Misplaced Pages

#211788

128-745: In 1875, G. K. Gilbert recognized that strata exposed near the bottom of parts of the Grand Canyon are similar to those known elsewhere in Arizona as the Tonto Group . Based upon this similarity, he assigned the strata that form the lowermost part of the Phanerozoic strata in the Grand Canyon to the Tonto Group . Without designating formal type localities , he also subdivided the Tonto Group into three subdivisions, from youngest to oldest,

256-653: 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

384-423: A mutation–selection balance . It is predicted that a viral quasispecies at a low but evolutionarily neutral and highly connected (that is, flat) region in the fitness landscape will outcompete a quasispecies located at a higher but narrower fitness peak in which the surrounding mutants are unfit, "the quasispecies effect" or the "survival of the flattest". There is no suggestion that a viral quasispecies resembles

512-475: 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

640-400: A "classical" method of determining species, such as with Linnaeus, early in evolutionary theory. However, different phenotypes are not necessarily different species (e.g. a four-winged Drosophila born to a two-winged mother is not a different species). Species named in this manner are called morphospecies . In the 1970s, Robert R. Sokal , Theodore J. Crovello and Peter Sneath proposed

768-424: A 'smallest clade' idea" (a phylogenetic species concept). Mishler and Wilkins and others concur with this approach, even though this would raise difficulties in biological nomenclature. Wilkins cited the ichthyologist Charles Tate Regan 's early 20th century remark that "a species is whatever a suitably qualified biologist chooses to call a species". Wilkins noted that the philosopher Philip Kitcher called this

896-541: 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

1024-428: A connected series of neighbouring populations, each of which can sexually interbreed with adjacent related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" population. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing

1152-507: 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

1280-432: A different species from its ancestors. Viruses have enormous populations, are doubtfully living since they consist of little more than a string of DNA or RNA in a protein coat, and mutate rapidly. All of these factors make conventional species concepts largely inapplicable. A viral quasispecies is a group of genotypes related by similar mutations, competing within a highly mutagenic environment, and hence governed by

1408-476: 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

SECTION 10

#1732772577212

1536-508: A genetic boundary suitable for defining a species concept is present. DNA barcoding has been proposed as a way to distinguish species suitable even for non-specialists to use. One of the barcodes is a region of mitochondrial DNA within the gene for cytochrome c oxidase . A database, Barcode of Life Data System , contains DNA barcode sequences from over 190,000 species. However, scientists such as Rob DeSalle have expressed concern that classical taxonomy and DNA barcoding, which they consider

1664-594: 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

1792-465: A lineage should be divided into multiple chronospecies , or when populations have diverged to have enough distinct character states to be described as cladistic species. Species and higher taxa were seen from the time of Aristotle until the 18th century as categories that could be arranged in a hierarchy, the great chain of being . In the 19th century, biologists grasped that species could evolve given sufficient time. Charles Darwin 's 1859 book On

1920-567: A locally scoured surface. In the Grand Canyon, the Bright Angel Shale is generally about 110 to 150 m (360 to 490 ft) thick. In the western Grand Canyon, the Bright Angel Shale is over 140 m (460 ft) thick and decrease eastward in thickness to only about 82 m (269 ft) at Toroweap in the central canyon. It is 100 m (330 ft) thick in Bright Angel Canyon. The complex interfingering of

2048-492: A misnomer, need to be reconciled, as they delimit species differently. Genetic introgression mediated by endosymbionts and other vectors can further make barcodes ineffective in the identification of species. A phylogenetic or cladistic species is "the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals (semaphoronts)". The empirical basis – observed character states – provides

2176-449: A particular species, including which genus (and higher taxa) it is placed in, is a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be corroborated or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as

2304-400: A perfect model of life, it is still a useful tool to scientists and conservationists for studying life on Earth, regardless of the theoretical difficulties. If species were fixed and clearly distinct from one another, there would be no problem, but evolutionary processes cause species to change. This obliges taxonomists to decide, for example, when enough change has occurred to declare that

2432-426: 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

2560-1029: 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

2688-400: A short way of saying that something applies to many species within a genus, but not to all. If scientists mean that something applies to all species within a genus, they use the genus name without the specific name or epithet. The names of genera and species are usually printed in italics . However, abbreviations such as "sp." should not be italicised. When a species' identity is not clear,

SECTION 20

#1732772577212

2816-404: A specialist may use "cf." before the epithet to indicate that confirmation is required. The abbreviations "nr." (near) or "aff." (affine) may be used when the identity is unclear but when the species appears to be similar to the species mentioned after. With the rise of online databases, codes have been devised to provide identifiers for species that are already defined, including: The naming of

