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 Jurassic period was a geologic period and system that spanned 56 million years from the end of the Triassic Period 201.3 million years ago to the beginning of the Cretaceous Period 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era known as the Age of Reptiles; the start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, the Tithonian event at the end; the Jurassic period is divided into three epochs: Early and Late. In stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, Upper Jurassic series of rock formations; the Jurassic is named after the Jura Mountains within the European Alps, where limestone strata from the period were first identified. By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses: Laurasia to the north, Gondwana to the south; this created more coastlines and shifted the continental climate from dry to humid, many of the arid deserts of the Triassic were replaced by lush rainforests.
On land, the fauna transitioned from the Triassic fauna, dominated by both dinosauromorph and crocodylomorph archosaurs, to one dominated by dinosaurs alone. The first birds appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Other major events include the appearance of the earliest lizards, the evolution of therian mammals, including primitive placentals. Crocodilians made the transition from a terrestrial to an aquatic mode of life; the oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates. The chronostratigraphic term "Jurassic" is directly linked to the Jura Mountains, a mountain range following the course of the France–Switzerland border. During a tour of the region in 1795, Alexander von Humboldt recognized the limestone dominated mountain range of the Jura Mountains as a separate formation that had not been included in the established stratigraphic system defined by Abraham Gottlob Werner, he named it "Jura-Kalkstein" in 1799.
The name "Jura" is derived from the Celtic root *jor via Gaulish *iuris "wooded mountain", borrowed into Latin as a place name, evolved into Juria and Jura. The Jurassic period is divided into three epochs: Early and Late. In stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, Upper Jurassic series of rock formations known as Lias and Malm in Europe; the separation of the term Jurassic into three sections originated with Leopold von Buch. The faunal stages from youngest to oldest are: During the early Jurassic period, the supercontinent Pangaea broke up into the northern supercontinent Laurasia and the southern supercontinent Gondwana; the Jurassic North Atlantic Ocean was narrow, while the South Atlantic did not open until the following Cretaceous period, when Gondwana itself rifted apart. The Tethys Sea closed, the Neotethys basin appeared. Climates were warm, with no evidence of a glacier having appeared; as in the Triassic, there was no land over either pole, no extensive ice caps existed.
The Jurassic geological record is good in western Europe, where extensive marine sequences indicate a time when much of that future landmass was submerged under shallow tropical seas. In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface. Though the epicontinental Sundance Sea left marine deposits in parts of the northern plains of the United States and Canada during the late Jurassic, most exposed sediments from this period are continental, such as the alluvial deposits of the Morrison Formation; the Jurassic was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate. Carbonate hardgrounds were thus common, along with calcitic ooids, calcitic cements, invertebrate faunas with dominantly calcitic skeletons; the first of several massive batholiths were emplaced in the northern American cordillera beginning in the mid-Jurassic, marking the Nevadan orogeny. Important Jurassic exposures are found in Russia, South America, Japan and the United Kingdom.
In Africa, Early Jurassic strata are distributed in a similar fashion to Late Triassic beds, with more common outcrops in the south and less common fossil beds which are predominated by tracks to the north. As the Jurassic proceeded and more iconic groups of dinosaurs like sauropods and ornithopods proliferated in Africa. Middle Jurassic strata are neither well studied in Africa. Late Jurassic strata are poorly represented apart from the spectacular Tendaguru fauna in Tanzania; the Late Jurassic life of Tendaguru is similar to that found in western North America's Morrison Formation. During the Jurassic period, the primary vertebrates living in the sea were marine reptiles; the latter include ichthyosaurs, which were at the peak of their diversity, plesiosaurs and marine crocodiles of the families Teleosauridae and Metriorhynchidae. Numerous turtles could be found in rivers. In the invertebrate world, several new groups appeared, including rudists (a reef-formi
Greenland is an autonomous constituent country of the Kingdom of Denmark between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for more than a millennium; the majority of its residents are Inuit, whose ancestors began migrating from the Canadian mainland in the 13th century settling across the island. Greenland is the world's largest island. Three-quarters of Greenland is covered by the only permanent ice sheet outside Antarctica. With a population of about 56,480, it is the least densely populated territory in the world. About a third of the population live in the capital and largest city; the Arctic Umiaq Line ferry acts as a lifeline for western Greenland, connecting the various cities and settlements. Greenland has been inhabited at intervals over at least the last 4,500 years by Arctic peoples whose forebears migrated there from what is now Canada.
