was Antarctic marine glaciation in

9230-416: The continents were affected, but Africa was impacted to a lesser extent and retained many large animals such as elephants, rhinoceros and hippopotamus . The extent to which Homo sapiens were involved in this megafaunal extinction is debated. The Holocene began 11,700 years ago at the end of Younger Dryas and lasts until the present day. All recorded history and so-called " human history " lies within

9360-406: The cooling climate even as more archaic forms died out. By the end of the Eocene, whales such as Basilosaurus had become fully aquatic. The late Eocene Epoch saw the rebirth of seasons, which caused the expansion of savanna-like areas with the earliest substantial grasslands . At the transition between the Eocene and Oligocene epochs there was a significant extinction event , the cause of which

9490-611: The current continental landmasses. The term "Phanerozoic" was coined in 1930 by the American geologist George Halcott Chadwick (1876–1953), deriving from the Ancient Greek words φανερός ( phanerós ), meaning "visible"; and ζωή ( zōḗ ), meaning "life". This is because it was once believed that life began in the Cambrian , the first period of this eon, due to the lack of Precambrian fossil record back then. However, trace fossils of booming complex life from

9620-576: The dawn of the Neogene 23 million years ago. It features three epochs : Paleocene , Eocene and Oligocene . The Paleocene Epoch began with the K–Pg extinction event, and the early part of the Paleocene saw the recovery of the Earth from that event. The continents began to take their modern shapes, but most continents (and India) remained separated from each other: Africa and Eurasia were separated by

9750-555: The earlier Archean eon. The Proterozoic –Phanerozoic boundary is at 538.8 million years ago. In the 19th century, the boundary was set at time of appearance of the first abundant animal ( metazoan ) fossils , but trace fossils of several hundred groups ( taxa ) of complex soft-bodied metazoa from the preceding Ediacaran period of the Proterozoic eon, known as the Avalon Explosion , have been identified since

9880-463: The earliest crown group birds. Acanthomorph fish, the most diverse group of modern vertebrates, appeared in aquatic habitats around the middle of the Cretaceous. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of previously widespread gymnosperm groups. The Cretaceous (along with

10010-544: The early and mid-Cretaceous (becoming extinct during the late Cretaceous Cenomanian-Turonian anoxic event ), plesiosaurs throughout the entire period, and mosasaurs appearing in the Late Cretaceous. Sea turtles in the form of Cheloniidae and Panchelonioidea lived during the period and survived the extinction event. Panchelonioidea is today represented by a single species; the leatherback sea turtle . The Hesperornithiformes were flightless, marine diving birds that swam like grebes . Baculites , an ammonite genus with

10140-554: The east, then receded late in the period, leaving thick marine deposits sandwiched between coal beds. Bivalve palaeobiogeography also indicates that Africa was split in half by a shallow sea during the Coniacian and Santonian, connecting the Tethys with the South Atlantic by way of the central Sahara and Central Africa, which were then underwater. Yet another shallow seaway ran between what is now Norway and Greenland, connecting

10270-747: The end of the AACS, which ended around 111 Ma with the Paquier/Urbino Thermal Maximum, giving way to the Mid-Cretaceous Hothouse (MKH), which lasted from the early Albian until the early Campanian. Faster rates of seafloor spreading and entry of carbon dioxide into the atmosphere are believed to have initiated this period of extreme warmth, along with high flood basalt activity. The MKH was punctuated by multiple thermal maxima of extreme warmth. The Leenhardt Thermal Event (LTE) occurred around 110 Ma, followed shortly by

10400-428: The end of the Cretaceous, alongside new forms such as the gigantic Quetzalcoatlus . Mammals diversified despite their small sizes, with metatherians ( marsupials and kin) and eutherians ( placentals and kin) coming into their own. In the oceans, mosasaurs diversified to fill the role of the now-extinct ichthyosaurs, alongside huge plesiosaurs such as Elasmosaurus . Also, the first flowering plants evolved. At

