Everything about Early Jurassic totally explained
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For general context see Jurassic.
The
Early Jurassic (in geology referred to as the
Lower Jurassic, originally (and still in Europe) the "
Lias") is the earliest of three
epochs of the
Jurassic period. Early Jurassic starts immediately after the
Triassic-Jurassic extinction event (200 mya) and ends at the start of
Middle Jurassic (176 mya).
Outcrops
There are extensive Liassic outcrops around the coast of the
UK, in particular in the
Dorset Jurassic coast, 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.
Perhaps the best example of a liassic coastline in the world are the superb cliffs of the
Vale of Glamorgan in southern
Wales. Stretching for around 14 miles just outside
Cardiff, the remarkable
strata of the cliffs, situated on the
Bristol Channel are a mixture of
carboniferous sandstone/
limestone,
shale and liassic
limestone, created during a geological upheaval known as the
variscan orogeny.
Stratigraphy
There has been some debate 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.
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 for example Davies, 1920, pp.173-5)
Marine Reptiles
The Early Jurassic was an important time in the evolution of the marine reptiles. The Hettangian saw the already existing
Rhaetian ichthyosaurs and
plesiosaurs continuing to flourish, while at the same time a number of new types of these
marine reptiles 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
thalattosuchians (marine "
crocodiles") appeared, as did new
genera of ichthyosaurs (
Stenopterygius,
Eurhinosaurus, and the persistently primitive
Suevoleviathan) and plesiosaurs (the
elasmosaurs (long-necked)
Microcleidus and
Occitanosaurus, and the
pliosaur Hauffiosaurus).
Terrestrial animals
On land, a number of new types of dinosaurs - the
heterodontosaurs,
scelidosaurs,
stegosaurs, and
tetanurae, appeared, and joined those groups like the
podokesaurs,
prosauropods and the
sauropods that had continued over from the Triassic. Accompanying them as small carnivores were the
sphenosuchid and
protosuchid crocodilians. In the air, new types of
pterosaurs replaced those that had died out at the end of the Triassic. While in the undergrowth were various types of early mammals, as well as
tritylodont mammal-like reptiles, lizard-like
sphenodonts, and early
Lissamphibians.
Origin of the name Lias
There are two possible reasons 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"; secondly,
sloops from
North Cornwall ports such as
Bude would sail to the
Vale of Glamorgan heritage coast to load up on lias limestone (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'.
Further Information
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