2944-523: A species as groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups. It has been argued that this definition is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection. Mayr's use of the adjective "potentially" has been a point of debate; some interpretations exclude unusual or artificial matings that occur only in captivity, or that involve animals capable of mating but that do not normally do so in

3072-400: A species as determined by a taxonomist. A typological species is a group of organisms in which individuals conform to certain fixed properties (a type), so that even pre-literate people often recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens (such as longer or shorter tails) would differentiate the species. This method was used as

3200-491: A species. All species definitions assume that an organism acquires its genes from one or two parents very like the "daughter" organism, but that is not what happens in HGT. There is strong evidence of HGT between very dissimilar groups of prokaryotes , and at least occasionally between dissimilar groups of eukaryotes , including some crustaceans and echinoderms . The evolutionary biologist James Mallet concludes that there

3328-685: A species. Generally the term includes the unknown element of a distinct act of creation. Many authors have argued that a simple textbook definition, following Mayr's concept, works well for most multi-celled organisms , but breaks down in several situations: Species identification is made difficult by discordance between molecular and morphological investigations; these can be categorised as two types: (i) one morphology, multiple lineages (e.g. morphological convergence , cryptic species ) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity , multiple life-cycle stages). In addition, horizontal gene transfer (HGT) makes it difficult to define

3456-532: 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

3584-528: A taxonomic decision at the discretion of cognizant specialists, is not governed by the Codes of Zoological or Botanical Nomenclature, in contrast to the PhyloCode , and contrary to what is done in several other fields, in which the definitions of technical terms, like geochronological units and geopolitical entities, are explicitly delimited. The nomenclatural codes that guide the naming of species, including

3712-517: A traditional biological species. The International Committee on Taxonomy of Viruses has since 1962 developed a universal taxonomic scheme for viruses; this has stabilised viral taxonomy. Most modern textbooks make use of Ernst Mayr 's 1942 definition, known as the Biological Species Concept as a basis for further discussion on the definition of species. It is also called a reproductive or isolation concept. This defines

3840-490: 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

3968-447: A variation on the morphological species concept, a phenetic species, defined as a set of organisms with a similar phenotype to each other, but a different phenotype from other sets of organisms. It differs from the morphological species concept in including a numerical measure of distance or similarity to cluster entities based on multivariate comparisons of a reasonably large number of phenotypic traits. A mate-recognition species

Bright Angel Shale - Misplaced Pages Continue

4096-515: A variety of reasons. Viruses are a special case, driven by a balance of mutation and selection , and can be treated as quasispecies . Biologists and taxonomists have made many attempts to define species, beginning from morphology and moving towards genetics . Early taxonomists such as Linnaeus had no option but to describe what they saw: this was later formalised as the typological or morphological species concept. Ernst Mayr emphasised reproductive isolation, but this, like other species concepts,

4224-438: Is "an entity composed of organisms which maintains its identity from other such entities through time and over space, and which has its own independent evolutionary fate and historical tendencies". This differs from the biological species concept in embodying persistence over time. Wiley and Mayden stated that they see the evolutionary species concept as "identical" to Willi Hennig 's species-as-lineages concept, and asserted that

4352-400: Is a group of sexually reproducing organisms that recognise one another as potential mates. Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms; no matter whether populations can hybridise successfully, they are still distinct cohesion species if

4480-429: 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

4608-458: Is a set of organisms adapted to a particular set of resources, called a niche, in the environment. According to this concept, populations form the discrete phenetic clusters that we recognise as species because the ecological and evolutionary processes controlling how resources are divided up tend to produce those clusters. A genetic species as defined by Robert Baker and Robert Bradley is a set of genetically isolated interbreeding populations. This

4736-579: Is based on the type of cross-bedding in sandstones and presence of wavy or lenticular heterolithic bedding, both of which indicate a changing tidal environment. Finally, the lack of acritarchs in the shales, the dominant lithology of the formation is regarded as evidence for minimal marine influence during their deposition. Isolated shale beds within the Bright Angel Shale that contain a well preserved and diverse fauna of trilobites, brachiopods and others fossils likely represent periods of shallow marine shelf deposition during periods of high, relative sea levels in

4864-600: 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 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

4992-414: Is called speciation . Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species . Speciation depends on a measure of reproductive isolation , a reduced gene flow. This occurs most easily in allopatric speciation, where populations are separated geographically and can diverge gradually as mutations accumulate. Reproductive isolation

