Avemetatarsalia is a clade name established by British palaeontologist Michael Benton in 1999 for all crown group archosaurs that are closer to birds than to crocodilians. An alternate name is Pan-Aves, or "all birds", in reference to its definition containing all animals, living or extinct, which are more related to birds than to crocodilians. All avemetatarsalians are members of a defined subgroup, Ornithodira. Ornithodira is defined as the last common ancestor of dinosaurs and pterosaurs, all of its descendants. Members of this group include the Dinosauromorpha, the genus Scleromochlus, Aphanosauria. Dinosauromorpha contains more basal forms, including Lagerpeton and Marasuchus, as well as more derived forms, including dinosaurs. Birds belong to the dinosaurs as members of the theropods. Pterosauromorpha contains Pterosauria. Aphanosauria is a Triassic group of gracile carnivorous quadrupeds, recognized in 2017; the foundational characteristic is the "advanced mesotarsal" ankles, which are characterized by a large astragalus and a small calcaneum.
This ankle orientation operated on a single hinge. As a result of this change, the common ancestor of the avemetatarsalians had an upright, bipedal posture, with their legs extending vertically, similar to the situation in mammals. Feathers and other filamentary structures are known across the avemetatarsalians, from the downy pycnofibers of pterosaurs, to quill-like structures present in ornithischian dinosaurs, such as Psittacosaurus and Tianyulong, to feathers in theropod dinosaurs and their descendants, birds. Two clades of avemetatarsalians,pterosaurs and birds, independently evolved flight. Pterosaurs are the earliest vertebrates known to have evolved powered flight, their wings are formed by a membrane of skin and other tissues stretching from the ankles to a lengthened fourth finger. Birds evolved flight much later, their wings formed from elongated fingers, their arms, all covered with flight feathers. Avemetatarsalians were lighter built than crocodile-line archosaurs, they had smaller heads and a complete lack of osteoderms.
Bird-line archosaurs appear in the fossil record by the Anisian stage of the Middle Triassic about 245 million years ago, represented by the dinosauriform Asilisaurus. However, Early Triassic fossil footprints reported in 2010 from the Świętokrzyskie Mountains of Poland may belong to a more primitive dinosauromorph. If so, the origin of avemetatarsalians would be pushed back into the early Olenekian age, around 249 Ma; the oldest Polish footprints are classified in the ichnogenus Prorotodactylus and were made by an unknown small quadrupedal animal, but footprints called Sphingopus, found from Early Anisian strata, show that moderately large bipedal dinosauromorphs had appeared by 246 Ma. The tracks show, their age suggests. The primitive traits found in the quadrupedal aphanosaur Teleocrater shows that the earliest avemetatarsalians had many pseudosuchian-like features, that the traits typical for the group evolved later. In 1986, Jacques Gauthier defined the name Ornithosuchia for a branch-based clade including all archosaurs more related to birds than to crocodiles.
In the same year, Gauthier coined and defined a more restrictive node-based clade, containing the last common ancestor of the dinosaurs and the pterosaurs and all of its descendants. Paul Sereno in 1991 gave a different definition of Ornithodira, one in which Scleromochlus was explicitly added, it was thus a larger group than the Ornithodira of Gauthier. In 1999 Michael Benton concluded that Scleromochlus was indeed outside Gauthier's original conception of Ornithodira, so he named a new branch-based clade for this purpose: Avemetatarsalia, named after the birds, the last surviving members of the clade, the metatarsal ankle joint, a typical character of the group. Avemetatarsalia was defined as all Avesuchia closer to Dinosauria than to Crocodylia. In 2005, Sereno stated the opinion that Ornithodira was not a useful concept, whereas Avemetatarsalia was. In 2001, the same clade was given the name Pan-Aves, coined by Jacques Gauthier, he defined it as the largest and most inclusive clade of archosaurs containing Aves but not Crocodylia.
