The cynodonts are therapsids that first appeared in the Late Permian. The group includes modern mammals as well as their extinct close relatives. Nonmammalian cynodonts spread throughout southern Pangea and are represented by fossils from South America, Africa and Antarctica. In the northern continents, fossils have been found in eastern North America as well as in Belgium and northwestern France. Cynodontia is one of the most diverse groups of therapsids. Richard Owen named Cynodontia in 1861. Robert Broom reranked Cynodonia as an infraorder, since retained by others, including Colbert and Kitching, Gauthier et al. and Rubidge and Cristian Sidor. Olson assigned Cynodontia to Theriodonta and Kitching to Theriodontia, Rubridge and Sidor to Eutheriodontia. William King Gregory, Carroll, Gauthier et al. Hopson and Kitching and Botha et al.. Botha et al. seems to have without specifying taxonomic rank. In 2001, James Allen Hopson e.a. defined a clade Cynodontia as the most inclusive group containing Mammalia but excluding Bauria.
Together with the extinct gorgonopsians and the therocephalians, the cynodonts themselves are part of a group of therapsids called theriodonts. The oldest and the most basal cynodont yet found. Other basal cynodonts were a family that includes Procynosuchus and Dvinia. Cynodonts were among the few groups of synapsids that survived the Permian–Triassic extinction event and had a slow recovery after the extinction; the most derived cynodonts are found within the clade Eucynodontia, which contains the members of Mammalia. Representative genera of nonmammalian cynodonts include the large carnivorous cynognathids, the large herbivorous traversodonts, the small mammal-like tritylodontids and ictidosaurs; the presence of respiratory turbinates suggests a rapid metabolism and endothermy. During their evolution, the number of cynodont jaw bones reduced; this move towards a single bone for the mandible paved the way for other bones in the jaw, the articular and angular, to migrate to the cranium, where they function as parts of the mammalian hearing system.
Cynodonts developed a secondary palate in the roof of the mouth. This caused air flow from the nostrils to travel to a position in the back of the mouth instead of directly through it, allowing cynodonts to chew and breathe at the same time; this characteristic is present in all mammals. Early cynodonts have many of the skeletal characteristics of mammals; the teeth were differentiated and the braincase bulged at the back of the head. Outside of some crown-group mammals, all cynodonts laid eggs; the temporal fenestrae were much larger than those of their ancestors, the widening of the zygomatic arch in a more mammal-like skull would have allowed for more robust jaw musculature. They have the secondary palate that other primitive therapsids lacked, except the therocephalians, who were the closest relatives of cynodonts; the dentary was the largest bone in their lower jaw. The cynodonts had some form of warm-blooded metabolism; this has led to many reconstructions of cynodonts as having fur. Being endothermic they may have needed it for thermoregulation, but fossil evidence of their fur has been elusive.
Modern mammals have Harderian glands secreting lipids to coat their fur, but the telltale imprint of this structure is only found from the primitive mammal Morganucodon and onwards. Nonetheless, recent studies on Permian synapsid coprolites show that more basal therapsids had fur, at any rate fur was present in Mammaliaformes such as Castorocauda and Megaconus. Marks in the upper and lower jaw of cynodonts have been interpreted as channels that supplied blood vessels and nerves to whiskers. Whiskers may have evolved in this group. Derived cynodonts developed epipubic bones; these served to strengthen the torso and support abdominal and hindlimb musculature, aiding them in the development of an erect gait, but at the expense of prolonged pregnancy, forcing these animals to give birth to larval young as in modern monotremes and marsupials. Only placentals, Megazostrodon and Erythrotherium, would lose these. A specimen of Kayentatherium does indeed demonstrate that at least tritylodontids had a fundamentally marsupial-like reproductive style, but produced much higher litters at around 38 perinates.
Cynodonts are the only known synapsid lineage to have produced aerial locomotors, with gliding and flying being known in haramiyidans and various mammal groups. Below is a cladogram from Ruta, Botha-Brink and Benton showing one hypothesis of cynodont relationships: Permian–Triassic extinction event Prehistoric mammal Tetrapod Triassic-Jurassic extinction event Hopson, J. A.. W.. "A probainognathian cynodont from South Africa and the phylogeny of non-mammalian cynodonts". Bull. Mus. Comp. Zool. 156: 5–35. Davis, Dwight. "Origin of the Mammalian Feeding Mechanism". Am. Zoologist, 1:229–234. Palaeos cynodonts Phylogeny of Theriodonts and Cynodonts Bennett and Ruben 1986; the Metabolic and Thermoregulatory Status of Therapsids BBC cynodonts
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
Gondwana, was a supercontinent that existed from the Neoproterozoic until the Jurassic. It was formed by the accretion of several cratons. Gondwana became the largest piece of continental crust of the Paleozoic Era, covering an area of about 100,000,000 km2. During the Carboniferous Period, it merged with Laurussia to form a larger supercontinent called Pangaea. Gondwana broke up during the Mesozoic Era; the remnants of Gondwana make up about two thirds of today's continental area, including South America, Antarctica and the Indian Subcontinent. The formation of Gondwana began c. 800 to 650 Ma with the East African Orogeny, the collision of India and Madagascar with East Africa,and was completed c. 600 to 530 Ma with the overlapping Brasiliano and Kuunga orogenies, the collision of South America with Africa and the addition of Australia and Antarctica, respectively. The continent of Gondwana was named by Austrian scientist Eduard Suess, after the Gondwana region of central India, derived from Sanskrit for "forest of the Gonds".
