The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, the longest period of the Phanerozoic Eon; the Cretaceous Period is abbreviated K, for its German translation Kreide. The Cretaceous was a period with a 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 and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared; the Cretaceous ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary, a geologic signature associated with the mass extinction which 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, using strata in the Paris Basin and named for the extensive beds of chalk, found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from Latin creta; 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 and Senonian. A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use; as with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact age of the system's base is uncertain by a few million years. No great extinction or burst of diversity separates the Cretaceous from the Jurassic. However, the top of the system is defined, being placed at an iridium-rich layer found worldwide, believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and into the Gulf of Mexico.
This layer has been dated at 66.043 Ma. A 140 Ma age for the Jurassic-Cretaceous boundary instead of the accepted 145 Ma was proposed in 2014 based on a stratigraphic study of Vaca Muerta Formation in Neuquén Basin, Argentina. Víctor Ramos, one of the authors of the study proposing the 140 Ma boundary age sees the study as a "first step" toward formally changing the age in the International Union of Geological Sciences. From youngest to oldest, the subdivisions of the Cretaceous period are: Late Cretaceous Maastrichtian – Campanian – Santonian – Coniacian – Turonian – Cenomanian – Early Cretaceous Albian – Aptian – Barremian – Hauterivian – Valanginian – Berriasian – 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, formed under warm, shallow marine circumstances.
Due to the high sea level, there was extensive space for such sedimentation. Because of the young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide. Chalk is a rock type characteristic for the Cretaceous, it consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas. 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 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 consolidated and the Chalk Group still consists of loose sediments in many places; the group has other limestones and arenites. Among the fossils it contains are sea urchins, belemnites and sea reptiles such as Mosasaurus. In southern Europe, the Cretaceous is 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. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half the worlds 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; these shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. During th
Afrotheria is a clade of mammals, the living members of which belong to groups that are either living in Africa or of African origin: golden moles, elephant shrews, aardvarks, elephants, sea cows, several extinct clades. Most groups of afrotheres share little or no superficial resemblance, their similarities have only become known in recent times because of genetics and molecular studies. Many afrothere groups are found or in Africa, reflecting the fact that Africa was an island continent from the early Cenozoic until around 25 million years ago when the Tethys Sea shrank; because Africa was isolated by water, Laurasian groups of mammals such as insectivores, rabbits and large herbivores could not reach Africa for much of the early to mid Cenozoic. Instead, the niches occupied by those groups on the northern continents were filled by various groups of afrotheres via the process of convergent evolution; the small insectivorous afrotheres such as elephant shrews, golden moles, tenrecs filled the niches of insectivores, the hyraxes filled the roles of rodents and rabbits, the aardvarks filled the roles of various medium size ant-eating mammals found on other continents throughout the Cenozoic, proboscideans filled the roles of large herbivores such as hippos and rhinos.
The sirenians became aquatic and started spreading to other parts of the world by water, evolving convergently with the other groups of marine mammals. In addition to their similarity with Laurasian mammals in North America and Asia, many afrotheres exhibit convergent evolution with groups of mammals that evolved and lived in South America, an island continent for much of the Cenozoic; the common ancestry of these animals was not recognized until the late 1990s. The Paenungulata had been linked to other ungulates. Continuing work on the molecular and morphological diversity of afrotherian mammals has provided increasing support for their common ancestry; the afrotherian clade was proposed in 1998 based on analyses of DNA sequence data. However, previous studies had hinted at the close interrelationships among subsets of endemic African mammals; the core of the Afrotheria consists of the Paenungulata, i.e. elephants, sea cows, hyraxes, a group with a long history among comparative anatomists. Hence, while DNA sequence data have proven essential to infer the existence of the Afrotheria as a whole, while the Afroinsectiphilia were not recognized as part of Afrotheria without DNA data, some precedent is found in the comparative anatomical literature for the idea that at least part of this group forms a clade.
