Portal:Jurassic/Natural world articles

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Selected articles covering the Jurassic and the natural world

Artist's reconstruction of Waptia fieldensis.
The evolutionary history of life on Earth traces the processes by which living and fossil organisms have evolved since life on the planet first originated until the present day. Earth formed about 4.5 Ga (billion years ago) and life appeared on its surface within one billion years. Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean. The evolution of oxygenic photosynthesis, around 3.5 Ga, eventually led to the oxygenation of the atmosphere, beginning around 2.4 Ga. The earliest evidence of eukaryotes (complex cells with organelles), dates from 1.85 Ga, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions. The earliest land plants date back to around 450 Ma (million years ago), although evidence suggests that algal scum formed on the land as early as 1.2 Ga. Land plants were so successful that they are thought to have contributed to the late Devonian extinction event. Invertebrate animals appear during the Vendian period, while vertebrates originated about525 Ma during the Cambrian explosion. During the Permian period, synapsids, including the ancestors of mammals, dominated the land, but the Permian–Triassic extinction event251 Ma came close to wiping out all complex life. (see more...)

Modern Halobacteria sp.
The Archaea (Listeni/ɑːrˈkə/ or /ɑːrˈkə/; singular archaeon) constitute a domain or kingdom of single-celled microorganisms. These microbes are prokaryotes, meaning that they have no cell nucleus or any other membrane-bound organelles within their cells.

The Archaea show many differences in their biochemistry from other forms of life, and so they are now classified as a separate domain in the three-domain system. So far, the Archaea have been further divided into four recognized phyla. Classification is still difficult, because the vast majority have never been studied in the laboratory.

Archaea and bacteria are quite similar in size and shape, but despite this visual similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes. Other aspects of archaean biochemistry are unique, such as their reliance on ether lipids in their cell membranes. Archaea use a much greater variety of sources of energy than eukaryotes: ranging from familiar organic compounds such as sugars, to ammonia, metal ions or even hydrogen gas. Salt-tolerant archaea use sunlight as an energy source, and other species of archaea fix carbon. Archaea reproduce asexually by binary fission, fragmentation, or budding.

Archaea are found in a broad range of habitats, includingsoils, oceans, marshlands and the human colon and navel. Archaea are now recognized as a major part of Earth's life and may play roles in both the carbon cycle and the nitrogen cycle. (see more...)

Scanning electron micrograph of modern Escherichia coli bacilli.
Bacteria (Listeni/bækˈtɪəriə/; singular: bacterium) constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in mosthabitats on the planet. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust. Bacteria also live in plants and animals and have flourished in manned space vehicles. There are approximately 5×1030 bacteria on Earth, forming a biomass that exceeds that of all plants and animals. Bacteria are vital in recycling nutrients, with many steps in nutrient cycles depending on these organisms, such as the fixation of nitrogen from the atmosphere and putrefaction. In the biological communities surrounding hydrothermal vents and cold seeps, bacteria provide the nutrients needed to sustain life by converting dissolved compounds such as hydrogen sulphide and methane to energy. Most bacteria have not been characterised, and only about half of the phyla of bacteria have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. (see more...)

The modern fungus Marasmius rotula had relatives that lived during the Mesozoic.
A fungus (/ˈfʌŋɡəs/; plural: fungi) is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants and animals. One major difference is that fungal cells have cell walls that contain chitin, unlike the cell walls of plants and some protists, which contain cellulose. These and other differences show that the fungi form a single group of related organisms, named the Eumycota. The discipline of biology devoted to the study of fungi is known as mycology. Genetic studies have shown that fungi are more closely related to animals than to plants. Abundant worldwide, most fungi are inconspicuous because of the small size of their structures, and their cryptic lifestyles in soil, on dead matter, and as symbionts of plants, animals, or other fungi. They may become noticeable when fruiting, either as mushrooms or molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. However, little is known of the true biodiversity of Kingdom Fungi, which has been estimated at 1.5 million to 5 million species. Phylogenetic studies published in the last decade have helped reshape the classification of Kingdom Fungi, which is divided into one subkingdom, seven phyla, and ten subphyla. A group of all the fungi present in a particular area or geographic region is known as mycobiota. (see more...)

A modern Rotavirus.
A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect members of every kingdom of life, but individual kinds of virus may specialize in certain types of host. About 5,000 viruses have been described in detail, although there are millions of different types. Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity. The study of viruses is known as virology.

Virus particles (known as virions) consist of two or three parts: i) the genetic material made from either DNA or RNA, long molecules that carry genetic information; ii) a proteincoat that protects these genes; and in some cases iii) an envelope of lipids that surrounds the protein coat when they are outside a cell. The shapes of viruses range from simplehelical and icosahedral forms to more complex structures. The average virus is about one one-hundredth the size of the average bacterium.

The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. Viruses are considered by some to be a life form, because they carry genetic material, reproduce, and evolve through natural selection. However they lack key characteristics (such as cell structure) that are generally considered necessary to count as life, so whether or not viruses are truly alive is controversial. (see more...)

Ernst Haeckel's "Acephala".
Bivalvia is a class of marine and freshwater molluscs with laterally compressed bodies enclosed by a shell in two hinged parts. Bivalves include clams, oysters, mussels, scallops, and numerous other families of shells. The majority are filter feeders. Most bivalves bury themselves in sediment on the seabed, where they are safe from predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. Some bivalves, such as the scallops, can swim.

The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. These are joined together along one edge by a flexible ligament that, in conjunction with interlocking "teeth" on each of the valves, forms the hinge. The shell is typically bilaterally symmetrical, with the hinge lying in the sagittal plane. Adult shell sizes vary from fractions of a millimetre to over a metre in length, but the majority of species do not exceed 10 cm (4 in).

Bivalves appear in the fossil record first in the early Cambrian more than 500 million years ago. The total number of living species is approximately 9,200. These species are placed within 1,260 genera and 106 families. Marine bivalves (including brackish water and estuarine species) represent about 8,000 species, combined in four subclasses and 99 families with 1,100 genera. The largestrecent marine family is the Veneridae, with more than 680 species. (see more...)

The annelids are a large phylum of segmented worms, with over 2,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments. The basic annelid form consists of multiple segments, each of which has the same sets of organs and, in most polychaetes, a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly defined or absent in some. Septa also enable annelids to change the shapes of individual segments, which facilitates movement by "ripples" that pass along the body or by undulations. Although many species can reproduce asexually and use similar mechanisms to regenerate after severe injuries, sexual reproduction is the normal method in species whose reproduction has been studied. Since annelids are soft-bodied, their fossils are rare – mostly jaws and the mineralized tubes that some of the species secreted. Although some late Ediacaran fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about 518 million years ago in the early Cambrian period. Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous, about 299 million years ago. Scientists disagree about whether some body fossils from the mid Ordovician, about 472 to 461 million years ago, are the remains of oligochaetes, and the earliest certain fossils of the group appear in the Tertiary period, which began 65 million years ago. (see more...)

Stenophlebia amphitrite.
Arthropods are members of the phylum Arthropoda, and include the insects, arachnids, and crustaceans. They are characterized by their jointed limbs and cuticles. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. The arthropod body plan consists of repeated segments, each with a pair of appendages. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80% of all described living animal species. They range in size from microscopic plankton up to forms a few meters long.

Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their vision relies on various combinations of compound eyes and pigment-pit ocelli. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles. Nearly all arthropods lay eggs. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form.

The evolutionary ancestry of arthropods dates back to the Cambrian period. The group is generally regarded as monophyletic, and many analyses support the placement of arthropods with cycloneuralians (or their constituent clades) in a superphylum Ecdysozoa. Overall however, the basal relationships of Metazoa are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated. (see more...)