10530-600: The end of the Cretaceous, the Deccan Traps and other volcanic eruptions were poisoning the atmosphere. As this was continued, it is thought that a large meteor smashed into Earth, creating the Chicxulub Crater and causing the event known as the K–Pg extinction , the fifth and most recent mass extinction event, during which 75% of life on Earth became extinct, including all non-avian dinosaurs. Every living thing with

10660-501: The end of the Cretaceous. The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms. The Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of marine limestone , a rock type that is formed under warm, shallow marine conditions. Due to

10790-463: The end of the period only three highly specialized families remained; Pteranodontidae , Nyctosauridae , and Azhdarchidae . The Liaoning lagerstätte ( Yixian Formation ) in China is an important site, full of preserved remains of numerous types of small dinosaurs, birds and mammals, that provides a glimpse of life in the Early Cretaceous. The coelurosaur dinosaurs found there represent types of

10920-407: The enlarged ridges—enriched the oceans in calcium ; this made the oceans more saturated, as well as increased the bioavailability of the element for calcareous nanoplankton . These widespread carbonates and other sedimentary deposits make the Cretaceous rock record especially fine. Famous formations from North America include the rich marine fossils of Kansas 's Smoky Hill Chalk Member and

11050-584: The evergreen trees. The long term cooling continued and seasonal rain patterns established. Mammals continued to grow larger. Paraceratherium , one of the largest land mammals to ever live, evolved during this epoch, along with many other perissodactyls . The Neogene spans from 23.03 million to 2.58 million years ago. It features two epochs: the Miocene and the Pliocene . The Miocene spans from 23.03 million to 5.333 million years ago and

11180-528: The extinction fed on insects , larvae , worms , and snails, which in turn fed on dead plant and animal matter. Scientists theorise that these organisms survived the collapse of plant-based food chains because they fed on detritus . In stream communities , few groups of animals became extinct. Stream communities rely less on food from living plants and more on detritus that washes in from land. This particular ecological niche buffered them from extinction. Similar, but more complex patterns have been found in

11310-460: The extinction is debated, but likely resulted from eruptions of the CAMP large igneous province . The Jurassic ranges from 201 million to 145 million years ago, and features three major epochs: Early Jurassic , Middle Jurassic and Late Jurassic . The Early Jurassic epoch spans from 201 million to 174 million years ago. The climate was much more humid than during the Triassic, and as

11440-678: The first flies. The Late Triassic spans from 237 million to 201 million years ago. Following the bloom of the Middle Triassic, the Late Triassic was initially warm and arid with a strong monsoon climate and with most precipitation limited to coastal regions and high latitudes. This changed late in the Carnian period with a 2 million years-long wet season which transformed the arid continental interior into lush alluvial forests . The first true dinosaurs appeared early in

11570-558: The first records of the gharial-like Neochoristodera , which appear to have evolved in the regional absence of aquatic neosuchian crocodyliformes. During the Late Cretaceous the neochoristodere Champsosaurus was widely distributed across western North America. Due to the extreme climatic warmth in the Arctic, choristoderans were able to colonise it too during the Late Cretaceous. In the seas, rays , modern sharks and teleosts became common. Marine reptiles included ichthyosaurs in

11700-1287: The formation are freshwater animals, and some are exclusively freshwater forms (for instance, frogs and salamanders). However, marine fossils are also found in the formation, suggesting that the sea was nearby. The bird fauna is mainly composed of orders still existing today. Apatornis A. retusus Reclassified as Palintropus retusus Ceramornis C. major A possible charadriiform bird Cimolopteryx C. petra Reclassified as Lamarqueavis minima C. rara A charadriiform C. retusa Reclassified as Palintropus retusus C. minima Reclassified as Lamarqueavis minima "Cimolopteryx" " C." maxima A charadriiform bird, not necessarily closely related to Cimolopteryx . Graculavus G. augustus A possible charadriiform Lamarqueavis L. minima A charadriiform L. petra A charadriiform Lonchodytes L. estesi A possible procellariiform "Lonchodytes" "L." pterygius A possible charadriiform " Palaeotringa " "P." vetus A bird similar to gruids , idiornithids and presbyornithids . Palintropus P. retusus A basal ornithuromorph belonging to Ambiortiformes . Potamornis P. skutchi A hesperornithiform possibly also present in

