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joanjournal · 25 days ago

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Surveying Mass Extinctions - Part 2

Postosuchus, one of the crocodylomorphs of the Triassic (FunkMonk, Public Domain)

Continuing my overview of the Earth's extinction events! Part 1 covered the 'Precambrian' and Paleozoic, and so Part 2 will conclude with the Mesozoic and Cenozoic Eras.

Smithian-Spathian Boundary Event

When? - ~249.9 Million Years Ago (Olenekian Age, Early Triassic)

Cause - The Siberian Traps still had a little juice in them, and briefly experienced a few more eruptions which were enough to recharge ocean anoxia and an increase in volcanic carbon leading to global warming (Du, etal. 2022).

Victims - Life was on the road to recovery following the Great Dying, even following a massive spike in global temperatures facilitated by the loss of forests (Xu, et al. 2025). Surveys of fossil communities point to the Smithian-Spathian being particularly disruptive to regrowth: many species survived the destruction only to perish almost immediately afterward. Lystrosaurus, one of the most common of the dicynodont protomammals, went extinct, as did contemporaneous tetrapods like the archosauriform Proterosuchus. In the seas, life was hit equally hard, with losses among conodonts, ammonoids, bivalves, and other marine invertebrates, while radiolarians (amoeba-like, shelled plankton) suffered extinctions.

Survivors - It appears that bony fishes and the ancestors of marine reptiles survived through the harsh conditions, and it's widely understood that ichthyosaurs and sauropterygians (placodonts & proto-plesiosaurs) experienced an adaptive radiation afterward, preying on the many newly evolving fishes. They would become some of the major marine vertebrates of the Mesozoic Era. Among marine invertebrates, the start of the Triassic marks the origin of the "Modern Fauna", mainly composed of bivalves, gastropods, echinoderms, and crustaceans.

Carnian Pluvial Episode

When? - ~234-232 Million Years Ago

Cause - Climate warming coincides with increased moisture in the atmosphere, and it appears that the hot-house conditions of the Triassic Period facilitated the appearance of megamonsoons which dumped rains across the Pangaean supercontinent for roughly 2 million years. Such persistent rainfall affected the rock and water cycles and turned large areas of the land into humid, wetland environments (Corso, et al. 2020).

Victims - Such a change in the water cycle shifted both marine and freshwater communities, and there were extinctions among mollusks, crinoids (isocrinids died out), corals, bryozoans, conodonts, bony fishes, and planktonic forms. On land, tetrapods experienced losses, primarily among herbivorous varieties like rhynchosaurs and dicynodonts.

Survivors - The spread of wetlands encouraged the growth of new forests, and ferns, conifers, and cycadeoids (stem-angiosperms) experienced increased biodiversity. Fossil evidence points to the rise of dinosaurs, crocodylomorphs, turtles, lepidosaurs (lizards & tuataras) and mammaliaforms during the Carnian Pluvial Episode, meaning that essentially all the modern land vertebrate groups evolved, perhaps, in response to the changes (Corso, et al. 2020). In the oceans new life took root with the first scleractinian corals (the group to which modern stony corals belong), though it wasn't until several million years later when they formed a symbiotic relationship with zooxanthellae algae that coral reefs developed. Importantly, zooxanthellae belong to the dinoflagellate group, which also shared an origin during the Carnian alongside the chalk-forming coccolithophore plankton. Neopterygians - ray-finned fishes with lightened scales and skeletons - and the proto-sharks and rays (or neoselachians) experienced an adaptive radiation as well.

End Triassic Mass Extinction

When? - 201.4 Million Years Ago

Cause - Pangaea was in the beginning stages of break-up, and one of the first areas of release was between the future continents of North America and Eurasia. In the proto-North Atlantic, massive lava flows marked the boundaries of separation and are recorded in the rock record as the Central Atlantic magmatic province or CAMP (Whalen, et al. 2015). Correlations between geologic dating and the timing of extinctions have pointed strongly to CAMP being one of the leading causes of the mass extinction (Deenen, et al. 2010; Bond & Grasby, 2017). As much as 720,000 cubic-miles of lava may have pooled across the northern hemisphere (Benton, 2023). Such enormous volcanic outpourings would have contributed to climatic warming and ocean acidification & anoxia as they erupted over an estimated 600,000 year period in four pulses (Bond & Grasby, 2017).