The name had been used in a geological context, first by H. B. Medlicott in 1872, from which the Gondwana sedimentary sequences are described; the term "Gondwanaland" is preferred by some scientists in order to make a clear distinction between the region and the supercontinent. The assembly of Gondwana was a protracted process during the Neoproterozoic and Paleozoic, which however remains incompletely understood because of the lack of paleo-magnetic data. Several orogenies, collectively known as the Pan-African orogeny, led to the amalgamation of most of the continental fragments of a much older supercontinent, Rodinia. One of those orogenic belts, the Mozambique Belt, formed 800 to 650 Ma and was interpreted as the suture between East and West Gondwana. Three orogenies were recognized during the 1990s: the East African Orogeny and Kuunga orogeny, the collision between East Gondwana and East Africa in two steps, the Brasiliano orogeny, the successive collision between South American and African cratons.
The final stages of Gondwanan assembly overlapped with the opening of the Iapetus Ocean between Laurentia and western Gondwana. During this interval, the Cambrian explosion occurred. Laurentia was docked against the western shores of a united Gondwana for a short period near the Precambrian/Cambrian boundary, forming the short-lived and still disputed supercontinent Pannotia; the Mozambique Ocean separated the Congo–Tanzania–Bangweulu Block of central Africa from Neoproterozoic India. The Azania continent was an island in the Mozambique Ocean; the Australia/Mawson continent was still separated from India, eastern Africa, Kalahari by c. 600 Ma, when most of western Gondwana had been amalgamated. By c. 550 Ma, India had reached its Gondwanan position. Meanwhile, on the other side of the newly-forming Africa, Kalahari collided with Congo and Rio de la Plata which closed the Adamastor Ocean. C. 540–530 Ma, the closure of the Mozambique Ocean brought India next to Australia–East Antarctica, both North and South China were located in proximity to Australia.
As the rest of Gondwana formed, a complex series of orogenic events assembled the eastern parts of Gondwana c. 750 to 530 Ma. First the Arabian-Nubian Shield collided with eastern Africa in the East African Orogeny c.750 to 620 Ma. Australia and East Antarctica were merged with the remaining Gondwana c. 570 to 530 Ma in the Kuunga Orogeny. The Malagasy orogeny at about 550–515 Mya affected Madagascar, eastern East Africa and southern India. In it, Neoproterozoic India collided with the combined Azania and Congo–Tanzania–Bangweulu Block, suturing along the Mozambique Belt; the 18,000 km -long Terra Australis Orogen developed along Gondwana's western and eastern margins. Proto-Gondwanan Cambrian arc belts from this margin have been found in eastern Australia, New Zealand, Antarctica. Though these belts formed a continuous arc chain, the direction of subduction was different between the Australian-Tasmanian and New Zealand-Antarctica arc segments. A large number of terranes were accreted to Eurasia during Gondwana's existence but the Cambrian or Precambrian origin of many of these terranes remains uncertain.
For example, some Palaeozoic terranes and microcontinents that now make up Central Asia called the "Kazakh" and "Mongolian terranes", were progressively amalgamated into the continent Kazakhstania in the Late Silurian. Whether these blocks originated on the shores of Gondwana is not known. In the Early Palaeozoic the Armorican terrane, which today form large parts of France, was part of either Peri-Gondwana or core Gondwana. Precambrian rocks from the Iberian Peninsula suggest it too formed part of core Gondwana before its detachment as an orocline in the Variscan orogeny close to the Carboniferous–Permian boundary. South-east Asia is made of Gondwanan and Cathaysian continental fragments that were assembled during the Mid-Palaeozoic and Cenozoic; this p
In the geological timescale, the Tithonian is the latest age of the Late Jurassic epoch or the uppermost stage of the Upper Jurassic series. It spans the time between 152.1 ± 4 145.0 ± 4 Ma. It is followed by the Berriasian stage; the Tithonian was introduced in scientific literature by German stratigrapher Albert Oppel in 1865. The name Tithonian is unusual in geological stage names. Tithonus was the son of Laomedon of Troy, he fell in love with Eos, the Greek goddess of dawn and finds his place in the stratigraphy because this stage, the Tithonian, finds itself hand in hand with the dawn of the Cretaceous. The base of the Tithonian stage is at the base of the ammonite biozone of Hybonoticeras hybonotum. A global reference profile for the base of the Tithonian had in 2009 not yet been established; the top of the Tithonian stage is marked by the first appearance of small globular calpionellids of the species Calpionella alpina, at the base of the Alpina Subzone. The Tithonian is subdivided into Lower/Early and Upper/Late substages or subages.
