Events like the Permian–Triassic and Triassic–Jurassic boundaries, which reshaped the course of life
1. The role of mass extinctions
Over 4.6 billion years of Earth's history, life has experienced multiple mass extinction crises, where a significant portion of global species disappeared over a relatively short geological period. Such events include:
- Removes dominant clades, opening ecological niches.
- Promotes rapid evolutionary radiation of surviving groups.
- Alters the composition of terrestrial and marine biotas.
While the "background extinction" occurs continuously (the baseline extinction rate), mass extinctions greatly exceed normal levels, leaving global "scars" in the fossil record. Of the "Big Five" events, the Permian–Triassic is the most catastrophic, but the Triassic–Jurassic transition also caused major faunal changes. Both show how significant ecological upheavals "shake up" Earth's history.
2. Permian–Triassic (P–Tr) extinction (~252 million years ago)
2.1 Scale of the crisis
The Late Permian period Permian–Triassic (P–Tr) mass extinction, also called the "Great Dying", is considered the largest known extinction event:
- In the seas: ~90–96% of marine species went extinct, including significant invertebrate groups like trilobites, rugose corals, and many brachiopods.
- On land: ~70% of terrestrial vertebrate species went extinct; a huge portion of plants also disappeared.
No other extinction matched this scale, essentially wiping out Paleozoic ecosystems and paving the way for the Mesozoic.
2.2 Possible causes
Likely many factors coincided, although their exact contributions are still debated:
- Siberian trap volcanism: Massive basalt flows in Siberia released abundant CO2, SO2, halogen, and aerosol emissions, causing global warming, ocean acidification, and possibly ozone layer depletion.
- Methane hydrate release: Warming oceans may have destabilized methane clathrates, further enhancing the greenhouse effect.
- Ocean anoxia: Water stagnation in the depths, elevated temperatures, and circulation changes led to widespread marine anoxia or euxinia (H2S presence).
- Impact?: There is less data on a large impact (unlike, for example, the Cretaceous–Paleogene event). Some suggest smaller bolide events, but volcanism and climate changes remain the main factors [1], [2].
2.3 Consequences: rise of archosaurs and Triassic renewal
After the extinction, ecosystems had to recover from very low diversity. Traditional Paleozoic groups (some "mammal-like reptile" synapsids) were heavily culled, so archosaur reptiles (from which dinosaurs, pterosaurs, crocodiles evolved) took dominant positions during the Triassic. New groups began to appear in marine environments (e.g., ichthyosaurs), as well as reef-building organisms being restructured. This "new start" is clearly visible in the abrupt fossil changes marking the transition from the Paleozoic to the Mesozoic.
3. Triassic–Jurassic (T–J) extinction (~201 million years ago)
3.1 Scale and affected groups
The Triassic–Jurassic boundary, though not as severe as the P–Tr event, was still significant: about 40–45% of marine genera went extinct, as well as many terrestrial groups. In the oceans, conodonts and certain large amphibians greatly declined, and several invertebrate groups, such as ammonites, suffered. On land, various archosaurs (phytosaurs, aetosaurs, rauisuchids) were heavily impacted, opening space for dinosaurs, which flourished during the Jurassic period [3], [4].
3.2 Possible causes
T–J causation hypotheses include:
- CAMP (Central Atlantic Magmatic Province) volcanism: Extensive basalt flows as Pangea split, releasing large amounts of greenhouse gases and causing global warming, ocean acidification, and other climate disruptions.
- Sea level changes: Tectonic shifts may have affected shallow sea habitats.
- Impact?: Less clear evidence of a large asteroid at the T–J boundary, unlike the K–Pg. Perhaps smaller impacts occurred, but volcanism and climatic disturbances seem to dominate.
3.3 The Rise of Dinosaurs
The T–J extinction severely affected many Triassic archosaurs, and dinosaurs – surviving in smaller forms – soon took advantage of the opportunity. The Early Jurassic shows a huge spread of familiar dinosaur groups (from sauropods to theropods), which for the next 135+ million years dominated the niches of large terrestrial herbivores and predators, thus cementing the full "Age of Reptiles".
4. Mechanisms of mass extinctions and ecological consequences
4.1 Carbon cycle and climate disruptions
Mass extinctions often coincide with sudden climate changes, such as intensified greenhouse conditions, ocean anoxia, or acidification. Volcanic CO2 emissions or methane from clathrates further increase warming, reduce dissolved oxygen in oceans, which hits marine invertebrates. On land, heat stress and ecosystem collapses occur. Under such radical conditions, species unable to adapt suddenly disappear, causing an extinction “avalanche.”