11830-718: The fossil record. Arthropods remained abundant, and some groups, such as eurypterids , became apex predators in the ocean. Fully terrestrial life established itself on land, including early fungi , arachnids , hexapods and myriapods . The evolution of vascular plants (mainly spore -producing ferns such as Cooksonia ) allowed land plants to gain a foothold further inland as well. During this time, there were four continents: Gondwana (Africa, South America, Australia, Antarctica, India), Laurentia (North America with parts of Europe), Baltica (the rest of Europe), and Siberia (Northern Asia). The Devonian spans from 419 million to 359 million years ago. Also informally known as

11960-503: The genus Berriasella , but its use as a stratigraphic indicator has been questioned, as its first appearance does not correlate with that of C. alpina . The boundary is officially considered by the International Commission on Stratigraphy to be approximately 145 million years ago, but other estimates have been proposed based on U-Pb geochronology, ranging as young as 140 million years ago. The upper boundary of

12090-609: The glaciation of Gondwana as much of it was situated around the South Pole . This event was known as the Permo-Carboniferous Glaciation and resulted in a major loss of coal forests, known as the Carboniferous rainforest collapse . The Permian spans from 299 million to 251 million years ago and was the last period of the Paleozoic era. At its beginning, all landmasses came together to form

12220-625: The group Maniraptora , which includes modern birds and their closest non-avian relatives, such as dromaeosaurs , oviraptorosaurs , therizinosaurs , troodontids along with other avialans . Fossils of these dinosaurs from the Liaoning lagerstätte are notable for the presence of hair-like feathers . Insects diversified during the Cretaceous, and the oldest known ants , termites and some lepidopterans , akin to butterflies and moths , appeared. Aphids , grasshoppers and gall wasps appeared. Rhynchocephalians (which today only includes

12350-412: The high sea level, there was extensive space for such sedimentation . Because of the relatively young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide. Chalk is a rock type characteristic for (but not restricted to) the Cretaceous. It consists of coccoliths , microscopically small calcite skeletons of coccolithophores , a type of algae that prospered in

12480-497: The highest rates of extinction and turnover. Thylacocephala , a class of crustaceans, went extinct in the Late Cretaceous. The first radiation of the diatoms (generally siliceous shelled, rather than calcareous ) in the oceans occurred during the Cretaceous; freshwater diatoms did not appear until the Miocene . Calcareous nannoplankton were important components of the marine microbiota and important as biostratigraphic markers and recorders of environmental change. The Cretaceous

12610-658: The last epoch of the Jurassic, the Tithonian, continued into the Berriasian, the first age of the Cretaceous. The North Atlantic seaway opened and enabled the flow of cool water from the Boreal Ocean into the Tethys. There is evidence that snowfalls were common in the higher latitudes during this age, and the tropics became wetter than during the Triassic and Jurassic. Glaciation was restricted to high- latitude mountains, though seasonal snow may have existed farther from

12740-473: The last of which includes fish , amphibians and the fully terrestrial amniotes ( synapsids and sauropsids ). The Mesozoic features the evolution of crocodilians , turtles , dinosaurs (including birds ), lepidosaurs ( lizards and snakes ) and mammals . The Cenozoic begins with the extinction of all non-avian dinosaurs , pterosaurs and marine reptiles , and features the great diversification in birds and mammals. Humans appeared and evolved during

12870-512: The late Valanginian (~ 134 million years ago) found in Israel and Italy, initially at low abundance. Molecular clock estimates conflict with fossil estimates, suggesting the diversification of crown-group angiosperms during the Late Triassic or the Jurassic, but such estimates are difficult to reconcile with the heavily sampled pollen record and the distinctive tricolpate to tricolporoidate (triple grooved) pollen of eudicot angiosperms. Among

13000-465: The late Cretaceous, and all else that depended on them suffered, as well. Herbivorous animals, which depended on plants and plankton as their food, died out as their food sources became scarce; consequently, the top predators , such as Tyrannosaurus rex , also perished. Yet only three major groups of tetrapods disappeared completely; the nonavian dinosaurs , the plesiosaurs and the pterosaurs . The other Cretaceous groups that did not survive into

