Events such as the Permian–Triassic and Triassic–Jurassic boundaries, which reshaped the course of life
1. The role of mass extinctions
Over the 4.6 billion years of Earth's history, life has experienced many mass extinction a crisis in which a significant proportion of the world's species became extinct within a relatively short geological period. Such events include:
- Removes dominant clades, opening up ecological niches.
- Encourages rapid evolutionary radiation of surviving groups.
- Changing composition of terrestrial and marine biota.
Until "background fade" is ongoing (the main indicator of extinction), mass extinctions far above normal levels, leaving global "scars" in the fossil record. Of the "Big Five" events Permian–Triassic is the most catastrophic, but Triassic–Jurassic The transition also caused major changes in fauna. Both show how Earth's history is "punctuated" by major ecological upheavals.
2. Permo-Triassic (P-Tr) extinction (~252 million years ago)
2.1 The scale of the crisis
Late Permian period successful Permian–Triassic (P–Tr) mass extinction, also called "The Great Dying", considered the largest known extinction event:
- In the seas: ~90–96 % of marine species became extinct, including significant groups of invertebrates such as trilobites, horn corals, and many brachiopods.
- On land: ~70 % of terrestrial vertebrate species became extinct; a large proportion of plants also disappeared.
No other extinction has matched this scale, essentially wiping out Paleozoic ecosystems and paving the way Mesozoic.
2.2 Possible causes
A number of factors likely converged, although their exact contribution is still disputed:
- Siberian Trap Volcanism: Giant basalt eruptions in Siberia released abundant CO2, SO2, halogen and aerosol emissions, causing global warming, ocean acidification and possibly ozone depletion.
- Methane hydrate release: Warming oceans may have destabilized methane clathrates, further enhancing the greenhouse effect.
- Ocean anoxia: Water stagnation in the depths, increased temperatures, and changes in circulation led to widespread marine anoxia or euxinia (H2S emergence).
- Impact?: There is less evidence for a large impact (as opposed to, for example, the Cretaceous–Paleogene). Some suggest smaller bolide events, but volcanism and climate change remain the most important [1], [2].
2.3 Consequences: the rise of archosaurs and the Triassic revival
After the extinction, ecosystems had to recover from very low diversity. Traditional Paleozoic groups (some "mammal-like reptilian" synapsids) were heavily pruned, so archosaurs reptiles (from which dinosaurs, pterosaurs, crocodiles evolved) took over the dominant positions during the Triassic. New groups began to appear in the marine environment (e.g. ichthyosaurs), as well as reshaped reef-building organisms. This "new start" is clearly visible in the abrupt changes in fossils that mark the transition from the Paleozoic to the Mesozoic.
3. Triassic–Jurassic (T–J) extinction (~201 million years ago)
3.1 Scope and affected groups
Triassic–Jurassic The boundary, although not as terrible as the P–Tr event, was nevertheless significant: about 40–45 % of marine genera, as well as many terrestrial groups. In the oceans, conodonts and certain large amphibians declined significantly, and several invertebrate groups, such as ammonites, also suffered.On land, various archosaurs (phytosaurs, aetosaurs, rauisuchids) suffered greatly, opening up space for dinosaurs, which flourished during the Jurassic period. [3], [4].
3.2 Possible causes
Versions of T–J causality include:
- CAMP (Central Atlantic Magmatic Province) volcanism: Extensive basalt outpouring during the breakup of Pangaea, releasing large amounts of greenhouse gases and causing global warming, ocean acidification, and other climate disruptions.
- Sea level changes: Tectonic changes may have affected shallow marine habitats.
- Impact?: Less clear evidence for a large asteroid at the T–J boundary than at K–Pg. There may have been small impacts, but volcanism and climatic disturbances seem to dominate.
3.3 Rise of the Dinosaurs
The T–J extinction severely affected many Triassic archosaurs, and dinosaurs – surviving in smaller forms – soon seized the opportunity. The Early Jurassic witnessed a massive expansion 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 establishing a complete “Age of Reptiles".