Gauthier referred Aves, all other Dinosauria, all Pterosauria, a variety of Triassic archosaurs, including Lagosuchus and Scleromochlus, to this group. In a 2005 review of archosaur classification, Phil Senter attempted to resolve this conflicting set of terminology by applying strict priority to names based on when and how they were first defined. Senter noted that Ornithosuchia, the earliest name used for the total group of archosaurs closer to birds than to crocodiles, should be the valid name for that group, have precedence over names with identical definitions, such as Avemetatarsalia and Pan-Aves. While this has been followed by some researchers, others have either continued to use Avemetatarsalia or Ornithodira, or have followed Senter only reluctantly. Mike Taylor for example noted that, while Senter is correct in stating that Ornithosuchia has priority, this is "undesirable" because it excludes the eponymous family Ornithosuchidae, questioned the utility of using priority before the PhyloCode is implemented to govern it.
In fact, the name Ornithosuchia may be "
Velociraptor is a genus of dromaeosaurid theropod dinosaur that lived 75 to 71 million years ago during the latter part of the Cretaceous Period. Two species are recognized, although others have been assigned in the past; the type species is V. mongoliensis. A second species, V. osmolskae, was named in 2008 for skull material from China. Smaller than other dromaeosaurids like Deinonychus and Achillobator, Velociraptor shared many of the same anatomical features, it was a bipedal, feathered carnivore with a long tail and an enlarged sickle-shaped claw on each hindfoot, thought to have been used to tackle and disembowel prey. Velociraptor can be distinguished from other dromaeosaurids by its long and low skull, with an upturned snout. Velociraptor is one of the dinosaur genera most familiar to the general public due to its prominent role in the Jurassic Park motion picture series. In real life, Velociraptor was the size of a turkey smaller than the 2 m tall and 80 kg reptiles seen in the films. Today, Velociraptor is well known to paleontologists, with over a dozen described fossil skeletons, the most of any dromaeosaurid.
One famous specimen preserves a Velociraptor locked in combat with a Protoceratops. Velociraptor was a mid-sized dromaeosaurid, with adults measuring up to 2.07 m long, 0.5 m high at the hip, weighing up to 15 kg, though there is a higher estimate of 19.7 kg. The skull, which grew up to 25 cm long, was uniquely up-curved, concave on the upper surface and convex on the lower; the jaws were lined with 26–28 spaced teeth on each side, each more serrated on the back edge than the front. Velociraptor, like other dromaeosaurids, had a large manus with three curved claws, which were similar in construction and flexibility to the wing bones of modern birds; the second digit was the longest of the three digits present. The structure of the carpal bones prevented pronation of the wrist and forced the'hands' to be held with the palmar surface facing inwards, not downwards; the first digit of the foot, as in other theropods, was a small dewclaw. However, whereas most theropods had feet with three digits contacting the ground, dromaeosaurids like Velociraptor walked on only their third and fourth digits.
The second digit, for which Velociraptor is most famous, was modified and held retracted off the ground. It bore a large, sickle-shaped claw, typical of dromaeosaurid and troodontid dinosaurs; this enlarged claw, which could grow to over 6.5 cm long around its outer edge, was most a predatory device used to tear into or restrain struggling prey. As in other dromaeosaurs, Velociraptor tails had long bony projections on the upper surfaces of the vertebrae, as well as ossified tendons underneath; the prezygapophyses began on the tenth tail vertebra and extended forward to brace four to ten additional vertebrae, depending on position in the tail. These were once thought to stiffen the tail, forcing the entire tail to act as a single rod-like unit. However, at least one specimen has preserved a series of intact tail vertebrae curved sideways into an S-shape, suggesting that there was more horizontal flexibility than once thought. In 2007, paleontologists reported the discovery of quill knobs on a well-preserved Velociraptor mongoliensis forearm from Mongolia, confirming the presence of feathers in this species.
Fossils of dromaeosaurids more primitive than Velociraptor are known to have had feathers covering their bodies and developed feathered wings. The fact that the ancestors of Velociraptor were feathered and capable of flight had long suggested to paleontologists that Velociraptor bore feathers as well, since flightless birds today retain most of their feathers. In September 2007, researchers found quill knobs on the forearm of a Velociraptor found in Mongolia; these bumps on bird wing bones show where feathers anchor, their presence on Velociraptor indicate it too had feathers. According to paleontologist Alan Turner, A lack of quill knobs does not mean that a dinosaur did not have feathers. Finding quill knobs on Velociraptor, means that it had feathers; this is something we'd long suspected. Co-author Mark Norell, Curator-in-Charge of fossil reptiles and birds at the American Museum of Natural History weighed in on the discovery, saying: The more that we learn about these animals the more we find that there is no difference between birds and their related dinosaur ancestors like velociraptor.
