Triassic-Jurassic Extinction Event
The
Triassic–Jurassic extinction event marks the boundary between the Triassic
and Jurassic periods, 199.6 million years
ago, and is one of the major extinction events of
the Phanerozoic eon,
profoundly affecting life on land and in the oceans. A whole
class (conodonts),
twenty percent of all marine families and all large
crurotarsans (non-dinosaurian
archosaurs), some remaining
therapsids, and many of the large
amphibians were wiped out. At least half of
the species now known to have been living on Earth at that time went
extinct. This event vacated ecological niches,
allowing the dinosaurs to assume the
dominant roles in the
Jurassic period. This event happened in less
than 10,000 years and occurred just before
Pangaea
started to break apart. This marked the divide between the Triassic dinosaurs and
the Jurassic dinosaurs.
Statistical analysis of marine losses at this time
suggests that the decrease in diversity was caused more by a decrease in
speciation than by an
increase in extinctions.
Several explanations for this event have been
suggested, but all have unanswered challenges:
 |
Gradual climate change or sea-level fluctuations during the late Triassic. However, this does not explain the suddenness of the extinctions in the marine realm. |
|
Asteroid impact, but no impact crater has been dated to coincide with the Triassic–Jurassic boundary. |
|
Massive volcanic eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province, would release carbon dioxide or sulfur dioxide which would cause either intense global warming (from the former) or cooling (from the latter). |
The isotopic
composition of fossil soils of
Late Triassic and
Early Jurassic show no evidence of any
change in the CO2 composition of the atmosphere. More recently however, some
evidence has been retrieved from near the Triassic–Jurassic boundary
suggesting that there was a rise in atmospheric CO2 and some researchers
have suggested that the cause of this rise, and of the mass extinction
itself, could have been a combination of volcanic CO2 outgassing and
catastrophic dissociation of
gas
hydrate. Gas hydrates have also been
suggested as one possible cause of the largest
mass extinction of all time; the so-called "Great
Dying" at the end of the Permian
Period.
Paleogeography
During the early Jurassic period,
the supercontinent
Pangaea broke up into the
northern
supercontinent
Laurasia
and the southern supercontinent
Gondwana; the
Gulf of Mexico
opened in the new rift between North America and what is now Mexico's
Yucatan Peninsula.
The Jurassic North
Atlantic Ocean was relatively narrow, while the South Atlantic did not
open until the following Cretaceous Period, when Gondwana itself rifted
apart. The
Tethys Sea
closed, and the
Neotethys basin appeared. Climates were warm, with no evidence of
glaciation. As in the
Triassic, there was apparently no land near either pole, and no extensive
ice caps existed.
The Jurassic geological record is good in western
Europe, where extensive
marine sequences indicate a time when much of the continent was submerged
under shallow tropical seas; famous locales include the
Jurassic Coast
World Heritage
Site and the renowned late Jurassic
lagerstätten of
Holzmaden and
Solnhofen. In
contrast, the North American Jurassic record is the poorest of the Mesozoic,
with few outcrops at the surface. Though the
epicontinental
Sundance Sea left
marine deposits in parts of the northern plains of the United States and
Canada during the late Jurassic, most exposed sediments from this period are
continental, such as the
alluvial deposits of the
Morrison Formation.
The Jurassic was a time of
calcite sea
geochemistry in which low-magnesium
calcite was the primary
inorganic marine precipitate of calcium carbonate.
Carbonate
hardgrounds were thus very common, along with calcitic
ooids, calcitic cements,
and invertebrate faunas with dominantly calcitic skeletons (Stanley and
Hardie, 1998, 1999).
In North America, the first of several massive
batholiths were
emplaced in the northern
Cordillera
beginning in the mid-Jurassic, marking the
Nevadan orogeny.
Important Jurassic exposures are also found in Russia, India, South America,
Japan, Australasia and the United Kingdom.
Aquatic and Marine Fauna
During the
Jurassic period, the primary vertebrates living in the seas were fish
and marine reptiles. The latter
include
ichthyosaurs who were
at the peak of their diversity,
plesiosaurs,
pliosaurs, marine
crocodiles, of the families
Teleosauridae and
Metriorhynchidae.
In the
invertebrate world,
several new groups appeared, including
rudists
(a
reef-forming variety of
bivalves) and
belemnites. The Jurassic also had diverse
encrusting and boring (sclerobiont) communities (see Taylor & Wilson, 2003),
and it saw a significant rise in the
bioerosion of carbonate shells and
hardgrounds. Especially common is the
ichnogenus (trace
fossil)
Gastrochaenolites.
During the Jurassic period about four or five of
the twelve
clades of planktonic
organisms that exist in the fossil record either experienced a massive
evolutionary radiation or appeared for the first time.
Terrestrial Fauna
On
land, large
archosaurian reptiles
remained dominant. The Jurassic was the golden age of the large herbivorous
dinosaurs known as the
sauropods—Camarasaurus,
Apatosaurus,
Diplodocus,
Brachiosaurus, and many others—that roamed
the land late in the period; their mainstays were either the prairies
of
ferns, palm-like
cycads
and
bennettitales, or the
higher coniferous growth, according to their adaptations. They were preyed
upon by large theropods as for
example
Ceratosaurus,
Megalosaurus,
Torvosaurus and
Allosaurus. All these belong to the 'lizard
hipped' or
saurischian branch of
the dinosaurs. During the
Late Jurassic, the first birds
evolved from small
coelurosaurian dinosaurs.
Ornithischian dinosaurs were less
predominant than saurischian dinosaurs, although some like
stegosaurs and small
ornithopods played important roles as small
and medium-to-large (but not sauropod-sized) herbivores. In the air,
pterosaurs were common; they ruled the
skies, filling many ecological roles now taken by birds.
The rest of the
Lissamphibia evolved in this period,
introducing the first
salamanders and
caecilians.
Flora
The arid,
continental conditions characteristic of the
Triassic steadily eased during the Jurassic
period, especially at higher latitudes; the warm, humid climate allowed lush
jungles to cover much of the landscape.
Gymnosperms were relatively diverse during
the Jurassic period. The
conifers in particular dominated the flora,
as during the Triassic; they were the most diverse group and constituted the
majority of large trees. Extant conifer families that flourished during the
Jurassic included the
Araucariaceae,
Cephalotaxaceae,
Pinaceae,
Podocarpaceae,
Taxaceae and
Taxodiaceae. The extinct Mesozoic conifer
family
Cheirolepidiaceae
dominated low latitude vegetation, as did the shrubby
Bennettitales.
Cycads were also
common, as were
ginkgos and
Dicksoniaceous tree ferns in the forest.
Smaller ferns were probably the
dominant undergrowth.
Caytoniaceous seed ferns
were another group of important plants during this time and are thought to
have been shrub to small-tree sized. Ginkgo plants were particularly common
in the mid- to high northern latitudes. In the
Southern Hemisphere,
podocarps were especially successful, while
Ginkgos and
Czekanowskiales were
rare.
In the oceans modern
coralline algae appeared for the first time.
End of Reading
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