Victims - In the oceans, the scleractinian corals faced their harshest extinction event before the present. Among the other marine losses was the total extinction of the lamprey-like conodonts, and ammonoid diversity plummeted so much that only one lineage survived to give rise to the famous ammonites of the Jurassic & Cretaceous oceans. Conulariids, a bizarre group related to jellyfish, also went extinct after having lived through the entire Paleozoic Era. On land, plants suffered a drop in biodiversity across several regions, and the CAMP was a decisive turning-point in amniote evolution. This extinction event was the death-nail for many of the crocodylomorphs and remaining therapsid protomammals (specifically dicynodonts and therocephalians), whose lines ended before the Triassic closed out. The giant temnospondyls would never recover their past diversity: metoposaurs and pelagiosaurids would go extinct. Many marine or coastal reptile groups died out, including the placodonts and long-necked tanystropheids. Reptiles as a whole were severely reduced in diversity, with parareptiles, many archosauromorphs, and the other so-called "Triassic weirdos" going extinct.

Survivors - It would be the dinosaurs who would take up the major terrestrial niches on land for the remainder of the Mesozoic, having outlasted the other reptile groups and having their own adaptive radiation. Of the therapsids, only the mammaliaforms survived to give rise to crown mammals in the Jurassic Period. Of the temnospondyls, the lissamphibians (frogs, salamanders, caecilians) would diversify across forested and wetland ecosystems. Of the marine reptiles, only the plesiosaurs and ichthyosaurs survived. Pterosaurs, the flying reptiles closely related to dinosaurs, had evolved prior to CAMP, and would go on to rule the skies.

Toarcian Oceanic Anoxic Event

When? - ~183 Million Years Ago (Early Jurassic Period)

Cause - A minor extinction event that occurred in about six pulses as a result of volcanic activity and ocean anoxia (Caruthers, et al. 2013). The eruptions of the Karoo and Ferrar Traps in South Africa have been linked, which may have also released methane as well as CO2 into the atmosphere. There is evidence to suggest extensive wildfires and acid rain affected life on land (Reolid, et al. 2022).

Victims - As a result of ocean anoxia, this event was particularly rough for shelled marine invertebrates, like ammonites, bivalves brachiopods, and ostracods. Forams, radiolarians, and dinoflagellate plankton were hit hard. Marine reptiles experienced severe losses, with a number of plesiosaur and ichthyosaur clades going extinct (including the giant predatory temnodontosaurids). Dinosaur and plant diversity appears to have been impacted by the Traps eruptions: fossil sites show a decline in gymnosperms, cycadeoids, seed ferns, true ferns, and lycopods, and there are notable extinctions of many lineages of early sauropodomorphs (traditionally known as "prosauropods") and thyreophorans (e.g. Scelidosaurus) as well as the coelophysoid predators who preyed on them (in addition to the related, double head-crested Dilophosaurus).

Survivors - Thalattosuchians, ocean-adapted crocodylomorphs seem to have taken over some of the niches of the lost marine reptile groups. Terrestrial ecosystems experienced a shift from the earlier high-diversity communities to substantially lower-diversity forests primarily composed of conifers and cycads (Slater, et al. 2019). Dinosaurs experienced a turnover in diversity, with the earlier predatory forms being replaced by allosauroids, megalosauroids, and tyrannosauroids; while the herbivores were replaced by larger and more derived sauropods, stegosaurs, and ankylosaurs (Reolid, et al. 2022). Recent evidence shows that insects may have benefited from the anoxic event by feasting on the dead fish corpses and newly-evolved plants (Swaby, et al. 2024).

Tithonian Extinction

When? - ~143 Million Years Ago (Late Jurassic Period)

Cause - An area of ongoing and controversial research, there is growing evidence of one or a number of events occurring at the end of the Jurassic Period that could be regarded as an extinction event, as there is evidence of both environmental change and faunal turnover at this time (Tennant, et al. 2016). Proposed causes include volcanic activity and even bolide impacts, which contributed to falling sea levels and a general cooling & drying of the global climate.