Both have wishbones, brooded their nests, possess hollow bones, were covered in feathers. If animals like velociraptor were alive today our first impression would be that they were just unusual looking birds. According to Turner and co-authors Norell and Peter Makovicky, quill knobs are not found in all prehistoric birds, their absence does not mean that an animal was not feathered – flamingos, for example, have no quill knobs. However, their presence confirms that Velociraptor bore modern-style wing feathers, with a rachis and vane formed by barbs; the forearm specimen on which the quill knobs were found represents an animal 1.5 meters in length and 15 kilograms in weight. Based on the spacing of the six preserved knobs in this specimen, the authors suggested that Velociraptor b
The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago, to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era; as with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when 60% of marine species were wiped out. A significant evolutionary milestone during the Silurian was the diversification of jawed fish and bony fish. Multi-cellular life began to appear on land in the form of small, bryophyte-like and vascular plants that grew beside lakes and coastlines, terrestrial arthropods are first found on land during the Silurian. However, terrestrial life would not diversify and affect the landscape until the Devonian; the Silurian system was first identified by British geologist Roderick Murchison, examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s.
He named the sequences for a Celtic tribe of Wales, the Silures, inspired by his friend Adam Sedgwick, who had named the period of his study the Cambrian, from the Latin name for Wales. This naming does not indicate any correlation between the occurrence of the Silurian rocks and the land inhabited by the Silures. In 1835 the two men presented a joint paper, under the title On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales, the germ of the modern geological time scale; as it was first identified, the "Silurian" series when traced farther afield came to overlap Sedgwick's "Cambrian" sequence, provoking furious disagreements that ended the friendship. Charles Lapworth resolved the conflict by defining a new Ordovician system including the contested beds. An early alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland; the French geologist Joachim Barrande, building on Murchison's work, used the term Silurian in a more comprehensive sense than was justified by subsequent knowledge.
He divided the Silurian rocks of Bohemia into eight stages. His interpretation was questioned in 1854 by Edward Forbes, the stages of Barrande, F, G and H, have since been shown to be Devonian. Despite these modifications in the original groupings of the strata, it is recognized that Barrande established Bohemia as a classic ground for the study of the earliest fossils; the Llandovery Epoch lasted from 443.8 ± 1.5 to 433.4 ± 2.8 mya, is subdivided into three stages: the Rhuddanian, lasting until 440.8 million years ago, the Aeronian, lasting to 438.5 million years ago, the Telychian. The epoch is named for the town of Llandovery in Wales; the Wenlock, which lasted from 433.4 ± 1.5 to 427.4 ± 2.8 mya, is subdivided into the Sheinwoodian and Homerian ages. It is named after Wenlock Edge in England. During the Wenlock, the oldest-known tracheophytes of the genus Cooksonia, appear; the complexity of later Gondwana plants like Baragwanathia, which resembled a modern clubmoss, indicates a much longer history for vascular plants, extending into the early Silurian or Ordovician.
The first terrestrial animals appear in the Wenlock, represented by air-breathing millipedes from Scotland. The Ludlow, lasting from 427.4 ± 1.5 to 423 ± 2.8 mya, comprises the Gorstian stage, lasting until 425.6 million years ago, the Ludfordian stage. It is named for the town of Ludlow in England; the Přídolí, lasting from 423 ± 1.5 to 419.2 ± 2.8 mya, is the final and shortest epoch of the Silurian. It is named after one locality at the Homolka a Přídolí nature reserve near the Prague suburb Slivenec in the Czech Republic. Přídolí is the old name of a cadastral field area. In North America a different suite of regional stages is sometimes used: Cayugan Lockportian Tonawandan Ontarian Alexandrian In Estonia the following suite of regional stages is used: Ohessaare stage Kaugatuma stage Kuressaare stage Paadla stage Rootsiküla stage Jaagarahu stage Jaani stage Adavere stage Raikküla stage Juuru stage With the supercontinent Gondwana covering the equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe.