4.2 Ecosystem collapse and recovery
When keystone species, reef communities, or important primary producers die, temporary “disaster faunas” form, dominated by opportunists or resistant organisms. Over tens of thousands or millions of years, new groups exploit free niches and diversify strongly, so mass extinctions have a double effect: tragic loss and subsequent evolutionary innovation. The dominance of archosaurs after P–Tr and the dinosaur surge after T–J are examples.
4.3 Domino effect and food webs
Mass extinctions emphasize the interdependence of food webs: when key producers (e.g., plankton) die, higher-level organisms perish, spreading the extinction. On land, the loss of large herbivores affects predators. Each extinction shows how ecosystems can collapse if key factors are exceeded.
5. Signs in the fossil record: how we recognize mass extinctions
5.1 Boundary zones and biostratigraphy
Geologists identify mass extinction events by boundary layers in rocks where a large portion of fossil species suddenly disappear. The P–Tr case is characterized by a global “boundary clay” with a distinctive carbon isotope (δ13C) shift and a sudden loss of fossil diversity. The T–J boundary similarly shows geochemical (carbon isotope) changes and fossil renewal.
5.2 Geochemical markers
Isotope anomalies (C, O, S), trace elements (e.g., iridium increase in the K–Pg layer), or sedimentary changes (black shales indicating anoxia) indicate environmental disturbances. At the P–Tr boundary, strong negative δ13C indicates a CO2/CH4 influx into the atmosphere; at the T–J boundary, CAMP volcanism may have left basalt layers and related climate traces.
5.3 Ongoing discussions and refined chronologies
Continuous paleontological studies detail the timing, rate, and selectivity of each event. Regarding P–Tr, some suggest multiple pulses rather than just one. For T–J, it is investigated whether extinctions occurred gradually or suddenly at the boundary. Our understanding is supplemented by new findings and improved dating methods.
6. Evolutionary legacy: faunal turnovers
6.1 From Permo–Triassic to Triassic
The P–Tr mass extinction ended Paleozoic dominance (e.g., trilobites, many synapsids, certain corals) and opened space for:
- For archosaur rise – emerging dinosaurs, pterosaurs, “crocodile” lineages.
- For marine reptile expansion – ichthyosaurs, nothosaurs, later plesiosaurs.
- For new reef builders – scleractinian corals, sea urchins, new bivalve dominances.
6.2 From Triassic–Jurassic to the “middle” Mesozoic
In the Triassic–Jurassic event, large Triassic crurotarsans and other archosaurs were affected, and dinosaurs became the dominant terrestrial animals, leading to the well-known Jurassic–Cretaceous dinosaur fauna. Marine ecosystems also reorganized: ammonites, modern corals, and new fish lineages flourished. This was the preparation for the “golden age” of dinosaurs in the Jurassic and Cretaceous periods.
6.3 Future insights on extinctions
Studying these ancient catastrophes helps us understand how life might respond to the anthropogenic climate crisis or current disturbances. Earth's past reveals that mass extinctions are truly exceptional but sometimes recurring events, after which the landscape of life is completely reorganized. This highlights both resilience and vulnerability.
7. Conclusion
Permo–Triassic and Triassic–Jurassic boundary extinctions fundamentally rebooted the evolution of life on Earth, wiping out entire groups and freeing up space for new clades (especially dinosaurs). Although the P–Tr event was the most severe, the T–J extinction is also very important because it removed Triassic competitors, enabling dinosaurs to dominate the rest of the Mesozoic. Each shows that mass extinctions, though catastrophic, act as turning points in evolutionary history, driving new waves of evolution and shaping Earth's biota for tens of millions of years.
Even now, paleontologists and geologists are refining the understanding of what causes these crises, how ecosystems collapse, and how survivors adapt. By studying ancient extinction histories, we gain valuable insights into the fragility and resilience of life, the interaction between geology and biology, and the continuous cycles of collapse and renewal that define Earth's dynamic history.
Links and further reading
- Erwin, D. H. (2006). Extinction: How Life on Earth Nearly Ended 250 Million Years Ago. Princeton University Press.
- Shen, S. Z., et al. (2011). “Calibrating the End-Permian Mass Extinction.” Science, 334, 1367–1372.
- Benton, M. J. (2003). When Life Nearly Died: The Greatest Mass Extinction of All Time. Thames & Hudson.
- Tanner, L. H., Lucas, S. G., & Chapman, M. G. (2004). “Assessing the record and causes of Late Triassic extinctions.” Earth-Science Reviews, 65, 103–139.