Triassic–Jurassic extinction event

Marine extinction intensity during the Phanerozoic

%

Millions of years ago

(H)

K–Pg

Tr–J

P–Tr

Late D

O–S

The blue graph shows the apparent percentage (not the absolute number) of marine animal genera becoming extinct during any given time interval. It does not represent all marine species, just those that are readily fossilized. The labels of the "Big Five" extinction events are clickable hyperlinks; see Extinction event for more details. (source and image info)

The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, 201.3 million years ago, and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. In the seas, a whole class (conodonts) and 34% of marine genera disappeared. On land, all pseudosuchians (non-dinosaurian archosaurs) other than crocodylomorphs (Sphenosuchia and Crocodyliformes), some remaining therapsids, and many of the large amphibians became extinct.

Impact

At least half of the species now known to have been living on Earth at that time went extinct. This event vacated terrestrial 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. In the area of Tübingen (Germany), a Triassic-Jurassic bonebed can be found, which is characteristic for this boundary.

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.

Ranges of families tetrapods through the Triassic and Early Jurassic.

Current theories

Several explanations for this event have been suggested, but all have unanswered challenges:

• Gradual climate change, sea-level fluctuations or a pulse of oceanic acidification during the late Triassic reached a tipping point. However, this does not explain the suddenness of the extinctions in the marine realm.
• Asteroid impact, but so far no impact crater of sufficient size has been dated to coincide with the Triassic–Jurassic boundary.

The eroded Rochechouart crater in France has most recently been dated to 201 ±2 million years ago, but at 25 km across (possibly up to 50 km across originally), appears to be too small. The impact responsible for the annular Manicouagan Reservoir occurred about 12 million years before the extinction event - the Rochechouart crater is now thought to have been caused by part of the same fragmented impactor.

• Massive volcanic eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province (CAMP), would release carbon dioxide or sulfur dioxide and aerosols, which would cause either intense global warming (from the former) or cooling (from the latter).

The isotopic composition of fossil soils of end Triassic and Early Jurassic has been tied to a large negative carbon isotope excursion (Whiteside et al. 2010). Carbon isotopes of lipids (n-alkanes) derived from leaf wax and lignin, and total organic carbon from two sections of lake sediments interbedded with the CAMP in eastern North America have shown carbon isotope excursions similar to those found in the mostly marine St. Audrie’s Bay section, Somerset, England; the correlation suggests that the end-Triassic extinction event began at the same time in marine and terrestrial environments, slightly before the oldest basalts in eastern North America but simultaneous with the eruption of the oldest flows in Morocco (Also suggested by Deenen et al., 2010), with both a critical CO₂ greenhouse and a marine biocalcification crisis.

Contemporaneous CAMP eruptions, mass extinction, and the carbon isotopic excursions are shown in the same places, making the case for a volcanic cause of a mass extinction. The catastrophic dissociation of gas hydrates (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) may have exacerbated greenhouse conditions.

Literature

• McHone, J.G. (2003), Volatile emissions of Central Atlantic Magmatic Province basalts: Mass assumptions and environmental consequences, in Hames, W.E. et al., eds., The Central Atlantic Magmatic Province: Insights from Fragments of Pangea. American Geophysical Union Monograph 136, p. 241-254.
• [1]

External links

• Theories on the Triassic-Jurassic Extinction
• The Triassic-Jurassic Mass Extinction
• 200 million year old mystery BBC News story, 12-Oct-2011