Liospiriferina rostrata.
Brachiopods are marine animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalvemolluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Two major groups are recognized, articulate and inarticulate. Articulate brachiopods have toothed hinges and simple opening and closing muscles, while inarticulate brachiopods have untoothed hinges and a more complex system of muscles used to keep the two halves aligned. In a typical brachiopod a stalk-like pedicle projects from an opening in one of the valves, known as the pedicle valve, attaching the animal to the seabed. Lineages of brachiopods that have both fossil and extant taxa appeared in the early Cambrian, Ordovician, and Carboniferous periods, respectively. Other lineages have arisen and then become extinct, sometimes during severe mass extinctions. At their peak in the Paleozoic era, the brachiopods were among the most abundant filter-feeders and reef-builders, and occupied other ecological niches, including swimming in the jet-propulsion style of scallops. However, after the Permian–Triassic extinction event, brachiopods recovered only a third of their former diversity. It was often thought that brachiopods were in decline after the Permian–Triassic extinction, and were out-competed by bivalves. However, a study in 1980 found both brachiopod and bivalve species increased from the Paleozoic to modern times, but bivalves increased faster; after the Permian–Triassic extinction, brachiopods for the first time became less diverse than bivalves. (see more...)

Cnidaria is a phylum of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids. Cnidarians' activities are coordinated by a decentralized nerve net. Many cnidarians have complex lifecycles with asexual polyp stages and sexual medusae, but some omit either the polyp or the medusa stage. Cnidarians are classified into four main groups: the almost wholly sessile Anthozoa (sea anemones, corals, sea pens); swimming Scyphozoa (jellyfish); Cubozoa (box jellies); and Hydrozoa, a diverse group that includes all the freshwater cnidarians as well as many marine forms. Fossil cnidarians have been found in rocks formed about 580 million years ago, and other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later. Fossils of cnidarians that do not build mineralized structures are very rare. Scientists currently think that cnidarians, ctenophores and bilaterians are more closely related to calcareous sponges than these are to other sponges, and that anthozoans are the evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians. (see more...)

Agatized coral fossil from Michigan.
Corals are marine invertebrates in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton. A coral "head" is a colony of myriad genetically identical polyps. Each polyp is a spineless animal typically only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. Over many generations, the colony thus creates a large skeleton that is characteristic of the species. Individual heads grow by asexual reproduction of polyps. Corals also breed sexually by spawning: polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon. Although some corals can catch small fish and plankton, using stinging cells on their tentacles, like those in sea anemone and jellyfish, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular algae that live within the coral's tissue called zooxanthella. Such corals require sunlight and grow in clear, shallow water. Corals can be major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia. Other corals do not have associated algae and can live in much deeper water. (see more...)

Dusa monocera.
Crustaceans (Crustacea) form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm (0.004 in), to the Japanese spider crabwith a leg span of up to 3.8 m (12.5 ft) and a mass of 20 kg (44 lb). Like other arthropods, crustaceans have an exoskeleton, which they moult to grow. They are distinguished from other groups of arthropods, such as insects, myriapods and chelicerates, by the possession of biramous (two-parted) limbs, and by the nauplius form of the larvae. Most crustaceans are free-living aquatic animals, but some are terrestrial (e.g. woodlice), some are parasitic (e.g. Rhizocephala, fish lice, tongue worms) and some are sessile (e.g. barnacles). The group has an extensive fossil record, reaching back to the Cambrian, and includes living fossils such as Triops cancriformis, which has existed apparently unchanged since the Triassic period. More than 10 million tons of crustaceans are produced by fishery or farming for human consumption, the majority of it being shrimp and prawns. Krill and copepods are not as widely fished, but may be the animals with the greatest biomass on the planet, and form a vital part of the food chain. The scientific study of crustaceans is known as carcinology, and a scientist who works in carcinology is a carcinologist. (see more...)

Artist's restoration of Ctenorhabdotus capulus.
Ctenophora is a phylum of marine animals characterized by "combs" consisting of cilia they use for swimming. Adults range from a few millimeters to 1.5 m (4 ft 11 in) in size. Their bodies consist of a mass of jelly, with one layer two cells thick on the outside and another lining the internal cavity. Almost all ctenophores consume tiny animal prey. The phylum has a wide range of body forms, including the egg-shaped cydippids with retractable tentacles that capture prey, the flat generally combless platyctenids, and the large-mouthed beroids, which prey on other ctenophores. Despite their soft, gelatinous bodies, fossils thought to represent ctenophores have been found in lagerstätten as far back as the early Cambrian, about 525 million years ago. The position of the ctenophores in the tree of life has long been debated, and the majority view at present, based on molecular phylogenetics, is that ctenophores are more primitive than the sponges, which are more primitive than the cnidarians and bilaterians. A recent molecular phylogenetics analysis concluded that the common ancestor of all modern ctenophores was cydippid-like, and that all the modern groups appeared relatively recently, probably after the Cretaceous–Paleogene extinction event 66 million years ago. Evidence accumulating since the 1980s indicates that the "cydippids" are not monophyletic, in other words do not include all and only the descendants of a single common ancestor, because all the other traditional ctenophore groups are descendants of various cydippids. (see more...)

Modern entoprocts.
Entoprocta is a phylum of mostly sessile marine animals, ranging from 0.1 to 7 millimetres (0.0039 to 0.2756 in) long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whose cilia generate water currents that draw food particles towards the mouth, and both the mouth and anus lie inside the "crown". Most families of entoprocts are colonial. Some species eject unfertilized ova into the water, while others keep their ova in brood chambers until they hatch, and some of these species use placenta-like organs to nourish the developing eggs. After hatching, the larvae swim for a short time and then settle on a surface. There they metamorphose, and the larval gut generally rotates by up to 180°, so that the mouth and anus face upwards. Both colonial and solitary species also reproduce by cloning – solitary species grow clones in the space between the tentacles and then release them when developed, while colonial ones produce new members from the stalks or from corridor-like stolons. Fossils of entoprocts are very rare, and the earliest specimens that have been identified with confidence date from the Late Jurassic. Most studies from 1996 onwards have regarded entoprocts as members of the Trochozoa, which also includes molluscs and annelids. However, a study in 2008 concluded that entoprocts are closely related to bryozoans. Recently, the Maotianshan Shales fossil,Cotyledion tylodes, has been reevaluated as being an ancient, sclerite-bearing entoproct. (see more...)

Artist's restoration of Leedsichthys.
A fish is any member of a paraphyletic group of organisms that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups. Most fish are ectothermic ("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change. Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams to even hadal depths of the deepest oceans. At 32,000 species, fish exhibit greater species diversity than any other group of vertebrates. Because the term "fish" is defined negatively, and excludes the tetrapods (i.e., the amphibians, reptiles, birds and mammals) which descend from within the same ancestry, it is paraphyletic, and is not considered a proper grouping in systematic biology. The earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessednotochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed external armor that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators rather than just the prey of arthropods. (see more...)

Artist's restoration of Latouchella costata.
The molluscs or mollusks, compose the large phylum of invertebrate animals known as the phylum Mollusca. Around 85,000 extant species of molluscs are recognized. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not only in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish and octopus, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species. The scientific study of molluscs is called malacology. The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula, and the structure of the nervous system. Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period 541 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists. (see more...)

Vauxia fossils.
Sponges are animals of the phylum Porifera (/pɒˈrɪfərə/; meaning "pore bearer"). They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. Sponges have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes. (see more...)

