The '''Early Jurassic''' [[Epoch (geology)|epoch]] (in [[chronostratigraphy]] corresponding to the '''Lower Jurassic''' [[series (stratigraphy)|series]]) is the earliest of three epochs of the [[Jurassic]] period. The Early Jurassic starts immediately after the [[Triassic-Jurassic extinction event]], 201.3 Ma(million years ago), and ends at the start of the [[Middle Jurassic]] 174.Ma.

Certain rocks of marine origin of this age in Europe are called "[[Lias Group|Lias]]" and that name was used for the period, as well, in 19th century geology.<ref>Rudwick, M.J.S (1992): Scenes from Deep Time: Early Pictorial Representations of the Prehistoric World, [[University of Chicago Press]], 280 pages. [ Except] from Google Books></ref> In southern Germany rocks of this age are called [[Black Jurassic]].

==Origin of the name Lias==

There are two possible origins for the name Lias: the first reason is it was taken by a [[geologist]] from an [[England|English]] [[quarryman]]'s [[dialect]] pronunciation of the word "layers"; secondly, [[sloops]] from north [[Cornwall|Cornish]] ports such as [[Bude]] would sail across the [[Bristol Channel]] to the [[Vale of Glamorgan]] to load up with rock from coastal [[limestone]] quarries (lias limestone from South Wales was used throughout North Devon/North Cornwall as it contains calcium carbonate to fertilise the poor quality Devonian soils of the West Country); the Cornish would pronounce the layers of limestone as 'laiyers' or 'lias'.


There are extensive Liassic outcrops around the coast of the [[United Kingdom]], in particular in [[Glamorgan]], [[North Yorkshire]] and [[Dorset]]. The [[Jurassic coast|'Jurassic Coast']] of Dorset is often associated with the pioneering work of [[Mary Anning]] of [[Lyme Regis]]. The [[facies]] of the Lower Jurassic in this area are predominantly of [[clays]], thin [[limestones]] and [[siltstones]], deposited under fully marine conditions.

[[File:Blue lias cliffs at Lyme Regis.jpg|thumb|left|Lias formations at [[Lyme Regis]], UK, known locally as [[Blue Lias]].]]

Lias Group [[stratum|strata]] form imposing cliffs on the [[Vale of Glamorgan]] coast, in southern [[Wales]]. Stretching for around {{convert|14|mi}} between [[Cardiff]] and [[Porthcawl]], the remarkable layers of these cliffs, situated on the [[Bristol Channel]] are a rhythmic decimetre scale repetition of limestone and [[mudstone]] formed as a late Triassic desert was inundated by the sea.<ref>Howe, S., Owen, G. & Sharpe, T. 2005 ''Walking the Rocks'' [[South Wales Geologists' Association|Geologists' Association - South Wales Group]]</ref>


There has been some debate<ref>[ International Subcommission on Jurassic Stratigraphy. Newsletter 35/1, December 2008, Edited by Nicol Morton and Stephen Hesselbo]</ref> over the actual base of the [[Hettangian]] stage, and so of the Jurassic system itself. [[Biostratigraphy|Biostratigraphically]], the first appearance of [[Psiloceratidae|psiloceratid]] [[ammonites]] 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 the [[Lias Group]]—a [[Lithostratigraphy|lithostratigraphical]] division—spans the Jurassic / [[Triassic]] boundary.



During this period, [[ammonoid]]s, 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 fossil]]s.  There were several distinct waves of ammonite evolution in Europe alone.<ref>See e.g. Davies, 1920, pp. 173–75</ref>

===Marine reptiles===

The Early Jurassic was an important time in the evolution of the marine reptiles.  The Hettangian saw the already existing [[Rhaetian]] [[ichthyosaur]]s and [[plesiosaur]]s continuing to flourish, while at the same time a number of new types of these [[marine reptile]]s appeared, such as ''[[Ichthyosaurus]]'' and ''[[Temnodontosaurus]]'' among the ichthyosaurs, and ''[[Eurycleidus]]'', ''[[Macroplata]]'', and ''[[Rhomaleosaurus]]'' among the plesiosaurs (all [[Rhomaleosauridae]], although as currently defined this group is probably [[paraphyletic]]).  All these plesiosaurs had medium-sized necks and large heads.  In the [[Toarcian]], at the end of the Early Jurassic, the [[thalattosuchia]]ns (marine "[[Crocodylomorpha|crocodiles]]") appeared, as did new [[genus|genera]] of ichthyosaurs (''[[Stenopterygius]]'', ''[[Eurhinosaurus]]'', and the persistently primitive ''[[Suevoleviathan]]'') and plesiosaurs (the [[Elasmosauridae|elasmosaurs]] (long-necked) ''[[Microcleidus]]'' and ''[[Occitanosaurus]]'', and the [[Pliosauridae|pliosaur]] ''[[Hauffiosaurus]]'').

===Terrestrial animals===

On land, a number of new types of dinosaurs—the [[Heterodontosauridae|heterodontosaurids]], [[Scelidosaurus|scelidosaurs]], [[stegosaur]]s, and [[tetanurae|tetanurans]]—appeared, and joined those groups like the [[coelophysoidea|coelophysoids]], [[prosauropoda|prosauropods]] and the [[sauropod]]s that had continued over from the Triassic. Accompanying them as small carnivores were the [[sphenosuchia]]n and [[Protosuchus|protosuchid]] crocodilians.  In the air, new types of [[pterosaur]]s replaced those that had died out at the end of the Triassic.  But in the undergrowth were various types of early mammals, as well as [[Tritylodontidae|tritylodont]] mammal-like reptiles, lizard-like [[Sphenodontia|sphenodonts]], and early [[Lissamphibia]]ns.

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