Chondrichthyes
Chondrichthyes is a class that contains the cartilaginous fishes: they are jawed vertebrates with paired fins, paired nares, scales, a heart with its chambers in series, skeletons made of cartilage rather than bone. The class is divided into two subclasses: Holocephali. Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates; the skeleton is cartilaginous. The notochord is replaced by a vertebral column during development, except in Holocephali, where the notochord stays intact. In some deepwater sharks, the column is reduced; as they do not have bone marrow, red blood cells are produced in the epigonal organ. They are produced in the Leydig's organ, only found in certain cartilaginous fishes; the subclass Holocephali, a specialized group, lacks both the Leydig's and epigonal organs. Apart from electric rays, which have a thick and flabby body, with soft, loose skin, chondrichthyans have tough skin covered with dermal teeth called placoid scales, making it feel like sandpaper.
In most species, all dermal denticles are oriented in one direction, making the skin feel smooth if rubbed in one direction and rough if rubbed in the other. The pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and pubioischiadic bars evolved. In rays, the pectoral fins have connected to the head and are flexible. One of the primary characteristics present in most sharks is the heterocercal tail, which aids in locomotion. Chondrichthyans have toothlike scales called placoid scales. Denticles provide protection, in most cases, streamlining. Mucous glands exist in some species, as well, it is assumed that their oral teeth evolved from dermal denticles that migrated into the mouth, but it could be the other way around, as the teleost bony fish Denticeps clupeoides has most of its head covered by dermal teeth. This is most a secondary evolved characteristic, which means there is not a connection between the teeth and the original dermal scales.
The old placoderms had sharp bony plates in their mouth. Thus, it is unknown whether the oral teeth evolved first, it has been suggested that the original bony plates of all vertebrates are now gone and that the present scales are just modified teeth if both the teeth and body armor had a common origin a long time ago. However, there is no evidence of this. All chondrichthyans breathe through five depending on the species. In general, pelagic species must keep swimming to keep oxygenated water moving through their gills, whilst demersal species can pump water in through their spiracles and out through their gills. However, this is only a general rule and many species differ. A spiracle is a small hole found behind each eye; these can be tiny and circular, such as found on the nurse shark, to extended and slit-like, such as found on the wobbegongs. Many larger, pelagic species, such as the mackerel sharks and the thresher sharks, no longer possess them. Chondrichthyes nervous system is composed of a small brain, 8-10 pairs of cranial nerves, a spinal chord with spinal nerves.
They have several sensory organs. Ampullae of Lorenzini are a network of small jelly filled pores called electroreceptors which help the fish sense electric fields in water; this aids in finding prey and sensing temperature. The Lateral line system has modified epithelial cells located externally which sense motion and pressure in the water around them. Most subspecies have large well-developed eyes, they have powerful nostrils and olfactory organs. Their inner ears consist of 3 large semicircular canals which aid in orientation, their sound detecting apparatus has limited range and is more powerful at lower frequencies. Some subspecies have electric organs which can be used for predation, they have simple brains with the forebrain not enlarged. The structure and formation of myelin in their nervous systems are nearly identical to that of tetrapods, which has led evolutionary biologists to believe that Chondrichthyes were a cornerstone group in the evolutionary timeline of myelin development. Like all other jawed vertebrates, members of Chondrichthyes have an adaptive immune system.
Fertilization is internal. Development is live birth but can be through eggs; some rare species are viviparous. There is no parental care after birth. Capture-induced premature birth and abortion occurs in sharks/rays when fished. Capture-induced parturition is mistaken for natural birth by recreational fishers and is considered in commercial fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays; the class Chondrichthyes has two subclasses: the subclass Elasmobranchii
Triassic
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
Temnospondyli
Temnospondyli is a diverse subclass of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carboniferous and Triassic periods. A few species continued into the Cretaceous. Fossils have been found on every continent. During about 210 million years of evolutionary history, they adapted to a wide range of habitats, including fresh water and coastal marine environments, their life history is well understood, with fossils known from the larval stage and maturity. Most temnospondyls were semiaquatic, although some were fully terrestrial, returning to the water only to breed; these temnospondyls were some of the first vertebrates adapted to life on land. Although temnospondyls are considered amphibians, many had characteristics, such as scales and armour-like bony plates, that distinguish them from modern amphibians. Temnospondyls have been known since the early 19th century, were thought to be reptiles, they were described at various times as batrachians and labyrinthodonts, although these names are now used.