The Paleocene genus Ocepeia, the most completely-known Paleocene African mammal and the oldest afrotherian known from a complete skull, shares similarities with both Paenungulata and Afroinsectiphilia, may help to characterize the ancestral body type of afrotherians. Since the 1990s, increasing molecular and anatomical data have been applied to the classification of animals. Both types of data support the idea that afrotherian mammals are descended from a single common ancestor to the exclusion of other mammals. On the anatomical side, features shared by most, if not all, afrotheres include high vertebral counts, aspects of placental membrane formation, the shape of the ankle bones, the late eruption of the permanent dentition; the snout is unusually mobile in several Afrotherian species. Studies of genomic data, including millions of aligned nucleotides sampled for a growing number of placental mammals support Afrotheria as a clade. Additionally, there might be some dental synapomorphies uniting afroinsectiphilians, if not afrotheres as a whole: p4 talonid and trigonid of similar breadth, a prominent p4 hypoconid, presence of a P4 metacone and absence of parastyles on M1–2.
Afrotheria is now recognized as one of the three major groups within the Eutheria. Relations within the three cohorts, Xenarthra and the identity of the placental root, remain somewhat controversial. Afrotheria as a clade has been discussed without a Linnaean rank, but has been assigned the rank of cohort and superorder. One reconstruction, which applies the molecular clock, proposes that the oldest split occurred between Afrotheria and the other three some 105 million years ago in the mid-Cretaceous, when the African continent was separated from other major land masses; this idea is consistent with the fossil record of Xenarthra, restricted to South America. However, Afrotheria itself does not have a fossil record restricted to Africa, appears in fact to have evolved in the continent's isolation. More recent, genomic-scale phylogenies favor the hypothesis that Afrotheria and Xenarthra comprise sister taxa at the base of the placental mammal radiation, suggesting an ancient Gondwanan clade of placental mammalsRelations between the various afrotherian orders are still being studied.
On the basis of molecular studies and manatees appear to be related, elephant shrews and aardvarks. These findings are compatible with the work of earlier anatomists. Many extant members of Afrotheria appe
The Miocene is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago. The Miocene was named by Charles Lyell; the Miocene is followed by the Pliocene. As the earth went from the Oligocene through the Miocene and into the Pliocene, the climate cooled towards a series of ice ages; the Miocene boundaries are not marked by a single distinct global event but consist rather of regionally defined boundaries between the warmer Oligocene and the cooler Pliocene Epoch. The Apes first evolved and diversified during the early Miocene, becoming widespread in the Old World. By the end of this epoch and the start of the following one, the ancestors of humans had split away from the ancestors of the chimpanzees to follow their own evolutionary path during the final Messinian stage of the Miocene; as in the Oligocene before it, grasslands continued to forests to dwindle in extent. In the seas of the Miocene, kelp forests made their first appearance and soon became one of Earth's most productive ecosystems.
The plants and animals of the Miocene were recognizably modern. Mammals and birds were well-established. Whales and kelp spread; the Miocene is of particular interest to geologists and palaeoclimatologists as major phases of the geology of the Himalaya occurred during the Miocene, affecting monsoonal patterns in Asia, which were interlinked with glacial periods in the northern hemisphere. The Miocene faunal stages from youngest to oldest are named according to the International Commission on Stratigraphy: Regionally, other systems are used, based on characteristic land mammals. Of the modern geologic features, only the land bridge between South America and North America was absent, although South America was approaching the western subduction zone in the Pacific Ocean, causing both the rise of the Andes and a southward extension of the Meso-American peninsula. Mountain building took place in western North America and East Asia. Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines.
Well studied continental exposures occur in Argentina. India continued creating dramatic new mountain ranges; the Tethys Seaway continued to shrink and disappeared as Africa collided with Eurasia in the Turkish–Arabian region between 19 and 12 Ma. The subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea near the end of the Miocene; the global trend was towards increasing aridity caused by global cooling reducing the ability of the atmosphere to absorb moisture. Uplift of East Africa in the late Miocene was responsible for the shrinking of tropical rain forests in that region, Australia got drier as it entered a zone of low rainfall in the Late Miocene. During the Oligocene and Early Miocene the coast of northern Brazil, south-central Peru, central Chile and large swathes of inland Patagonia were subject to a marine transgression; the transgressions in the west coast of South America is thought to be caused by a regional phenomenon while the rising central segment of the Andes represents an exception.
