Biology:Early Jurassic
| Early/Lower Jurassic | |
|---|---|
| 201.3 ± 0.2 – 174.7 ± 0.8 Ma | |
.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 the Ammonite Psiloceras spelae tirolicum. |
| Lower boundary GSSP | Kuhjoch section, Karwendel mountains, Northern Calcareous Alps, Austria [ ⚑ ] 47°29′02″N 11°31′50″E / 47.4839°N 11.5306°E |
| GSSP ratified | 2010[1] |
| Upper boundary definition | FAD of the Ammonites Leioceras opalinum and Leioceras lineatum |
| Upper boundary GSSP | Fuentelsaz, Spain [ ⚑ ] 41°10′15″N 1°50′00″W / 41.1708°N 1.8333°W |
| GSSP ratified | 2000[2] |
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" and that name was used for the period, as well, in 19th-century geology.[3] 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 English quarryman's dialect pronunciation of the word "layers";[4] secondly, sloops from north 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 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".[4]
Geology
Stratigraphy
There has been some debate[5] over the actual base of the Hettangian Stage, and so of the Jurassic System itself. Biostratigraphically, the first appearance of 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 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 clays, thin limestones and siltstones, deposited under fully marine conditions.
Lias Group strata form imposing cliffs on the Vale of Glamorgan coast, in southern Wales. Stretching for around 14 miles (23 km) 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.[6]
Life
Ammonites
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.[7]
Marine reptiles
Terrestrial animals
See also
References
- ↑ Hillebrandt, A.v.; Krystyn, L.; Kürschner, W.M.; Bonis, N.R.; Ruhl, M.; Richoz, S.; Schobben, M. A. N.; Urlichs, M. et al. (September 2013). "The Global Stratotype Sections and Point (GSSP) for the base of the Jurassic System at Kuhjoch (Karwendel Mountains, Northern Calcareous Alps, Tyrol, Austria)". Episodes 36 (3): 162–198. doi:10.18814/epiiugs/2013/v36i3/001.
- ↑ Cresta, S.; Goy, A.; Arias, C.; Barrón, E.; Bernad, J.; Canales, M.; García-Joral, F.; García-Romero, E et al. (September 2001). "The Global Boundary Stratotype Section and Point (GSSP) of the Toarcian-Aalenian Boundary (Lower-Middle Jurassic)". Episodes 24 (3): 166–175. doi:10.18814/epiiugs/2001/v24i3/003. https://timescalefoundation.org/references/Aalenian.pdf. Retrieved 13 December 2020.
- ↑ 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
- ↑ 4.0 4.1 Cite error: Invalid
<ref>tag; no text was provided for refs namedEB1911 - ↑ International Subcommission on Jurassic Stratigraphy. Newsletter 35/1, December 2008, Edited by Nicol Morton and Stephen Hesselbo
- ↑ Howe, S., Owen, G. & Sharpe, T. 2005 Walking the Rocks Geologists' Association - South Wales Group
- ↑ See e.g. Davies, 1920, pp. 173–75
Sources
- Davies, A. M., An Introduction to Palaeontology, Thomas Murby & Co., London
- House, M.R. (1993) Geology of The Dorset Coast, The Geologists' Association.
- 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
- Early Jurassic Period – The Lias epoch. Palaeos – overall presentation.
- Lecture 12 – Early Jurassic. Informative lecture notes by Paul Olsen.
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