Victims - There is evidence of diversity loss at regional levels across many taxa, from marine mollusks to dinosaurs. Scleractinian coral reefs were badly hit, and there was a large decline in decapod crustaceans, ammonites, and bivalves; among the latter, heteroconchs (the cockle & unionid mussle clade) and lucinids (hatchet clams) experienced losses. Lineages of freshwater fishes, turtles, and plesiosaurs died out. The long-tailed pterosaurs (traditionally known as "rhamphorhynchoids") died out. Among dinosaurs, there was a particularly noticable turnover in forms: predatory ceratosaurids, megalosaurids, and allosaurids went extinct; many clades of sauropods suffered losses; and the stegosaurs lost considerable diversity.

Survivors - Rudists, a clade of bivalves, replaced the scleractinians as the ocean's major reef-building organisms for the remainder of the Mesozoic Era. Both gastropods and brachiopods seemed to have been unaffected by the changes. Marine fishes, and sharks & rays as a whole, seemed to do well, as did the reptilian ichthyosaurs and thalattosuchians. Notosuchian crocodylomorphs evolved right after the Jurassic extinction, encompassing a diverse group of land-living forms. Pterodactyloids - the short-tailed pterosaurs - persisted and underwent a burst in evolutionary change. Though the major groups of dinosaurs were more or less established long before the end of the Jurassic, it wasn't until afterward that they experienced another burst of evolution. New species of ceratopsians, ornithopods, ankylosaurs, titanosaurs, allosauroids, megalosauroids, and coelurosaurs (including new lineages of birds) evolved across the continents.

Cycadeoid, a type of stem-angiosperm (Matteo De Stefano/MUSE, CC BY-SA 3.0)

Aptian-Albian Extinction

When? - ~117-113 Million Years Ago (Early Cretaceous Epoch)

Cause - Volcanic eruptions in the South Asian Rahjamal Traps appear to have spurred a period of high global temperatures and ocean anoxia, designated at "OAE1" (Benton, 2023). The oceans, at least, experienced a cooling trend by the early Albian Age (Balestra, et al. 2025).

Victims - The Aptian Event is said to have been one of the major die-offs of foraminifera in the Earth's history, impacting populations from the Atlantic to the Tethys (Balestra, et al. 2025). Extinctions on land appear to have been more regional in scope and mainly affected plant communities: a number of ginkgo, conifer, and cycadeloid genera went extinct (Archangelsky, 2001).

Survivors - While gymnosperms, cycadeoids, and ferns as a whole did not suffer tremendous losses, the early angiosperms (flowering plants) would ultimately experience a burst in biodiversity. By 100 million years ago, the "Angiosperm Terrestrial Revolution" would bring a significant shift in the ecological relationships between plants and animals, particularly among insects and herbivorous tetrapods (Benton, et al. 2021).

Cenomanian-Turonian Extinction

When? - ~94.5-90.3 Million Years Ago (Late Cretaceous Epoch)

Cause - As in the Aptian-Albian, the oceans underwent another anoxic event - "OAE2" - that was paralleled by a sharp rise in global sea levels and temperatures (Petrizzo, et al. 2022; Arthur, et al. 1988). These are linked to volcanic lava flows in both the newly-forming Caribbean Sea and in Madagascar as it separated from the Indian Subcontinent (Kuroda, et al. 2007), but other areas of the world seem to have contributed with their own eruptions (Petrizzo, et al. 2022). The transition from the Early to Late Cretaceous Epochs was thus marked by a dynamic shift in the Earth's climate and oceans.

Victims - There was a significant turnover of Mesozoic marine life, particularly among plankton (many coccoliths, forams, radiolarians, and dinoflagellates went extinct), mollusks (many rudists and ammonites went extinct), and sea-going reptiles (ichthyosaurs and the large-headed plesiosaurs called pliosaurs went extinct). So far as can be deduced, there were no significant extinctions on land at a global level.

Survivors - The Cenomanian-Turonian would be the last time in Earth's history that a major ocean-anoxia event occurred across the world's oceans (Petrizzo, et al. 2022). Mosasaurs, a lineage of paddle-limbed lizards, would evolve to take over the role of the ocean's great predators from the pliosaurs and ichthyosaurs. On land, the drying of the climate facilitated the spread of mid-latitude open forest environments dominated by flowering trees of the Fagales clade (the oaks, birches, alders, etc) while closed-conifer araucaria & cypress forests reduced in size (Heimhofer, et al. 2018).