The Late Tithonian is coeval with the Portlandian stage of British stratigraphy. The Tithonian stage contains seven ammonite biozones in the Tethys domain, from top to base: zone of Durangites zone of Micracanthoceras micranthum zone of Micracanthoceras ponti or Burckardticeras peroni zone of Semiformiceras fallauxi zone of Semiformiceras semiforme zone of Semiformiceras darwini zone of Hybonoticeras hybonotum In the ocean of Tethys, the Tithonian has a calcareous facies with a typical cephalopod fauna; the Solnhofen limestone of southern Germany, known for its fossils, is of Tithonian age. Gradstein, F. M.. G. & Smith, A. G.. Oppel, C. A.. GeoWhen Database - Tithonian Jurassic-Cretaceous timescale, at the website of the subcommission for stratigraphic information of the ICS Stratigraphic chart of the Upper Jurassic, at the website of Norges Network of offshore records of geology and stratigraphy
In the geologic timescale, the Sinemurian is an age and stage in the Early or Lower Jurassic epoch or series. It spans the time between 199.3 ± 2 Ma and 190.8 ± 1.5 Ma. The Sinemurian is followed by the Pliensbachian. In Europe the Sinemurian age, together with the Hettangian age, saw the deposition of the lower Lias, in Great Britain known as the Blue Lias; the Sinemurian stage was defined and introduced into scientific literature by French palaeontologist Alcide d'Orbigny in 1842. It takes its name from the French town near Dijon; the calcareous soil formed from the Jurassic limestone of the region is in part responsible for the character of the classic Sancerre wines. The base of the Sinemurian stage is at the first appearance of the ammonite genera Vermiceras and Metophioceras in the stratigraphic record. A global reference profile for the Sinemurian stage is located in a cliff north of the hamlet of East Quantoxhead, 6 kilometres east of Watchet, England; the top of the Sinemurian is at the first appearances of the ammonite species Bifericeras donovani and ammonite genus Apoderoceras.
The Sinemurian contains six ammonite biozones in the Tethys domain: zone of Echioceras raricostatum zone of Oxynotoceras oxynotum zone of Asteroceras obtusum zone of Caenisites turneri zone of Arnioceras semicostatum zone of Arietites bucklandi Bloos, G. & Page, K. N.. M.. G. & Smith, A. G.. D´Orbigny, A. C. V. M. D.. 1. Terrains oolitiques ou jurassiques, Paris. Komlosaurus carbonis GeoWhen Database - Sinemurian Lower Jurassic timescale, at the website of the subcommission for stratigraphic information of the ICS Stratigraphic chart of the Lower Jurassic, at the website of Norges Network of offshore records of geology and stratigraphy
The Plesiosauria or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period in the Rhaetian stage, about 203 million years ago, they became common during the Jurassic Period, thriving until their disappearance due to the Cretaceous–Paleogene extinction event at the end of the Cretaceous Period, about 66 million years ago. They had a worldwide oceanic distribution. Plesiosaurs were among the first fossil reptiles discovered. In the beginning of the nineteenth century, scientists realised how distinctive their build was and they were named as a separate order in 1835; the first plesiosaurian genus, the eponymous Plesiosaurus, was named in 1821. Since more than a hundred valid species have been described. In the early twenty-first century, the number of discoveries has increased, leading to an improved understanding of their anatomy and way of life. Plesiosaurs had a short tail, their limbs had evolved into four long flippers, which were powered by strong muscles attached to wide bony plates formed by the shoulder girdle and the pelvis.