Norsemen settled the uninhabited southern part of Greenland beginning in the 10th century, having settled Iceland to escape persecution from the King of Norway and his central government. These Norsemen would set sail from Greenland and Iceland, with Leif Erikson becoming the first known European to reach North America nearly 500 years before Columbus reached the Caribbean islands. Inuit peoples arrived in the 13th century. Though under continuous influence of Norway and Norwegians, Greenland was not formally under the Norwegian crown until 1262; the Norse colonies disappeared in the late 15th century when Norway was hit by the Black Death and entered a severe decline. Soon after their demise, beginning in 1499, the Portuguese explored and claimed the island, naming it Terra do Lavrador. In the early 18th century, Danish explorers reached Greenland again. To strengthen trading and power, Denmark–Norway affirmed sovereignty over the island; because of Norway's weak status, it lost sovereignty over Greenland in 1814 when the union was dissolved.
Greenland became Danish in 1814, was integrated in the Danish state in 1953 under the Constitution of Denmark. In 1973, Greenland joined the European Economic Community with Denmark. However, in a referendum in 1982, a majority of the population voted for Greenland to withdraw from the EEC, effected in 1985. Greenland contains the world's largest and most northerly national park, Northeast Greenland National Park. Established in 1974, expanded to its present size in 1988, it protects 972,001 square kilometres of the interior and northeastern coast of Greenland and is bigger than all but twenty-nine countries in the world. Greenland is divided into five municipalities – Sermersooq, Qeqertalik and Avannaata. Greenland does not have an independent seat at the United Nations. In 1979, Denmark granted home rule to Greenland, in 2008, Greenlanders voted in favor of the Self-Government Act, which transferred more power from the Danish government to the local Greenlandic government. Under the new structure, in effect since 21 June 2009, Greenland can assume responsibility for policing, judicial system, company law and auditing.
It retains control of monetary policy, providing an initial annual subsidy of DKK 3.4 billion, planned to diminish over time. Greenland expects to grow its economy based on increased income from the extraction of natural resources; the capital, held the 2016 Arctic Winter Games. At 70%, Greenland has one of the highest shares of renewable energy in the world coming from hydropower; the early Norse settlers named the island as Greenland. In the Icelandic sagas, the Norwegian-born Icelander Erik the Red was said to be exiled from Iceland for manslaughter. Along with his extended family and his thralls, he set out in ships to explore an icy land known to lie to the northwest. After finding a habitable area and settling there, he named it Grœnland in the hope that the pleasant name would attract settlers; the Saga of Erik the Red states: "In the summer, Erik left to settle in the country he had found, which he called Greenland, as he said people would be attracted there if it had a favorable name."The name of the country in the indigenous Greenlandic language is Kalaallit Nunaat.
The Kalaallit are the indigenous Greenlandic Inuit people. In prehistoric times, Greenland was home to several successive Paleo-Eskimo cultures known today through archaeological finds; the earliest entry of the Paleo-Eskimo into Greenland is thought to have occurred about 2500 BC. From around 2500 BC to 800 BC, southern and western Greenland were inhabited by the Saqqaq culture. Most finds of Saqqaq-period archaeological remains have been around Disko Bay, including the site of Saqqaq, after which the culture is named. From 2400 BC to 1300 BC, the Independence I culture existed in northern Greenland, it was a part of the Arctic small tool tradition. Towns, including Deltaterrassern
Pinnipeds known as seals, are a distributed and diverse clade of carnivorous, fin-footed, semiaquatic marine mammals. They comprise the extant families Odobenidae and Phocidae. There are 33 extant species of pinnipeds, more than 50 extinct species have been described from fossils. While seals were thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic lineage. Pinnipeds belong to the order Carnivora and their closest living relatives are believed to be bears and the superfamily of musteloids, having diverged about 50 million years ago. Seals range in size from the 1 m and 45 kg Baikal seal to the 5 m and 3,200 kg southern elephant seal, the largest member of the order Carnivora. Several species exhibit sexual dimorphism, they have four limbs that are modified into flippers. Though not as fast in the water as dolphins, seals are more agile. Otariids use their front limbs to propel themselves through the water, while phocids and walruses use their hind limbs.