The high sea levels of the Silurian and the flat land resulted in a number of island chains, thus a rich diversity of environmental settings. During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late-Ordovician glaciation; the southern continents remained united during this period. The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity; the continents of Avalonia and Laurentia drifted together near the equator, starting the formation of a second supercontinent known as Euramerica. When the proto-Europe coll
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago, although the exact origin and timing of the evolution of dinosaurs is the subject of active research. They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201 million years ago. Reverse genetic engineering and the fossil record both demonstrate that birds are modern feathered dinosaurs, having evolved from earlier theropods during the late Jurassic Period; as such, birds were the only dinosaur lineage to survive the Cretaceous–Paleogene extinction event 66 million years ago. Dinosaurs can therefore be divided into birds; this article deals with non-avian dinosaurs. Dinosaurs are a varied group of animals from taxonomic and ecological standpoints. Birds, at over 10,000 living species, are the most diverse group of vertebrates besides perciform fish. Using fossil evidence, paleontologists have identified over 500 distinct genera and more than 1,000 different species of non-avian dinosaurs.
Dinosaurs are represented on every continent by fossil remains. Through the first half of the 20th century, before birds were recognized to be dinosaurs, most of the scientific community believed dinosaurs to have been sluggish and cold-blooded. Most research conducted since the 1970s, has indicated that all dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction; some were herbivorous, others carnivorous. Evidence suggests that egg-laying and nest-building are additional traits shared by all dinosaurs and non-avian alike. While dinosaurs were ancestrally bipedal, many extinct groups included quadrupedal species, some were able to shift between these stances. Elaborate display structures such as horns or crests are common to all dinosaur groups, some extinct groups developed skeletal modifications such as bony armor and spines. While the dinosaurs' modern-day surviving avian lineage are small due to the constraints of flight, many prehistoric dinosaurs were large-bodied—the largest sauropod dinosaurs are estimated to have reached lengths of 39.7 meters and heights of 18 meters and were the largest land animals of all time.
Still, the idea that non-avian dinosaurs were uniformly gigantic is a misconception based in part on preservation bias, as large, sturdy bones are more to last until they are fossilized. Many dinosaurs were quite small: Xixianykus, for example, was only about 50 cm long. Since the first dinosaur fossils were recognized in the early 19th century, mounted fossil dinosaur skeletons have been major attractions at museums around the world, dinosaurs have become an enduring part of world culture; the large sizes of some dinosaur groups, as well as their monstrous and fantastic nature, have ensured dinosaurs' regular appearance in best-selling books and films, such as Jurassic Park. Persistent public enthusiasm for the animals has resulted in significant funding for dinosaur science, new discoveries are covered by the media; the taxon'Dinosauria' was formally named in 1841 by paleontologist Sir Richard Owen, who used it to refer to the "distinct tribe or sub-order of Saurian Reptiles" that were being recognized in England and around the world.
The term is derived from Ancient Greek δεινός, meaning'terrible, potent or fearfully great', σαῦρος, meaning'lizard or reptile'. Though the taxonomic name has been interpreted as a reference to dinosaurs' teeth and other fearsome characteristics, Owen intended it to evoke their size and majesty. Other prehistoric animals, including pterosaurs, ichthyosaurs and Dimetrodon, while popularly conceived of as dinosaurs, are not taxonomically classified as dinosaurs. Pterosaurs are distantly related to dinosaurs; the other groups mentioned are, like dinosaurs and pterosaurs, members of Sauropsida, except Dimetrodon. Under phylogenetic nomenclature, dinosaurs are defined as the group consisting of the most recent common ancestor of Triceratops and Neornithes, all its descendants, it has been suggested that Dinosauria be defined with respect to the MRCA of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria. Both definitions result in the same set of animals being defined as dinosaurs: "Dinosauria = Ornithischia + Saurischia", encompassing ankylosaurians, ceratopsians, ornithopods and sauropodomorphs.
