Triassic–Jurassic extinction

The Triassic–Jurassic 'extinction event, often called the Triassic–Jurassic mass extinction or end-Triassic extinction', marks the boundary between the Triassic and Jurassic periods,. It represents one of five major extinction events during the Phanerozoic, profoundly affecting life on land and in the oceans.
In the seas, about 23–34% of marine genera disappeared; corals, bivalves, brachiopods, bryozoans, and radiolarians suffered severe losses of diversity and conodonts were completely wiped out, while marine vertebrates, gastropods, and benthic foraminifera were relatively unaffected. On land, all archosauromorph reptiles other than crocodylomorphs, dinosaurs, and pterosaurs became extinct. Crocodylomorphs, dinosaurs, pterosaurs, and mammals were left largely untouched, allowing them to become the dominant land animals for the next 135 million years. Plants were likewise significantly affected by the crisis, with floral communities undergoing radical ecological restructuring across the extinction event.
The cause of the TJME is generally considered to have been extensive volcanic eruptions in the Central Atlantic Magmatic Province, a large igneous province whose emplacement released large amounts of carbon dioxide into the Earth's atmosphere, causing profound global warming and ocean acidification, and discharged immense quantities of toxic mercury into the environment. Older hypotheses have proposed that gradual changes in climate and sea levels may have been the cause, or perhaps one or more asteroid strikes.

Research history

The earliest research on the TJME was conducted in the mid-20th century, when events in earth history were widely assumed to have been gradual, a paradigm known as uniformitarianism, while comparatively rapid cataclysms as causes of extinction events were dismissed as catastrophism, which had been associated with biblical creationism. Consequently, most researchers believed gradual environmental changes were the best explanation of the extinction; prominent vertebrate palaeontologist Edwin H. Colbert suggested gradual changes in the seasonality of rainfall and eustatic sea level rise that decreased the available land area above sea level were the culprit. In the 1980s, Jack Sepkoski identified the Triassic-Jurassic boundary drop in biodiversity as one of the "Big 5" mass extinction events. After the discovery that the Cretaceous-Paleogene extinction event was caused by an impact event, the TJME had also been suggested to have been caused by such an impact in the 1980s and 1990s. The theory that the TJME was caused by massive volcanism in the Central Atlantic Magmatic Province first emerged in the 1990s after similar research examining the Permian-Triassic extinction event found it to have been caused by volcanic activity and the emplacement of the CAMP was found to have occurred around the time of the Triassic-Jurassic transition. Despite some early objections, this paradigm remains the scientific consensus in the present day.