Animals now grouped in Temnospondyli were spread out among several amphibian groups until the early 20th century, when they were found to belong to a distinct taxon based on the structure of their vertebrae. Temnospondyli means "cut vertebrae". Experts disagree over whether temnospondyls were ancestral to modern amphibians, or whether the whole group died out without leaving any descendants. Different hypotheses have placed modern amphibians as the descendants of temnospondyls, another group of early tetrapods called lepospondyls, or as descendants of both groups. Recent studies place a family of temnospondyls called the amphibamids as the closest relatives of modern amphibians. Similarities in teeth and hearing structures link the two groups. Many temnospondyls are much larger than living amphibians, superficially resemble crocodiles. Others resemble salamanders. Most have flat heads that are either blunt or elongated; the skulls are rounded or triangular in shape when viewed from above, are covered in pits and ridges.
The rugged surfaces of bones may have supported blood vessels, which could transfer carbon dioxide to the bones to neutralize acidic build up in the blood. Many temnospondyls have canal-like grooves in their skulls called sensory sulci; the sulci, which run around the nostrils and eye sockets, are part of a lateral line system used to detect vibrations in water. As semiaquatic animals, all known temnospondyls have small limbs with no more than four toes on each front foot and five on each hind foot. Terrestrial temnospondyls have larger, thicker limbs, some have claws. One unusual terrestrial temnospondyl, has long limbs for its body, lived as an active runner able to chase prey. Homologues of most of the bones of temnospondyls are seen in other early tetrapods, aside from a few bones in the skull, such as interfrontals and interparietals, that have developed in some temnospondyl taxa. Most temnospondyls have tabular horns in the backs of their skulls, rounded projections of bone separated from the rest of the skull by indentations called otic notches.
Among the most distinguishing features of temnospondyls are the interpterygoid vacuities, two large holes in the back of the palate. Another pair of holes, are present in front of these vacuities, connect the nasal passage with the mouth. Temnospondyls have teeth on their palates, as well as in their jaws; some of these teeth are so large, they are referred to as tusks. In some temnospondyls, such as Nigerpeton, tusks in the lower jaw pierce the palate and emerge through openings in the top of the skull. Little is known of the soft tissue of temnospondyls. A block of sandstone, described in 2007 from the Early Carboniferous Mauch Chunk Formation of Pennsylvania, included impressions of the bodies of three temnospondyls; these impressions show, when alive, they had smooth skin, robust limbs with webbed feet, a ridge of skin on their undersides. Trackways referable to small temnospondyls have been found in Carboniferous and Permian rocks; the trackways, called batrachichni, are found in strata deposited around freshwater environments, suggesting the animals had some ties to the water.
Unlike modern amphibians, many temnospondyls are covered in small packed scales. The undersides of most temnospondyls are covered in rows of large ventral plates. During early stages of development, they first have only rounded scales. Fossils show, as the animals grew, the scales on the undersides of their bodies developed into large, wide ventral plates; the plates overlap each other in a way. Semiaquatic temnospondyls, such as trematosaurs and capitosaurs, have no evidence of scales, they may have lost scales to make movement easier under water or to allow cutaneous respiration, the absorption of oxygen through the skin. Several groups of temnospondyls have large bony plates on their backs. One temnospondyl, has armour-like plating that covers both its back and underside; the temnospondyl Laidleria has
Transcription (biology)
Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA by the enzyme RNA polymerase. Both DNA and RNA are nucleic acids. During transcription, a DNA sequence is read by an RNA polymerase, which produces a complementary, antiparallel RNA strand called a primary transcript. Transcription proceeds in the following general steps: RNA polymerase, together with one or more general transcription factors, binds to promoter DNA. RNA polymerase creates a transcription bubble; this is done by breaking the hydrogen bonds between complementary DNA nucleotides. RNA polymerase adds RNA nucleotides. RNA sugar-phosphate backbone forms with assistance from RNA polymerase to form an RNA strand. Hydrogen bonds of the RNA–DNA helix break, freeing the newly synthesized RNA strand. If the cell has a nucleus, the RNA may be further processed; this may include polyadenylation and splicing. The RNA may exit to the cytoplasm through the nuclear pore complex; the stretch of DNA transcribed into an RNA molecule is called a transcription unit and encodes at least one gene.