Modern tunicates.
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata which is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords. Some tunicates live as solitary individuals but others replicate by budding and become colonies, each unit being known as a zooid. They are marine filter feeders with a water-filled, sac-like body structure and two tubular openings, known as siphons, through which they draw in and expel water. During their respiration and feeding they take in water through the incurrent (or inhalant) siphon and expel the filtered water through the excurrent (or exhalant) siphon. Most adult tunicates are sessile and are permanently attached to rocks or other hard surfaces on the ocean floor; others such as salps, doliolids and pyrosomes swim in the pelagic zone of the sea as adults. Various species are commonly known as sea squirts, sea pork, sea liver or sea tulips. The Tunicata first appear in the fossil record in the early Cambrian period. Despite their simple appearance and very different adult form, their close relationship to the vertebrates is shown by the fact that during their mobile larval stage, they possess a notochord or stiffening rod and resemble a tadpole. Their name derives from their unique outer covering or "tunic" which is formed from proteins and carbohydrates and acts as an exoskeleton. In some species it is thin, translucent and gelatinous while in others it is thick, tough and stiff. (see more...)

Geologic map of Scotland.
The geology of Scotland is unusually varied for a country of its size, with a large number of differing geological features. There are three main geographical sub-divisions: the Highlands and Islands is a diverse area which lies to the north and west of the Highland Boundary Fault; the Central Lowlands is a rift valley mainly comprising Paleozoic formations; and the Southern Uplands, which lie south of the Southern Uplands Fault, are largely composed of Silurian deposits.

The existing bedrock includes very ancient Archean gneiss, metamorphic beds interspersed with granite intrusions created during the Caledonian mountain building period (the Caledonian orogeny), commercially important coal, oil and iron bearing carboniferous deposits and the remains of substantial Paleogene volcanoes. During their formation, tectonic movements created climatic conditions ranging from polar to desert to tropical and a resultant diversity of fossil remains.

Scotland has also had a role to play in many significant discoveries such as plate tectonics and the development of theories about the formation of rocks and was the home of important figures in the development of the science including James Hutton, (the "father of modern geology") Hugh Miller and Archibald Geikie. Various locations such as 'Hutton's Unconformity' at Siccar Point in Berwickshire and the Moine Thrust in the north west were also important in the development of geological science. (see more...)

Fossil Astropecten.
Starfish or sea stars are echinoderms belonging to the class Asteroidea. About 1,500 living species of starfish occur on the seabed in all the world's oceans, from the tropics to subzero polar waters. They are found from the intertidal zone down to abyssal depths, 6,000 m (20,000 ft) below the surface.

Starfishes typically have a central disc and five arms, though some species have more than this. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface. They are opportunistic feeders and are mostly predators on benthicinvertebrates. Several species having specialized feeding behaviours including eversion of their stomachs and suspension feeding. They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged parts or lost arms and they can shed arms as a means of defence. The Asteroidea occupy several significant ecological roles.

The fossil record for starfish is ancient, dating back to the Ordovician around 450 million years ago, but it is rather poor, as starfish tend to disintegrate after death. Only the ossicles and spines of the animal are likely to be preserved, making remains hard to locate. (see more...)

Ordovician bryozoans.
The Bryozoa are a phylum of aquatic invertebrate animals. Typically about 0.5 millimetres (0.020 in) long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia. Individuals in bryozoan colonies are called zooids, since they are not fully independent animals. All colonies contain autozooids, which are responsible for feeding and excretion. Colonies of some classes have various types of non-feeding specialist zooids. Zooids consist of a cystid that provides the body wall and produces the exoskeleton and a polypide that contains the internal organs and the lophophore or other specialist extensions. Colonies take a variety of forms, including fans, bushes and sheets. Mineralized skeletons of bryozoans first appear in rocks from Early Ordovician period, making it the last major phylum to appear in the fossil record. This has led researchers to suspect that bryozoans had arisen earlier but were initially unmineralized, and may have differed significantly from fossilized and modern forms. Early fossils are mainly of erect forms, but encrusting forms gradually became dominant. It is uncertain whether the phylum is monophyletic. Bryozoans' evolutionary relationships to other phyla are also unclear, partly because scientists' view of the family tree of animals is mainly influenced by better-known phyla. Both morphological and molecular phylogeny analyses disagree over bryozoans' relationships with entoprocts, about whether bryozoans should be grouped with brachiopods and phoronids in Lophophorata, and whether bryozoans should be considered protostomes or deuterostomes. (see more...)

Fossil Mesolimulus.
The subphylum (or phylum) Chelicerata constitutes one of the major subdivisions of the phylum (or superphylum) Arthropoda, and includes horseshoe crabs, scorpions, spiders, mites, harvestmen, ticks, and Solifugae. Like all arthropods, chelicerates have segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins. The chelicerate bauplan consists of two tagmata, the cephalothorax and the abdomen. The group is named for their chelicerae, appendages near the mouth generally used to feed. The group has the open circulatory system typical of arthropods, in which a tube-like heart pumps blood through the hemocoel, which is the major body cavity.

Chelicerates were originally predators, but the group has diversified to use all the major feeding strategies. The guts of most modern chelicerates are too narrow for solid food, and they generally liquidize their food by grinding it with their chelicerae and pedipalps and flooding it with digestive enzymes. Most lay eggs that hatch as what look like miniature adults. In most chelicerate species the young have to fend for themselves, but in scorpions and some species of spider the females protect and feed their young.

The chelicerata originated as marine animals, possibly in the Cambrian period, but the first confirmed chelicerate fossils, eurypterids, date from 445 million years ago in the Late Ordovician period.

The surviving marine species include the four species of xiphosurans (horseshoe crabs), and possibly the 1,300 species of pycnogonids (sea spiders), if the latter are chelicerates. (see more...)

Modern marchantiophytes.
The Marchantiophyta are a division of non-vascular bryophyte land plants commonly referred to as hepatics or liverworts. Like other bryophytes, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information.

It is estimated that there are about 9000 species of liverworts. Some of the more familiar species grow as a flattened leafless thallus, but most species are leafy with a form very much like a flattenedmoss. Leafy species can be distinguished from the apparently similar mosses on the basis of a number of features, including their single-celled rhizoids. Leafy liverworts also differ from most (but not all) mosses in that their leaves never have a costa and may bear marginal cilia (very rare in mosses). Other differences are not universal for all mosses and liverworts, but the occurrence of leaves arranged in three ranks, the presence of deep lobes or segmented leaves, or a lack of clearly differentiated stem and leaves all point to the plant being a liverwort.

Liverworts are typically small, usually from 2–20 mm wide with individual plants less than 10 cm long, and are therefore often overlooked. However, certain species may cover large patches of ground, rocks, trees or any other reasonably firm substrate on which they occur. They are distributed globally in almost every available habitat, most often in humid locations although there are desert and arctic species as well. (see more...)

Modern phoronids.
Phoronids (sometimes called horseshoe worms) are a phylum of marine animals that filter-feed with a "crown" of tentacles, and build upright tubes of chitin to support and protect their soft bodies. Most adult phoronids are 2 cm long and about 1.5 mm wide, although the largest are 50 cm long. The bottom end of the body is an a flask-like swelling, which anchors the animal in the tube and enables it to retract its body very quickly when threatened. When the lophophore is extended at the top of the body, little hairs on the sides of the tentacles draw food particles to the mouth, which is inside and slightly to one side of the base of the lophophore. The food then moves down to the stomach, which is in the ampulla. Solid wastes are moved up the intestine and out through the anus, which is outside and slightly below the lophophore. As of 2010 there are no indisputable body fossils of phoronids. There is good evidence that phoronids created trace fossils found in the Silurian, Devonian, Permian, Jurassic and Cretaceous periods, and possibly in the Ordovician and Triassic. Phoronids, brachiopods and bryozoans have collectively been called lophophorates, because all use lophophores to feed. Most researchers now regard phoronids as members of the protostome super-phylum Lophotrochozoa. The relationships between lophotrochozoans are still unclear. Some analyses regard phoronids and brachiopods as sister-groups, while others place phoronids as a sub-group within brachiopoda. (see more...)

Artist's restoration of Hybodus.
Sharks are a group of fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus. Under this broader definition, the earliest known sharks date from more than 420 million years ago. Since then, sharks have diversified into over 470 species. They range in size from the small dwarf lanternshark (Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (39 ft). Sharks are found in all seas and are common to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive in both seawater and freshwater. They breathe through five to seven gill slits. Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have several sets of replaceable teeth. (see more...)