13130-574: The latest Albian. Approximately 94 Ma, the Cenomanian-Turonian Thermal Maximum occurred, with this hyperthermal being the most extreme hothouse interval of the Cretaceous and being associated with a sea level highstand. Temperatures cooled down slightly over the next few million years, but then another thermal maximum, the Coniacian Thermal Maximum, happened, with this thermal event being dated to around 87 Ma. Atmospheric CO 2 levels may have varied by thousands of ppm throughout

13260-536: The latter of which include two clades — the actinopterygians ( ray-finned fish ) and sarcopterygians ( lobe-finned fish ). One lineage of sarcopterygians, Rhipidistia , evolved the first four-limbed vertebrates , which would eventually become tetrapods . On land, plant groups diversified after the Silurian-Devonian Terrestrial Revolution ; the first woody ferns and the earliest seed plants evolved during this period. By

13390-711: The l’Arboudeyesse Thermal Event (ATE) a million years later. Following these two hyperthermals was the Amadeus Thermal Maximum around 106 Ma, during the middle Albian. Then, around a million years after that, occurred the Petite Verol Thermal Event (PVTE). Afterwards, around 102.5 Ma, the Event 6 Thermal Event (EV6) took place; this event was itself followed by the Breistroffer Thermal Maximum around 101 Ma, during

13520-517: The middle Cretaceous, becoming the dominant group of land plants by the end of the period, coincident with the decline of previously dominant groups such as conifers. The oldest known fossils of grasses are from the Albian , with the family having diversified into modern groups by the end of the Cretaceous. The oldest large angiosperm trees are known from the Turonian (c. 90 Mya) of New Jersey, with

13650-470: The most promising candidates for fixing the Jurassic–Cretaceous boundary. In particular, the first appearance Calpionella alpina , coinciding with the base of the eponymous Alpina subzone, has been proposed as the definition of the base of the Cretaceous. The working definition for the boundary has often been placed as the first appearance of the ammonite Strambergella jacobi , formerly placed in

13780-405: The most recent part of the Cenozoic. The Paleozoic is a time in Earth's history when active complex life forms evolved, took their first foothold on dry land, and when the forerunners of all multicellular life on Earth began to diversify. There are six periods in the Paleozoic era: Cambrian , Ordovician , Silurian , Devonian , Carboniferous and Permian . The Cambrian is the first period of

13910-824: The oceans. Extinction was more severe among animals living in the water column than among animals living on or in the seafloor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton, while animals living on or in the ocean floor feed on detritus or can switch to detritus feeding. The largest air-breathing survivors of the event, crocodilians and champsosaurs , were semiaquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food and go into hibernation when conditions are unfavorable, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms or fragments of organisms for their first few years. These characteristics have been linked to crocodilian survival at

14040-601: The oldest records of Angiosperm macrofossils are Montsechia from the Barremian aged Las Hoyas beds of Spain and Archaefructus from the Barremian-Aptian boundary Yixian Formation in China. Tricolpate pollen distinctive of eudicots first appears in the Late Barremian, while the earliest remains of monocots are known from the Aptian. Flowering plants underwent a rapid radiation beginning during

14170-514: The phosphorus cycle, wherein increased phosphorus input into the ocean would increase surficial biological productivity that would in turn enhance iron redox cycling and thus remove phosphorus from seawater; this maintained a relatively stable rate of removal of carbon from the atmosphere and ocean via organic carbon burial. The climate also controlled the availability of phosphate through its regulation of rates of continental and seafloor weathering. Major global temperature variations of >7 °C during

14300-523: The poles. After the end of the first age, however, temperatures began to increase again, with a number of thermal excursions, such as the middle Valanginian Weissert Thermal Excursion (WTX), which was caused by the Paraná-Etendeka Large Igneous Province's activity. It was followed by the middle Hauterivian Faraoni Thermal Excursion (FTX) and the early Barremian Hauptblatterton Thermal Event (HTE). The HTE marked