The flippers made a flying movement through the water. Plesiosaurs breathed air, bore live young. Plesiosaurs showed two main morphological types; some species, with the "plesiosauromorph" build, had small heads. Other species, some of them reaching a length of up to seventeen metres, had the "pliosauromorph" build with a short neck and a large head; the two types are related to the traditional strict division of the Plesiosauria into two suborders, the long-necked Plesiosauroidea and the short-neck Pliosauroidea. Modern research, indicates that several "long-necked" groups might have had some short-necked members or vice versa. Therefore, the purely descriptive terms "plesiosauromorph" and "pliosauromorph" have been introduced, which do not imply a direct relationship. "Plesiosauroidea" and "Pliosauroidea" today have a more limited meaning. The term "plesiosaur" is properly used to refer to the Plesiosauria as a whole, but informally it is sometimes meant to indicate only the long-necked forms, the old Plesiosauroidea.
Skeletal elements of plesiosaurs are among the first fossils of extinct reptiles recognised as such. In 1605, Richard Verstegen of Antwerp illustrated in his A Restitution of Decayed Intelligence plesiosaur vertebrae that he referred to fishes and saw as proof that Great Britain was once connected to the European continent; the Welshman Edward Lhuyd in his Lithophylacii Brittannici Ichnographia from 1699 included depictions of plesiosaur vertebrae that again were considered fish vertebrae or Ichthyospondyli. Other naturalists during the seventeenth century added plesiosaur remains to their collections, such as John Woodward. In 1719, William Stukeley described a partial skeleton of a plesiosaur, brought to his attention by the great-grandfather of Charles Darwin, Robert Darwin of Elston; the stone plate came from a quarry at Fulbeck in Lincolnshire and had been used, with the fossil at its underside, to reinforce the slope of a watering-hole in Elston in Nottinghamshire. After the strange bones it contained had been discovered, it was displayed in the local vicarage as the remains of a sinner drowned in the Great Flood.
Stukely affirmed its "diluvial" nature but understood it represented some sea creature a crocodile or dolphin. The specimen is today preserved in the Natural History Museum, its inventory number being BMNH R.1330. It is the earliest discovered less complete fossil reptile skeleton in a museum collection, it can be referred to Plesiosaurus dolichodeirus. During the eighteenth century, the number of English plesiosaur discoveries increased, although these were all of a more or less fragmentary nature. Important collectors were the reverends William Mounsey and Baptist Noel Turner, active in the Vale of Belvoir, whose collections were in 1795 described by John Nicholls in the first part of his The History and Antiquities of the County of Leicestershire. One of Turner's partial plesiosaur skeletons is still preserved as specimen BMNH R.45 in the British Museum of Natural History. In the early nineteenth century, plesiosaurs were still poorly known and their special build was not understood. No systematic distinction was made with ichthyosaurs, so the fossils of one group were sometimes combined with those of the other to obtain a more complete specimen.
In 1821, a partial skeleton discovered in the collection of Colonel Thomas James Birch, was described by William Conybeare and Henry Thomas De la Beche, recognised as representing a distinctive group. A new genus was named, Plesiosaurus; the generic name was derived from the Greek πλήσιος, plèsios, "closer to" and the Latinised saurus, in the meaning of "saurian", to express that Plesiosaurus was in the Chain of Being more positioned to the Sauria the crocodile, than Ichthyosaurus, which had the form of a more lowly fish. The name should thus be rather read as "approaching the Sauria" or "near reptile" than as "near lizard". Parts of the specimen are still present in the Oxford University Museum of Natural History. Soon afterwards, the morphology became much better known. In 1823, Thomas Clark reported an complete skull belonging to Thalassiodracon, which i
The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 174.1 Ma. It is followed by the Aalenian; the Toarcian age began with the Toarcian turnover, the extinction event that sets its fossil faunas apart from the previous Pliensbachian age. The Toarcian takes its name from the city of Thouars, just south of Saumur in the Loire Valley of France; the stage was introduced by French palaeontologist Alcide d'Orbigny in 1842, after examining rock strata of this age in a quarry near Thouars. In Europe this period is represented by the upper part of the Lias; the base of the Toarcian is defined as the place in the stratigraphic record where the ammonite genus Eodactylites first appears. A global reference profile for the base is located at Portugal; the top of the stage is at the first appearance of ammonite genus Leioceras. In the Tethys domain, the Toarcian contains the following ammonite biozones: zone of Pleydellia aalensis zone of Dumortieria pseudoradiosa zone of Phlyseogrammoceras dispansum zone of Grammoceras thouarcense zone of Haugia variabilis zone of Hildoceras bifrons zone of Harpoceras serpentinum zone of Dactylioceras tenuicostatum Gradstein, F.
M.. G. & Smith, A. G.. D´Orbigny, A. C. V. M. D.. 1. Terrains oolitiques ou jurassiques, Paris. GeoWhen Database - Toarcian Lower Jurassic timescale, at the website of the subcommission for stratigraphic information of the ICS Stratigraphic chart of the Lower Jurassic, at the website of Norges Network of offshore records of geology and stratigraphy