Location of Somerset within England.
The geology of Somerset refers to the study of the structure, composition and history of the rocks in Somerset, a rural county in the southwest of England. Somerset covers4,171 square kilometres (1,610 sq mi). It is bounded on the north-west by the Bristol Channel, on the north by Bristol and Gloucestershire, on the north-east by Wiltshire, on the south-east by Dorset, and on the south west and west by Devon. It has broad central plains with several ranges of low hills. The landscape divides into four main geological sections from the Silurian through the Devonian and Carboniferous to the Permian which influence the landscape, together with water-related features. The low lying areas of the North Somerset Levels and Somerset Levels have been subject to thousands of years of flooding and man's attempts to control the flow of water. In the north of the county the Limestone of the Mendip Hills dominates the landscape, while in the south the Blackdown and Quantock Hills rise out of the levels. The highest areas are on Exmoor. The wide variety of landscapes has led to several areas being designated as Sites of Special Scientific Interest for geological reasons, and support a range of flora and fauna as can be seen from the List of Sites of Special Scientific Interest in Somerset (see more...)

Life restoration of Prosalirus.
Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia (Greek ἀμφí, amphi, "both" + βíος, bios, "life"). They inhabit a wide variety of habitats with most species living within terrestrial, fossorial, arboreal or freshwater aquatic ecosystems. Amphibians typically start out as larvaliving in water, but some species have developed behavioural adaptations to bypass this. The young generally undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. The earliest amphibians evolved in the Devonian Period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia. The three modern orders of amphibians are Anura (the frogs and toads), Caudata/Urodela (the salamanders), and Gymnophiona/Apoda (the caecilians). The total number of known amphibian species is approximately 7,000, of which nearly 90% are frogs. The largest living amphibian is the 1.8 m (5 ft 11 in) Chinese giant salamander (Andrias davidianus) but this is dwarfed by the extinct 9 m (30 ft) Prionosuchus from the middle Permian of Brazil. The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology. (see more...)

Insects are a class of invertebrates within the arthropod phylum that have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed legs, compound eyes and one pair of antennae. They are among the most diverse groups of animals on the planet, including more than a million described species and representing more than half of all known living organisms. Insects may be found in nearly all environments, although only a small number of species reside in the oceans.

The life cycles of insects vary but most insects hatch from eggs. Insect growth is constrained by the inelastic exoskeleton and development involves a series of molts. The immature stages can differ from the adults in structure, habit and habitat. Insects that undergoincomplete metamorphosis lack a pupal stage and adults develop through a series of nymphal stages. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonflies with wingspans of 55 to 70 cm (22–28 in). The most diverse insect groups appear to have coevolved with flowering plants.

Adult insects typically move about by walking, flying, or sometimes swimming. Insects are the only invertebrates to have evolved flight. Many insects spend at least part of their lives under water, with larval adaptations that include gills, and some adult insects are aquatic and have adaptations for swimming. Insects are mostly solitary, but some, such as certain bees, ants and termites, are social and live in large, well-organized colonies. (see more...)

The Jurassic temnospondyl Siderops.
The Temnospondyli are a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few species continued into the Cretaceous. Fossils have been found on every continent. During about 210 million years of evolutionary history, they adapted to a wide range of habitats, including fresh water, terrestrial, and even coastal marine environments. Their life history is well understood, with fossils known from the larval stage, metamorphosis, and maturity. Most temnospondyls were semiaquatic, although some were almost fully terrestrial, returning to the water only to breed. These temnospondyls were some of the first vertebrates fully adapted to life on land. Although temnospondyls are considered amphibians, many had characteristics, such as scales, claws, and armor-like bony plates, that distinguish them from modern amphibians. Authorities disagree over whether temnospondyls were ancestral to modern amphibians (frogs, salamanders, and caecilians), or whether the whole group died out without leaving any descendants. Different hypotheses have placed modern amphibians as the descendants of temnospondyls, another group of early tetrapods called lepospondyls, or even as descendants of both groups (with caecilians evolving from lepospondyls and frogs and salamanders evolving from temnospondyls). Recent studies place a family of temnosondyls called the amphibamids as the closest relatives of modern amphibians. Similarities in teeth, skulls, and hearing structures link the two groups. (see more...)

A modern mesothelan spider.
Spiders (order Araneae) are air-breathing arthropods that have eight legs and chelicerae with fangs that inject venom. They are the largest order of arachnids. Spiders are found nearly worldwide in nearly every habitat with the exception of air and sea. Anatomically, spiders differ from other arthropods in that the usual body segments are fused into two tagmata, the cephalothorax and abdomen, and joined by a small, cylindrical pedicel. Spiders generally have very centralized nervous systems for arthropods. Their abdomens bear appendages that have been modified into spinnerets that extrude silk from up to six types of silk glands within their abdomen. Spider webs vary widely in size, shape and the amount of sticky thread used. Spiders' guts are too narrow to take solids, and they liquidize their food by flooding it with digestive enzymes and grinding it with the bases of their pedipalps. Spider-like arachnids with silk-producing spigots appeared in the Devonian period about 386 million years ago, but these animals apparently lacked spinnerets. True spiders have been found in Carboniferous rocks from 318 to 299 million years ago, and are very similar to the most primitive surviving order, the Mesothelae. The main groups of modern spiders, Mygalomorphae and Araneomorphae, first appeared in the Triassic period, before 200 million years ago. (see more...)

Geology of the Capitol Reef area.
The exposed geology of the Capitol Reef area presents a record of mostly Mesozoic-aged sedimentation in an area of North America in and around Capitol Reef National Park. Nearly 10,000 feet (3,000 m) of sedimentary strata are found in the Capitol Reef area, representing nearly 200 million years of geologic history of the south-central part of the U.S. state of Utah. These rocks range in age from Permian (as old as 270 million years old) to Cretaceous (as young as 80 million years old.) Rock layers in the area reveal ancient climates as varied as rivers and swamps (Chinle Formation), Sahara-like deserts (Navajo Sandstone), and shallow ocean (Mancos Shale). The area's first known sediments were laid down as a shallow sea invaded the land in the Permian. At first sandstone was deposited but limestone followed as the sea deepened. After the sea retreated in the Triassic, streams deposited silt before the area was uplifted and underwent erosion. Conglomerate followed by logs, sand, mud and wind-transportedvolcanic ash were later added. Mid to Late Triassic time saw increasing aridity, during which vast amounts of sandstone were laid down along with some deposits from slow-moving streams. As another sea started to return it periodically flooded the area and left evaporite deposits. Barrier islands, sand bars and later, tidal flats, contributed sand for sandstone, followed by cobbles for conglomerate and mud for shale. The sea retreated, leaving streams, lakes and swampy plains. (see more...)

Kolob Canyons from the end of Kolob Canyons Road.

The geology of the Zion and Kolob canyons area includes nine known exposed formations, all visible in Zion National Park in the U.S. state of Utah. Together, these formations represent about 150 million years of local sedimentation from the Late Permian to Early Cretaceous. Part of a super-sequence of rock units called the Grand Staircase, the formations exposed in the Zion and Kolob area were deposited in several different environments that range from the warm shallow seas of the Kaibab and Moenkopi formations, streams and lakes of the Chinle, Moenave, and Kayenta formations to the large deserts of the Navajo and Temple Cap formations and dry near shore environments of the Carmel Formation.

Subsequent uplift of the Colorado Plateau slowly raised these formations much higher than where they were deposited. This steepened the stream gradient of the ancestral rivers and other streams on the plateau. The faster-moving streams took advantage of uplift-created joints in the rocks to remove all Cenozoic-aged formations and cut gorges into the plateaus. Zion Canyon was cut by the North Fork of the Virgin River in this way. Lava flows and cinder cones covered parts of the area during the later part of this process. (see more...)