Otariids and walruses have hind limbs that can be used as legs on land. By comparison, terrestrial locomotion by phocids is more cumbersome. Otariids have visible external ears, while walruses lack these. Pinnipeds have well-developed senses—their eyesight and hearing are adapted for both air and water, they have an advanced tactile system in their whiskers or vibrissae; some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in the cold water, other than the walrus, all species are covered in fur. Although pinnipeds are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres, they spend most of their lives in the water, but come ashore to mate, give birth, molt or escape from predators, such as sharks and killer whales. They feed on fish and marine invertebrates. Walruses are specialized for feeding on bottom-dwelling mollusks. Male pinnipeds mate with more than one female, although the degree of polygyny varies with the species.
The males of land-breeding species tend to mate with a greater number of females than those of ice breeding species. Male pinniped strategies for reproductive success vary between defending females, defending territories that attract females and performing ritual displays or lek mating. Pups are born in the spring and summer months and females bear all the responsibility for raising them. Mothers of some species fast and nurse their young for a short period of time while others take foraging trips at sea between nursing bouts. Walruses are known to nurse their young while at sea. Seals produce a number of vocalizations, notably the barks of California sea lions, the gong-like calls of walruses and the complex songs of Weddell seals; the meat and fur coats of pinnipeds have traditionally been used by indigenous peoples of the Arctic. Seals have been depicted in various cultures worldwide, they are kept in captivity and are sometimes trained to perform tricks and tasks. Once relentlessly hunted by commercial industries for their products and walruses are now protected by international law.
The Japanese sea lion and the Caribbean monk seal have become extinct in the past century, while the Mediterranean monk seal and Hawaiian monk seal are ranked endangered by the International Union for Conservation of Nature. Besides hunting, pinnipeds face threats from accidental trapping, marine pollution, conflicts with local people; the German naturalist Johann Karl Wilhelm Illiger was the first to recognize the pinnipeds as a distinct taxonomic unit. American zoologist Joel Asaph Allen reviewed the world's pinnipeds in an 1880 monograph, History of North American pinnipeds, a monograph of the walruses, sea-lions, sea-bears and seals of North America. In this publication, he traced the history of names, gave keys to families and genera, described North American species and provided synopses of species in other parts of the world. In 1989, Annalisa Berta and colleagues proposed the unranked clade Pinnipedimorpha to contain the fossil genus Enaliarctos and modern seals as a sister group. Pinnipeds belong to the suborder Caniformia.
Pinnipedia was considered its own suborder under Carnivora. Of the three extant families, the Otariidae and Odobenidae are grouped in the superfamily Otarioidea, while the Phocidae belong to the superfamily Phocoidea. Otariids are known as eared seals due to the presence of pinnae; these animals rely on their well-developed fore-flippers to propel themselves through the water. They can turn their hind-flippers forward and "walk" on land; the anterior end of an otariid's frontal bones extends between the nasal bones, the supraorbital foramen is large and flat horizontally. The supraspinatous fossas are divided by a "secondary spine" and the bronchi are divided anteriorly. Otariids consist of two types: fur seals. Sea lions are distinguished by their rounder snouts and shorter, rougher pelage, while fur seals have more pointed snouts, longer fore-flippers and thicker fur coats that include an undercoat and guard hairs; the former tend to be larger than the latter. Five genera and seven species of
The Lissamphibia are a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia, the Caudata, the Gymnophiona. A fourth group, the Allocaudata, was moderately successful, spanning 160 million years from the Middle Jurassic to the Early Pliocene, but became extinct 2.5 million years ago. For several decades, this name has been used for a group that includes all living amphibians, but excludes all the main groups of Paleozoic tetrapods, such as Temnospondyli, Lepospondyli and Seymouriamorpha; some scientists have concluded that all of the primary groups of modern amphibians—frogs and caecilians—are related. Some writers have argued that the early Permian dissorophoid Gerobatrachus hottoni is a lissamphibian. If it is not, the earliest known lissamphibians are Triadobatrachus and Czatkobatrachus from the Early Triassic. Some, if not all, lissamphibians share the following characteristics; some of these apply to the soft body parts, hence do not appear in fossils.
