HOME

TheInfoList



OR:

The Early Jurassic
Epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...
(in
chronostratigraphy Chronostratigraphy is the branch of stratigraphy that studies the ages of rock strata in relation to time. The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological ...
corresponding to the Lower Jurassic
Series Series may refer to: People with the name * Caroline Series (born 1951), English mathematician, daughter of George Series * George Series (1920–1995), English physicist Arts, entertainment, and media Music * Series, the ordered sets used i ...
) is the earliest of three epochs of the
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
Period. The Early Jurassic starts immediately after the
Triassic–Jurassic extinction event The Triassic–Jurassic (Tr-J) extinction event (TJME), often called the end-Triassic extinction, marks the boundary between the Triassic and Jurassic periods, . It represents one of five major extinction events during the Phanerozoic, profoundly ...
, 201.3 Ma (million years ago), and ends at the start of the
Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...
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. 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 A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...
from an English
quarryman A quarry is a type of open-pit mining, open-pit mine in which dimension stone, rock (geology), rock, construction aggregate, riprap, sand, gravel, or slate is excavated from the ground. The operation of quarries is regulated in some juri ...
's
dialect A dialect is a Variety (linguistics), variety of language spoken by a particular group of people. This may include dominant and standard language, standardized varieties as well as Vernacular language, vernacular, unwritten, or non-standardize ...
pronunciation of the word "layers"; secondly, sloops from north Cornish ports such as
Bude Bude (, locally or ; Cornish language, Cornish ) is a seaside town in north Cornwall, England, in the civil parish of Bude-Stratton and at the mouth of the River Neet (also known locally as the River Strat). It was sometimes formerly known as ...
would sail across the
Bristol Channel The Bristol Channel (, literal translation: "Severn Sea") is a major inlet in the island of Great Britain, separating South Wales (from Pembrokeshire to the Vale of Glamorgan) and South West England (from Devon to North Somerset). It extends ...
to the
Vale of Glamorgan The Vale of Glamorgan ( ), locally referred to as ''The Vale'', is a Principal areas of Wales, county borough in the South East Wales, south-east of Wales. It borders Bridgend County Borough to the west, Cardiff to the east, Rhondda Cynon Taf t ...
to load up with rock from coastal
limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
quarries (lias and
Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...
limestone from South Wales was used throughout North Devon/North Cornwall as it contains
calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...
to 'sweeten' (i.e.neutralise) the acidic Devonian and Carboniferous soils of the
West Country The West Country is a loosely defined area within southwest England, usually taken to include the counties of Cornwall, Devon, Dorset, Somerset and Bristol, with some considering it to extend to all or parts of Wiltshire, Gloucestershire and ...
); the Cornish would pronounce the layers of limestone as '' or ''; ''leac '' is
Gaelic Gaelic (pronounced for Irish Gaelic and for Scots Gaelic) is an adjective that means "pertaining to the Gaels". It may refer to: Languages * Gaelic languages or Goidelic languages, a linguistic group that is one of the two branches of the Insul ...
for "flat stone".


Geology


Stratigraphy

There has been some debate over the actual base of the
Hettangian The Hettangian is the earliest age and lowest stage of the Jurassic Period of the geologic timescale. It spans the time between 201.3 ± 0.2 Ma and 199.3 ± 0.3 Ma (million years ago). The Hettangian follows the Rhaetian (part of the Triass ...
Stage, and so of the Jurassic System itself. Biostratigraphically, the first appearance of psiloceratid
ammonites Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...
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 The Lias Group or Lias is a lithostratigraphic unit (a sequence of rock strata) found in a large area of western Europe, including the British Isles, the North Sea, the Low Countries and the north of Germany. It consists of marine limestones, ...
—a lithostratigraphical division—spans the Jurassic /
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
boundary.


United Kingdom

There are extensive Liassic outcrops around the coast of the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...
, in particular in
Glamorgan Glamorgan (), or sometimes Glamorganshire ( or ), was Historic counties of Wales, one of the thirteen counties of Wales that existed from 1536 until their abolishment in 1974. It is located in the South Wales, south of Wales. Originally an ea ...
,
North Yorkshire North Yorkshire is a Ceremonial counties of England, ceremonial county in Northern England.The Unitary authorities of England, unitary authority areas of City of York, York and North Yorkshire (district), North Yorkshire are in Yorkshire and t ...
and
Dorset Dorset ( ; Archaism, archaically: Dorsetshire , ) is a Ceremonial counties of England, ceremonial county in South West England. It is bordered by Somerset to the north-west, Wiltshire to the north and the north-east, Hampshire to the east, t ...
. The 'Jurassic Coast' of Dorset is often associated with the pioneering work of
Mary Anning Mary Anning (21 May 1799 – 9 March 1847) was an English fossil collector, fossil trade, dealer, and palaeontologist. She became known internationally for her discoveries in Jurassic marine fossil beds in the cliffs along the English Cha ...
of
Lyme Regis Lyme Regis ( ) is a town in west Dorset, England, west of Dorchester, Dorset, Dorchester and east of Exeter. Sometimes dubbed the "Pearl of Dorset", it lies by the English Channel at the Dorset–Devon border. It has noted fossils in cliffs and ...
. The
facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...
of the Lower Jurassic in this area are predominantly of
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...
s, thin
limestones Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Limestone forms when these ...
and
siltstones Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed ...
, deposited under fully marine conditions. Lias Group
strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...
form imposing cliffs on the
Vale of Glamorgan The Vale of Glamorgan ( ), locally referred to as ''The Vale'', is a Principal areas of Wales, county borough in the South East Wales, south-east of Wales. It borders Bridgend County Borough to the west, Cardiff to the east, Rhondda Cynon Taf t ...
coast, in southern
Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...
. Stretching for around between
Cardiff Cardiff (; ) is the capital city, capital and List of urban areas in the United Kingdom, largest city of Wales. Cardiff had a population of in and forms a Principal areas of Wales, principal area officially known as the City and County of Ca ...
and
Porthcawl Porthcawl () is a town and community in the Bridgend County Borough of Wales. It is located on the south coast, west of Cardiff and south-east of Swansea. Historically part of Glamorgan and situated on a low limestone headland on the South ...
, the remarkable layers of these cliffs, situated on the
Bristol Channel The Bristol Channel (, literal translation: "Severn Sea") is a major inlet in the island of Great Britain, separating South Wales (from Pembrokeshire to the Vale of Glamorgan) and South West England (from Devon to North Somerset). It extends ...
are a rhythmic decimetre scale repetition of limestone and
mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...
formed as a late Triassic desert was inundated by the sea.