Varied flatworm species fromKunstformen der Natur (1904), plate 75.
The flatworms, or Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates, (having no body cavity), and no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both the ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, the food cannot be processed continuously. Over half of all known flatworm species are parasitic. Free-living flatworms are mostly predators, and live in water or in shaded, humid terrestrial environments such as leaf litter. Analyses since the mid-1980s have separated out one subgroup, the Acoelomorpha, as basal bilaterians (animals with bilateral symmetry and hence with distinct front and rear ends). The remaining Platyhelminthes form a monophyletic group - one that contains all and only descendants of a common ancestor that is itself a member of the group. The redefined Platyhelminthes is part of the Lophotrochozoa, one of the three main groups of more complex bilaterians. These analyses had concluded the redefined Platyhelminthes, excluding Acoelomorpha, consists of two monophyletic subgroups, Catenulida and Rhabditida, with Cestoda, Trematoda and Monogenea forming a monophyletic subgroup within one branch of the Rhabditophora. Hence, the traditional platyhelminth subgroup "Turbellaria" is now regarded as paraphyletic, since it excludes the wholly parasitic groups, although these are descended from one group of "turbellarians". (see more...)

Allosaurus skeletal mount.
Allosaurus is a genus of large theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period (Kimmeridgian to early Tithonian). The name "Allosaurus" means "different lizard". The first fossil remains that can definitely be ascribed to this genus were described in 1877 by paleontologist Othniel Charles Marsh. As one of the first well-known theropod dinosaurs, it has long attracted attention outside of paleontological circles.

Allosaurus was a large bipedal predator. Its skull was large and equipped with dozens of large, sharp teeth. It averaged 8.5 m (28 ft) in length, though fragmentary remains suggest it could have reached over 12 m (39 ft). It is classified as an allosaurid, a type of carnosaurian theropod dinosaur. The genus has a complicated taxonomy, and includes an uncertain number of valid species, the best known of which is A. fragilis. The bulk of Allosaurus remains have come from North America's Morrison Formation, with material also known from Portugal and possibly Tanzania.

As the most abundant large predator in the Morrison Formation, Allosaurus was at the top of the food chain, probably preying on contemporaneous large herbivorous dinosaurs and perhaps even other predators. Potential prey included ornithopods, stegosaurids, and sauropods. Some paleontologists interpret Allosaurus as having had cooperative social behavior, and hunting in packs, while others believe individuals may have been aggressive toward each other. It may have attacked large prey by ambush, using its upper jaw like a hatchet. (see more...)

The Berlin specimen of Archaeopteryx.
Archaeopteryx is a genus of early bird that is transitional between feathered dinosaurs and modern birds. Since the late nineteenth century, it had been generally accepted by palaeontologists, and celebrated in lay reference works, as being the oldest known bird. Archaeopteryx lived in the Late Jurassic period around 150 million years ago, in what is now southern Germany during a time when Europe was an archipelago of islands in a shallow tropical sea. Similar in shape to a European Magpie, with the largest individuals possibly attaining the size of a raven, Archaeopteryx could grow to about 0.5 m (1 ft 8 in) in length. Despite its small size, broad wings, and inferred ability to fly or glide, Archaeopteryx has more in common with other small Mesozoic dinosaurs than it does with modern birds. In particular, it shares the following features with the deinonychosaurs (dromaeosaurs and troodontids): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which also suggest homeothermy), and various skeletal features. These features make Archaeopteryx a clear candidate for a transitional fossil between dinosaurs and birds. The type specimen of Archaeopteryx was discovered just two years after Charles Darwin published On the Origin of Species. Archaeopteryx seemed to confirm Darwin's theories and has since become a key piece of evidence for the origin of birds, the transitional fossils debate, and confirmation of evolution. (see more...)

Skeletal mount of Compsognathus.
Compsognathus is a genus of small, bipedal, carnivorous theropod dinosaurs. Members of its single species Compsognathus longipes could grow to the size of a turkey. They lived around 150 million years ago, the latest Kimmeridgian age of the late Jurassic period, in what is now Europe. Paleontologists have found two well-preserved fossils, one in Germany in the 1850s and the second in France more than a century later. Today, C. longipes is the only recognized species, although the larger specimen discovered in France in the 1970s was once thought to belong to a separate species and named C. corallestris.

Many presentations still describe Compsognathus as "chicken-sized" dinosaurs because of the small size of the German specimen, which is now believed to be a juvenile. Compsognathus longipes is one of the few dinosaur species for which diet is known with certainty: the remains of small, agile lizards are preserved in the bellies of both specimens. Teeth discovered in Portugal may be further fossil remains of the genus.

Although not recognized as such at the time of its discovery, Compsognathus is the first theropod dinosaur known from a reasonably complete fossil skeleton. Until the 1990s, it was the smallest known non-avialan dinosaur; earlier it was the closest supposed relative of the early bird Archaeopteryx. (see more...)

Specifiers for Dinosauria.
Dinosaurs are a diverse group of animals that first appeared during the Triassic period, 231.4 million years ago, and were the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic until the end of the Cretaceous (66 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of most dinosaur groups. The fossil record indicates that birds evolved from theropod dinosaurs and, consequently, they are considered a subgroup of dinosaurs by many paleontologists. Some birds survived the extinction event and their descendants continue the dinosaur lineage to the present day. Using fossil evidence, paleontologists have identified over 500 distinct genera of non-avian dinosaurs. Dinosaurs are represented on every continent. Some are herbivorous, others carnivorous. While dinosaurs were ancestrally bipedal, many extinct groups included quadrupedal species. Elaborate display structures such as horns or crests are common to all dinosaur groups, and some extinct groups developed skeletal modifications such as bony armor and spines. Evidence suggests that egg laying and nest building are additional traits shared by all dinosaurs. While modern birds are generally small due to the constraints of flight, many prehistoric dinosaurs were large-bodied—the largest sauropod dinosaurs may have achieved lengths of 58 meters (190 feet). Many dinosaurs were quite small: Xixianykus, for example, was only about 50 cm (20 in) long. (see more...)

Restoration of two Diplodocus longus.
Diplodocus is an extinct genus of diplodocid sauropod dinosaur whose fossils were first discovered in 1877 by S. W. Williston. The generic name, coined by Othniel Charles Marsh in 1878, is a Neo-Latin term derived from Greek διπλός (diplos) "double" and δοκός (dokos) "beam", in reference to its double-beamed chevron bones located in the underside of the tail. These bones were initially believed to be unique to Diplodocus; however, they have since then been discovered in other members of the diplodocid family and in non-diplodocid sauropods such as Mamenchisaurus.

This genus of dinosaurs lived in what is now western North America at the end of the Jurassic Period. Diplodocus is one of the more common dinosaur fossils found in the Upper Morrison Formation, a sequence of shallow marine and alluvial sediments deposited about 155 to 148 million years ago, in what is now termed the Kimmeridgian and Tithonian stages (Diplodocus itself ranged from about 154 to 150 million years ago). The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs such as Camarasaurus, Barosaurus, Apatosaurus and Brachiosaurus.

Diplodocus is among the most easily identifiable dinosaurs, with its classic dinosaur shape, long neck and tail and four sturdy legs. For many years, it was the longest dinosaur known. Its great size may have been a deterrent to the predators Allosaurus and Ceratosaurus: their remains have been found in the same strata, which suggests they coexisted with Diplodocus. (see more...)

Artist's restoration of Massospondylus carinatus.
Massospondylus is a genus of prosauropod dinosaur from the early Jurassic Period (Hettangian to Pliensbachian ages, ca. 200–183 million years ago). It was described by Sir Richard Owen in 1854 from remains found in South Africa, and is thus one of the first dinosaurs to have been named. Fossils have since been found at other locations in South Africa, Lesotho, and Zimbabwe.

