The tusk shells or tooth shells referred to by the more-technical term scaphopods, are members of a class of shelled marine mollusc with worldwide distribution, are the only class of infaunal marine molluscs. Shells of species within this class range from about 0.5 to 15 cm in length. Members of the order Dentaliida tend to be larger than those of the order Gadilida; these molluscs live in soft substrates offshore. Because of this subtidal habitat and the small size of most species, many beachcombers are unfamiliar with them. Molecular data suggest that the scaphopods are a sister group to the cephalopods, although higher-level molluscan phylogeny remains somewhat unresolved; the morphological shape of the scaphopod body makes it difficult to orient it satisfactorily. As a result, researchers have disagreed as to which direction is anterior/ posterior and, ventral/ dorsal. According to Shimek and Steiner, "he apex of the shell and mantle are anatomically dorsal, the large aperture is ventral and anterior.
The concave side of the shell and viscera are anatomically dorsal. The convex side has to be divided into anteriorly ventral and dorsally posterior portions, with the anus as the demarcation. Functionally, as in cephalopods, the large aperture with the foot is anterior, the apical area posterior, the concave side dorsal and the convex side ventral." The shells of the members of the Gadilida are glassy-smooth in addition to being quite narrow and with a reduced aperture. This along with other structures of their anatomy allows them to move with surprising speed through loose sediment to escape potential bottom-dwelling predators; the Dentalids, on the other hand, tend to have ribbed and rather rough shells. When they sense vibrations anywhere around them, their defensive response is to freeze; this makes them harder to detect by animals such as ratfish which can sense the electrical signals given off by the most minute muscle movement. The mantle of a scaphopod is within the shell; the foot extends from the larger end of the shell, is used to burrow through the substrate.
The scaphopod positions itself head down in the substrate, with the apical end of the shell projecting upward. This end appears above the level of the substrate, however, as doing so exposes the animal to numerous predators. Most adult scaphopods live their lives buried within the substrate. Water enters the mantle cavity through the apical aperture, is wafted along the body surface by cilia. There are no gills. Unlike most other molluscs, there is no continuous flow of water with a separate exhalant stream. Instead, deoxygenated water is expelled back through the apical aperture through muscular action once every ten to twelve minutes. A number of minute tentacles around the foot, called captacula, sift through the sediment and latch onto bits of food, which they convey to the mouth; the mouth has a grinding radula. The radulae and cartilaginous oral bolsters of the Gadilidae are structured like zippers where the teeth crush the prey by opening and closing on it while the radulae and bolsters of the Dentaliidae work rachet-like to pull the prey into the esophagus, sometimes whole.
The massive radula of the scaphopods is the largest such organ relative to body size of any mollusc. The remainder of the digestive system consists of a digestive diverticulum, esophagus and intestine. A digestive gland secretes enzymes into the stomach, unlike some other molluscs, does not digest the food directly itself; the anus opens on the ventral/ underside of the animal in the middle of the mantle cavity. The scaphopod vascular system is rudimentary lacking heart auricles as well as corresponding ctenidia and blood vessels; the heart, a characteristic feature of all other groups of mollusca, has been considered lost or reduced to a thin fold of the pericardium. Metabolic waste is excreted through a pair of nephridia close to the anus; the tusk shells appear to be the only extant molluscs which lack the otherwise standard molluscan reno-pericardial apertures. Furthermore, they appear to be the only molluscs with openings that directly connect the hemocoel with the surrounding water; these openings may serve to allow the animal to relieve internal pressure by ejecting body fluid during moments of extreme muscular contraction of the foot.
The nervous system is similar to that of gastropods. One pair each of cerebral and pleural ganglia lie close to the oesophagus, form the animal's brain. A separate set of pedal ganglia lie in the foot, a pair of visceral ganglia are set further back in the body, connect to pavilion ganglia via long connectives. Radular and sub-radular ganglia are present, as are statocysts with staticonia. Scaphopods have no osphradia, or other distinct sensory organs. Scaphopods have separate sexes, external fertilisation
The Triassic is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.9 million years ago, to the beginning of the Jurassic Period 201.3 Mya. The Triassic is the shortest period of the Mesozoic Era. Both the start and end of the period are marked by major extinction events. Triassic began in the wake of the Permian–Triassic extinction event, which left the Earth's biosphere impoverished. Therapsids and archosaurs were the chief terrestrial vertebrates during this time. A specialized subgroup of archosaurs, called dinosaurs, first appeared in the Late Triassic but did not become dominant until the succeeding Jurassic Period; the first true mammals, themselves a specialized subgroup of therapsids evolved during this period, as well as the first flying vertebrates, the pterosaurs, like the dinosaurs, were a specialized subgroup of archosaurs. The vast supercontinent of Pangaea existed until the mid-Triassic, after which it began to rift into two separate landmasses, Laurasia to the north and Gondwana to the south.