Birds are now recognized as being the sole surviving lineage of theropod dinosaurs. In traditional taxonomy, birds were considered a separate class that had evolved from dinosaurs, a distinct superorder. However, a majority of contemporary paleontologists concerned with dinosaurs reject the traditional style of classification in favor of phylogenetic taxonomy. Birds are thus considered to be dinosaurs and dinosaurs are, not extinct. Birds are classified as belonging to the subgroup M
The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago, to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō and ferō, was coined by geologists William Conybeare and William Phillips in 1822. Based on a study of the British rock succession, it was the first of the modern'system' names to be employed, reflects the fact that many coal beds were formed globally during that time; the Carboniferous is treated in North America as two geological periods, the earlier Mississippian and the Pennsylvanian. Terrestrial animal life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates, of which one branch would evolve into amniotes, the first terrestrial vertebrates. Arthropods were very common, many were much larger than those of today. Vast swaths of forest covered the land, which would be laid down and become the coal beds characteristic of the Carboniferous stratigraphy evident today.
The atmospheric content of oxygen reached its highest levels in geological history during the period, 35% compared with 21% today, allowing terrestrial invertebrates to evolve to great size. The half of the period experienced glaciations, low sea level, mountain building as the continents collided to form Pangaea. A minor marine and terrestrial extinction event, the Carboniferous rainforest collapse, occurred at the end of the period, caused by climate change. In the United States the Carboniferous is broken into Mississippian and Pennsylvanian subperiods; the Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal-bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal in duration. In Europe the Lower Carboniferous sub-system is known as the Dinantian, comprising the Tournaisian and Visean Series, dated at 362.5-332.9 Ma, the Upper Carboniferous sub-system is known as the Silesian, comprising the Namurian and Stephanian Series, dated at 332.9-298.9 Ma.
The Silesian is contemporaneous with the late Mississippian Serpukhovian plus the Pennsylvanian. In Britain the Dinantian is traditionally known as the Carboniferous Limestone, the Namurian as the Millstone Grit, the Westphalian as the Coal Measures and Pennant Sandstone; the International Commission on Stratigraphy faunal stages from youngest to oldest, together with some of their regional subdivisions, are: A global drop in sea level at the end of the Devonian reversed early in the Carboniferous. There was a drop in south polar temperatures; these conditions had little effect in the deep tropics, where lush swamps to become coal, flourished to within 30 degrees of the northernmost glaciers. Mid-Carboniferous, a drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites hard; this sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod. This happened about 323 million years ago, at the onset of the Permo-Carboniferous Glaciation.
The Carboniferous was a time of active mountain-building as the supercontinent Pangaea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America–Europe along the present line of eastern North America; this continental collision resulted in the Hercynian orogeny in Europe, the Alleghenian orogeny in North America. In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural Mountains. Most of the Mesozoic supercontinent of Pangea was now assembled, although North China, South China continents were still separated from Laurasia; the Late Carboniferous Pangaea was shaped like an "O." There were two major oceans in the Carboniferous—Panthalassa and Paleo-Tethys, inside the "O" in the Carboniferous Pangaea. Other minor oceans were shrinking and closed - Rheic Ocean, the small, shallow Ural Ocean and Proto-Tethys Ocean. Average global temperatures in the Early Carboniferous Period were high: 20 °C.