5120-412: Is called the specific name or the specific epithet (in botanical nomenclature , also sometimes in zoological nomenclature ). For example, Boa constrictor is one of the species of the genus Boa , with constrictor being the species' epithet. While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic species concepts. For example,

5248-538: 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

Bright Angel Shale - Misplaced Pages Continue

5376-403: Is described formally, in a publication that assigns it a unique scientific name. The description typically provides means for identifying the new species, which may not be based solely on morphology (see cryptic species ), differentiating it from other previously described and related or confusable species and provides a validly published name (in botany) or an available name (in zoology) when

5504-671: Is further weakened by the existence of microspecies , groups of organisms, including many plants, with very little genetic variability, usually forming species aggregates . For example, the dandelion Taraxacum officinale and the blackberry Rubus fruticosus are aggregates with many microspecies—perhaps 400 in the case of the blackberry and over 200 in the dandelion, complicated by hybridisation , apomixis and polyploidy , making gene flow between populations difficult to determine, and their taxonomy debatable. Species complexes occur in insects such as Heliconius butterflies, vertebrates such as Hypsiboas treefrogs, and fungi such as

5632-726: Is hard or even impossible to test. Later biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others. Many of the concepts are quite similar or overlap, so they are not easy to count: the biologist R. L. Mayden recorded about 24 concepts, and the philosopher of science John Wilkins counted 26. Wilkins further grouped the species concepts into seven basic kinds of concepts: (1) agamospecies for asexual organisms (2) biospecies for reproductively isolated sexual organisms (3) ecospecies based on ecological niches (4) evolutionary species based on lineage (5) genetic species based on gene pool (6) morphospecies based on form or phenotype and (7) taxonomic species,

5760-509: 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

5888-550: 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

6016-403: Is no easy way to tell whether related geographic or temporal forms belong to the same or different species. Species gaps can be verified only locally and at a point of time. One is forced to admit that Darwin's insight is correct: any local reality or integrity of species is greatly reduced over large geographic ranges and time periods. The botanist Brent Mishler argued that the species concept

6144-478: Is not valid, notably because gene flux decreases gradually rather than in discrete steps, which hampers objective delimitation of species. Indeed, complex and unstable patterns of gene flux have been observed in cichlid teleosts of the East African Great Lakes . Wilkins argued that "if we were being true to evolution and the consequent phylogenetic approach to taxa, we should replace it with

6272-770: Is rich in trace fossils . Overall, when compared with other Middle Cambrian formations in the region, the Bright Angel Shale body fossils are fragmentary and rare. Although it is relatively barren regionally of body fossils, individual beds can be rich in complete body fossils. Individual beds within the Bright Angel Shale are as productive as many of the other Cambrian formations in the Great Basin and Rocky Mountain regions. The Bright Angel Shale has yielded at least 15 species of trilobites and four genera of brachiopods . In addition, one genus of hyoliths , four genera of possible marine arthropods , and two species of eocrinoids have been found in it. The trace fossils found in

6400-586: Is similar to Mayr's Biological Species Concept, but stresses genetic rather than reproductive isolation. In the 21st century, a genetic species could be established by comparing DNA sequences. Earlier, other methods were available, such as comparing karyotypes (sets of chromosomes ) and allozymes ( enzyme variants). An evolutionarily significant unit (ESU) or "wildlife species" is a population of organisms considered distinct for purposes of conservation. In palaeontology , with only comparative anatomy (morphology) and histology from fossils as evidence,

6528-509: Is the first period of 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

SECTION 50

#1732772577212

6656-594: Is threatened by hybridisation, but this can be selected against once a pair of populations have incompatible alleles of the same gene, as described in the Bateson–Dobzhansky–Muller model . A different mechanism, phyletic speciation, involves one lineage gradually changing over time into a new and distinct form (a chronospecies ), without increasing the number of resultant species. Horizontal gene transfer between organisms of different species, either through hybridisation , antigenic shift , or reassortment ,

6784-530: The ICZN for animals and the ICN for plants, do not make rules for defining the boundaries of the species. Research can change the boundaries, also known as circumscription, based on new evidence. Species may then need to be distinguished by the boundary definitions used, and in such cases the names may be qualified with sensu stricto ("in the narrow sense") to denote usage in the exact meaning given by an author such as

