The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic 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 174.7 ±0.8 Ma.
Certain rocks of marine origin of this age in Europe are called "Lias Group" and that name was used for the period, as well, in 19th-century geology.[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] 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 quarryman's
dialect pronunciation of the word "layers";
secondly,
sloops from north
Cornwall 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 and
Carboniferous limestone from South Wales was used throughout North Devon/North Cornwall as it contains calcium carbonate to 'sweeten' (i.e.neutralise) the acidic Devonian and Carboniferous soils of the
West Country); the Cornish would pronounce the layers of limestone as 'laiyers' or 'lias';
leac is Gaelic for "flat stone".
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]
Geology
Stratigraphy
There has been some debate[ International Subcommission on Jurassic Stratigraphy. Newsletter 35/1, December 2008, Edited by Nicol Morton and Stephen Hesselbo ] over the actual base of the Hettangian Stage, and so of the Jurassic System itself. Biostratigraphy, the first appearance of Psiloceratidae 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 lithostratigraphical division—spans the Jurassic / Triassic boundary.
United Kingdom
There are extensive Liassic outcrops around the coast of the United Kingdom, in particular in Glamorgan, North Yorkshire and Dorset. The 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 , thin limestones and siltstones, deposited under fully marine conditions.
Lias Group stratum form imposing cliffs on the Vale of Glamorgan coast, in southern Wales. Stretching for around 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.[Howe, S., Owen, G. & Sharpe, T. 2005 Walking the Rocks Geologists' Association - South Wales Group]
Life
Ammonites
During this period, , 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.[See e.g. Davies, 1920, pp. 173–75]
Marine reptiles
The Early Jurassic was an important time in the evolution of the marine reptiles. The Hettangian saw the already existing Rhaetian Ichthyosauria and continuing to flourish, while at the same time a number of new types of these 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 (marine "Crocodylomorpha") appeared, as did new genus of ichthyosaurs ( Stenopterygius, Eurhinosaurus, and the persistently primitive Suevoleviathan) and plesiosaurs (the Elasmosauridae (long-necked) Microcleidus and Occitanosaurus, and the Pliosauridae Hauffiosaurus).
Terrestrial animals
On land, a number of new types of dinosaurs—the heterodontosaurids, dilophosaurus, Scelidosaurus, , and tetanurae—appeared, and joined those groups like the coelophysoidea, prosauropoda and the that had continued over from the Triassic. Accompanying them as small carnivores were the and Protosuchus crocodilians. In the air, new types of 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 , lizard-like Sphenodontia, and early .
See also
Sources
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Davies, A. M., An Introduction to Palaeontology, Thomas Murby & Co., London
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House, M.R. (1993) Geology of The Dorset Coast, The Geologists' Association.
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Simms, M.J., Chidlaw, N., Morton, N. and Page, K.N. (2004) British Lower Jurassic Stratigraphy, Geological Conservation Review Series, No. 30, Joint Nature Conservation Committee, Peterborough.
External links
