Triassic–Jurassic extinction event
The Triassic–Jurassic (Tr-J) extinction event (TJME), often called the end-Triassic extinction, was a Mesozoic extinction event that marks the boundary between the Triassic and Jurassic periods, 201.4 million years ago,[1] and is one of the top five major extinction events of the Phanerozoic eon,[2] profoundly affecting life on land and in the oceans. In the seas, the entire class of conodonts[3][4] and 23–34% of marine genera disappeared.[5][6] On land, all archosauromorphs other than crocodylomorphs, pterosaurs, and dinosaurs became extinct; some of the groups which died out were previously abundant, such as aetosaurs, phytosaurs, and rauisuchids. Some remaining non-mammalian therapsids and many of the large temnospondyl amphibians had become extinct prior to the Jurassic as well. However, there is still much uncertainty regarding a connection between the Tr-J boundary and terrestrial vertebrates, due to a lack of terrestrial fossils from the Rhaetian (latest) stage of the Triassic.[7] Plants, crocodylomorphs, dinosaurs, pterosaurs and mammals were left largely untouched;[8][9][10] this allowed the dinosaurs, pterosaurs, and crocodylomorphs to become the dominant land animals for the next 135 million years.[11][9]
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.[12] Nevertheless, a pronounced turnover in plant spores and a collapse of coral reef communities indicates that an ecological catastrophe did occur at the Triassic-Jurassic boundary.[13][14] Older hypotheses on extinction have proposed that gradual climate or sea level change may be the culprit,[15] or perhaps one or more asteroid strikes.[16][17][18] However, the most well-supported and widely-held theory for the cause of the Tr-J extinction places the blame on the start of volcanic eruptions in the Central Atlantic Magmatic Province (CAMP),[19] which was responsible for outputting a high amount of carbon dioxide into Earth's atmosphere,[20][21] inducing profound global warming,[22] along with ocean acidification.[23]
Possible causes[edit]
Gradual climate change[edit]
Gradual climate change, sea-level fluctuations, or a pulse of oceanic acidification during the late Triassic may have reached a tipping point. However, the effect of such processes on Triassic animal and plant groups is not well understood.
The extinctions at the end of the Triassic were initially attributed to gradually changing environments. Within his 1958 study recognizing biological turnover between the Triassic and Jurassic, Edwin H. Colbert's proposal was that this extinction was a result of geological processes decreasing the diversity of land biomes. He considered the Triassic period to be an era of the world experiencing a variety of environments, from towering highlands to arid deserts to tropical marshes. In contrast, the Jurassic period was much more uniform both in climate and elevation due to excursions by shallow seas.[15]
Later studies noted a clear trend towards increased aridification towards the end of the Triassic. Although high-latitude areas like Greenland and Australia actually became wetter, most of the world experienced more drastic changes in climate as indicated by geological evidence. This evidence includes an increase in carbonate and evaporite deposits (which are most abundant in dry climates) and a decrease in coal deposits (which primarily form in humid environments such as coal forests).[7] In addition, the climate may have become much more seasonal, with long droughts interrupted by severe monsoons.[71] The world gradually got warmer over this time as well; from the late Norian to the Rhaetian, mean annual temperatures rose by 7 to 9 °C.[72] The site of Hochalm in Austria preserves evidence of carbon cycle perturbations during the Rhaetian preceding the Triassic-Jurassic boundary, potentially having a role in the ecological crisis.[73]
Sea level fall[edit]
Geological formations in Europe seem to indicate a drop in sea levels in the late Triassic, and then a rise in the early Jurassic. Although falling sea levels have sometimes been considered a culprit for marine extinctions, evidence is inconclusive since many sea level drops in geological history are not correlated with increased extinctions. However, there is still some evidence that marine life was affected by secondary processes related to falling sea levels, such as decreased oxygenation (caused by sluggish circulation), or increased acidification. These processes do not seem to have been worldwide, with the sea level fall observed in European sediments believed to be not global but regional,[74] but they may explain local extinctions in European marine fauna.[7] A pronounced sea level in latest Triassic records from Lake Williston in northeastern British Columbia, which was then the northeastern margin of Panthalassa, resulted in an extinction event of infaunal (sediment-dwelling) bivalves, though not epifaunal ones.[75]
Comparisons to present climate change[edit]
The extremely rapid, centuries-long timescale of carbon emissions and global warming caused by pulses of CAMP volcanism has drawn comparisons between the Triassic-Jurassic mass extinction and anthropogenic global warming, currently causing the Holocene extinction.[20] The current rate of carbon dioxide emissions is around 50 gigatonnes per year, hundreds of times faster than during the latest Triassic, although the lack of extremely detailed stratigraphic resolution and pulsed nature of CAMP volcanism means that individual pulses of greenhouse gas emissions likely occurred on comparable timescales to human release of warming gases since the Industrial Revolution.[21] The degassing rate of the first pulse of CAMP volcanism is estimated to have been around half of the rate of modern anthropogenic emissions.[20] Palaeontologists studying the TJME and its impacts warn that a major reduction in humanity's carbon dioxide emissions to slow down climate change is of critical importance for preventing a catastrophe similar to the TJME from befalling the modern biosphere.[21] If human-induced climate change persists as is, predictions can be made as to how various aspects of the biosphere will respond based on records of the TJME. For example, current conditions such the increased carbon dioxide levels, ocean acidification, and ocean deoxygenation create a similar climate to that of the Triassic-Jurassic boundary for marine life, so it is the common assumption that should the trends continue, modern reef-building taxa and skeletal benthic organisms will be preferentially impacted.[181]