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 ...

(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 geologic ...

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 in ...

) is the earliest of three epochs of the

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

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 The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively ...

174.1 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 The Black Jurassic or Black Jura (german: Schwarzer Jura) in earth history refers to the lowest of the three lithostratigraphic units of the South German Jurassic, the latter being understood not as a geographical, but a geological term in the ...

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 solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...

from an

English English usually refers to: * English language * English people English may also refer to: Peoples, culture, and language * ''English'', an adjective for something of, from, or related to England ** English national ...

quarryman A quarry is a type of open-pit mine in which dimension stone, rock, construction aggregate, riprap, sand, gravel, or slate is excavated from the ground. The operation of quarries is regulated in some jurisdictions to reduce their enviro ...

dialect The term dialect (from Latin , , from the Ancient Greek word , 'discourse', from , 'through' and , 'I speak') can refer to either of two distinctly different types of linguistic phenomena: One usage refers to a variety of a language that is ...

pronunciation of the word "layers"; secondly,

sloops A sloop is a sailboat with a single mast typically having only one headsail in front of the mast and one mainsail aft of (behind) the mast. Such an arrangement is called a fore-and-aft rig, and can be rigged as a Bermuda rig with triangular ...

from north Cornish ports such as

Bude Bude (; kw, Porthbud) is a seaside town in north east 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 Bude Haven.''Corn ...

would sail across the

Bristol Channel The Bristol Channel ( cy, Môr Hafren, literal translation: "Severn Sea") is a major inlet in the island of Great Britain, separating South Wales from Devon and Somerset in South West England. It extends from the lower estuary of the River Se ...

to the

Vale of Glamorgan The Vale of Glamorgan ( cy, Bro Morgannwg ), often referred to as The Vale, is a county borough in the south-east of Wales. It borders Bridgend County Borough to the west, Cardiff to the east, Rhondda Cynon Taf to the north, and the Bristol ...

to load up with rock from coastal

limestone Limestone ( calcium carbonate ) 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 . Limestone forms w ...

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 '' or ''; ''leac '' is

Gaelic Gaelic is an adjective that means "pertaining to the Gaels". As a noun it refers to the group of languages spoken by the Gaels, or to any one of the languages individually. Gaelic languages are spoken in Ireland, Scotland, the Isle of Man, an ...

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 Triassi ...

Stage, and so of the Jurassic System itself. Biostratigraphically, the first appearance of psiloceratid

ammonites Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefis ...

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, sh ...

—a lithostratigraphical division—spans the Jurassic /

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...

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 Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and ...

, in particular in

Glamorgan , HQ = Cardiff , Government = Glamorgan County Council (1889–1974) , Origin= , Code = GLA , CodeName = Chapman code , Replace = * West Glamorgan * Mid Glamorgan * South Glamorgan , Mot ...

North Yorkshire North Yorkshire is the largest ceremonial county (lieutenancy area) in England, covering an area of . Around 40% of the county is covered by national parks, including most of the Yorkshire Dales and the North York Moors. It is one of four co ...

and

Dorset Dorset ( ; archaically: Dorsetshire , ) is a county in South West England on the English Channel coast. The ceremonial county comprises the unitary authority areas of Bournemouth, Christchurch and Poole and Dorset. Covering an area of , ...

. 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, dealer, and palaeontologist who became known around the world for the discoveries she made in Jurassic marine fossil beds in the cliffs along the English Channel ...

Lyme Regis Lyme Regis is a town in west Dorset, England, west of 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 beaches on the He ...

. The

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

of the Lower Jurassic in this area are predominantly of

clays Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

, thin

limestones Limestone ( calcium carbonate ) 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 . Limestone forms when t ...

and

siltstones Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

, deposited under fully marine conditions. Lias Group

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...

form imposing cliffs on the

coast, in southern

Wales Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...

. Stretching for around between

Cardiff Cardiff (; cy, Caerdydd ) is the capital city, capital and List of urban areas in the United Kingdom, largest city of Wales. It forms a Principal areas of Wales, principal area, officially known as the City and County of Cardiff ( cy, Dinas a ...

and

Porthcawl Porthcawl (, ) is a town and community on the south coast of Wales in the county borough of Bridgend, west of the capital city, Cardiff and southeast of Swansea. Historically part of Glamorgan and situated on a low limestone headland on the S ...

, the remarkable layers of these cliffs, situated on the

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 (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology.' ...

formed as a late Triassic desert was inundated by the sea.

Life

Ammonites

During this period,

ammonoid Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

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 Index fossils (also known as guide fossils or indicator fossils) are fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, sh ...

s. 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 of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest a ...

ichthyosaur Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, alt ...

s and

plesiosaur The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appea ...

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. The earliest marine reptile mesosaurus (not to be confused with mosasaurus), arose in the Permian period during th ...

s appeared, such as ''

Ichthyosaurus ''Ichthyosaurus'' (derived from Greek ' () meaning 'fish' and ' () meaning 'lizard') is a genus of ichthyosaurs from the Early Jurassic (Hettangian - Pliensbachian), with possible Late Triassic record, from Europe ( Belgium, England, Germany, ...

'' and ''

Temnodontosaurus ''Temnodontosaurus'' (Greek for "cutting-tooth lizard"temno, meaning "to cut", odont meaning "tooth" and sauros meaning "lizard") is an extinct genus of ichthyosaur from the Early Jurassic period. They lived between 200 and 175 million years a ...