The type species is M. carinatus; seven other species have been named during the past 150 years, but only M. kaalae among these is still considered valid. Prosauropod systematics have undergone numerous revisions during the last several years, and many scientists disagree where exactly Massospondylus lies on the dinosaur evolutionary tree. The family name Massospondylidae was once coined for the genus, but because knowledge of prosauropod relationships is in a state of flux, it is unclear which other dinosaurs—if any—belong in a natural grouping of massospondylids.

Although Massospondylus was long depicted as quadrupedal, a 2007 study found it to be bipedal. It was probably a plant eater (herbivore), although it is speculated that the prosauropods may have been omnivorous. This animal, 4–6 metres (13–20 ft) long, had a long neck and tail, with a small head and slender body. On each of its forefeet, it bore a sharp thumb claw that was used in defense or feeding. Recent studies indicate Massospondylus grew steadily throughout its lifespan, possessed air sacs similar to those of birds, and may have cared for its young. (see more...)

Artist's restoration of Stegosaurus stenops.
Stegosaurus (/ˌstɛɡˈsɔːrəs/, meaning "roof lizard" or "covered lizard" in reference to its bony plates) is a genus of armored stegosaurid dinosaur. They lived during the Late Jurassic period (Kimmeridgian to early Tithonian), some 155 to 150 million years ago in what is now western North America. In 2006, a specimen of Stegosaurus was announced from Portugal, showing that they were present in Europe as well. Due to its distinctive tail spikes and plates, Stegosaurus is one of the most recognizable dinosaurs. At least three species have been identified in the upper Morrison Formation and are known from the remains of about 80 individuals. A large, heavily built, herbivorous quadruped, Stegosaurus had a distinctive and unusual posture, with a heavily rounded back, short forelimbs, head held low to the ground and a stiffened tail held high in the air. Its array of plates and spikes has been the subject of much speculation. The spikes were most likely used for defense, while the plates have also been proposed as a defensive mechanism, as well as having display and thermoregulatory functions. Stegosaurus had a relatively low brain-to-body mass ratio. It had a short neck and small head, meaning it most likely ate low-lying bushes and shrubs. It was the largest of all the stegosaurians (bigger than genera such as Kentrosaurus and Huayangosaurus) and, although roughly bus-sized, it nonetheless shared many anatomical features (including the tail spines and plates) with the other stegosaurian genera. (see more...)

Artist's restoration of Rugops .
Abelisauridae (meaning "Abel's lizards") is a family (or clade) of ceratosaurian theropod dinosaurs. Abelisaurids thrived during the Cretaceous Period, on the ancient southern supercontinent of Gondwana, and today their fossil remains are found on the modern continents of Africa and South America, as well as on the Indian subcontinent and the island of Madagascar. Abelisaurids first appear in the fossil record of the early middle Jurassic period, and at least one species (Majungasaurus crenatissimus) survived until the end of the Mesozoic era 66 million years ago. Like most theropods, abelisaurids were carnivorous bipeds. They were characterized by stocky hindlimbs and extensive ornamentation of the skull bones, with grooves and pits. In many abelisaurids, like Carnotaurus, the forelimbs are vestigial, the skull is shorter and bony crests grows above the eyes. Most of the known abelisaurids would have been between 5 to 9 meters (17 to 30 ft) in length, from snout to tip of tail, with a new and as yet unnamed specimen from northwestern Turkana in Kenya, Africa reaching a possible length of 11–12 meters (36 to 39 feet). Before becoming well known, fragmentary abelisaurid remains were occasionally misidentified as possible South American tyrannosaurids. (see more...)

Scale image of both Amphicoelias species.
Amphicoelias (/ˌæmfˈsliəs/, meaning "biconcave", from the Greek αμφι, amphi: "on both sides", and κοιλος, koilos: "hollow, concave") is a genus of herbivorous sauropod dinosaur that includes what may be the largest dinosaur ever discovered, A. fragillimus. Based on surviving descriptions of a single fossil bone, A. fragillimus may have been the longest known vertebrate at 40 to 60 metres (130 to 200 ft) in length, and may have had a mass of up to 122 tonnes (135 short tons). However, because the only fossil remains were lost at some point after being studied and described in the 1870s, evidence survives only in drawings and field notes. The fossil was found in the Morrison Formation. (see more...)

Artist's restration of Zuniceratops.
Ceratopsia or Ceratopia (/ˌsɛrəˈtɒpsiə/ or /ˌsɛrəˈtpiə/; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7 million years ago. The last ceratopsian species became extinct in the Cretaceous–Paleogene extinction event, 66 million years ago.

Early members of the ceratopsian group, such as Psittacosaurus, were small bipedal animals. Later members, including ceratopsids like Centrosaurus and Triceratops, became very large quadrupeds and developed elaborate facial horns and frills extending over the neck. While these frills might have served to protect the vulnerable neck from predators, they may also have been used for display, thermoregulation, the attachment of large neck and chewing muscles or some combination of the above. Ceratopsians ranged in size from 1 meter (3 ft) and 23 kilograms (50 lb) to over 9 meters (30 ft) and 5,400 kg (12,000 lb).

Triceratops are by far the best-known ceratopsians to the general public. It is traditional for ceratopsian genus names to end in "-ceratops", although this is not always the case. One of the first named genera was Ceratops itself, which lent its name to the group, although it is considered a nomen dubium today as its fossil remains have no distinguishing characteristics that are not also found in other ceratopsians. (see more...)

Holotype specimen of Cetiosauriscus.
Cetiosauriscus (/sθsɒrɪskʌs/ meaning "whale-lizard-like" i.e. "Cetiosaurus-like") is a genus of sauropod dinosaur. It was perhaps a diplodocid, a relative of Diplodocus, and lived in the Callovian (Middle to Late Jurassic Period) of England (about 165 to 161 million years ago [mya]). Cetiosauriscus was a quadrupedal, herbivorous, saurischian. It was named by Friedrich von Huene in 1927, the species name being C. leedsi. Later it was shown that C. leedsi was not a Cetiosauriscus species, so, in 1993, Alan J. Charig sent a petition to the ICZN to designate C. stewarti as the type species. The remains include a series of vertebra, a hind leg, a possible whiplash tail, a partial sacrum, and a front leg. Cetiosauriscus has, over time, been classified in Cardiodontinae within Cetiosauridae, Diplodocidae, and Mamenchisauridae. It lived alongside Eustreptospondylus, Sarcolestes, Callovosaurus, Lexovisaurus, and possibly Megalosaurus, and Cetiosaurus. (see more...)

Artist's restoration of Coelurus.
Coelurus (/sˈljʊərəs/ si-LEWR-əs) is a genus of coelurosaur dinosaur from the Late Jurassic period (mid-late Kimmeridgian faunal stage, 153–150 million years ago). The name means "hollow tail", referring to its hollow tail vertebrae (Greek κοιλος, koilos = hollow + ουρα, oura = tail). Although its name is linked to one of the main divisions of theropods (Coelurosauria), it has historically been poorly understood, and sometimes confused with its better-known contemporary Ornitholestes. Like many dinosaurs studied in the early years of paleontology, it has had a confusing taxonomic history, with several species being named and later transferred to other genera or abandoned. Only one species is currently recognized as valid: the type species, C. fragilis, described by Othniel Charles Marsh in 1879. It is known from one partial skeleton found in the Morrison Formation of Wyoming, United States. It was a small bipedal carnivore with elongate legs. (see more...)

Artist's restoration of Cryolophosaurus.
Cryolophosaurus (/ˌkr.ˌlɒfˈsɔːrəs/ or /krˌɒlfəˈsɔːrəs/; "CRY-oh-loaf-oh-SAWR-us") is a genus of large theropods known from only a single species Cryolophosaurus ellioti, known from the early Jurassic period of Antarctica. It was about 6 to 7 metres (20 to 23 ft) long and 460 kilograms (1,010 lb) in weight, making it one of the largest theropods of its time. Individuals of this species may have grown even larger, because the only known specimen probably represents a sub-adult.