The global climate during the Triassic was hot and dry, with deserts spanning much of Pangaea's interior. However, the climate became more humid as Pangaea began to drift apart; the end of the period was marked by yet another major mass extinction, the Triassic–Jurassic extinction event, that wiped out many groups and allowed dinosaurs to assume dominance in the Jurassic. The Triassic was named in 1834 by Friedrich von Alberti, after the three distinct rock layers that are found throughout Germany and northwestern Europe—red beds, capped by marine limestone, followed by a series of terrestrial mud- and sandstones—called the "Trias"; the Triassic is separated into Early and Late Triassic Epochs, the corresponding rocks are referred to as Lower, Middle, or Upper Triassic. The faunal stages from the youngest to oldest are: During the Triassic all the Earth's land mass was concentrated into a single supercontinent centered more or less on the equator and spanning from pole to pole, called Pangaea.
From the east, along the equator, the Tethys sea penetrated Pangaea, causing the Paleo-Tethys Ocean to be closed. In the mid-Triassic a similar sea penetrated along the equator from the west; the remaining shores were surrounded by the world-ocean known as Panthalassa. All the deep-ocean sediments laid down during the Triassic have disappeared through subduction of oceanic plates; the supercontinent Pangaea was rifting during the Triassic—especially late in that period—but had not yet separated. The first nonmarine sediments in the rift that marks the initial break-up of Pangaea, which separated New Jersey from Morocco, are of Late Triassic age. S. these thick sediments comprise the Newark Group. Because a super-continental mass has less shoreline compared to one broken up, Triassic marine deposits are globally rare, despite their prominence in Western Europe, where the Triassic was first studied. In North America, for example, marine deposits are limited to a few exposures in the west, thus Triassic stratigraphy is based on organisms that lived in lagoons and hypersaline environments, such as Estheria crustaceans.
At the beginning of the Mesozoic Era, Africa was joined with Earth's other continents in Pangaea. Africa shared the supercontinent's uniform fauna, dominated by theropods and primitive ornithischians by the close of the Triassic period. Late Triassic fossils are more common in the south than north; the time boundary separating the Permian and Triassic marks the advent of an extinction event with global impact, although African strata from this time period have not been studied. During the Triassic peneplains are thought to have formed in what is now southern Sweden. Remnants of this peneplain can be traced as a tilted summit accordance in the Swedish West Coast. In northern Norway Triassic peneplains may have been buried in sediments to be re-exposed as coastal plains called strandflats. Dating of illite clay from a strandflat of Bømlo, southern Norway, have shown that landscape there became weathered in Late Triassic times with the landscape also being shaped during that time. At Paleorrota geopark, located in Rio Grande do Sul, the Santa Maria Formation and Caturrita Formations are exposed.
In these formations, one of the earliest dinosaurs, Staurikosaurus, as well as the mammal ancestors Brasilitherium and Brasilodon have been discovered. The Triassic continental interior climate was hot and dry, so that typical deposits are red bed sandstones and evaporites. There is no evidence of glaciation near either pole. Pangaea's large size limited the moderating effect of the global ocean; the strong contrast between the Pangea supercontinent and the global ocean triggered intense cross-equatorial monsoons. The Triassic may have been a dry period, but evidence exists that it was punctuated by several episodes of increased rainfall in tropical and subtropical latitudes of the Tethys Sea and its surrounding land. Sediments and fossils suggestive of a more humid climate are known from the Anisian to Ladinian of the Tethysian domain, from the Carnian and Rhaetian of a larger area that includes the Boreal domain, the North
The Oncocerida comprise a diverse group of small nautiloid cephalopods known from the Middle Ordovician to the Mississippian, in which the connecting rings are thin and siphuncle segments are variably expanded. At present the order consists of some 16 families, a few of which, such as the Oncoceratidae and Acleistoceratidae contain a fair number of genera each while others like the Trimeroceratidae and Archiacoceratidae are represented by only two or three; the shells of oncocerids are somewhat compressed cyrtoconic brevicones. More advanced forms include gyrocones, serpenticones and elongate orthocones and cyrtocones, reflective of the different families and genera; the siphuncle in the Oncocerida is located at or near the ventral margin. Connecting rings are most thin and structureless but in certain derived forms may become actinosiphonate with inwardly projecting radial lamellae; the juvenile segments in early genera are straight and tubular, with short orthochoantic septal necks inherited from the Bassleroceratidae.