14430-412: The principal food of mosasaurs , a group of giant marine lizards related to snakes that became extinct at the boundary. Omnivores , insectivores , and carrion -eaters survived the extinction event, perhaps because of the increased availability of their food sources. At the end of the Cretaceous, there seem to have been no purely herbivorous or carnivorous mammals . Mammals and birds that survived

14560-501: The punctuation mark at the end of a progressive decline in biodiversity during the Maastrichtian age. The result was the extinction of three-quarters of Earth's plant and animal species. The impact created the sharp break known as the K–Pg boundary (formerly known as the K–T boundary). Earth's biodiversity required substantial time to recover from this event, despite the probable existence of an abundance of vacant ecological niches . Despite

14690-744: The rise of angiosperms, during the Jurassic and the Early Cretaceous, the higher flora was dominated by gymnosperm groups, including cycads , conifers , ginkgophytes , gnetophytes and close relatives, as well as the extinct Bennettitales . Other groups of plants included pteridosperms or "seed ferns", a collective term that refers to disparate groups of extinct seed plants with fern-like foliage, including groups such as Corystospermaceae and Caytoniales . The exact origins of angiosperms are uncertain, although molecular evidence suggests that they are not closely related to any living group of gymnosperms. The earliest widely accepted evidence of flowering plants are monosulcate (single-grooved) pollen grains from

14820-400: The severity of the K-Pg extinction event, there were significant variations in the rate of extinction between and within different clades . Species that depended on photosynthesis declined or became extinct as atmospheric particles blocked solar energy . As is the case today, photosynthesizing organisms, such as phytoplankton and land plants , formed the primary part of the food chain in

14950-422: The strong regionality of most biostratigraphic markers, and the lack of any chemostratigraphic events, such as isotope excursions (large sudden changes in ratios of isotopes ) that could be used to define or correlate a boundary. Calpionellids , an enigmatic group of planktonic protists with urn-shaped calcitic tests briefly abundant during the latest Jurassic to earliest Cretaceous, have been suggested as

15080-424: The supercontinent Pangaea , surrounded by one expansive ocean called Panthalassa . The Earth was relatively dry compared to the Carboniferous, with harsh seasons , as the climate of the interior of Pangaea was not moderated by large bodies of water. Amniotes still flourished and diversified in the new dry climate, particularly synapsids such as Dimetrodon , Edaphosaurus and therapsids , which gave rise to

15210-507: The systematic study of those forms started in the 1950s. The transition from the largely sessile Precambrian biota to the active mobile Cambrian biota occurred early in the Phanerozoic. The Phanerozoic is divided into three eras : the Paleozoic , Mesozoic and Cenozoic , which are further subdivided into 12 periods . The Paleozoic features the evolution of the three most prominent animal phyla, arthropods , molluscs and chordates ,

15340-573: The terrestrial fauna of the late Cretaceous Hell Creek Formation . Other important Cretaceous exposures occur in Europe (e.g., the Weald ) and China (the Yixian Formation ). In the area that is now India, massive lava beds called the Deccan Traps were erupted in the very late Cretaceous and early Paleocene. Palynological evidence indicates the Cretaceous climate had three broad phases:

15470-579: The thinner equivalent formations such as the Hell Creek Formation, the base of which has been estimated at 66.8 million years old. The formation is described by W.G. Pierce as thick-bedded, buff-colored sandstone, and drab to green shale. It is Upper Cretaceous in age. The formation varies in thickness from about 90 m (300 ft.) in North Dakota , to almost 600 m (2,000 ft.) in parts of Wyoming. The Lance Formation

15600-483: The trunk having a preserved diameter of 1.8 metres (5.9 ft) and an estimated height of 50 metres (160 ft). During the Cretaceous, ferns in the order Polypodiales , which make up 80% of living fern species, would also begin to diversify. On land, mammals were generally small sized, but a very relevant component of the fauna , with cimolodont multituberculates outnumbering dinosaurs in some sites. Neither true marsupials nor placentals existed until