Impression of the K-Pg Bolide Hitting the Earth (Donald Davis, Public Domain)

End-Cretaceous Mass Extinction Event

When? - ~66.043 Million Years Ago

Cause - Since the pioneering research done by the Alvarez team in 1980, the sheer volume of studies on the world of the very latest Cretaceous Period have converged on one major consensus: a ~6.2 mile-long bolide (space rock) collided with the Earth and it was this that ultimately caused a mass extinction (Chiarenza, et al. 2020). It struck the Yucatán Peninsula with the power of over a billion nuclear bombs, landing in an area rich in carbonate and sulfate minerals (Schulte, et al. 2010). Immediate effects from the blast included enormous earthquakes and megatsunamis. It is argued that the rain of returning debris in the form of superheated glass may have triggered surface conditions comparable to an oven, and the land would have cooked and burned under a global firestorm (Robertson, et al. 2013). All this would have occurred on the first day of the impact. By having landed in carbonate & sulfate-rich terrain, particularly strong aerosols had also been ejected across the atmosphere to produce a blanket against solar radiation. This then shifted the atmosphere from fire to ice, and a prolonged impact winter would unfold over a period of years to decades (Brugger, et al. 2016). The release of aerosols also contributed to ocean acidification through acid rains. Though there is evidence of intensive and substantial volcanism on the Indian Subcontinent at the end of the Cretaceous - these being the Deccan Traps eruptions - its effects on extinctions appear to have been minimal, and in fact they may even have helped life survive in the wake of the devastation (Chiarenza, et al. 2020).

Victims - The species losses span the entire breadth of the tree of life. The most prominent extinctions were, of course, among the non-avian dinosaurs (including all but one lineage of birds), marine reptiles, and pterosaurs. There were also losses among modern reptile groups, with lizards, tuataras, and crocodylomorphs being hard hit. Mammals - typically thought of as hardy survivors - actually suffered significant extinctions too, with marsupials and multituberculates experiencing the biggest die-offs. There were broken connections between plants and insects, leading to a decline in numbers for both (Labandeira, et al. 2022). Several clades of oceanic sharks and rays, and bony fishes, went extinct. The great rudist bivalves and the reefs they formed died out, and there were large losses among the scleractinian corals too. Fellow marine invertebrates suffered both declines and extinctions, most notably the inoceramid clams (now extinct), the squid-like belemnites (now extinct), and the ammonites (extinct within about a million years of the event). As with most mass extinctions, planktonic forms suffered greatly, with coccoliths and forams experiencing another wave of extinctions.

Survivors - Freshwater animals, particularly turtles, frogs, and bony fishes, seem to have buffered against the devastation, with some forms growing exceptionally large fairly soon into the Paleogene Period. In the seas, many marine forms survived by either retreating to the depths or seeking shelter in refugia. On land, all major animal groups ultimately survived, including the dinosaurs: neornithine birds (the group to which our living forms belong) seem to have made it out against the other bird clades because they were ground-dwelling generalists. Mammals and other small tetrapods also survived by being generalists, as well as having burrowing or subsurface behaviors that allowed them to ride out the head & cold. Total recovery is estimated to have taken a few hundred thousand years as global temperatures climbed back to hothouse conditions, and the increased ash from the global wildfires appear to have fertilized the soil enough to spur a rapid growth in broadleaved angiosperms. The bolide impact gave birth to the first tropical rainforests (Carvalho, et al. 2021).

Eocene Marine Events

When? - ~41.5-37.71 Million Years Ago (Eocene Epoch)

Cause - Two little known extinction events, which appear to be linked to changes in ocean circulation (MacLeod, 2015). In the first at the Lutetian-Bartonian boundary, there is evidence of a brief period of global warming which increased runoff into the oceans, though there is as yet no evidence of increased CO2 outgassing (Intxauspe-Zubiaurre, et al. 2018). During the second at the Bartonian-Priabonian boundary, cooler waters from temperate zones flooded into warmer, tropical waters (MacLeod, 2015).

