TRIASSIC-JURASSIC EXTINCTION

201.3 Ma – TRIASSIC-JURASSIC EXTINCTION

The following impacts “may” be related to the Triassic–Jurassic Extinction:

The red dot represents the approximate area of the Red Wing impact approximately 200 million years ago in the Triassic Period.
The red dot represents the approximate area of the Viewfield impact 190 million years ago in the Jurassic Period.

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 twenty percent of all marine families disappeared. On land, all large crurotarsans (non-dinosaurian archosaurs) other than crocodilians, 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 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 (Wikipedia).

Evidence for Impact: “Analysis of tetrapod footprints and skeletal material from more than 70 localities in eastern North America shows that large theropod dinosaurs appeared less than 10,000 years after the Triassic-Jurassic boundary and less than 30,000 years after the last Triassic taxa, synchronous with a terrestrial mass extinction. This extraordinary turnover is associated with an iridium anomaly (up to 285 parts per trillion, with an average maximum of 141 parts per trillion) and a fern spore spike, suggesting that a bolide impact was the cause. Eastern North American dinosaurian diversity reached a stable maximum less than 100,000 years after the boundary, marking the establishment of dinosaur-dominated communities that prevailed for the next 135 million years” (Olsen et al 2002).

P. E. Olsen, D. V. Kent, H.-D. Sues, C. Koeberl, H. Huber, A. Montanari, E. C. Rainforth, S. J. Fowell, M. J. Szajna, B. W. Hartline ASCENT OF DINOSAURS LINKED TO AN IRIDIUM ANOMALY AT THE TRIASSIC JURASSIC BOUNDARY Science, 17 May 2002

A Late Triassic Impact Ejecta Layer in Southwestern Britain

Gordon Walkden 1*, Julian Parker 1, Simon Kelley 2

1 Department of Geology and Petroleum Geology, Kings College, University of Aberdeen, Aberdeen AB24 3UD, UK.
2 Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, UK.

* To whom correspondence should be addressed. E-mail: spherules@abdn.ac.uk.

Despite the 160 or so known terrestrial impact craters of Phanerozoic age, equivalent ejecta deposits within distal sedimentary successions are rare. We have recognized a Triassic deposit in southwestern Britain that contains spherules and shocked quartz, characteristic of an impact ejecta layer. Inter- and intragranular potassium feldspar from the deposit yields an Ar-Ar age of 214 ± 2.5 million years old. This is within the age range of several known Triassic impact craters, the two closest of which, both in age and location, are Manicouagan in northeastern Canada and Rochechouart in central France. The ejecta deposit provides an important sedimentary record of an extraterrestrial impact in the Mesozoic that will help to decipher the number and effect of impact events, the source and dynamics of the event that left this distinctive sedimentary marker, and the relation of this ejecta layer to the timing of extinctions in the fossil record.

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Asteroid impacts may not always cause mass extinctions, say geologists in the US. They believe that the the impact that created the 100-kilometre Manicouagan crater in Quebec did not coincide with any extinction event.

Some scientists had claimed the Manicouagan impact occurred at the same time as the mass extinctions which ended the Triassic period about 202 million years ago. But Joseph Hodych and G. Dunning of Memorial University of Newfoundland in St John’s have now dated zircon crystals from the site. They find them to be 214 million years old.

Evidence that the impact of an asteroid caused the mass extinctions at the end of the Cretaceous period 65 million years ago made many geologists suspect that a similar impact at the end of the Triassic period caused the mass extinctions then. The Manicouagan crater, one of the largest impact structures on the Earth’s surface, was an obvious candidate for the impact site. Previous estimates of its date came with errors that were large enough to overlap with the boundary between the Triassic and Jurassic periods.

The best evidence for an impact at the end of the Triassic was reported by David Bice of Carleton College in Northfield, Minnesota, in late 1990. At the Triassic-Jurassic boundary in Italy, he found ‘shocked’ quartz grains, which are considered evidence of an impact. Bice and others believed that the extinctions occurred rapidly.

Hodych and Dunning’s date, obtained by analysing uranium and lead isotopes, is the most precise yet (Geology, January, p 51). Bice remains unconvinced.

From issue 1807 of New Scientist magazine, 08 February 1992, page 23