If the gene encodes a protein, the transcription produces messenger RNA. Alternatively, the transcribed gene may encode for non-coding RNA such as microRNA, ribosomal RNA, transfer RNA, or enzymatic RNA molecules called ribozymes. Overall, RNA helps synthesize and process proteins. In virology, the term may be used when referring to mRNA synthesis from an RNA molecule. For instance, the genome of a negative-sense single-stranded RNA virus may be template for a positive-sense single-stranded RNA; this is because the positive-sense strand contains the information needed to translate the viral proteins for viral replication afterwards. This process is catalyzed by a viral RNA replicase. A DNA transcription unit encoding for a protein may contain both a coding sequence, which will be translated into the protein, regulatory sequences, which direct and regulate the synthesis of that protein; the regulatory sequence before the coding sequence is called the five prime untranslated region. As opposed to DNA replication, transcription results in an RNA complement that includes the nucleotide uracil in all instances where thymine would have occurred in a DNA complement.
Only one of the two DNA strands serve as a template for transcription. The antisense strand of DNA is read by RNA polymerase from the 3' end to the 5' end during transcription; the complementary RNA is created in the opposite direction, in the 5' → 3' direction, matching the sequence of the sense strand with the exception of switching uracil for thymine. This directionality is because RNA polymerase can only add nucleotides to the 3' end of the growing mRNA chain; this use of only the 3' → 5' DNA strand eliminates the need for the Okazaki fragments that are seen in DNA replication. This removes the need for an RNA primer to initiate RNA synthesis, as is the case in DNA replication; the non-template strand of DNA is called the coding strand, because its sequence is the same as the newly created RNA transcript. This is the strand, used by convention when presenting a DNA sequence. Transcription has some proofreading mechanisms, but they are fewer and less effective than the controls for copying DNA.
As a result, transcription has a lower copying fidelity than DNA replication. Transcription is divided into initiation, promoter escape and termination. Transcription begins with the binding of RNA polymerase, together with one or more general transcription factors, to a specific DNA sequence referred to as a "promoter" to form an RNA polymerase-promoter "closed complex". In the "closed complex" the promoter DNA is still double-stranded. RNA polymerase, assisted by one or more general transcription factors unwinds 14 base pairs of DNA to form an RNA polymerase-promoter "open complex". In the "open complex" the promoter DNA is unwound and single-stranded; the exposed, single-stranded DNA is referred to as the "transcription bubble."RNA polymerase, assisted by one or more general transcription factors selects a transcription start site in the transcription bubble, binds to an initiating NTP and an extending NTP complementary to the transcription start site sequence, catalyzes bond formation to yield an initial RNA product.
In bacteria, RNA polymerase holoenzyme consists of five subunits: 2 α subunits, 1 β subunit, 1 β' subunit, 1 ω subunit. In bacteria, there is one general RNA transcription factor: sigma. RNA polymerase core enzyme binds to the bacterial general transcription factor sigma to form RNA polymerase holoenzyme and binds to a promoter. In archaea and eukaryotes, RNA polymerase contains subunits homologous to each of the five RNA polymerase subunits in bacteria and contains additional subunits. In archaea and eukaryotes, the functions of the bacterial general transcription factor sigma are performed by multiple general transcription factors that work together. In archaea, there ar
Brain
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head close to the sensory organs for senses such as vision; the brain is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains 14–16 billion neurons, the estimated number of neurons in the cerebellum is 55–70 billion; each neuron is connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells. Physiologically, the function of the brain is to exert centralized control over the other organs of the body; the brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows coordinated responses to changes in the environment.
Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain. The operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. Recent models in modern neuroscience treat the brain as a biological computer different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, processes it in a variety of ways; this article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human brain insofar; the ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context.
The most important is brain disease and the effects of brain damage, that are covered in the human brain article. The shape and size of the brain varies between species, identifying common features is difficult. There are a number of principles of brain architecture that apply across a wide range of species; some aspects of brain structure are common to the entire range of animal species. The simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, sliced apart for examination of the interior. Visually, the interior of the brain consists of areas of so-called grey matter, with a dark color, separated by areas of white matter, with a lighter color. Further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations.
It is possible to examine the microstructure of brain tissue using a microscope, to trace the pattern of connections from one brain area to another. The brains of all species are composed of two broad classes of cells: neurons and glial cells. Glial cells come in several types, perform a number of critical functions, including structural support, metabolic support and guidance of development. Neurons, are considered the most important cells in the brain; the property that makes neurons unique is their ability to send signals to specific target cells over long distances. They send these signals by means of an axon, a thin protoplasmic fiber that extends from the cell body and projects with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body; the length of an axon can be extraordinary: for example, if a pyramidal cell of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon magnified, would become a cable a few centimeters in diameter, extending more than a kilometer.
These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials at rates of 10–100 per second in irregular patterns. Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells; when an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell. Synapses are the key functional elements of the brain; the essential function of the brain is cell-to-cell communication, synapses are the points at which communication occurs. The human brain has been estimated to contain 100 trillion synapses; the functions of these synapses are diverse: some are excitatory.
Cretaceous
The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, the longest period of the Phanerozoic Eon; the Cretaceous Period is abbreviated K, for its German translation Kreide. The Cretaceous was a period with a warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas; these oceans and seas were populated with now-extinct marine reptiles and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared; the Cretaceous ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary, a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras.
The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris Basin and named for the extensive beds of chalk, found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from Latin creta; the Cretaceous is divided into Early and Late Cretaceous epochs, or Lower and Upper Cretaceous series. In older literature the Cretaceous is sometimes divided into three series: Neocomian and Senonian. A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use; as with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact age of the system's base is uncertain by a few million years. No great extinction or burst of diversity separates the Cretaceous from the Jurassic. However, the top of the system is defined, being placed at an iridium-rich layer found worldwide, believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and into the Gulf of Mexico.
This layer has been dated at 66.043 Ma. A 140 Ma age for the Jurassic-Cretaceous boundary instead of the accepted 145 Ma was proposed in 2014 based on a stratigraphic study of Vaca Muerta Formation in Neuquén Basin, Argentina. Víctor Ramos, one of the authors of the study proposing the 140 Ma boundary age sees the study as a "first step" toward formally changing the age in the International Union of Geological Sciences. From youngest to oldest, the subdivisions of the Cretaceous period are: Late Cretaceous Maastrichtian – Campanian – Santonian – Coniacian – Turonian – Cenomanian – Early Cretaceous Albian – Aptian – Barremian – Hauterivian – Valanginian – Berriasian – The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms; the Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of marine limestone, a rock type, formed under warm, shallow marine circumstances.
Due to the high sea level, there was extensive space for such sedimentation. Because of the young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide. Chalk is a rock type characteristic for the Cretaceous, it consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas. In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast; the group is found in England, northern France, the low countries, northern Germany, Denmark and in the subsurface of the southern part of the North Sea. Chalk is not consolidated and the Chalk Group still consists of loose sediments in many places; the group has other limestones and arenites. Among the fossils it contains are sea urchins, belemnites and sea reptiles such as Mosasaurus. In southern Europe, the Cretaceous is a marine system consisting of competent limestone beds or incompetent marls.
Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European continental shelf, at the margin of the Tethys Ocean. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half the worlds petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic shales were formed during this interval; these shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. During th
Placodermi
Placodermi is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish. Placoderms are paraphyletic, consist of several distinct outgroups or sister taxa to all living jawed vertebrates, which originated among their ranks; this is illustrated by a 419-million-year-old fossil, from China, the only known placoderm with a type of bony jaw like that found in modern bony fishes. This includes a dentary bone, found in humans and other tetrapods. A recent analysis shows placodermi to be monophyletic; the jaws in other placoderms were consisted of a single bone. Placoderms were the first fish to develop pelvic fins, the precursor to hindlimbs in tetrapods, as well as true teeth. Paraphyletic groupings are problematic, as one can not talk about their phylogenic relationships, their characteristic traits and literal extinction.
380-million-year-old fossils of three other genera, Incisoscutum and Austroptyctodus, represent the oldest known examples of live birth. The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian; the various groups of placoderms were diverse and abundant during the Devonian, but became extinct at the end-Devonian Hangenberg event 358.9 million years ago Many placoderms the Rhenanida, Petalichthyida and Antiarchi, were bottom-dwellers. In particular, the antiarchs, with their modified, jointed bony pectoral fins, were successful inhabitants of Middle-Late Devonian freshwater and shallow marine habitats, with the Middle to Late Devonian genus, known from over 100 valid species; the vast majority of placoderms were predators, many of which lived near the substrate. Many the Arthrodira, were active, nektonic predators that dwelled in the middle to upper portions of the water column. A study of the arthrodire Compagopiscis published in 2012 concluded that placoderms possessed true teeth contrary to some early studies.
The teeth were made of both bone and dentine. However, the tooth and jaw development were not as integrated as in modern gnathostomes; these teeth were homologous to the teeth of other gnathostomes. One of the largest known arthrodires, Dunkleosteus terrelli, was 6 m long, is presumed to have had a large distribution, as its remains have been found in Europe, North America and Morocco; some paleontologists regard it as the world's first vertebrate "superpredator", preying upon other predators. Other, smaller arthrodires, such as Fallacosteus and Rolfosteus, both of the Gogo Formation of Western Australia, had streamlined, bullet-shaped head armor supporting the idea that many, if not most, arthrodires were active swimmers, rather than passive ambush-hunters whose armor anchored them to the sea floor; some placoderms were herbivorous, such as the Middle to Late Devonian arthrodire Holonema, some were planktivores, such as the gigantic, 8 m long arthrodire, Titanichthys. Extraordinary evidence of internal fertilization in a placoderm was afforded by the discovery in the Gogo Formation, near Fitzroy Crossing, Western Australia, of a small female placoderm, about 25 cm in length, which died in the process of giving birth to a 6 cm offspring and was fossilized with the umbilical cord intact.
The fossil, named Materpiscis attenboroughi, had eggs which were fertilized internally, the mother providing nourishment to the embryo and giving birth to live young. With this discovery, the placoderm became the oldest vertebrate known to have given birth to live young, pushing the date of first viviparity back some 200 million years earlier than had been known. Specimens of the arthrodire Incisoscutum ritchei from the Gogo Formation, have been found with embryos inside them indicating this group had live bearing ability; the males reproduced by inserting a long clasper into the female. Elongated basipterygia are found on the phyllolepid placoderms, such as Austrophyllolepis and Cowralepis, both from the Middle Devonian of Australia, suggesting that the basiptergia were used in copulation; the placoderm claspers are not homologous with the claspers in cartilaginous fishes. The similarities between the structures has been revealed to be an example of convergent evolution. While the claspers in cartilaginous fishes are specialized parts of their paired pelvic fins that have been modified for copulation due to changes in the hox genes hoxd13, the origin of the mating organs in placoderms most relied on different sets of hox genes and were structures that developed further down the body as an extra and independent pair of appendages, but which during development turned into body parts used for reproduction only.
Because they were not attached to the pelvic fins, as are the claspers in fish like sharks, they were much more flexible and could be rotated forward. It was thought for a time that placoderms became extinct due to competition from the first bony fish and early sharks, given a combination of the supposed inherent superiority of bony fish and the presumed sluggishness of placoderms. With more accurate summaries of prehistoric organisms, it is now thought that they systematically died out as marine and freshwater ecologi
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