Cryolophosaurus is known from a skull, a femur and other material, the skull. C. ellioti possessed a distinctive crest on its head that spanned the head from side to side. Based on evidence from related species and studies of bone texture, it is thought that this bizarre crest was used for intra-species recognition. Since its original description, the consensus is that Cryolophosaurus is either a primitive member of the Tetanurae or a close relative of that group.

Cryolophosaurus was first excavated from Antarctica's Early Jurassic, Sinemurian to Pliensbachian aged Hanson Formation, formerly the upper Falla Formation, by paleontologist Dr. William Hammer in 1991. It was the first carnivorous dinosaur to be discovered in Antarctica and the first non-avian dinosaur from the continent to be officially named. The sediments in which its fossils were found have been dated at ~194 to 188 million years ago, representing the Early Jurassic Period. (see more...)

Artist's restoration of Yurgovuchia.
Dromaeosauridae is a family of bird-like theropod dinosaurs. They were small- to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek dromeus (δρομευς) meaning 'runner' and sauros (σαυρος) meaning 'lizard'. In informal usage they are often called raptors (after Velociraptor), a term popularized by the film Jurassic Park; a few types include the term "raptor" directly in their name and have come to emphasize their supposed bird-like habits. Dromaeosaurid fossils have been found in North America, Europe, Africa, Japan, China, Mongolia, Madagascar, Argentina, and Antarctica. They first appeared in the mid-Jurassic Period (late Bathonian stage, about 164 million years ago) and survived until the end of the Cretaceous (Maastrichtian stage, 66 ma), existing for over 100 million years, up until the Cretaceous–Paleogene extinction event. The presence of dromaeosaurs as early as the Middle Jurassic has been confirmed by the discovery of isolated fossil teeth, though no dromaeosaurid body fossils have been found from this period. (see more...)

Artist's reconstruction of Heterodontosaurus.
Heterodontosauridae ("different-toothed lizards") is a family of early ornithischian dinosaurs that were likely among the most basal (primitive) members of the group. Although their fossils are rare, they lived around the globe beginning in the late Triassic Period, and a few late-surviving species persisted into the Early Cretaceous. Heterodontosaurids were fox-sized dinosaurs less than 2 meters (6.6 ft) in length, including a long tail. They are known mainly for their characteristic teeth, including enlarged canine-like tusks and cheek teeth adapted for chewing, analogous to those of Cretaceous hadrosaurids. Their diet was herbivorous or possibly omnivorous. (see more...)

Artist's restoaration of two Megalosaurus.
Megalosaurus is a genus of large meat-eating theropod dinosaurs of the Middle Jurassic period (Bathonian stage, 166 million years ago) of Southern England. Although fossils from other areas have been assigned to the genus, the only certain remains of Megalosaurus come from the late Middle Jurassic of the Oxfordshire.

Megalosaurus was, in 1824, the first genus of dinosaur to be validly named, apart from birds. The type species is Megalosaurus bucklandii, named in 1827. In 1842, Megalosaurus was one of three genera on which Richard Owen based his Dinosauria. On Owen's directions a model was made as one of the Crystal Palace Dinosaurs, which greatly increased the public interest for prehistoric reptiles. Subsequently, over fifty other species would be classified under the genus, originally because dinosaurs were not well known, but even during the 20th century after many dinosaurs had been discovered. Today it is understood these additional species were not directly related to M. bucklandii, which is the only true Megalosaurus species.

Megalosaurus was about seven metres long, weighing roughly 1.5 tonnes. It was bipedal, walking on stout hindlimbs, its horizontal torso balanced by a horizontal tail. Its forelimbs were short, though very robust. Megalosaurus had a rather large head, equipped with long curved teeth. It was generally a robust and heavily muscled animal. (see more...)

Artist's restoration of Othnielosaurus.
Othnielosaurus is a genus of ornithischian dinosaur that lived about 155 to 148 million years ago, during the Late Jurassic-age Morrison Formation of the western United States. It is named in honor of famed paleontologist Othniel Charles Marsh, and was formerly assigned to the genus Laosaurus. This genus was coined to hold fossils formerly included in Othnielia, which is based on remains that may be too sparse to hold a name; as such, it is part of decades of research to untangle the taxonomy left behind by Marsh and his rival Edward Drinker Cope from the Bone Wars. Othnielosaurus has usually been classified as a hypsilophodont, a type of generalized small bipedal herbivore or omnivore, although recent research has called this and the existence of a distinct group of hypsilophodonts into question. (see more...)

Artist's restoration of Scelidosaurus.
Scelidosaurus is a genus of herbivorous armoured ornithischian dinosaur from England. Scelidosaurus lived during the Early Jurassic Period, during the Sinemurian to Pliensbachian stages around 191 million years ago. This genus and related genera at the time lived on the supercontinent Laurasia. Its fossils have been found near Charmouth in Dorset, England, and are known for their excellent preservation. Scelidosaurus has been called the earliest complete dinosaur. It is the most completely known dinosaur of the British Isles.

Scelidosaurus was about 4 metres (13 ft) long. It was a largely quadrupedal animal, feeding on low scrubby plants, the parts of which were bitten off by the small, elongated, head to be processed in the large gut. Scelidosaurus was lightly armoured, protected by long horizontal rows of keeled oval scutes, that stretched along the neck, back and tail.

One of the oldest known and most "primitive" of the thyreophorans, the exact placement of Scelidosaurus within this group has been the subject of debate for nearly 150 years. This was not helped by the limited additional knowledge about the early evolution of armoured dinosaurs. Today most evidence indicates that Scelidosaurus is a basal member of the Thyreophora, lower placed in the evolutionary tree of life than the Ankylosauridae or the Stegosauridae. (see more...)

Artist's restoration of Appalachiosaurus.
Tyrannosauroidea (meaning 'tyrant lizard forms') is a superfamily (or clade) of coelurosaurian theropod dinosaurs that includes the family Tyrannosauridae as well as more basal relatives. Tyrannosauroids lived on the Laurasian supercontinent beginning in the Jurassic Period. By the end of the Cretaceous Period, tyrannosauroids were the dominant large predators in the Northern Hemisphere, culminating in the gigantic Tyrannosaurus itself. Fossils of tyrannosauroids have been recovered on what are now the continents of North America, Europe, Asia, possibly South America and Australia. Tyrannosauroids were bipedal carnivores, as were most theropods, and were characterized by numerous skeletal features, especially of the skull and pelvis. Early in their existence, tyrannosauroids were small predators with long, three-fingered forelimbs. Late Cretaceous genera became much larger, including some of the largest land-based predators ever to exist, but most of these later genera had proportionately small forelimbs with only two digits. Primitive feathers have been identified in fossils of two species, and may have been present in other tyrannosauroids as well. Prominent bony crests in a variety of shapes and sizes on the skulls of many tyrannosauroids may have served display functions. (see more...)

Sculptures of Vulcanodon karibaensis.
Vulcanodon (meaning "volcano tooth") is an extinct genus of sauropod dinosaur from the Early Jurassic of southern Africa. The only known species is V. karibaensis. Discovered in 1969 in Rhodesia (now known as Zimbabwe), it was regarded as the earliest known sauropod for decades, and is still one of the most primitive sauropods that has been discovered. As a quadrupedal, ground-dwelling herbivore, Vulcanodon already showed the typical sauropod body plan with column-like legs and a long neck and tail. It was much smaller than most other sauropods, measuring approximately 6.5 metres (20 ft) in length. Vulcanodon is known from a fragmentary skeleton including much of the pelvic girdle, hind limbs, forearms, and tail, but lacking the trunk and neck vertebrae as well as the skull. Originally, this genus was believed to be a prosauropod because of the knife-shaped teeth found near its fossils, which fit in with the idea that prosauropods were omnivorous. Scientists now know that the teeth belonged to an unidentified theropod that may have scavenged on the Vulcanodon carcass. Vulcanodon is now known to be a true sauropod. Upon the discovery of the related Tazoudasaurus, both animals were unified in the family Vulcanodontidae, though this has not been universally accepted. (see more...)