In the mature stages of early forms and throughout in the more advanced the connecting rings are inflated with cyrtochoanitic septal necks, giving what can be described as a "beaded" or "ellipsoidal" appearance. The Oncocerida are thought to be derived from the Bassleroceratidae through Graciloceras as a result of a thinning of the connecting rings in the siphuncle. Oncocerids reached their greatest generic diversity in the Middle Silurian with some 43 genera representing nine families, the most at any time. Of these 43 or so genera, about 38 were new, a recovery from a precipitous decline in the Late Ordovician and Early Silurian. A second period of greater diversity occurred in the Middle Devonian with eight families represented by some 37 genera, following a second decline after the Middle Silurian. After this the order declined until its extinction in the Early Carboniferous. Near the beginning of the Devonian and well before its end, the Oncocerida gave rise to the Rutoceratidae, which form the root stock of the Nautilida, which among its members includes the modern Nautilus and Allonautilus.
Oncocerids are well known as fossils from the Ordovician and Devonian in North America and Australia, to a lesser extent from parts of Asia, after which the order declined into the Mississippian and reached its end by the Pennsylvanian. Families in the Oncocerida, according to the Treatise on Invertebrate Paleontology, follow with the number of genera in each shown in parentheses, along with the stratigraphic range. Graciloceratidae M-U Ord Tripteroceratidae M-U Ord Valcourocratidae M-U Ord Diestoceratidae M-U Ord Oncoceratidae M Ord - U Sil Jovellaniidae? U Ord, M Sil - L Dev. Nothoceratidae L Sil - U Dev Karoceratidae L - M Sil,? L Dev Hemiphragmoceratidae M-U Sil,? M Dev Acleistoceratidae M Sil - M Dev Polyelasmoceratidae M Sil - U Dev Brevicoceratidae M Sil - U Dev Poterioceratidae L Dev - L Carb Tripleuroceratidae? L Dev, M Dev - L Carb Archiacoceratidae M Dev/ According to more current thinking, e.g. Flower, Kümmel, the Oncocerida gave rise to the Rutoceratidae which form the root stock of the Nautilida, which after a number of iterations, ends up with the modern Nautilus and Allonautilus.
Flower, R. H. in Flower and Kümmel Jr 1950. H. 1976. G. Ed. Kümmel,B. 1964. C. Moore Ed. Sweet, W. C. 1964. C. Moore Ed
Jean Louis Rodolphe Agassiz was a Swiss-American biologist and geologist recognized as an innovative and prodigious scholar of Earth's natural history. Agassiz grew up in Switzerland, he received Doctor of Philosophy and medical degrees at Munich, respectively. After studying with Cuvier and Humboldt in Paris, Agassiz was appointed professor of natural history at the University of Neuchâtel, he emigrated to the United States in 1847 after visiting Harvard University. He went on to become professor of zoology and geology at Harvard, to head its Lawrence Scientific School, to found its Museum of Comparative Zoology. Agassiz is known for his regimen of observational data analysis, he made vast institutional and scientific contributions to zoology and related areas, including writing multi-volume research books running to thousands of pages. He is known for his contributions to ichthyological classification, including of extinct species, to the study of geological history, including to the founding of glaciology.
In the 20th and 21st centuries, Agassiz's resistance to Darwinian evolution, belief in creationism, the scientific racism implicit in his writings on human polygenism, have tarnished his reputation and led to controversies over his legacy. Louis Agassiz was born in Môtier in the Swiss canton of Fribourg; the son of a pastor, Agassiz was educated first at home, he spent four years of secondary school in Bienne, entering in 1818 and completing his elementary studies in Lausanne. Agassiz studied successively at the universities of Zürich and Munich. In 1829 he received the degree of doctor of philosophy at Erlangen, in 1830 that of doctor of medicine at Munich. Moving to Paris, he came under the tutelage of Alexander von Humboldt. Humboldt and Georges Cuvier launched him on his careers of zoology respectively. Ichthyology soon became a focus of his life's work. In 1819–1820, the German biologists Johann Baptist von Spix and Carl Friedrich Philipp von Martius undertook an expedition to Brazil, they returned home to Europe with many natural objects, including an important collection of the freshwater fish of Brazil of the Amazon River.