4. Mechanisms and ecological consequences of mass extinctions
4.1 Disturbances in the carbon cycle and climate
Mass extinctions often coincide with sudden climate change, such as greenhouse intensification, ocean anoxia, or acidification. Volcanic CO2 Emissions of methane from clathrates further increase warming, reducing dissolved oxygen levels in the oceans, which is detrimental to marine invertebrates. On land, heat stress and ecosystem collapse occur. In such radical conditions, species that can no longer adapt suddenly disappear, triggering an "avalanche" of extinctions.
4.2 Ecosystem collapse and recovery
When keystone species, reef communities, or important primary producers die, temporary “catastrophic fauna", in which opportunists or resistant organisms take over. Over tens of thousands or millions of years, new groups exploit the vacant niches and expand rapidly, so mass extinctions have a dual effect: tragic loss and subsequent evolutionary innovation. The dominance of archosaurs after the P–Tr and the dinosaur leap after the T–J are examples of this.
4.3 Domino effect and food webs
Mass extinctions highlight food webs interdependence: when primary producers (e.g. plankton) die, higher-level organisms die, spreading extinctions. On land, the loss of large herbivores affects predators. Each extinction illustrates how ecosystems can collapse if key drivers are overridden.
5. Fossil Record Signs: How We Recognize Mass Extinctions
5.1 Marginal zones and biostratigraphy
Geologists identify mass extinction events based on boundary layers in rocks where a large proportion of fossil species suddenly disappear. The P–Tr case is characterized by a global “marginal clay” with a characteristic carbon isotope (δ13C) change and a sudden loss of fossil diversity. The T–J boundary similarly has 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 scales indicating anoxia) indicate environmental shocks. Strong negative δ at the P–Tr boundary13C indicates CO2/CH4 influx into the atmosphere; at the T–J boundary, CAMP volcanism may have left basalt layers and associated climate traces.
5.3 Ongoing debates and revised chronologies
Continuing paleontological research is detailing the timing, speed, and selectivity of each event. For P–Tr, some suggest multiple pulses rather than a single one. For T–J, the question is whether the extinctions occurred gradually or abruptly at a boundary. Our understanding is being expanded by new finds and improved dating methods.
6. Evolutionary legacy: faunal transformations
6.1 Permian–Triassic to Triassic
The P–Tr mass extinction ended the Paleozoic dominance (e.g., trilobites, many synapsids, certain corals) and made way for:
- The rise of archosaurs – dinosaurs, pterosaurs, and "crocodilian" branches appeared.
- For the expansion of marine reptiles – ichthyosaurs, notosaurs, later plesiosaurs.
- For new reef builders – scleractinian corals, urchins, new bivalve dominances.
6.2 From the Triassic–Jurassic to the “middle” Mesozoic
In the Triassic–Jurassic event, the large Triassic crurotarsans and other archosaurs were affected, and dinosaurs became the dominant land animals, leading to the well-known Jurassic–Cretaceous dinosaur fauna. Marine ecosystems also underwent a reorganization, with ammonites, modern corals, and new lineages of fish flourishing. This was in preparation for the "golden age" of dinosaurs in the Jurassic and Cretaceous periods.
6.3 Future insights into extinctions
Studying these ancient catastrophes helps us understand how life would respond to anthropogenic climate crisis or current disruptions. Earth's past reveals that mass extinctions – truly extraordinary, but sometimes recurring phenomena that leave a completely transformed landscape of life. This highlights both resilience and vulnerability.
7. Conclusion
Permian–Triassic and Triassic–Jurassic the disappearance of boundaries from the foundations reloaded The P–Tr event was the most devastating, but the T–J extinction was also very important because it eliminated the Triassic competitors, freeing up the dinosaur dominance for the rest of the Mesozoic. Each demonstrates that mass extinctions, while catastrophic, act as turning points in evolutionary history, triggering new evolutionary waves and shaping Earth's biota for tens of millions of years to come.
Even now, paleontologists and geologists are improving our understanding of what causes these crises, how ecosystems collapse, and how survivors adapt. By deciphering the stories of ancient extinctions, we gain valuable insights into the fragility and resilience of life, the interplay of geology and biology, and the endless cycles of collapse and renewal that define Earth's dynamic history.
References 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, LH, Lucas, SG, & Chapman, MG (2004). "Assessing the record and causes of Late Triassic extinctions." Earth-Science Reviews, 65, 103–139.