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 (199.6 Ma (million years ago) and ends at the start
of the
Middle Jurassic
(175.6 Ma).
Rocks of this age are in Europe called
Lias, a
name that is sometimes still erroneously applied to the time in
which they were deposited as well.
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"; 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'.
Geology
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.
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
heterodontosaurids,
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.
See also
References
- See e.g. Davies, 1920, pp.173-5
Literature
- 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.
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