Spix, who died in 1826, did not live long enough to work out the history of these fish, Martius selected Agassiz for this project. Agassiz threw himself into the work with an enthusiasm that would go on to characterize the rest of life's work; the task of describing the Brazilian fish was completed and published in 1829. This was followed by research into the history of fish found in Lake Neuchâtel. Enlarging his plans, in 1830 he issued a prospectus of a History of the Freshwater Fish of Central Europe, it was only in 1839, that the first part of this publication appeared, it was completed in 1842. In 1832, Agassiz was appointed professor of natural history at the University of Neuchâtel; the fossil fish in the rock of the surrounding region, the slates of Glarus and the limestones of Monte Bolca, soon attracted his attention. At the time little had been accomplished in their scientific study. Agassiz, as early as 1829, planned the publication of a work which, more than any other, laid the foundation of his worldwide fame.
Five volumes of his Recherches sur les poissons fossiles were published from 1833 to 1843. They were magnificently illustrated, chiefly by Joseph Dinkel. In gathering materials for this work Agassiz visited the principal museums in Europe, meeting Cuvier in Paris, he received much encouragement and assistance from him, they had known him for seven years at the time. Agassiz found; the fossils he examined showed any traces of the soft tissues of fish, instead, consisted chiefly of the teeth and fins, with the bones being preserved in comparatively few instances. He, adopted a classification that divided fish into four groups: Ganoids, Placoids and Ctenoids, based on the nature of the scales and other dermal appendages; this did much to improve fish taxonomy. Agassiz needed financial support to continue his work; the British Association and the Earl of Ellesmere—then Lord Francis Egerton—stepped in to help. The 1,290 original drawings made for the work were purchased by the Earl, presented by him to the Geological Society of London.
In 1836, the Wollaston Medal was awarded to Agassiz by the council of that society for his work on fossil ichthyology. Meanwhile, invertebrate animals engaged his attention. In 1837, he issued the "Prodrome" of a monograph on the recent and fossil Echinodermata, the first part of which appeared in 1838. Before Agassiz's first visit to England in 1834, Hugh Miller and other geologists had brought to light the remarkable fossil fish of the Old Red Sandstone of the northeast of Scotland; the strange forms of the Pterichthys, the Coccosteus and other genera were made known to geologists for the first time. They were of intense interest to Agassiz, formed the subject of a monograph by him published in 1844–45: Monographie des poissons fossiles du Vieux Grès Rouge, ou Système Dévonien des Îles Britanniques et de Russie ("Monograph on Fossil Fish of the Old Red Sandstone, or Devonian
The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago, to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozoic era; the concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the city of Perm. The Permian witnessed the diversification of the early amniotes into the ancestral groups of the mammals, turtles and archosaurs; the world at the time was dominated by two continents known as Pangaea and Siberia, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Amniotes, who could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors; the Permian ended with the Permian–Triassic extinction event, the largest mass extinction in Earth's history, in which nearly 96% of marine species and 70% of terrestrial species died out.
It would take well into the Triassic for life to recover from this catastrophe. Recovery from the Permian–Triassic extinction event was protracted; the term "Permian" was introduced into geology in 1841 by Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Édouard de Verneuil; the region now lies in the Perm Krai of Russia. Official ICS 2017 subdivisions of the Permian System from most recent to most ancient rock layers are: Lopingian epoch Changhsingian Wuchiapingian Others: Waiitian Makabewan Ochoan Guadalupian epoch Capitanian stage Wordian stage Roadian stage Others: Kazanian or Maokovian Braxtonian stage Cisuralian epoch Kungurian stage Artinskian stage Sakmarian stage Asselian stage Others: Telfordian Mangapirian Sea levels in the Permian remained low, near-shore environments were reduced as all major landmasses collected into a single continent—Pangaea; this could have in part caused the widespread extinctions of marine species at the end of the period by reducing shallow coastal areas preferred by many marine organisms.
During the Permian, all the Earth's major landmasses were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean, the Paleo-Tethys Ocean, a large ocean that existed between Asia and Gondwana; the Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys Ocean to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic era. Large continental landmass interiors experience climates with extreme variations of heat and cold and monsoon conditions with seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea; such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores in a wetter environment. The first modern trees appeared in the Permian. Three general areas are noted for their extensive Permian deposits—the Ural Mountains and the southwest of North America, including the Texas red beds.
The Permian Basin in the U. S. states of Texas and New Mexico is so named because it has one of the thickest deposits of Permian rocks in the world. The climate in the Permian was quite varied. At the start of the Permian, the Earth was still in an ice age. Glaciers receded around the mid-Permian period as the climate warmed, drying the continent's interiors. In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles. Permian marine deposits are rich in fossil mollusks and brachiopods. Fossilized shells of two kinds of invertebrates are used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist, one of the foraminiferans, ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct. Terrestrial life in the Permian included diverse plants, fungi and various types of tetrapods; the period saw a massive desert covering the interior of Pangaea.
