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| Early/Lower Jurassic | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| 201.3 ± 0.2 – 174.7 ± 0.8Ma | |||||||||
.png) A map of Earth as it appeared 190 million years ago during the Early Jurassic Epoch, Pliensbachian Age | |||||||||
| Chronology | |||||||||
| |||||||||
| Etymology | |||||||||
| Chronostratigraphic name | Lower Jurassic | ||||||||
| Geochronological name | Early Jurassic | ||||||||
| Name formality | Formal | ||||||||
| Usage information | |||||||||
| Celestial body | Earth | ||||||||
| Regional usage | Global (ICS) | ||||||||
| Time scale(s) used | ICS Time Scale | ||||||||
| Definition | |||||||||
| Chronological unit | Epoch | ||||||||
| Stratigraphic unit | Series | ||||||||
| Time span formality | Formal | ||||||||
| Lower boundary definition | FAD of theAmmonitePsiloceras spelae tirolicum. | ||||||||
| Lower boundary GSSP | Kuhjoch section,Karwendel mountains,Northern Calcareous Alps,Austria 47°29′02″N11°31′50″E / 47.4839°N 11.5306°E /47.4839; 11.5306 | ||||||||
| Lower GSSP ratified | 2010[2] | ||||||||
| Upper boundary definition | FAD of the AmmonitesLeioceras opalinum andLeioceras lineatum | ||||||||
| Upper boundary GSSP | Fuentelsaz,Spain 41°10′15″N1°50′00″W / 41.1708°N 1.8333°W /41.1708; -1.8333 | ||||||||
| Upper GSSP ratified | 2000[3] | ||||||||
TheEarly JurassicEpoch (inchronostratigraphy corresponding to theLower JurassicSeries) is the earliest of three epochs of theJurassic Period. The Early Jurassic starts immediately after theTriassic–Jurassic extinction event, 201.3 Ma (million years ago), and ends at the start of theMiddle Jurassic 174.7 ±0.8 Ma.
Certain rocks of marine origin of this age in Europe are called "Lias" and that name was used for the period, as well, in 19th-century geology.[4] In southern Germany rocks of this age are calledBlack Jurassic.
There are two possible origins for the name Lias: the first reason is it was taken by ageologist from anEnglishquarryman'sdialect pronunciation of the word "layers";[5] secondly,sloops from northCornish ports such asBude would sail across theBristol Channel to theVale of Glamorgan to load up with rock from coastallimestone quarries (lias andCarboniferous limestone from South Wales was used throughout North Devon/North Cornwall as it containscalcium carbonate to 'sweeten' (i.e.neutralise) the acidic Devonian and Carboniferous soils of theWest Country); the Cornish would pronounce the layers of limestone as 'laiyers' or 'lias';leac isGaelic for "flat stone".[5]
There has been some debate[6] over the actual base of theHettangian Stage, and so of the Jurassic System itself.Biostratigraphically, the first appearance ofpsiloceratidammonites has been used; but this depends on relatively complete ammonite faunas being present, a problem that makes correlation between sections in different parts of the world difficult. If this biostratigraphical indicator is used, then technically theLias Group—alithostratigraphical division—spans the Jurassic /Triassic boundary.
There are extensive Liassic outcrops around the coast of theUnited Kingdom, in particular inGlamorgan,North Yorkshire andDorset. The'Jurassic Coast' of Dorset is often associated with the pioneering work ofMary Anning ofLyme Regis. Thefacies of the Lower Jurassic in this area are predominantly ofclays, thinlimestones andsiltstones, deposited under fully marine conditions.
Lias Groupstrata form imposing cliffs on theVale of Glamorgan coast, in southernWales. Stretching for around 14 miles (23 km) betweenCardiff andPorthcawl, the remarkable layers of these cliffs, situated on theBristol Channel are a rhythmic decimetre scale repetition of limestone andmudstone formed as a late Triassic desert was inundated by the sea.[7]
During this period,ammonoids, which had almost died out at the end-of-Triassic extinction, radiated out into a huge diversity of new forms with complex suture patterns (the ammonites proper). Ammonites evolved so rapidly, and their shells are so often preserved, that they serve as important zone fossils. There were several distinct waves of ammonite evolution in Europe alone.[8]
The Early Jurassic was an important time in the evolution of the marine reptiles. The Hettangian saw the already existingRhaetianichthyosaurs andplesiosaurs continuing to flourish, while at the same time a number of new types of thesemarine reptiles appeared, such asIchthyosaurus andTemnodontosaurus among the ichthyosaurs, andEurycleidus,Macroplata, andRhomaleosaurus among the plesiosaurs (allRhomaleosauridae, although as currently defined this group is probablyparaphyletic). All these plesiosaurs had medium-sized necks and large heads. In theToarcian, at the end of the Early Jurassic, thethalattosuchians (marine "crocodiles") appeared, as did newgenera of ichthyosaurs (Stenopterygius,Eurhinosaurus, and the persistently primitiveSuevoleviathan) and plesiosaurs (theelasmosaurs (long-necked)Microcleidus andOccitanosaurus, and thepliosaurHauffiosaurus).[citation needed]
On land, a number of new types of dinosaurs—theheterodontosaurids,dilophosaurus,scelidosaurs,stegosaurs, andtetanurans—appeared, and joined those groups like thecoelophysoids,prosauropods and thesauropods that had continued over from the Triassic. Accompanying them as small carnivores were thesphenosuchian andprotosuchid crocodilians. In the air, new types ofpterosaurs replaced those that had died out at the end of the Triassic. But in the undergrowth were various types of early mammals, as well astritylodontsynapsids, lizard-likesphenodonts, and earlylissamphibians.[citation needed]
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