While there are numerous registers of Oligo-Miocene transgressions around the world it is doubtful that these correlate. It is thought that the Oligo-Miocene transgression in Patagonia could have temporarily linked the Pacific and Atlantic Oceans, as inferred from the findings of marine invertebrate fossils of both Atlantic and Pacific affinity in La Cascada Formation. Connection would have occurred through narrow epicontinental seaways that formed channels in a dissected topography; the Antarctic Plate started to subduct beneath South America 14 million years ago in the Miocene, forming the Chile Triple Junction. At first the Antarctic Plate subducted only in the southernmost tip of Patagonia, meaning that the Chile Triple Junction lay near the Strait of Magellan; as the southern part of Nazca Plate and the Chile Rise became consumed by subduction the more northerly regions of the Antarctic Plate begun to subduct beneath Patagonia so that the Chile Triple Junction advanced to the north over time.
The asthenospheric window associated to the triple junction disturbed previous patterns of mantle convection beneath Patagonia inducing an uplift of ca. 1 km that reversed the Oligocene–Miocene transgression. Climates remained moderately warm, although the slow global cooling that led to the Pleistocene glaciations continued. Although a long-term cooling trend was well underway, there is evidence of a warm period during the Miocene when the global climate rivalled that of the Oligocene; the Miocene warming b
Taeniodonta is an extinct early group of cimolestid mammals known from the Palaeocene to the Eocene. Taeniodonts evolved into specialized digging animals, varied in size, from rat-sized to species as large as a bear. Species developed prominent front teeth and huge claws for digging and rooting; some genera, like Stylinodon, had ever-growing teeth. Two families belong to this group and Conoryctidae, they were endemic to North America. The scarcity of taeniodont fossils can be explained by the fact that these animals lived in dry or arid climates unconductive to fossilization. Taeniodonts are unambiguously Eutherians, part of Cimolesta. From Thomas E. Williamson and Stephen L. Brusatte: Order CimolestaSuborder TaeniodontaAlveugena Schowalteria Onychodectes Family ConoryctidaeConoryctella Huerfanodon ConoryctesFamily Stylinodontidae Chungchienia Wortmania Psittacotherium Ectoganus Stylinodon
Placentalia is one of the three extant subdivisions of the class of animals Mammalia. The Placentals are distinguishable from other mammals in that the fetus is carried in the uterus of its mother to a late stage of development, it is somewhat of a misnomer since marsupials nourish their fetuses via a placenta. Placental mammals are anatomically distinguished from other mammals by: a sufficiently wide opening at the bottom of the pelvis to allow the birth of a large baby relative to the size of the mother; the absence of epipubic bones extending forward from the pelvis, which are found in all other mammals. The rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula, forming a complete mortise and tenon upper ankle joint; the presence of a malleolus at the bottom of the fibula. Analysis of retroposon presence/absence patterns has provided a rapid, unequivocal means for revealing the evolutionary history of organisms: this has resulted in a revision in the classification of placentals.
There are now thought to be three major subdivisions or lineages of placental mammals: Boreoeutheria and Afrotheria, all of which diverged from common ancestors. The orders of placental mammals in the three groups are: Magnorder Afrotheria Superorder Afroinsectiphilia Order Afrosoricida Order Macroscelidea Order Tubulidentata Superorder Paenungulata Order Hyracoidea Mirorder Tethytheria Order Proboscidea Order Sirenia Magnorder Boreoeutheria Superorder Euarchontoglires Grandorder Gliriformes Mirorder Glires Order Lagomorpha Order Rodentia Grandorder Euarchonta Order Scandentia Mirorder Primatomorpha Order Dermoptera Order Primates Superorder Laurasiatheria Order Eulipotyphla Order Chiroptera Order Cetartiodactyla Order Perissodactyla Mirorder Ferae Order Pholidota Order Carnivora Magnorder Xenarthra Order Cingulata Order Pilosa The exact relationships among these three lineages is a subject of debate, three different hypotheses have been proposed with respect to which group is basal or diverged first from other placentals.