Victims - The first thermal maximum period appears to have wiped out a large diversity of gastropods, bivalves, and foraminifera, while the latter saw extinctions among foraminifera (including the last Nummulites), gastropods, bivalves, and echinoids (the urchin clade) (Less & Özcan, 2012). There are correlated extinctions on land during these times, particularly with the loss of various placental mammal groups (e.g. dinoceratans, hyopsodontids), and these may be tied to possible changes in sea levels (MacLeod, 2015).

Survivors - Beyond the losses of mollusks and plankton, most marine life appears to have made it through this period. Fossil records at European sites point to remarkable fish diversity in the following ages.

The "Grande Coupure"

When? - ~33.9-33.4 Million Years Ago (Eocene-Oligocene Epoch)

Cause - At the end of the Eocene Epoch, Antarctica had begun to acquire its permanent ice sheets following its separation from the other southern hemisphere continents and the beginnings of circumpolar ocean currents. This would have cascading effects down through the Cenozoic Era, as the Earth's average global climate would gradually shift towards cooler temperatures and more marked seasonality. Research supports a link between the increasingly rapid seasonal changes (including colder winters) and a high rate of extinctions (Ivany, et al. 2000). As sea-levels shrunk due to the expanding ice sheets, the Turgai Strait which had separated the European and greater Eurasian landmasses closed up, allowing different animals and plants to spread into new lands.

Victims - The name "Grande Coupure" means "Great Cut" and refers to a clear turnover in mammalian diversity in Europe, as their original faunas were replaced by greater-Eurasian migrants (Costa, et al. 2011). European artiodactyls or even-toed hoofed mammals (e.g. xiphodontids, amphimerycids), perissodactyls or odd-toed hoofed mammals (e.g. palaeotheres), early primates (e.g. adapids & omomyids), rodents, and lipotyphlans all suffered extinctions. There is evidence in North America of a significant turnover in smaller animals like reptiles and snails (Zanazzi, et al. 2007). In the oceans, there was a notable decline in the diversity of mollusks and other marine invertebrates (Ivany, et al. 2000), and several varieites of stem-whales (like Basilosaurus) went extinct likely as a result of poor catches.

Survivors - In Europe, the new mammals that tookover and diversified include rhinoceroses, anthracotheres & entelodonts (clades related to hippos), ancestral hedgehogs, and various rodent lineages (e.g. cricrtids, castorids), while several of the native European forms also survived relatively unscathred. In the oceans where was an increase in predatory shell-drilling snail species upon the surviving bivalves (Kelly & Hanson, et al. 1996). Both toothed and baleen whales evolved and diversified in astonishing variety.

Middle Miocene Disruption

When? - ~14 Million Years Ago

Cause - Throughout the Earth's history, Milankovitch cycles have shifted the axis and orbital eccentricity of the planet, and these shifts become noticeably pronounced when there are large areas of surface ice. It's widely believed that the most famous Ice Ages of the Quaternary Period were influenced by these cycles, but in earlier periods they had similar power (Halbourn, et al. 2005). During the preceding Neogene Period, such orbital changes shifted the circulation of ocean currents, which in-turn shifted heat transfer and spurred increased global cooling (Shevenell, et al. 2004). Further influence on ocean currents was the final closure of the Tethys, which once linked separate oceans (Hamon, et al. 2013).

Victims - Statistically, it has been argued that this was one of the largest extinction events of the Cenozoic Era (at least regarding regional extinctions) but so far concrete evidence has been lacking for many groups (MacLeod, 2015). The warm-living faunas of the northern hemisphere suffered die-offs, with losses among crocodylomorphs, turtles, and lizards (Böhme, 2003). Tundra ecosystems in Antarctica, some of the last major land communities on the continent, experienced their last breath (Lewis, et al. 2008). In the oceans, a number of foram genera went extinct, but there is little evidence for marine invertebrate extinctions.

Survivors - Those plants and animals that could migrated to more accomdating clines and adapted to the cooling of the climate, eventually giving rise in a few million years time to extensive grassland ecosystems and their herding and pack-hunting faunas.