Effects

Marine invertebrates

The Triassic-Jurassic extinction completed the transition from the Palaeozoic evolutionary fauna to the Modern evolutionary fauna that continues to dominate the oceans in the present, a change that began in the aftermath of the end-Guadalupian extinction and continued following the Permian-Triassic extinction event. Between 23% and 34.1% of marine genera went extinct. Plankton diversity dropped suddenly, but it was relatively mildly impacted at the Triassic-Jurassic boundary, although extinction rates among radiolarians rose significantly. Early Hettangian radiolarian communities became depauperate as a result of the TJME and consisted mainly of spumellarians and entactiniids. Benthic foraminifera suffered relatively minor losses of diversity. Some opportunistic foraminifera such as Triasina hantkeni increased in abundance as they thrived in oxygen-depleted waters. Ammonites were affected substantially by the Triassic-Jurassic extinction and were nearly wiped out. Ceratitidans, the most prominent group of ammonites in the Triassic, became extinct at the end of the Rhaetian after having their diversity reduced significantly in the Norian, while other ammonite groups such as the Ammonitina, Lytoceratina, and Phylloceratina diversified from the Early Jurassic onward. Bivalves suffered heavy losses, although the extinction was highly selective, with some bivalve clades escaping substantial diversity losses. The Lilliput effect, a term coined to describe a phenomenon wherein organisms shrink in size following a mass extinction, affected megalodontid bivalves, whereas file shell bivalves experienced the Brobdingnag effect, the reverse of the Lilliput effect. There is some evidence of a bivalve cosmopolitanism event during the mass extinction. Additionally, following the TJME, mobile bivalve taxa outnumbered stationary bivalve taxa. Gastropod diversity was barely affected at the Triassic-Jurassic boundary, although gastropods gradually suffered numerous losses over the late Norian and Rhaetian, during the leadup to the TJME. Brachiopods declined in diversity at the end of the Triassic before rediversifying in the Sinemurian and Pliensbachian; the dielasmatoid, athyridoid, and spondylospiroid brachiopods experienced particularly severe declines. Bryozoans, particularly taxa that lived in offshore settings, had already been in decline since the Norian and suffered further losses in the TJME. Ostracods also suffered significant losses, although opportunistic ostracod forms thrived in the eutrophic conditions of the TJME. Conulariids seemingly completely died out at the end of the Triassic. Around 96% of coral genera died out, with integrated corals being especially devastated. Corals practically disappeared from the Tethys Ocean at the end of the Triassic except for its northernmost reaches, resulting in an early Hettangian "coral gap". There is good evidence for a collapse in the reef community, which was likely driven by ocean acidification resulting from supplied to the atmosphere by the CAMP eruptions.
Most evidence points to a relatively fast recovery from the mass extinction. Benthic ecosystems recovered far more rapidly after the TJME than they did after the PTME. British Early Jurassic benthic marine environments display a relatively rapid recovery that began almost immediately after the end of the mass extinction despite numerous relapses into anoxic conditions during the earliest Jurassic. In the Neuquén Basin, recovery began in the late early Hettangian and lasted until a new biodiversity equilibrium in the late Hettangian. Also despite recurrent anoxic episodes, large bivalves began to reappear shortly after the extinction event. Siliceous sponges dominated the immediate aftermath interval thanks to the enormous influx of silica into the oceans, a consequence of the aerial extent of the CAMP basalts that were exposed to surficial weathering processes. In some regions, recovery was slow; in the northern Tethys, carbonate platforms in the TJME's aftermath became dominated by microbial carbonate producers and r-selected calcitic taxa such as Thaumatoporella parvovesiculifera, while dasycladacean algae did not reappear until the Sinemurian stage.

Marine vertebrates

Fish did not suffer a mass extinction at the end of the Triassic. The Late Triassic in general did experience a gradual drop in actinopterygiian diversity after an evolutionary explosion in the Middle Triassic. Though this may have been due to falling sea levels or the Carnian Pluvial Event, it may instead be a result of sampling bias considering that Middle Triassic fish have been more extensively studied than Late Triassic fish. Despite the apparent drop in diversity, neopterygiians suffered less than more "primitive" actinopterygiians, indicating a biological turnover where modern groups of fish started to supplant earlier groups. Pycnodontiform fish were insignificantly affected. Conodonts, which were prominent index fossils throughout the Paleozoic and Triassic, finally became extinct at the T-J boundary following declining diversity.
Like fish, marine reptiles experienced a substantial drop in diversity between the Middle Triassic and the Jurassic. However, their extinction rate at the Triassic–Jurassic boundary was not elevated. The highest extinction rates experienced by Mesozoic marine reptiles actually occurred at the end of the Ladinian stage, which corresponds to the end of the Middle Triassic. The only marine reptile families which became extinct at or slightly before the Triassic–Jurassic boundary were the placochelyids, making plesiosaurs the only surviving sauropterygians, and giant ichthyosaurs such as shastasaurids. Some authors have argued that the end of the Triassic acted as a genetic "bottleneck" for ichthyosaurs, which never regained the level of anatomical diversity and disparity which they possessed during the Triassic, although analysis of ichthyosaurian and eosauropterygian disparity across the Triassic-Jurassic transition has shown no evidence for such a bottleneck. The high diversity of rhomaelosaurids immediately after the TJME points to a gradual extinction of marine reptiles rather than an abrupt one.

