JURASSIC

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JURASSIC

201.4 - 143.1 mya

The Jurassic is a geologic period that lasted about 58.3 million years, starting 201.4 mya, with the end of the Triassic, and ending 143.1 mya, with the start of the Cretaceous. By the beginning of the Jurassic, the supercontinent of Pangaea was starting to separate itself into two landmasses, with Laurasia to the north and Gondwana to the south. The average climate across the period was warm, with plenty of forests growing next to the poles and large arid expanses covering much of the lower latitudes. It was a period of evolutionary innovation, where plenty of animals were starting to dominate in new environments, such as crocodylomorphs, which were beginning to turn into predominant reptiles in aquatic environments.

The Jurassic was initially divided into a three-fold division by the German geologist Leopold von Buch in 1839, composing the Black Jurassic (199-175 mya), the Brown Jurassic (175-161 mya) and the White Jurassic (161-150 mya). The term "Lias" had previously been used for strata of equivalent age to the Black Jurassic in England by William Conybeare and William Phillips in 1822. The French palaeontologist Alcide d'Orbigny, during the 1840-50s, divided the Jurassic into ten stages based on ammonite and other fossil assemblages in England and France, seven of which are still used today, though none retains its original definition.

Currently, the Jurassic is divided into three epochs: Early (201.4 - 174.7 mya), Middle (174.7 - 161.5 mya) and Late (161.5 - 143.1 mya). The first stage of the Early Jurassic is the Hettangian (201.4 - 199.5 mya), named by Swiss palaeontologist Eugène Renevier in 1864 after Hettange-Grande in northeastern France. It is followed by the Sinemurian (199.5 - 192.9 mya) which is then followed by the Pliensbachian (192.9 - 184.2 mya), this last one being named after the hamlet of Pliensbach in the community of Zell unter Aichelberg in the Swabian Alb, near Stuttgart, Germany. The early Jurassic ends with the Toarcian (184.2 - 174.7 mya), with a GSSP for its base being located in Peniche, Portugal. The Middle Jurassic starts with the Aalenian (174.7 - 170.9 mya), whose lower boundary was originally positioned between the dark clays of the Black Jurassic and the overlying clayey sandstone and ferruginous oolite of the Brown Jurassic sequences of southwestern Germany, dated to around 175 mya. The base of the Aalenian is now defined by the first appearance of the ammonite Leioceras opalinum. Following this one is the Bajocian (170.9 - 168.2 mya), whose base is defined by the first appearance of the ammonite Hyperlioceras mundum. After this one is the Bathonian (168.2 - 165.3 mya) and then there's the Callovian (165.3 - 161.5 mya), whose boundary was initially placed at the contact between the Forest Marble Formation and the Cornbrash Formation, but that is now agreed to be part od the late Bathonian instead. The Late Jurassic is divided into the Oxfordian (161.5 - 154.8 mya), the Kimmeridgian (154.8 - 149.2 mya) and the Tithonian (149.2 - 143.1 mya), the latter of which has an unresolved upper end, in its border with the Cretaceous, with some working definitions existing, such as thefirst appearance of the ammonite Strambergella jacobi, formerly placed in the genus Berriasella, but its use as a stratigraphic indicator has been questioned, as its first appearance does not correlate with that of the alveolate Calpionella alpina, often argued as the index fossil for the start of the Cretaceous period. The Jurassic is home to the formation of some major extraterrestrial impact structures, such as the Morokweng impact structure, located in the Kalahari desert in South Africa, about 70 kilometers in diameter. The impact occured around 146 mya, and it may have, curiously, been caused by an object nearing the size of the one that caused the end Cretaceous extinction event, but curiously doesn't seem to have resulted in a recognizable mass extinction. Another major structure is the Puchezh-Katunki crater, 40 kilometres in diameter, in western Russia, dating to about 195.9 mya in the Sinemurian.

During the Jurassic, the continents of the world were surrounded by the massive Panthalassa ocean, with the Tethys ocean separating Gondwana from Asia. With the end of the Triassic, marine transgressions into Europe turned it into a massive archipelago, which stayed that way throughout the entirety of the Jurassic period. Starting in the early Jurassic, the Boreal Ocean was connected by the proto-Atlantic via the Viking corridor, also known as Transcontinental Laurasian Seaway, a passage between the Baltic Shield and Greenland several hundred kilometers wide. As part of the Nevadan orogeny, which began during the Triassic, the Cache Creek Ocean closed, and various terranes including the large Wrangellia Terrane accreted onto the western margin of North America. The sea levels across the period were quite variable, ranging from levels similar to nowadays up until a peak of possibly 140 meters above modern days during the Kimmeridgian-Tithonian boundary (149.2 mya) before falling in the late Tithonian to about 100 meters, and later rebounding to about 110 meters in the Jurassic-Cretaceous boundary (143.1 mya). The sea level within the long-term trends across the Jurassic was cyclical, with 64 fluctuations, 15 of which were over 75 metres, with the most noted cyclicity in Jurassic rocks is fourth order, with a periodicity of approximately 410,000 years.