However, the skeletal characteristics appear in several types of Palaeozoic amphibians: Double or paired occipital condyles Two types of skin glands Fat bodies associated with gonads Double-channeled sensory papillae in the inner ear Green rods Ribs do not encircle body. Anurans do not have ribs. Ability to elevate the eyes Forced-pump respiratory mechanism, the primitive breathing system found in labyrinthodont amphibians Cylindrical centra Pedicellate teeth Bicuspid teeth Operculum Loss of posterior skull bones Small separated pterygoid bones Wide cultriform process of the parasphenoid The features uniting the Lissamphibia were first noted by Ernst Haeckel though in Haeckel's work, Lissamphibia excluded the caecilians. Haeckel considered the caecilians to be related to what he called Lissamphibia, now called Batrachia and includes frogs and salamanders. In the early to mid 20th century, a biphyletic origin of amphibians was favoured. In the late 20th century, a flood of new fossil evidence mapped out in some detail the nature of the transition between the elpistostegalid fish and the early amphibians, most paleontologists no longer accept the view that amphibians have arisen twice, from two related but separate groups of fish.
With the single origin of amphibians being accepted by most herpetologists and paleontologists, it was assumed that Lissamphibia was monophyletic as well. However, the origin and relationships of the various lissamphibian groups both with each other and among other early tetrapods remains controversial. Not all paleontologists today are convinced that Lissamphibia is indeed a natural group, as there are important characteristics shared with some non-lissamphibian Palaeozoic amphibians; the two prevailing theories of lissamphibian origin are: Monophyletic within the temnospondyli Monophyletic within lepospondyliOne of the hypotheses regarding their ancestors is that they evolved by paedomorphosis and miniaturization from early tetrapods. Recent molecular studies of extant amphibians based on multiple-locus data favor one or the other of the monophyletic alternatives and indicate a Late Carboniferous date for the divergence of the lineage leading to caecilians from the one leading to frogs and salamanders, an early Permian date for the separation of the frog and salamander groups.
Benton, M. J. Vertebrate Palaeontology, 3rd ed. Blackwell. Carroll, R. L. Vertebrate Paleontology and Evolution, WH Freeman & Co. San Mauro, Diego. "Initial diversification of living amphibians predated the breakup of Pangaea". American Naturalist. 165: 590–599. Doi:10.1086/429523. PMID 15795855. Biology 356 - Major Features of Vertebrate Evolution by Dr. Robert Reisz, University of Toronto
Stegocephalia is a name used for four-limbed stem-tetrapods, their amphibian-grade descendants, in phylogenetic nomenclature for all tetrapods. The term was coined in 1868 by American palaentologist Edward Drinker Cope and comes from Greek στεγοκεφαλια - "roofed head", refer to the copious amounts of dermal armour some of the larger primitive forms evidently had. Canadian paleontologist Michel Laurin gave the group its first formal phylogenetic definition including all vertebrates with toes rather than fins; the term was used as a systematic unit at the rank of order. The term rose to prominence in American and British science in the 19th century, though the equivalent term Labyrinthodontia had been coined 18 years earlier by Hermann Burmeister in reference to the tooth structure; the terms were used interchangeably during the early 20th century divided into three orders. However, the Stegocephalia being paraphyletic, the name is now used in an informal way to denote the early non-piscine vertebrates, excluding amniotes and modern lissamphibians).