6912-763: The Marbled limestone , the Tonto shale , and the Tonto sandstone . In 1914, L F. Noble renamed Gilbert's subdivisions of the Tonto Group. He renamed the Tonto sandstone as the Tapeats Sandstone and the Tonto shale as the Bright Angel Shale . He named the Bright Angel Shale after Bright Angel Canyon because in the walls of this canyon, this formation is well exposed. Later in 1922, the Marbled limestone

7040-520: 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

7168-632: 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

7296-812: 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

7424-511: The Tonian period and the earliest known life forms (in the form of simple prokaryotic microbial mats ) started in the ocean floor during 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

7552-597: 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

7680-399: The fly agaric . Natural hybridisation presents a challenge to the concept of a reproductively isolated species, as fertile hybrids permit gene flow between two populations. For example, the carrion crow Corvus corone and the hooded crow Corvus cornix appear and are classified as separate species, yet they can hybridise where their geographical ranges overlap. A ring species is

7808-507: The jaguar ( Panthera onca ) of Latin America or the leopard ( Panthera pardus ) of Africa and Asia. In contrast, the scientific names of species are chosen to be unique and universal (except for some inter-code homonyms ); they are in two parts used together : the genus as in Puma , and the specific epithet as in concolor . A species is given a taxonomic name when a type specimen

SECTION 60

#1732772577212

7936-512: 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),

8064-406: The "cynical species concept", and arguing that far from being cynical, it usefully leads to an empirical taxonomy for any given group, based on taxonomists' experience. Other biologists have gone further and argued that we should abandon species entirely, and refer to the "Least Inclusive Taxonomic Units" (LITUs), a view that would be coherent with current evolutionary theory. The species concept

8192-472: 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

8320-465: The Bright Angel Shale consist of arthropod tracks and trails, worm burrows , and dwelling structures. Some of these have been identified at the ichnogenus and ichnospecies level. In addition, wrinkle structures have been found within the Bright Angel Shale in the Sumner Butte area . They are classified as microbially induced sedimentary structures. Initially, the Bright Angel Shale, largely on

8448-564: The Bright Angel Shale into a number of subdivisions, members , based upon prominent beds of limestone and sandstone within it. However, these members have found to be confusing and unhelpful in understanding the stratigraphy of the Bright Angel Shale and are typically ignored and rarely used in the published literature. In the Grand Canyon, the Bright Angel Shale is typically a heterogenous, sometimes fossiliferous , mixture of interbedded shale, mudstone, siltstone, fine-grained sandstone, and isolated beds of limestone. The dominant lithology within

8576-431: The Bright Angel Shale is greenish shale that is composed largely of illite and varying amounts of chlorite and kaolinite . A reddish brown coloration is imparted to a number of the sandstone and siltstone beds by the high percentage of hematitic ooids and iron oxide cements that they contain. Isolated beds of coarse-grained sandstone with either basal conglomerate or conglomeratic sandstone also locally occur within

8704-473: The Bright Angel Shale with the overlying Muav Limestone is the main cause of the variability in its thickness. South the Grand Canyon, decreases in thickness only a few feet thick in the Juniper Mountains north of Prescott, Arizona. Within the Grand Canyon, the Bright Angel Shale is the most fossiliferous of the five formations that comprise the Tonto Group. It contains a variety of body fossils and

8832-407: The Bright Angel Shale. The gravel fraction of these beds contain quartz , minor amounts of potassium feldspars , sedimentary rock fragments, and glauconite . Glauconite also occurs in and imparts a greenish color to many of the siltstones and sandstones. Towards its base, the Bright Angel Shale becomes increasingly coarser with increasing frequency of sandstone beds until it grades gradually into

8960-500: 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

9088-514: 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

9216-407: The Grand Canyon region. In 1945, E. D. McKee and C. E. Resser used trilobite fossil assemblages to determine age of the Bright Angel Shale. They interpreted the trilobite biostratigraphy to indicate that it crosses time lines and becomes younger toward the east. In their interpretation, the base of the Bright Angel Shale and its lower third of it is late Early Cambrian whereas in the eastern part of

9344-599: 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

9472-544: 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

9600-600: 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

9728-626: The Origin of Species explained how species could arise by natural selection . That understanding was greatly extended in the 20th century through genetics and population ecology . Genetic variability arises from mutations and recombination , while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures . Genes can sometimes be exchanged between species by horizontal gene transfer ; new species can arise rapidly through hybridisation and polyploidy ; and species may become extinct for

9856-584: 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

9984-401: 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

10112-463: The Phanerozoic starts with the sudden appearance of fossilised evidence of a number of animal phyla ; 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

