第四次生物大灭绝，又称三叠纪大灭绝。

　　三叠纪(Triassic period)是中生代的第一纪，爬行动物和裸子植物崛起，位于二叠纪(Permian)和侏罗纪(Jurassic)之间。

　　始于距今2.5亿年至2.03亿年，延续了约5000万年。海西运动以后，许多地槽转化为山系，陆地面积扩大，地台区产生了一些内陆盆地。新的古地理条件导致沉积相及生物界的变化。从三叠纪起，陆相沉积在世界各地，尤其在中国及亚洲其它地区都有大量分布。古气候方面，三叠纪初期继承了二叠纪末期干旱的特点;到中、晚期之后，气候向湿热过渡，由此出现了红色岩层含煤沉积、旱生性植物向湿热性植物发展的现象。植物地理区也同时发生了分异。

　　距今1.95亿年前的三叠纪末期，估计有76%的物种，其中主要是海洋生物灭绝，此次灾难并无特别明显的标志，只发现海平面下降之后又上升，出现大面积缺氧的海水。

Triassic-Jurassic extinction

　　The Triassic-Jurassic extinction event occurred 200 million years ago and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. Twenty percent of all marine families and all large Crurotarsi (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 opened an ecological niche 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 Pangea started to break apart.

　　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. As yet, no impact crater can be dated to coincide with the Triassic-Jurassic boundary.

　　▪ Massive volcanic eruptions. Such 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). However, 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 hydrates. 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 era.

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