The Jurassic witnessed the decline of the Pangaea megamonsoon, so characteristic of the earlier Permian and Triassic periods. Variation in the frequency of wildfire activity in the Jurassic was governed by the 405 thousand years eccentricity cycle. Thanks to the breakup of Pangaea, the hydrological cycle during the Jurassic was significantly enhanced. The beginning of the Jurassic was marked by a thermal spike responsible by the Triassic-Jurassic extinction event, as the Central Atlantic magmatic province (CAMP) erupted. It was followed by a cool interval in the early Jurassic (199 - 183 mya), with glaciation being proposed to have occured in the early and late Pliensbachian. There was later a spike in global temperatures of about 4 to 8ºC in the early Toarcian that corresponds to the Toarcian Oceanic Anoxic Event (TOAE), around 183 mya, caused by the Karoo-Ferrar large igneous provinces in southern Gondwana, causing a warm period that lasts until around 174 mya. By the late Bajocian, a transient ice age possibly occured. A prominent drop in temperatures occurred during the Tithonian, known as the Early Tithonian Cooling Event (ETCE). This marked the beginning of the Tithonian–early Barremian Cool Interval (TBCI), starting 150 mya and continuing into the Early Cretaceous. The Toarcian Oceanic Anoxic Event affected several groups of organisms, including ammonites, ostracods, foraminiferans, bivalves and cnidarians, but one of the most affected were the brachiopods, causing one of the most severe extinction events for this group. Despite being very impactful for marine invertebrates, the marine reptiles were little affected by this event. During this event, the Sichuan Basin was transformed into a gigantic freshwater lake, three times the size of modern Lake Superior, well represented by the Ziliujing Formation.

With the end of the Triassic, flora has changed dramatically, but the extinction rates appear to have been minimal, with overturns in flora being mostly caused by local eccological successions. For instance, Dicroidium, a successful corystosperm seed fern in the Triassic, was dominant in the gondwanan floral communities until the end Triassic extinction, where it later survived as a relict in Antarctica during the early Jurassic. Araucariaceans first appear in the Jurassic period, with stem members being known from fossils as early as from the early Jurassic, with members attributed to the modern Araucaria genus being found in both hemispheres since the Middle Jurassic. Also abundant in the Jurassic are the Cheirolepidiaceae, characterized by their highly distinctive Classopolis pollen. Jurassic representatives include the pollen cone Classostrobus and the seed cone Pararaucaria. Araucarian and Cheirolepidiaceae conifers often occured in association. The earliest fossil cupressacean is Austrohamia from the Pliensbachian of Patagonia, whose reproductive structures bare strong resemblance to the living Taiwania and Cunninghamia. Members of the extinct genus Schizolepidopsis, which could represent a stem-group to the pine family (Pinaceae), were widely distributed across Eurasia during the Jurassic. The oldest unambiguous members of Podocarpaceae are known from the Jurassic, found across both hemispheres, including Scarburgia and Harrisiocarpus from the Middle Jurassic of England, as well as unnamed species from the Middle-Late Jurassic of Patagonia. The extinct Podozamites was a deciduous broad leaf conifer whose range extended into the northern polar regions of Siberia, contracting northwards in the middle and late Jurassic, in responde to increase aridity in the region. The Jurassic was also home to an array of ginkgoaleans, such as Yimaia, Grenana, Nagrenia and Karkenia. During the Jurassic there was also an abundance of Bennettitales, a group so similar to the unrelated cycads that they can't reliably be distinguished on the basis of morphology alone. These were plants that supported an array of unique pollinators, such as the extinct kalligrammatids, lacewings with enormous proboscis, and the acrocerid flies that still exist today, feeding on the nectar of bennettitalean cones. Cycadophytes also existed in the Jurassic, including genera such as the widespread Ctenis, distantly related to modern cycads. Abundant too in this period were the paraphyletic seed ferns, "fern-like" plants that produced seeds, with nebulous affinities with modern seed plant groups. One of them was Czekanowskiales, also known as Leptostrobales, a group of seed plants of uncertain affinities with persistent heavily dissected leaves borne on deciduous short shoots, subtended by scale-like leaves, known from fossils the Late Triassic up until the late Cretaceous. The genus Phoenicopsis was a widespread czekanowskialean genus in Early-Middle Jurassic floras of Eastern Asia and Siberia. Living families of ferns existed back in the Jurassic, such as Gleicheniaceae (forked ferns), Osmundaceae and Marattiaceae. The earliest fossil horsetails of the genus Equisetum appear in the early Jurassic in the form of Equisetum dimorphum of Patagonia and Equisetum laterale of the early-mid Jurassic of Australia. Quillworts were well present in the Jurassic, with primitive forms from this period persisting into the Cretaceous in the form of genera such as Nathorstiana. Liverworts too were well represented in this period.