10240-570: The Phanerozoic were strongly associated with mass extinctions. Species A species ( pl. : species) is a population of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring , typically by sexual reproduction . It is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity . Other ways of defining species include their karyotype , DNA sequence, morphology , behaviour, or ecological niche . In addition, paleontologists use

10368-494: The Tapeats Sandstone. The siltstones and sandstones of the Bright Angel Shale exhibit sedimentary structures throughout the Grand Canyon. These sedimentary structures include horizontal laminations, small- to large-scale planar tabular and trough cross-stratification, and wavy and lenticular bedding. The isolated coarse-grained and conglomeratic sandstones are typically structureless and crudely stratified and overlie

10496-405: The abbreviation "sp." in the singular or "spp." (standing for species pluralis , Latin for "multiple species") in the plural in place of the specific name or epithet (e.g. Canis sp.). This commonly occurs when authors are confident that some individuals belong to a particular genus but are not sure to which exact species they belong, as is common in paleontology . Authors may also use "spp." as

10624-570: The amount of hybridisation is insufficient to completely mix their respective gene pools . A further development of the recognition concept is provided by the biosemiotic concept of species. In microbiology , genes can move freely even between distantly related bacteria, possibly extending to the whole bacterial domain. As a rule of thumb, microbiologists have assumed that members of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA–DNA hybridisation to decide if they belong to

10752-812: 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

10880-458: 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

11008-697: 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

11136-887: 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

11264-474: The biological species concept, "the several versions" of the phylogenetic species concept, and the idea that species are of the same kind as higher taxa are not suitable for biodiversity studies (with the intention of estimating the number of species accurately). They further suggested that the concept works for both asexual and sexually-reproducing species. A version of the concept is Kevin de Queiroz 's "General Lineage Concept of Species". An ecological species

11392-505: The biological species concept, a cladistic species does not rely on reproductive isolation – its criteria are independent of processes that are integral in other concepts. Therefore, it applies to asexual lineages. However, it does not always provide clear cut and intuitively satisfying boundaries between taxa, and may require multiple sources of evidence, such as more than one polymorphic locus, to give plausible results. An evolutionary species, suggested by George Gaylord Simpson in 1951,

11520-428: The boundaries between closely related species become unclear with hybridisation , in a species complex of hundreds of similar microspecies , and in a ring species . Also, among organisms that reproduce only asexually , the concept of a reproductive species breaks down, and each clone is potentially a microspecies. Although none of these are entirely satisfactory definitions, and while the concept of species may not be

11648-627: 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

11776-464: The canyon its base and entire thickness is Middle Cambrian. In 2018, K. Karlstrom and others found that the Tapeats Sandstone is younger than previously interpreted and does not get younger to the east. As a result, the Bright Angel Shale lies entirely in the Middle Cambrian. The units of the Tonto Group and the colorful Bright Angel Shale are easily identified as a geological sequence beneath

11904-564: 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

12032-433: The concept of a chronospecies can be applied. During anagenesis (evolution, not necessarily involving branching), some palaeontologists seek to identify a sequence of species, each one derived from the phyletically extinct one before through continuous, slow and more or less uniform change. In such a time sequence, some palaeontologists assess how much change is required for a morphologically distinct form to be considered

12160-416: The concept of the chronospecies since fossil reproduction cannot be examined. The most recent rigorous estimate for the total number of species of eukaryotes is between 8 and 8.7 million. About 14% of these had been described by 2011. All species (except viruses ) are given a two-part name , called a "binomial". The first part of a binomial is the genus to which the species belongs. The second part

12288-533: The continents had collected them into a single landmass known as Pangaea (the most recent supercontinent ), which then separated into 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

12416-416: 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

12544-406: 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

12672-519: 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

12800-428: 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

12928-600: 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

13056-584: 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

13184-435: The evidence to support hypotheses about evolutionarily divergent lineages that have maintained their hereditary integrity through time and space. Molecular markers may be used to determine diagnostic genetic differences in the nuclear or mitochondrial DNA of various species. For example, in a study done on fungi , studying the nucleotide characters using cladistic species produced the most accurate results in recognising

13312-486: The evolution of the three most prominent animal phyla, arthropods , molluscs and chordates , 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

13440-460: 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

13568-550: 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

13696-718: 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

13824-609: 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

13952-523: The great diversification in birds and mammals. Humans appeared and evolved during 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