These hypotheses are Atlantogenata and Exafroplacentalia. Estimates for the divergence times among these three placental groups range from 105 to 120 million years ago, depending on the type of DNA and varying interpretations of paleogeographic data. Cladogram based on Amrine-Madsen, H. et al. and Asher, R. J. et al. True placental mammals arose from stem-group members of the clade Eutheria, which had existed since at least the Middle Jurassic period, about 170 MYA); these early eutherians were nocturnal insect eaters, with adaptations for life in trees. True placentals may have originated in the Late Cretaceous around 90 MYA, but the earliest undisputed fossils are from the early Paleocene, 66 MYA, following the Cretaceous–Paleogene extinction event; the species Protungulatum donnae was thought to be a stem-ungulate known 1 meter above the Cretaceous-Paleogene boundary in the geological stratum that marks the Cretaceous–Paleogene extinction event and Purgatorius considered a stem-primate, appears no more than 300,000 years after the K-Pg boundary.
The rapid appearance of placentals after the mass extinction at the end of the Cretaceous suggests that the group had originated and undergone an initial diversification in the Late Cretaceous, as suggested by molecular clocks. The lineages leading to Xenarthra and Afrotheria originated around 90 MYA, Boreoeutheria underwent an initial diversification around 70-80 MYA, producing the lineages that would lead to modern primates, insectivores and carnivorans. However, modern members of the placental orders originated in the Paleogene around 66 to 23 MYA, following the Cretaceous–Paleogene extinction event; the evolution of crown orders such modern primates and carnivores appears to be part of an adaptive radiation that took place as mammals evolved to take advantage of ecological niches that were left open when most dinosaurs and other animals disappeared following the Chicxulub asteroid impact. As they occupied new niches, mammals increased in body size, began to take over the large herbivore and large carnivore niches, left open by the decimation of the dinosaurs.
Mammals exploited niches that the dinosaurs had never touched: for example, bats evolved flight and echolocation, allowing them to be effective nocturnal, aerial insectivores.
The opossum is a marsupial of the order Didelphimorphia endemic to the Americas. The largest order of marsupials in the Western Hemisphere, it comprises 103 or more species in 19 genera. Opossums originated in South America and entered North America in the Great American Interchange following the connection of the two continents, their unspecialized biology, flexible diet, reproductive habits make them successful colonizers and survivors in diverse locations and conditions. Although the animal is called a possum in North America, which would refer to the Virginia opossum species, it should not be confused with the suborder Phalangeriformes, which are arboreal marsupials in the Eastern Hemisphere called "possums" because of their resemblance to Didelphimorphia; the word "opossum" is borrowed from the Powhatan language and was first recorded between 1607 and 1611 by John Smith and William Strachey. Both men encountered the language at the British settlement of Jamestown, which Smith helped to found and where Strachey served as its first secretary.
Strachey's notes describe the opossum as a "beast in bigness of a pig and in taste alike," while Smith recorded it "hath an head like a swine... tail like a rat... of the bigness of a cat." The Powhatan word derives from a Proto-Algonquian word meaning "white dog or dog-like beast."Following the arrival of Europeans in Australia, the term "possum" was borrowed to describe distantly related Australian marsupials of the suborder Phalangeriformes, which are more related to other Australian marsupials such as kangaroos. "Didelphimorphia" refers to the fact. Didelphimorphs are small to medium-sized marsupials, they tend to be semi-arboreal omnivores. Most members of this taxon have long snouts, a narrow braincase, a prominent sagittal crest; the dental formula is: 22.214.171.124.1.3.4 × 2 = 50 teeth. By mammalian standards, this is an unusually full jaw; the incisors are small, the canines large, the molars are tricuspid. Didelphimorphs have a plantigrade stance and the hind feet have an opposable digit with no claw.
Like some New World monkeys, opossums have prehensile tails. Like that of all marsupials, the fur consists of awn hair only, the females have a pouch; the tail and parts of the feet bear scutes. The stomach is simple, with a small cecum. Like most marsupials, the male opossum has a forked penis bearing twin glandes. Although all living opossums are opportunistic omnivores, different species vary in the amount of meat and vegetation they include in their diet. Members of the Caluromyinae are frugivorous; the yapok is unusual, as it is the only living semi-aquatic marsupial, using its webbed hindlimbs to dive in search of freshwater mollusks and crayfish. The extinct Thylophorops, the largest known opossum at 4–7 kg, was a macropredator. Most opossums are scansorial, well-adapted to life in the trees or on the ground, but members of the Caluromyinae and Glironiinae are arboreal, whereas species of Metachirus, to a lesser degree Didelphis show adaptations for life on the ground; the Metachirus nudicaudatus, found in the upper Amazon basin, consumes fruit seeds, small vertebrate creatures like birds and reptiles and invertebrates like crayfish and snails, but seems to be most insectivorous.