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.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 The Rhaetian is the latest age (geology), age of the Triassic period (geology), Period (in geochronology) or the uppermost stage (stratigraphy), stage of the Triassic system (stratigraphy), System (in chronostratigraphy). It was preceded by the N ...
ichthyosaur Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides. Ichthyosaurians thrived during much of the Mesozoic era; based on fo ...
s and
plesiosaur The Plesiosauria or plesiosaurs are an Order (biology), order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period (geology), Period, possibly in the Rhaetian st ...
s continuing to flourish, while at the same time a number of new types of these
marine reptile Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semiaquatic life in a marine environment. Only about 100 of the 12,000 extant reptile species and subspecies are classed as marine reptiles, including mari ...
s appeared, such as '' Ichthyosaurus'' and '' Temnodontosaurus'' among the ichthyosaurs, and '' Eurycleidus'', '' Macroplata'', and ''
Rhomaleosaurus ''Rhomaleosaurus'' (meaning "strong lizard") is an extinct genus of Early Jurassic (Toarcian Faunal stage, age, about 183 to 175.6 million years ago) rhomaleosaurid pliosauroid known from Northamptonshire and from Yorkshire of the United Kingdom. ...
'' among the plesiosaurs (all Rhomaleosauridae, although as currently defined this group is probably
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
). All these plesiosaurs had medium-sized necks and large heads. In the
Toarcian The Toarcian is, in the International Commission on Stratigraphy, ICS' geologic timescale, an age (geology), age and stage (stratigraphy), stage in the Early Jurassic, Early or Lower Jurassic. It spans the time between 184.2 Megaannum, Ma (million ...
, at the end of the Early Jurassic, the thalattosuchians (marine " crocodiles") appeared, as did new
genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...
of ichthyosaurs ('' Stenopterygius'', ''
Eurhinosaurus ''Eurhinosaurus'' (Greek for 'well-nosed lizard'- eu meaning 'well or good', rhino meaning 'nose' and sauros meaning 'lizard') is an extinct genus of ichthyosaur from the Early Jurassic (Toarcian), ranging between 183 and 175 million years. Fossi ...
'', and the persistently primitive ''
Suevoleviathan ''Suevoleviathan'' is an extinct genus of primitive ichthyosaur found in the Early Jurassic (Toarcian) of Holzmaden, Germany. Taxonomy The genus was named in 1998 by Michael Maisch for ''Leptopterygius disinteger'' and ''Ichthyosaurus integer' ...
'') and plesiosaurs (the elasmosaurs (long-necked) '' Microcleidus'' and '' Occitanosaurus'', and the pliosaur ''
Hauffiosaurus ''Hauffiosaurus'' is an extinct genus of Early Jurassic (early Toarcian stage) pliosaurid plesiosaur known from Holzmaden of Germany and from Yorkshire of the United Kingdom. It was first named by Frank Robin O’Keefe in 2001 and the type sp ...
'').


Terrestrial animals

On land, a number of new types of dinosaurs—the
heterodontosaurids Heterodontosauridae is a family (biology), family of ornithischian dinosaurs that were likely among the most Basal (phylogenetics), basal (primitive) members of the group. Their phylogenetic placement is uncertain but they are most commonly fou ...
,
dilophosaurus ''Dilophosaurus'' ( ) is a genus of theropod dinosaurs that lived in what is now North America during the Early Jurassic, about 186 million years ago. Three skeletons were discovered in northern Arizona in 1940, and the two best preserv ...
, scelidosaurs, stegosaurs, and tetanurans—appeared, and joined those groups like the coelophysoids, prosauropods and the
sauropod Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their b ...
s that had continued over from the Triassic. Accompanying them as small carnivores were the sphenosuchian and protosuchid crocodilians. In the air, new types of
pterosaur Pterosaurs are an extinct clade of flying reptiles in the order Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 million to 66 million years ago). Pterosaurs are the earli ...
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 tritylodont
synapsid Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...
s, lizard-like sphenodonts, and early
lissamphibia The Lissamphibia (from Greek λισσός (lissós, "smooth") + ἀμφίβια (amphíbia), meaning "smooth amphibians") is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia ( ...
ns.


See also

*
Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...
*
Toarcian turnover The Toarcian extinction event, also called the Pliensbachian-Toarcian extinction event, the Early Toarcian mass extinction, the Early Toarcian palaeoenvironmental crisis, or the Jenkyns Event, was an extinction event that occurred during the early ...


References


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.

Informative lecture notes by Paul Olsen. {{Authority control Geological epochs Jurassic geochronology