Jaw fragment of Ambondro mahabo.
Ambondro mahabo is a mammal from the middle Jurassic (about 167 million years ago) of Madagascar. The only species of the genus Ambondro, it is known from a fragmentary lower jaw with three teeth, interpreted as the last premolar and the first two molars. Features of the talonid suggest that Ambondro had tribosphenic molars, the basic arrangement of molar features also present in marsupial and placental mammals. It is the oldest known mammal with putatively tribosphenic teeth; at the time of its discovery it antedated the second oldest example by about 25 million years. Upon its description in 1999, Ambondro was interpreted as a primitive relative of Tribosphenida (marsupials, placentals, and their extinct tribosphenic-toothed relatives). In 2001, however, an alternative suggestion was published that united it with the Cretaceous Australian Ausktribosphenos and the monotremes (the echidnas, the platypus, and their extinct relatives) into the clade Australosphenida, which would have acquired tribosphenic molars independently from marsupials and placentals. The Jurassic Argentinean Asfaltomylos and Henosferus and the Cretaceous Australian Bishops were later added to Australosphenida, and new work on wear in australosphenidan teeth has called into question whether these animals, including Ambondro, did in fact have tribosphenic teeth. Other paleontologists have challenged this concept of Australosphenida, and instead proposed that Ambondro is not closely related to Ausktribosphenos plus monotremes, or that monotremes are not australosphenidans and that the remaining australosphenidans are related to placentals. (see more...)

Artist's restoration of Pterodactylus.
Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period (228 to 66 million years ago). Pterosaurs are the earliest vertebrates known to have evolved powered flight. Their wings were formed by a membrane of skin, muscle, and other tissues stretching from the ankles to a dramatically lengthened fourth finger. Early species had long, fully toothed jaws and long tails, while later forms had a highly reduced tail, and some lacked teeth. Many sported furry coats made up of hair-like filaments known as pycnofibres, which covered their bodies and parts of their wings. Pterosaurs spanned a wide range of adult sizes, from the very small Nemicolopterus to the largest known flying creatures of all time, including Quetzalcoatlus and Hatzegopteryx. Pterosaurs are often referred to in the popular media and by the general public as flying dinosaurs, but this is incorrect. However, like the dinosaurs, pterosaurs are more closely related to birds than to any living reptile. Pterosaurs are also incorrectly referred to as pterodactyls, particularly by journalists. "Pterodactyl" refers specifically to members of the genus Pterodactylus, and more broadly to members of the suborder Pterodactyloidea of the pterosaurs. (see more...)

A modern male bluetail damselfly.
Damselflies (suborder Zygoptera) are insects in the order Odonata. They are similar to dragonflies, but are smaller, have slimmer bodies, and most species fold the wings along the body when at rest. Damselflies are an ancient group, having existed since at least the Lower Permian. Today they are found on every continent except Antarctica.

All damselflies are predatory; both nymphs and adults eat other insects. The nymphs are aquatic, with different species living in a variety of freshwater habitats including acid bogs, ponds, lakes and rivers. The nymphs moult repeatedly, at the last moult climbing out of the water to undergo metamorphosis. The skin splits down the back, they emerge and inflate their wings and abdomen to gain their adult form.

Some species of damselfly have elaborate courtship behaviours. Many species are sexually dimorphic, the males often being more brightly coloured than the females. Like dragonflies, they reproduce using indirect insemination and delayed fertilisation. A mating pair form a shape known as a "heart" or "wheel", the male clasping the female at the back of the head, the female curling her abdomen down to pick up sperm from secondary genitalia at the base of the male's abdomen. The pair often remain together with the male still clasping the female while laying eggs within the tissue of plants in or near water using a robust ovipositor. (see more...)

Jaw fragment of Ambondro mahabo.
Several mammals are known from the Mesozoic of Madagascar. The Bathonian (middle Jurassic) Ambondro, known from a piece of jaw with three teeth, is the earliest known mammal with molars showing the modern, tribosphenic pattern that is characteristic of marsupial and placental mammals. Interpretations of its affinities have differed; one proposal places it in a group known as Australosphenida with other Mesozoic tribosphenic mammals from the southern continents (Gondwana) as well as the monotremes, while others favor closer affinities with northern (Laurasian) tribosphenic mammals or specifically with placentals. At least five species are known from the Maastrichtian (late Cretaceous), including a yet undescribed species known from a nearly complete skeleton that may represent a completely new group of mammals. The gondwanathere Lavanify, known from two teeth, is most closely related to other gondwanatheres found in India and Argentina. Two other teeth may represent another gondwanathere or a different kind of mammal. One molar fragment is one of the few known remains of a multituberculate mammal from Gondwana and another (UA 8699) has been interpreted as either a marsupial or a placental. (see more...)

Dinheirosaurus with a human to scale
Dinheirosaurus is a genus of diplodocid sauropod dinosaur that is known from fossils uncovered in modern-day Portugal. It may represent a species of Supersaurus. The only species is Dinheirosaurus lourinhanensis, first described by José Bonaparte and Octávio Mateus in 1999 for vertebrae and some other material from the Lourinhã Formation. Although the precise age of the formation is not known, it can be dated around the early Tithonian of the Late Jurassic.

The known material includes two cervical vertebrae, nine dorsal vertebrae, a few ribs, a fragment of a pubis, and many gastroliths. Of the material, only the vertebrae are diagnostic, with the ribs and pubis being too fragmentary or general to distinguish Dinheirosaurus. This material was first described as in the genus Lourinhasaurus, but differences were noticed and in 1999 Bonaparte and Mateus redescribed the material under the new binomial Dinheirosaurus lourinhanensis. Another specimen, ML 418, thought to be Dinheirosaurus, is now known to be from another Portuguese diplodocid. This means that Dinheirosaurus lived alongside many theropods, sauropods, thyreophorans and ornithopods, as well as at least one other diplodocid.

Dinheirosaurus is a diplodocid, a relative of Apatosaurus, Diplodocus, Barosaurus, Supersaurus, and Tornieria. Among those, the closest relative to Dinheirosaurus is Supersaurus, and together they form a clade of primitive diplodocids. While they were once considered to be diplodocines they are likely more basal than Apatosaurus. (see more...)

Artist's restoration of Apatosaurus louisae
Apatosaurus is a genus of extinct sauropod dinosaurs that lived in North America during the Late Jurassic period. Othniel Charles Marsh described and named the first-known species, A. ajax in 1877, and a second species, A. louisae, was discovered and named by William H. Holland in 1916. Fossils are known from the Morrison Formation of modern-day Colorado, Oklahoma, and Utah, in the United States. Apatosaurus had an average length of 22.8 m (75 ft), and an average mass of at least 16.4 metric tons (18.1 short tons).

The cervical vertebrae of Apatosaurus are less elongated and more heavily constructed than those of Diplodocus, and the bones of the leg are much stockier despite being longer, implying that Apatosaurus was a more robust animal. The tail was held above the ground during normal locomotion. Apatosaurus had a single claw on each forelimb and three on each hindlimb. Apatosaurus was a generalized browser that likely held its head elevated. To lighten its vertebrae, Apatosaurus had air sacs that made the bones internally full of holes. Like that of other diplodocids, its tail may have been used as a whip to create loud noises.

Apatosaurus is a genus in the family Diplodocidae. It is one of the more basal genera. Brontosaurus has long been considered a junior synonym of Apatosaurus; its only species was reclassified as A. excelsus in 1903. However, the 2015 study concluded that Brontosaurus was a valid genus of sauropod distinct from Apatosaurus. (see more...)

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