14080-536: 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

14208-483: The numerous fungi species of all the concepts studied. Versions of the phylogenetic species concept that emphasise monophyly or diagnosability may lead to splitting of existing species, for example in Bovidae , by recognising old subspecies as species, despite the fact that there are no reproductive barriers, and populations may intergrade morphologically. Others have called this approach taxonomic inflation , diluting

14336-593: The paper is accepted for publication. The type material is usually held in a permanent repository, often the research collection of a major museum or university, that allows independent verification and the means to compare specimens. Describers of new species are asked to choose names that, in the words of the International Code of Zoological Nomenclature , are "appropriate, compact, euphonious, memorable, and do not cause offence". Books and articles sometimes intentionally do not identify species fully, using

14464-674: The person who named the species, while the antonym sensu lato ("in the broad sense") denotes a wider usage, for instance including other subspecies. Other abbreviations such as "auct." ("author"), and qualifiers such as "non" ("not") may be used to further clarify the sense in which the specified authors delineated or described the species. Species are subject to change, whether by evolving into new species, exchanging genes with other species, merging with other species or by becoming extinct. The evolutionary process by which biological populations of sexually-reproducing organisms evolve to become distinct or reproductively isolated as species

14592-514: 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

14720-652: The preceding Ediacaran period of the Proterozoic eon, known as the Avalon Explosion , have been identified since 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

14848-430: The presence of glauconite, was regarded to have accumulated within the deeper part of a marine shelf. Later, the Bright Angel Shale is typically interpreted as a shallow marine shelf unit and its various members are interpreted to be to minor transgressions and regressions. However, parts of it have also been more recently interpreted as including estuary and tidal flat settings influenced by storm events. This interpretation

14976-625: The present. It is the eon during which abundant animal and plant life has proliferated , diversified and colonized various niches on 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

15104-487: The result of misclassification leading to questions on whether there really are any ring species. The commonly used names for kinds of organisms are often ambiguous: "cat" could mean the domestic cat, Felis catus , or the cat family, Felidae . Another problem with common names is that they often vary from place to place, so that puma, cougar, catamount, panther, painter and mountain lion all mean Puma concolor in various parts of America, while "panther" may also mean

15232-586: The ring. Ring species thus present a difficulty for any species concept that relies on reproductive isolation. However, ring species are at best rare. Proposed examples include the herring gull – lesser black-backed gull complex around the North pole, the Ensatina eschscholtzii group of 19 populations of salamanders in America, and the greenish warbler in Asia, but many so-called ring species have turned out to be

15360-508: The same species. This concept was narrowed in 2006 to a similarity of 98.7%. The average nucleotide identity (ANI) method quantifies genetic distance between entire genomes , using regions of about 10,000 base pairs . With enough data from genomes of one genus, algorithms can be used to categorize species, as for Pseudomonas avellanae in 2013, and for all sequenced bacteria and archaea since 2020. Observed ANI values among sequences appear to have an "ANI gap" at 85–95%, suggesting that

15488-529: The same species. When two species names are discovered to apply to the same species, the older species name is given priority and usually retained, and the newer name considered as a junior synonym, a process called synonymy . Dividing a taxon into multiple, often new, taxa is called splitting . Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognising differences or commonalities between organisms. The circumscription of taxa, considered

15616-506: The species concept and making taxonomy unstable. Yet others defend this approach, considering "taxonomic inflation" pejorative and labelling the opposing view as "taxonomic conservatism"; claiming it is politically expedient to split species and recognise smaller populations at the species level, because this means they can more easily be included as endangered in the IUCN red list and can attract conservation legislation and funding. Unlike

15744-424: 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

15872-722: The tall cliffs of the Redwall Limestone (the Redwall sits upon a short resistant cliff of Muav Limestone); the Tonto Group is also easily seen beside Granite Gorge of the Colorado River and the Vishnu Basement Rocks 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

16000-540: The wild. It is difficult to define a species in a way that applies to all organisms. The debate about species concepts is called the species problem. The problem was recognised even in 1859, when Darwin wrote in On the Origin of Species : I was much struck how entirely vague and arbitrary is the distinction between species and varieties. He went on to write: No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of

16128-654: 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

16256-669: Was redefined and renamed as the Muav Limestone by L. F. Noble. According to Noble's definition, the Muav Limestone consisted of an upper set of dolomite beds and a lower set of limestone beds. Noble's lower set of limestone beds is now defined as the Muav Limestone and the Frenchman Mountain Dolostone is now defined as Noble's upper set of dolomite beds. E. D. McKee and C. E. Resser largely retained Noble's nomenclature. However, they did subdivide

16384-635: 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

#211788