15730-789: The ultimate end of the Tithonian-early Barremian Cool Interval (TEBCI). During this interval, precession was the dominant orbital driver of environmental changes in the Vocontian Basin. For much of the TEBCI, northern Gondwana experienced a monsoonal climate. A shallow thermocline existed in the mid-latitude Tethys. The TEBCI was followed by the Barremian-Aptian Warm Interval (BAWI). This hot climatic interval coincides with Manihiki and Ontong Java Plateau volcanism and with

15860-643: The upper Cretaceous of Western Europe . The name Cretaceous was derived from the Latin creta , meaning chalk . The twofold division of the Cretaceous was implemented by Conybeare and Phillips in 1822. Alcide d'Orbigny in 1840 divided the French Cretaceous into five étages (stages): the Neocomian , Aptian, Albian, Turonian, and Senonian, later adding the Urgonian between Neocomian and Aptian and

15990-628: The very end, but a variety of non-marsupial metatherians and non-placental eutherians had already begun to diversify greatly, ranging as carnivores ( Deltatheroida ), aquatic foragers ( Stagodontidae ) and herbivores ( Schowalteria , Zhelestidae ). Various "archaic" groups like eutriconodonts were common in the Early Cretaceous, but by the Late Cretaceous northern mammalian faunas were dominated by multituberculates and therians , with dryolestoids dominating South America . The apex predators were archosaurian reptiles , especially dinosaurs , which were at their most diverse stage. Avians such as

16120-407: The world, alternative local subdivisions are still in use. From youngest to oldest, the subdivisions of the Cretaceous period are: The lower boundary of the Cretaceous is currently undefined, and the Jurassic–Cretaceous boundary is currently the only system boundary to lack a defined Global Boundary Stratotype Section and Point (GSSP). Placing a GSSP for this boundary has been difficult because of

16250-654: The world, as did grazers. The Tethys Sea finally closed with the creation of the Arabian Peninsula and in its wake left the Black , Red , Mediterranean and Caspian seas. This only increased aridity. Many new plants evolved, and 95% of modern seed plants evolved in the mid-Miocene. The Pliocene lasted from 5.333 million to 2.58 million years ago. The Pliocene featured dramatic climatic changes, which ultimately led to modern species and plants. The Mediterranean Sea dried up for hundreds of thousand years in

16380-493: Was also an important interval in the evolution of bioerosion , the production of borings and scrapings in rocks, hardgrounds and shells. Phanerozoic The Phanerozoic is the current and the latest of the four geologic eons in the Earth 's geologic time scale , covering the time period from 538.8 million years ago to the present. It is the eon during which abundant animal and plant life has proliferated , diversified and colonized various niches on

16510-654: Was followed by the Aptian-Albian Cold Snap (AACS) that began about 118 Ma. A short, relatively minor ice age may have occurred during this so-called "cold snap", as evidenced by glacial dropstones in the western parts of the Tethys Ocean and the expansion of calcareous nannofossils that dwelt in cold water into lower latitudes. The AACS is associated with an arid period in the Iberian Peninsula . Temperatures increased drastically after

16640-544: Was laid down by streams, on a coastal plain along the edge of the Western Interior Seaway . The climate was subtropical; there was no cold season and probably ample precipitation . At least tens of thousands of Late Cretaceous vertebrate remains have been recovered from the Lance Formation. Fossils ranging from microscopic elements to extensive bonebeds , with nearly complete, sometimes articulated dinosaur skeletons, have been found. Most other animals known from

16770-454: Was largely ice-free, although there is some evidence of brief periods of glaciation during the cooler first half, and forests extended to the poles. Many of the dominant taxonomic groups present in modern times can be ultimately traced back to origins in the Cretaceous. During this time, new groups of mammals and birds appeared, including the earliest relatives of placentals & marsupials ( Eutheria and Metatheria respectively), and

16900-635: Was warm and humid, with little temperature gradient from pole to pole. In the Middle Eocene Epoch, the Antarctic Circumpolar Current formed when South America and Australia both separated from Antarctica to open up the Drake Passage and Tasmanian Passage , disrupting ocean currents worldwide, resulting in global cooling and causing the jungles to shrink. More modern forms of mammals continued to diversify with

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