The warm zone spread in the northern hemisphere. The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. A number of older types of plants and animals became marginal elements; the Permian began with the Carboniferous flora still flourishing. About the middle of the Permian a major transition in vegetation began; the swamp-loving
Cladistics is an approach to biological classification in which organisms are categorized in groups based on the most recent common ancestor. Hypothesized relationships are based on shared derived characteristics that can be traced to the most recent common ancestor and are not present in more distant groups and ancestors. A key feature of a clade is that all its descendants are part of the clade. All descendants stay in their overarching ancestral clade. For example, if within a strict cladistic framework the terms animals, bilateria/worms, fishes/vertebrata, or monkeys/anthropoidea were used, these terms would include humans. Many of these terms are used paraphyletically, outside of cladistics, e.g. as a'grade'. Radiation results in the generation of new subclades by bifurcation; the techniques and nomenclature of cladistics have been applied to other disciplines. Cladistics is now the most used method to classify organisms; the original methods used in cladistic analysis and the school of taxonomy derived from the work of the German entomologist Willi Hennig, who referred to it as phylogenetic systematics.
Cladistics in the original sense refers to a particular set of methods used in phylogenetic analysis, although it is now sometimes used to refer to the whole field. What is now called the cladistic method appeared as early as 1901 with a work by Peter Chalmers Mitchell for birds and subsequently by Robert John Tillyard in 1921, W. Zimmermann in 1943; the term "clade" was introduced in 1958 by Julian Huxley after having been coined by Lucien Cuénot in 1940, "cladogenesis" in 1958, "cladistic" by Cain and Harrison in 1960, "cladist" by Mayr in 1965, "cladistics" in 1966. Hennig referred to his own approach as "phylogenetic systematics". From the time of his original formulation until the end of the 1970s, cladistics competed as an analytical and philosophical approach to systematics with phenetics and so-called evolutionary taxonomy. Phenetics was championed at this time by the numerical taxonomists Peter Sneath and Robert Sokal, evolutionary taxonomy by Ernst Mayr. Conceived, if only in essence, by Willi Hennig in a book published in 1950, cladistics did not flourish until its translation into English in 1966.
Today, cladistics is the most popular method for constructing phylogenies from morphological data. In the 1990s, the development of effective polymerase chain reaction techniques allowed the application of cladistic methods to biochemical and molecular genetic traits of organisms, vastly expanding the amount of data available for phylogenetics. At the same time, cladistics became popular in evolutionary biology, because computers made it possible to process large quantities of data about organisms and their characteristics; the cladistic method interprets each character state transformation implied by the distribution of shared character states among taxa as a potential piece of evidence for grouping. The outcome of a cladistic analysis is a cladogram – a tree-shaped diagram, interpreted to represent the best hypothesis of phylogenetic relationships. Although traditionally such cladograms were generated on the basis of morphological characters and calculated by hand, genetic sequencing data and computational phylogenetics are now used in phylogenetic analyses, the parsimony criterion has been abandoned by many phylogeneticists in favor of more "sophisticated" but less parsimonious evolutionary models of character state transformation.
Cladists contend. Every cladogram is based on a particular dataset analyzed with a particular method. Datasets are tables consisting of molecular, ethological and/or other characters and a list of operational taxonomic units, which may be genes, populations, species, or larger taxa that are presumed to be monophyletic and therefore to form, all together, one large clade. Different datasets and different methods, not to mention violations of the mentioned assumptions result in different cladograms. Only scientific investigation can show, more to be correct; until for example, cladograms like the following have been accepted as accurate representations of the ancestral relations among turtles, lizards and birds: If this phylogenetic hypothesis is correct the last common ancestor of turtles and birds, at the branch near the ▼ lived earlier than the last common ancestor of lizards and birds, near the ♦. Most molecular evidence, produces cladograms more like this: If this is accurate the last common ancestor of turtles and birds lived than the last common ancestor of lizards and birds.
Since the cladograms provide competing accounts of real events, at most one of them is correct. The cladogram to the right represents the current universally accepted hypothesis that all primates, including strepsirrhines like the lemurs and lorises, had a common ancestor all of whose descendants were primates, so form a clade. Within the primates, all anthropoids are hypothesized to have had a common ancestor all of whose descendants were anthropoids, so they form the clade called Anthropoidea; the "prosimians", on the other hand, form a paraphyletic taxon. The name Prosimii is not used in phylogenetic nomenclature, whic