However, cooling during the Middle Carboniferous reduced average global temperatures to about 12 °C. Lack of growth rings of fossilized trees suggest a lack of seasons of a tropical climate. Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are referred to as Permo-Carboniferous in age; the cooling and drying of the climate led to the Carboniferous Rainforest Collapse during the late Carboniferous. Tropical rainforests fragmented and were devastated by climate change. Carboniferous rocks in Europe and eastern North America consist of a repeated sequence of limestone, sandstone and coal beds. In North America, the early Carboniferous is marine
Yangchuanosaurus is an extinct genus of metriacanthosaurid theropod dinosaur that lived in China during the Bathonian and Callovian stages of the Middle Jurassic, was similar in size and appearance to its North American and European relative, Allosaurus. It hails from the Upper Shaximiao Formation and was the largest predator in a landscape that included the sauropods Mamenchisaurus and Omeisaurus and the stegosaurs Chialingosaurus and Chungkingosaurus, it was named after the area in, discovered, Yongchuan, in China. The type specimen of Y. shangyouensis had a skull 82 cm long, its total body length was estimated at about 8 m. Another specimen, assigned to the new species Y. magnus, was larger, with a skull length of 1.11 m. It may have been up to 10.8 m long, weighed as much as 3.4 metric tons. There was a bony ridge on its nose and multiple ridges, similar to Ceratosaurus. Yangchuanosaurus was a powerful meat-eater, it walked on two large, muscular legs, had short arms, a strong, short neck, a big head with powerful jaws, large, serrated teeth.
It had a long, massive tail, about half of its length. Its arms were short; the first digit of its foot was a small dewclaw. The three outer toes were used to bear weight and each was equipped with a large claw. Dong et al. named Yangchuanosaurus shangyouensis on the basis of CV 00215, a complete skull and skeleton, collected from the Shangshaximiao Formation, near Yongchuan, Yongchuan District, Sichuan. It dates to the Bathonian to Callovian stage of the Middle Jurassic period, about 161.2-154 million years ago. It was discovered in June 1977 by a construction worker during the construction of the Shangyou Reservoir Dam. A second species from the same locality, Y. magnus, was named by Dong et al. on the basis of CV 00216, another complete skull and skeleton. A detailed revision of tetanuran phylogeny by Carrano, Benson & Sampson revealed that both species are conspecific. Dong et al. and Dong et al. differentiated these species on the basis of size. In addition, Dong et al. noted that the maxilla of Y. magnus has an additional fenestra within the antorbital fossa, whereas Y. shangyouensis possessed only a fossa in this location.
However, it is considered to be an intraspecific ontogenetic, variation. Furthermore, the apparent difference in cervical vertebral morphology can be explained by comparing different positions within the column. Hence, the holotypes of the two species of Yangchuanosaurus are identical, their codings are identical in Carrano et al. matrix. Gregory S. Paul regarded this genus the same as Metriacanthosaurus. Carrano et al. assigned a third specimen to Y. shangyouensis. CV 00214 is represented by a partial postcranial skeleton lacking the skull, it was collected in the Wujiaba Quarry, near Zigong city, from the lower part of the Shangshaximiao Formation. CV 00214 was listed by Dong et al. in a faunal list as a new species of Szechuanosaurus, Szechuanosaurus "yandonensis". There is no illustration of it, making S. "yandonensis" a nomen nudum. Dong et al. described it, assigned it to Szechuanosaurus campi, a dubious species, known only from four teeth. Carrano et al. noted that CV 00214 can't be assigned to S. campi because the holotype materials of S. campi are non-diagnostic and no teeth are preserved in CV 00214.
A recent restudy of CV 00214 by Daniel Chure concluded that it represented a new taxon, informally named "Szechuanoraptor dongi", into which Szechuanosaurus zigongensis should be subsumed. However, the most recent revision suggested that CV 00214 and "S." zigongensis cannot be cospecific as there are no autapomorphies shared between them, the latter derives from the underlying Xiashaximiao Formation. A phylogenetic analysis found CV 00214 to be most related to Y. shangyouensis, thus the former is assignable to it. Furthermore, Szechuanosaurus zigongensis was found to be related to Y. shangyouensis and therefore was designated as the second species of Yangchuanosaurus. Yangchuanosaurus zigongensis is known from four specimens including ZDM 9011, a partial postcranial skeleton, it was first described by Gao, all specimens were collected from the Middle Jurassic Xiashaximiao Formation in the Dashanpu Dinosaur Quarry of Zigong, Sichuan. A phylogenetic analysis by Carrano et al. found Yangchuanosaurus to be the basalmost known metriacanthosaurid and the only non-metriacanthosaurine metriacanthosaurid.
The cladogram presented below follows their study. The cladogram presented below follows Zanno & Makovicky