Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
Fossilworks is a portal which provides query and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. Fossilworks is housed at Macquarie University, it includes many analysis and data visualization tools included in the Paleobiology Database. "Fossilworks". Retrieved 2010-04-08
Crinozoa is a subphylum of sessile echinoderms, of which the crinoids, or sea lilies, are the only extant members. Crinozoans have an extensive fossil history which may or may not extend into the Precambrian; the classes contained within Crinozoa include Crinoidea, Cystoidea and Rhombifera. List of echinoderm orders Blastoids, superficially similar-appearing echinoderms that belong to a different echinoderm subphylum
Echinoderm is the common name given to any member of the phylum Echinodermata of marine animals. The adults are recognizable by their radial symmetry, include such well-known animals as sea stars, sea urchins, sand dollars, sea cucumbers, as well as the sea lilies or "stone lilies". Echinoderms are found from the intertidal zone to the abyssal zone; the phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes, after the chordates. Echinoderms are the largest phylum that has no freshwater or terrestrial representatives. Aside from the hard-to-classify Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian. One group of Cambrian echinoderms, the cinctans, which are close to the base of the echinoderm origin, have been found to possess external gills used for filter feeding, similar to those possessed by chordates and hemichordates; the echinoderms are important both geologically. Ecologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as shallower oceans.
Most echinoderms are able to reproduce asexually and regenerate tissue and limbs. Geologically, the value of echinoderms is in their ossified skeletons, which are major contributors to many limestone formations, can provide valuable clues as to the geological environment, they were the most used species in regenerative research in the 20th centuries. Further, it is held by some scientists that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution. Along with the chordates and hemichordates, echinoderms are deuterostomes, one of the two major divisions of the bilaterians, the other being the protostomes. During the early development of the embryo, in deuterostomes, the blastopore becomes the anus whereas in the protostomes, it becomes the mouth. In deuterostomes, the mouth develops at a stage, at the opposite end of the blastula from the blastopore, a gut forms connecting the two; the larvae of echinoderms have bilateral symmetry but this is lost during metamorphosis when their bodies are reorganised and develop the characteristic radial symmetry of the echinoderm pentamerism.
The characteristics of adult echinoderms are the possession of a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles connected by a mesh of collagen fibres. A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree. There are a total of about 7,000 extant species of echinoderm as well as about 13,000 extinct species, they are found in habitats ranging from shallow intertidal areas to abyssal depths. Two main subdivisions are traditionally recognised: the more familiar motile Eleutherozoa, which encompasses the Asteroidea, Ophiuroidea and Holothuroidea; these consist of the extinct blastoids and Paracrinoids. A fifth class of Eleutherozoa consisting of just three species, the Concentricycloidea, were merged into the Asteroidea; the fossil record includes a large number of other classes which do not appear to fall into any extant crown group. All echinoderms are marine and nearly all are benthic; the oldest known echinoderm fossil may be Arkarua from the Precambrian of Australia.
It is a disc-like fossil with radial ridges on the rim and a five-pointed central depression marked with radial lines. However, no stereom or internal structure showing a water vascular system is present and the identification is inconclusive; the first universally accepted echinoderms appear in the Lower Cambrian period, asterozoans appeared in the Ordovician and the crinoids were a dominant group in the Paleozoic. Echinoderms left behind an extensive fossil record, it is hypothesised that the ancestor of all echinoderms was a simple, bilaterally symmetrical animal with a mouth and anus. This ancestral stock adopted an attached mode of life and suspension feeding, developed radial symmetry as this was more advantageous for such an existence; the larvae of all echinoderms are now bilaterally symmetrical and all develop radial symmetry at metamorphosis. The starfish and crinoids still attach themselves to the seabed while changing to their adult form; the first echinoderms gave rise to free-moving groups.