Paleontologist Michel Laurin took up the older term and defined stegocephalians cladistically as all vertebrates more related to temnospondyli than to Panderichthys Therefore, Stegocephalia includes all vertebrate groups that have toes rather than fins, a few that may have retained paired fins. Contrary to the old usage of this term, the Stegocephali refers to a clade in this scheme; this concept of the clade Stegocephalia was chosen to substitute for the name Tetrapoda by those who sought to restrict Tetrapoda to the crown group. As such, it encompasses all presently living land vertebrates as well as their early amphibious ancestors. Below is an evolutionary tree of stegocephalians, as recovered from an analysis by Swartz in 2012. Tree of Life: Terrestrial Vertebrates
A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, petrified wood, coal, DNA remnants; the totality of fossils is known as the fossil record. Paleontology is the study of fossils: their age, method of formation, evolutionary significance. Specimens are considered to be fossils if they are over 10,000 years old; the oldest fossils are around 3.48 billion years old to 4.1 billion years old. The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils; the development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host. There are many processes that lead to fossilization, including permineralization and molds, authigenic mineralization and recrystallization, adpression and bioimmuration.
Fossils vary in size from one-micrometre bacteria to dinosaurs and trees, many meters long and weighing many tons. A fossil preserves only a portion of the deceased organism that portion, mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates. Fossils may consist of the marks left behind by the organism while it was alive, such as animal tracks or feces; these types of fossil are called trace ichnofossils, as opposed to body fossils. Some fossils are called chemofossils or biosignatures; the process of fossilization varies according to external conditions. Permineralization is a process of fossilization; the empty spaces within an organism become filled with mineral-rich groundwater. Minerals precipitate from the groundwater; this process can occur in small spaces, such as within the cell wall of a plant cell. Small scale permineralization can produce detailed fossils. For permineralization to occur, the organism must become covered by sediment soon after death, otherwise decay commences.
The degree to which the remains are decayed when covered determines the details of the fossil. Some fossils consist only of skeletal teeth; this is a form of diagenesis. In some cases, the original remains of the organism dissolve or are otherwise destroyed; the remaining organism-shaped hole in the rock is called an external mold. If this hole is filled with other minerals, it is a cast. An endocast, or internal mold, is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull; this is a special form of mold formation. If the chemistry is right, the organism can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it. If this happens before significant decay to the organic tissue fine three-dimensional morphological detail can be preserved. Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, USA, are among the best documented examples of such mineralization.
Replacement occurs. In some cases mineral replacement of the original shell occurs so and at such fine scales that microstructural features are preserved despite the total loss of original material. A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite. Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues. In this case the fossil consists of original material, albeit in a geochemically altered state; this chemical change is an expression of diagenesis. What remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression. However, the phytoleim is lost and all that remains is an impression of the organism in the rock—an impression fossil. In many cases, however and impressions occur together. For instance, when the rock is broken open, the phytoleim will be attached to one part, whereas the counterpart will just be an impression.
For this reason, one term covers the two modes of preservation: adpression. Because of their antiquity, an unexpected exception to the alteration of an organism's tissues by chemical reduction of the complex organic molecules during fossilization has been the discovery of soft tissue in dinosaur fossils, including blood vessels, the isolation of proteins and evidence for DNA fragments. In 2014, Mary Schweitzer and her colleagues reported the presence of iron particles associated with soft tissues recovered from dinosaur fossils. Based on various experiments that studied the interaction of iron in haemoglobin with blood vessel tissue they proposed that solution hypoxia coupled with iron chelation enhances the stability and preservation of soft tissue and provides the basis for an explanation for the unforeseen preservation of fossil soft tissues. However, a older study based on eight taxa ranging in time from the Devonian to the Jurassic found that reasonably well-preserved fibrils that represent collagen were preser