Terrestrial animals

Terrestrial fauna was affected by the TJME much more severely than marine fauna. One of the earliest pieces of evidence for a Late Triassic extinction was a major turnover in terrestrial tetrapods such as amphibians, reptiles, and synapsids. Edwin H. Colbert drew parallels between the system of extinction and adaptation between the Triassic–Jurassic and Cretaceous–Paleogene boundaries. He recognized how dinosaurs, lepidosaurs, and crocodyliforms filled the niches of more ancient groups of amphibians and reptiles which were extinct by the start of the Jurassic. Olsen estimated that 42% of all terrestrial tetrapods became extinct at the end of the Triassic, based on his studies of faunal changes in the Newark Supergroup of eastern North America. In contrast to the end-Cretaceous extinction, the TJME substantially affected freshwater ecosystems, and it further differed from the former in that body size did not affect extinction risk. More modern studies have debated whether the turnover in Triassic tetrapods was abrupt at the end of the Triassic, or instead more gradual.
During the Triassic, amphibians were mainly represented by large, crocodile-like members of the order Temnospondyli. Although the earliest lissamphibians did appear during the Triassic, they would become more common in the Jurassic while the temnospondyls diminished in diversity past the Triassic–Jurassic boundary. Although the decline of temnospondyls did send shockwaves through freshwater ecosystems, it was probably not as abrupt as some authors have suggested. Brachyopoids, for example, survived until the Cretaceous according to new discoveries in the 1990s. Several temnospondyl groups did become extinct near the end of the Triassic despite earlier abundance, but it is uncertain how close their extinctions were to the end of the Triassic. The last known metoposaurids were from the Redonda Formation, which may have been early Rhaetian or late Norian. Gerrothorax, the last known plagiosaurid, has been found in rocks which are probably Rhaetian, while a capitosaur humerus was found in Rhaetian-age deposits in 2018. Therefore, plagiosaurids and capitosaurs were likely victims of an extinction at the very end of the Triassic, while most other temnospondyls were already extinct.
File:Machaeroprosopus IMG 0720.jpg|left|thumb|Reptile extinction at the end of the Triassic is poorly understood, but phytosaurs went from abundant to extinct by the end of the Rhaetian.
Terrestrial reptile faunas were dominated by archosauromorphs during the Triassic, particularly phytosaurs and members of Pseudosuchia. In the Early Jurassic and onwards, dinosaurs and pterosaurs became the most common land reptiles, while small reptiles were mostly represented by lepidosauromorphs. Among pseudosuchians, only small crocodylomorphs did not become extinct by the end of the Triassic, with both dominant herbivorous subgroups and carnivorous ones having died out. Phytosaurs, drepanosaurs, trilophosaurids, tanystropheids, and procolophonids, which were other common reptiles in the Late Triassic, had also become extinct by the start of the Jurassic. However, pinpointing the extinction of these different land reptile groups is difficult, as the last stage of the Triassic, the Rhaetian, and the first stage of the Jurassic, the Hettangian, each have few records of large land animals; some paleontologists have considered only phytosaurs and procolophonids to have become extinct at the Triassic–Jurassic boundary, with other groups having become extinct earlier. However, it is likely that many other groups survived up until the boundary according to British fissure deposits from the Rhaetian. Aetosaurs, kuehneosaurids, drepanosaurs, thecodontosaurids, "saltoposuchids", trilophosaurids, and various non-crocodylomorph pseudosuchians are all examples of Rhaetian reptiles which may have become extinct at the Triassic–Jurassic boundary.
In the TJME's aftermath, dinosaurs experienced a major radiation, filling some of the niches vacated by the victims of the extinction. Crocodylomorphs likewise underwent a very rapid and major adaptive radiation. Surviving non-mammalian synapsid clades similarly played a role in the post-TJME adaptive radiation during the Early Jurassic.
Herbivorous insects were minimally affected by the TJME; evidence from the Sichuan Basin shows they were overall able to quickly adapt to the floristic turnover by exploiting newly abundant plants. Odonates suffered highly selective losses, and their morphospace was heavily restructured as a result.