The evolution of endoskeletal plates with stereom structure and of external ciliary grooves for feeding were early echinoderm developments. The Paleozoic echinoderms were globular, attached to the substrate and were orientated with their oral surfaces upwards; the fossil echinoderms had ambulacral grooves extending down the side of the body, fringed on either side by brachioles, structures similar to the pinnules of a modern crinoid. It seems probable that the mouth-upward orientation is the primitive state and that at some stage, all the classes of echinoderms except the crinoids reversed this to become mouth-downward. Before this happened, the podia had a feeding function as they do in the crinoids today, their locomotor function came after the re-orientation of the mouth when the podia were in contact with the substrate for the firs
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago to the start of the Silurian Period 443.8 Mya. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, placed them in a system of their own; the Ordovician received international approval in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events.
Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today; the Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya. It lasted for about 42 million years and ended with the Ordovician–Silurian extinction events, about 443.8 Mya which wiped out 60% of marine genera. The dates given are recent radiometric dates and vary from those found in other sources; this second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs. The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is significant, it correlates well with the occurrence of widespread graptolite and trilobite species.
The base of the Tremadocian allows scientists to relate these species not only to each other, but to species that occur with them in other areas. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period. A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions. There exist Baltoscandic, Siberian, North American, Chinese Mediterranean and North-Gondwanan regional stratigraphic schemes; the Ordovician Period in Britain was traditionally broken into Early and Late epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column; the faunal stages from youngest to oldest are: Late Ordovician Hirnantian/Gamach Rawtheyan/Richmond Cautleyan/Richmond Pusgillian/Maysville/Richmond Middle Ordovician Trenton Onnian/Maysville/Eden Actonian/Eden Marshbrookian/Sherman Longvillian/Sherman Soudleyan/Kirkfield Harnagian/Rockland Costonian/Black River Chazy Llandeilo Whiterock Llanvirn Early Ordovician Cassinian Arenig/Jefferson/Castleman Tremadoc/Deming/Gaconadian The Tremadoc corresponds to the Tremadocian.
The Floian corresponds to the lower Arenig. The Llanvirn occupies the rest of the Darriwilian, terminates with it at the base of the Late Ordovician; the Sandbian represents the first half of the Caradoc. During the Ordovician, the southern continents were collected into Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents of Laurentia and Baltica were still independent continents, but Baltica began to move towards Laurentia in the period, causing the Iapetus Ocean between them to shrink; the small continent Avalonia separated from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia. The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times. In the early and middle Ordovician, temperatures were mild, but at the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere, turning the planet into a hothouse.
Sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and sea levels dropped. At first, low-lying sea beds increased diversity, but glaciation led to mass extinctions as the seas drained and continental shelves became dry land. During the Ordovician, in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved; these volcanic island arcs collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician the volcanic emissions had stopped. Gondwana had by that time neared the South Pole and was glaciated
Cystoidea is a class of extinct crinozoan echinoderms, termed cystoids, that lived attached to the sea floor by stalks. They existed during the Paleozoic Era, in the Middle Ordovician and Silurian Periods, until their extinction in the Devonian Period. Cystoids are distinguished from other echinoderms by triangular pore openings. Superficially, cystoids resembled crinoids; the mouth was at the upper pole of the body, with the opposite end attached to the substratum by a stalk, although some stalkless species did exist. The anus lay on the side of the body. Five, or less three, ambulacral areas ran along the outside of the body, radiating outwards from the mouth. A number of small tentacles either surrounded the mouth, or projected outwards in a row from the ambulacral areas, depending on species; the most distinctive feature of cystoids was the presence of a number of pores in the rigid skeleton encasing the body. These were most respiratory in nature, allowing fluid to flow in or out of the body.
In some species, the pores were clustered in distinct regions, but in others they were distributed quite over the body surface. See List of echinodermata orders. Prehistoric echinoderms Paleozoic echinoderms Barnes, Robert D.. Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. Pp. 1008–1009. ISBN 0-03-056747-5. Clarkson, E. N. K.. Invertebrate Paleontology and Evolution. Oxford, UK: Blackwell Science. P. 262. ISBN 0-632-05238-4