As a marsupial, the female opossum has a reproductive system that includes a bifurcated vagina, a divided uterus and a marsupium, her pouch. The average estrous cycle of the opossum is about 28 days. Opossums do possess a placenta, but it is short-lived, simple in structure, unlike that of placental mammals, not functional; the young are therefore born at a early stage, although the gestation period is similar to that of many other small marsupials, at only 12 to 14 days. Once born, the offspring must find their way into the marsupium to hold on to and nurse from a teat. Baby opossums, like their Australian cousins, are called joeys. Female opossums give birth to large numbers of young, most of which fail to attach to a teat, although as many as thirteen young can attach, therefore survive, depending on species; the young are weaned between 125 days, when they detach from the teat and leave the pouch. The opossum lifespan is unusually short for a mammal of its size only one to two years in the wild and as long as four or more years in captivity.
Senescence is rapid. The species are moderately sexually dimorphic with males being larger, much heavier, having larger canines than females; the largest difference between the opossum and non-marsupial mammals is the bifurcated penis of the male and bifurcated vagina of the female. Opossum spermatozoa exhibit sperm-pairing; this may ensure that flagella movement can be coordinated for maximal motility. Conjugate pairs dissociate into separate spermatozoa before fertilization. Opossums are solitary and nomadic, staying in one area as long as food and water are available; some families will group together in ready-made burrows or under houses. Though they will temporarily occupy abandoned burrows, they do not dig or put much effort into building their own; as nocturnal animals, they favor secure areas. These areas may be below ground or above; when threatened or harmed, they will "play possum", mimicking the app
A chordate is an animal constituting the phylum Chordata. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, a post-anal tail: these five anatomical features define this phylum. Chordates are bilaterally symmetric; the Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata. There are extinct taxa such as the Vetulicolia. Hemichordata has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Osteichthyes. Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Cladistically, vertebrates - chordates with the notochord replaced by a vertebral column during development - are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull.
The Craniata and Tunicata compose the clade Olfactores. Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: A notochord, a stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, in wholly aquatic species this helps the animal to swim by flexing its tail. A dorsal neural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system. Pharyngeal slits; the pharynx is the part of the throat behind the mouth. In fish, the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live. Post-anal tail. A muscular tail that extends backwards behind the anus. An endostyle; this is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus.
It stores iodine, may be a precursor of the vertebrate thyroid gland. There are soft constraints that separate chordates from certain other biological lineages, but are not part of the formal definition: All chordates are deuterostomes; this means. All chordates are based on a bilateral body plan. All chordates are coelomates, have a fluid filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm; the following schema is from the third edition of Vertebrate Palaeontology. The invertebrate chordate classes are from Fishes of the World. While it is structured so as to reflect evolutionary relationships, it retains the traditional ranks used in Linnaean taxonomy. Phylum Chordata †Vetulicolia? Subphylum Cephalochordata – Class Leptocardii Clade Olfactores Subphylum Tunicata – Class Ascidiacea Class Thaliacea Class Appendicularia Class Sorberacea Subphylum Vertebrata Infraphylum incertae sedis Cyclostomata Superclass'Agnatha' paraphyletic Class Myxini Class Petromyzontida or Hyperoartia Class †Conodonta Class †Myllokunmingiida Class †Pteraspidomorphi Class †Thelodonti Class †Anaspida Class †Cephalaspidomorphi Infraphylum Gnathostomata Class †Placodermi Class Chondrichthyes Class †Acanthodii Superclass Osteichthyes Class Actinopterygii Class Sarcopterygii Superclass Tetrapoda Class Amphibia Class Sauropsida Class Synapsida Craniates, one of the three subdivisions of chordates, all have distinct skulls.
They include the hagfish. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or all deuterostomes, are by their tails". Most craniates are vertebrates; these consist of a series of bony or cartilaginous cylindrical vertebrae with neural arches that protect the spinal cord, with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, are therefore not regarded as vertebrates, but as members of the craniates, the group from which vertebrates are thought to have evolved; however the cladistic exclusion of hagfish from the vertebrates is controversial, as they ma