Judging only by the fossil data, the only dinosaurs represented in this period were theropods, sauropodomorphs and ornithischians, though Chilesaurus, a strange herbivorous dinosaur from the late Jurassic of South America, has had conflicting taxonomic positionings, though it likely represents an aberrant herbivorous theropod. The Jurassic shows the earliest fossil representatives of Averostra, with the appearance of the ceratosaur genus Saltriovenator from the early Jurassic of Italy, living 199.3 - 197.5 mya in the Sinemurian. The earliest fossil representatives of tetanurans appear in the early-middle Jurassic. This period also delineates the earliest fossil appearance of several coelurosaur groups, though some groups are dubiously represented, such as alvarezsaurs, where the late Jurassic chinese Shishugounykus and Haplocheirus may be another type of coelurosaur despite frequently being grouped with alvarezsaurs. The Jurassic shows the earliest fossil appearance of the ankylopollexian ornithischian dinosaurs, firstly represented by bipedal forms, such as Camptosaurus. Its also in this period that the earliest fossil neosauropods appear, including the macronarians and the diplodocoids, first unequivocally represented in the middle Jurassic, becoming nearly globally distributed by the late Jurassic. Jurassic pseudosuchians are almost, if not entirely, represented by crocodylomorphs (according to fossils), with aetosaurs and non-crocodylomorph loricatans appearing to disappear at large from the fossil record before the start of the Jurassic. A striking Jurassic group of crocodylomorphs are the thalattosuchians, a group of mostly marine pseudosuchians, including the metriorhynchids that became highly adapted for a fully marine life, with flipper-like limbs, scaleless skin and a fluked tail. Turtles appear to first originate in the Jurassic, with the Pleurodira and Cryptodira groups already represented by the middle-late Jurassic. Lepidosaurs too diversify a lot in this period, with some modern groups already seemingly appearing, such as the late Jurassic Dorsetisaurus as a stem-anguimorph from North America and Europe. Choristoderes also are represented in the Jurassic, with the genus Cteniogenys being one of them, a small lizard-like reptile from the mid-late Jurassic of North America and potentially also Eurasia. Ichthyosaurs are abundant in the Jurassic oceans, with the most successful family being the ophthalmosaurids from the middle-late Jurassic, which nevertheless remain mostly static in general appearance and niches, in comparison to the more diverse ichthyosaurs of the early Jurassic. Among sauropterygians, only plesiosaurs appear in the Jurassic fossil record, with the Triassic placodonts and nothosaurs not showing up preserved. Most early plesiosaurs were generally small, but their body size increased in some lineages into the Toarcian. The earliest record of freshwater plesiosaur fossils appear in this period, represented in sediments in China and Australia. Monofenestratan pterosaurs first appear in the fossil record in the early Jurassic, with more derived members, such as wukongopterids, appearing as fossils in the mid-late Jurassic. The pterodactyloids first appear as fossils in this period too, characterized by their short tails and more erect limbs, and including some notable families such as the ctenochasmatids, where many species were well adapted filter feeders with needle-like teeth. Modern lineages of lissamphibians were diversifying wildly in this period. Stem and crown frogs were well represented in the Jurassic, such as the stem-frog Notobatrachus of the early-mid Jurassic of South America, and the crown frogs Enneabatrachus and Rhadinosteus from the late Jurassic. Jurassic stem and crown salamanders were also present, with some stem-salamanders, such as Marmorerpeton and Kokartus, even displaying signs of neoteny. Salamanders earliest fossil presence in North America is evidenced by the late Jurassic Iridotriton from the Morrison Formation. Mammaliaforms were also well represented in the Jurassic, some were so unique that they had patagium between their limbs to glide like modern flying squirrels, a tactic adopted by the Jurassic volaticotheres and euharamiyidans. It would also be during the Jurassic that the first therian mammals appeared, with the genus Juramaia being suggested as the earliest known fossil eutherian. Despite this, Juramaia may actually be from the early Cretaceous instead, making any undisputable fossil Jurassic therians unknown to the scientific community, for now. Outside of mammaliamorphs, other synapsids known from the Jurassic are the possibly insectivorous trithelodonts, which are represented from fossils in the early Jurassic. Conodont fish were only represented by a handful of fossil species in the very end of the Triassic, with