'' among the ichthyosaurs, and ''

Eurycleidus ''Eurycleidus'' is an extinct genus of large-bodied rhomaleosaurid known from the Early Jurassic period (most likely earliest Hettangian stage) of the United Kingdom. It contains a single species, ''E. arcuatus''. Like other plesiosaurs, ''Euryc ...

'', ''

Macroplata ''Macroplata'' (meaning "big plate") is an extinct genus of Early Jurassic rhomaleosaurid plesiosaur which grew up to in length and weighed up to . Like other plesiosaurs, ''Macroplata'' probably lived on a diet of fish, using its sharp needle-l ...

'', and ''

Rhomaleosaurus ''Rhomaleosaurus'' (meaning "strong lizard") is an extinct genus of Early Jurassic (Toarcian age, about 183 to 175.6 million years ago) rhomaleosaurid pliosauroid known from Northamptonshire and from Yorkshire of the United Kingdom. It was ...

'' among the plesiosaurs (all

Rhomaleosauridae Rhomaleosauridae is a family of plesiosaurs from the Earliest Jurassic to the latest Middle Jurassic (Hettangian to Callovian stages) of Europe, North America, South America and possibly Asia. Most rhomaleosaurids are known from England, many ...

, although as currently defined this group is probably

paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

). All these plesiosaurs had medium-sized necks and large heads. In the

Toarcian The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toar ...

, at the end of the Early Jurassic, the

thalattosuchia Thalattosuchia is a clade of marine crocodylomorphs from the Early Jurassic to the Early Cretaceous that had a cosmopolitan distribution. They are colloquially referred to as marine crocodiles or sea crocodiles, though they are not members of Cro ...

ns (marine "

crocodiles Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant memb ...

") appeared, as did new

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

of ichthyosaurs (''

Stenopterygius ''Stenopterygius'' is an extinct genus of thunnosaur ichthyosaur known from Europe ( England, France, Germany, Luxembourg and Switzerland). This genus of ichthyosaur was about long and weighed .Huene F. von 1939. Ein ganzes Ichthyosaurier-Sk ...

'', ''

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. Foss ...

'', 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 integ ...

'') and plesiosaurs (the elasmosaurs (long-necked) ''

Microcleidus ''Microcleidus'' is an extinct genus of sauropterygian reptile belonging to the Plesiosauroidea. The species has 40 neck vertebrae and a short tail of 28 vertebrae. Fossils of the genus have been found in France, the Posidonia Shale in Germany a ...

'' and ''

Occitanosaurus ''Microcleidus'' is an extinct genus of sauropterygian reptile belonging to the Plesiosauroidea. The species has 40 neck vertebrae and a short tail of 28 vertebrae. Fossils of the genus have been found in France, the Posidonia Shale in Germany an ...

'', and the

pliosaur Pliosauroidea is an extinct clade of plesiosaurs, known from the earliest Jurassic to early Late Cretaceous. They are best known for the subclade Thalassophonea, which contained crocodile-like short-necked forms with large heads and massive toot ...

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 specie ...

'').

Terrestrial animals

On land, a number of new types of dinosaurs—the

heterodontosaurids Heterodontosauridae is a family of ornithischian dinosaurs that were likely among the most basal (primitive) members of the group. Their phylogenetic placement is uncertain but they are most commonly found to be primitive, outside of the gr ...

, scelidosaurs,

stegosaur Stegosauria is a group of herbivorous ornithischian dinosaurs that lived during the Jurassic and early Cretaceous periods. Stegosaurian fossils have been found mostly in the Northern Hemisphere, predominantly in what is now North America, Euro ...

s, and

tetanurans Tetanurae (/ˌtɛtəˈnjuːriː/ or "stiff tails") is a clade that includes most theropod dinosaurs, including megalosauroids, allosauroids, tyrannosauroids, ornithomimosaurs, compsognathids and maniraptorans (including birds). Tetanurans ar ...

—appeared, and joined those groups like the

coelophysoids Coelophysoidea were common dinosaurs of the Late Triassic and Early Jurassic periods. They were widespread geographically, probably living on all continents. Coelophysoids were all slender, carnivorous forms with a superficial similarity to the ...

, 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 ...

s that had continued over from the Triassic. Accompanying them as small carnivores were the

sphenosuchia Sphenosuchia is a suborder of basal crocodylomorphs that first appeared in the Triassic and occurred into the Middle Jurassic. Most were small, gracile animals with an erect limb posture. They are now thought to be ancestral to crocodyliforms, ...

n and

protosuchid Protosuchidae was a family of crocodylomorph reptiles from the Late Triassic, Jurassic, and Cretaceous time periods. They were closely related to the Gobiosuchidae. Distribution Triassic protosuchids are known from Lesotho, Argentina, and Arizon ...

crocodilians. In the air, new types of

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is 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 ...

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 Tritylodontidae ("three-knob teeth", named after the shape of their cheek teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, bearing several mammalian traits like erect limbs, endothermy and details o ...

synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...

s, lizard-like sphenodonts, and early

lissamphibia The Lissamphibia is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia (frogs, toads, and their extinct relatives), the Caudata (salamanders, newts, and their extinct relativ ...

ns.

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 Dr. Paul Olsen. {{Authority control Geological epochs Jurassic geochronology