the group progressingly declining since the mid-late Triassic, with the most recent fossil species surviving extremelly shortly after the start of the Jurassic. Coelacanthiiforms are well represented in the Jurassic, including the family Latimeriidae that includes the extant coelacanths from our timeline's today, with this group appearing first as fossils in the Triassic, with Jurassic representatives including the european Swenzia from the late Jurassic. Among actinopterygiians, the archaic "palaeoniscoid" fish, once abundant in both sea and freshwater during the Triassic, were declining in the Jurassic as they were replaced by other actinopterygiians fish. Pycnodontiforms, which first appear as fossils in the late Triassic of the western Tethys, expand to South America and southeast Asia by the end of the Jurassic, achieving a great diversity in Europe. The oldest fossil lepisosteid fish appear in the Jurassic, already similar to our timeline's modern gars. The oldest known fossil rays appear in the early Jurassic, in the form of Antiquaobatis from the Pliensbachian of Germany. The oldest known fossil members of the shark order Heterodontiformes appear in the Early Jurassic, with the genus Heterodontus, including our timeline's modern bullhead sharks, first appearing as fossils in the Late Jurassic. Chimaeras already were around during the Jurassic period as well. Insect diversity in the Jurassic stagnated throughout the Early and Middle Jurassic, but the latter third of the period saw an increase in clade origination rates while extinction rates remained flat. Weevils first appear as fossils in the Late Jurassic, but they could have originated earlier in the period or in the Late Triassic. Dragonflies first appear in the Jurassic and the earliest fossil damselflies also appear in the Jurassic. Related to fleas, the Jurassic had less specialized and larger ectoparasitic insects known as the pseudopulicids. Other groups of insects, such as Phasmatodea, Mantophasmatodea, Embioptera and Raphidioptera have their first fossil appearance in the Jurassic as well. The early Jurassic Seppo may represent a stem-palpimanoid spider. The only known Jurassic fossil scorpion is Liassoscorpionides from the early Jurassic of Germany, having an unclear taxonomic placement. Marine invertebrate diversity in this period was influenced by the effects of the Triassic-Jurassic extinction, when the diversity of these animals was mostly affected in tropical latitudes, and especially more in Panthalassa, rather than in the Tethys and Boreal oceans. At the end of the Jurassic, there was a massive collapse in reefs, causing an accentuated drop in diversity of decapod crustaceans. Coccolithophores and dinoflagellates, which had first appear as fossils during the Triassic, radiated during the Early to Middle Jurassic, becoming prominent members of the phytoplankton. Rates of diversification in echinoderms sharply dropped during the Late Jurassic. In the Jurassic, members of the crab lineage are poorly represented as fossils, known mostly from carapace pieces which complicates their taxonomic placement. The earliest fossil hermit crabs appear in the form of Schobertella from the early Jurassic of Germany. Barnacles had a low diversity, compared to our timeline's today, but important evolutionary achievements were taken in this lineage, including the first known appearances of calcite shelled forms and species with an epiplanktonic mode of life. Brachiopod diversity declined heavily with the Triassic-Jurassic extinction event. Spire-bearing brachiopods (Spiriferinida and Athyridida) did not recover their biodiversity, disappearing from the fossil record in the TOAE. Like with the Triassic, bryozoan diversity in the Jurassic was low compared to the Paleozoic. Bivalve family level diversity after the Early Jurassic was static, though genus diversity experienced a gradual increase throughout the period. Rudist bivalves first appear as fossils in the Late Jurassic, around the middle Oxfordian in the northwestern margin of the Tethys Ocean, expanding to the eastern Tethys by the end of the Jurassic, foreshadowing the reef building dominance of this group in the following Cretaceous period. The squid-like belemnites, originating in the fossil record during the Triassic, diversify immensely starting in the Early Jurassic of Eurasia and expand worldwide throughout the rest of the period. Belemnites were very important components of marine ecosystems, being both important prey and predators in their environments. The earliest fossils of vampyromorph cephalopods first appear in the Early Jurassic. Octopi were already present during the Jurassic period, maybe evolving first from animals that lived away from the water surface, near the benthos.

main source: Wikipedia

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