Pliocene-Pleistocene Extinction

When? - ~3-2 Million Years Ago

Cause - As the Neogene passed into the Quaternary, and the dawn of the last Ice Age was approaching, there was a time of dynamic climatic and environmental changes. The Greenland Ice Sheet and the greater Arctic Polar Cap were forming, and the Antarctic glaciers had by then more-or-less formed. Much like in the Middle Miocene, there were shifts in ocean circulation and heat-exchange which affected the globe, and shallow coastal seas were diminished (Pimiento, et al. 2017). This was assisted by the closing of the Isthmus of Panama, which connected North and South America and further closed off ocean currents.

Victims - Marine invertebrates suffered tremendous losses, particularly in the Atlantic and Pacific Oceans as cooler conditions took over warm-adapted species. In the Caribbean Sea, coral reefs declined in spread and diversity after 2 million years ago (Budd, 2000). Recent work shows that large marine vertebrates experienced a dramatic extinction of species, with losses among whales, sharks, penguins, and sea turtles (Pimiento, et al. 2017). The connection of the western hemisphere facilitated a prolongued exchange of animals and plants between the two Americas, the Great American Biotic Interchange, which contributed to the eventual turnover (and decline) of native South American mammals by incoming North American species. In the newly-formed Afro-Eurasian landmass, there were also an increased extinction of proboscideans and other mammal groups (Cantalapiedra, et al. 2021).

Survivors - The shift to the Quaternary Ice Age promoted the evolution and adaptation of cold-adapted organisms, both on land and in the oceans. It's not until after this time that the great rorquals and other baleen whales evolved in response to the decline in larger marine predators (Slater, et al. 2017).

Late Pleistocene Megafaunal Extinctions

When? - Began roughly 80,000 Years Ago

Cause - Homo sapiens evolved across Africa by 300,000 Years Ago and there is evidence of periodic dispersals into Eurasia since that time, but it isn't until between 80 and 50,000 years ago that a wave of humans migrated from East Africa, into the Iranian Plateau, and out across the rest of the world. In contrast to other hominins like Neanderthals, early Homo sapiens lived in extensive social networks, could adapt remarkably fast to unique environments, bred fast, and relied on domestic dogs as hunting assistants. This made them very effective predators, and organisms which bred slow & few or were previously unaccustomed to a human presence (as was the case in regions like the Americas or Oceania) were the most vulnerable. Though it is true that there was a shift from glacial to interglacial conditions between the Pleistocene and Holocene epochs, so far as can be discerned from the patterns of extinction, it was human beings who ultimately wiped out the world's megafauna, the largest animals of a given ecosystem (Lemoine, et al. 2023).

Victims - Far from being considered a mass extinction, the decline consisted mainly of giant mammals, birds, and assorted non-avian reptiles. Wherever a firm human presence was established, within a few thousand years, giant marsupials, mammoths, mastodons, ground sloths, glyptodonts, horses, camels, bovids, bears, big cats, flightless birds, giant tortoises, and meiolaniid turtles went extinct. Islands were also particularly hard-hit and a tenuious link with the extinction of the New Zealand moa and Madagascan hippos, giant lemurs, and elephant birds to these earlier human dispersals, even though they occurrred within the last 2,000 years.

Survivors - Though there is even evidence of significant declines among mammals which survived into the present day (Bergman, et al. 2023), ultimately the factors in their favor were many. Some wild mammals dispersed alongside humans and settled where now-extinct forms roamed, like the moose (Meiri, et al. 2020). Others had co-evolved with humans or other hominins in the first place: hence the lack of die-offs in Africa or South Asia (Turvey, et al. 2021). Subsequent environmental changes unfolded in a world depleted of large animals, given their extensive influence on ecosystem engineering, and habitats that survived were a shell of their former selves (Svenning, et al. 2024).

Earth System Trends of the last 300 Years (Bryanmackinnon, CC BY-SA 4.0) Larger Image

Late Holocene Mass Extinction

When? - At least 500 Years Ago, but arguably earlier

Cause - As human populations increased and the need/desire for resources like food, minerals, and territory increased, the global environment took on greater and greater stresses. While there is ample evidence of positive and negative ecological management across human history, the last 500 years of global capitalism and colonialism have had a proportionally-devastating effect on habitat destruction, defaunation, and extinction. Particularly strong drivers include (and have included) intensive agriculture, overhunting, overfishing, overharvesting, the wildlife trade, pollution & runoff, added input from CO2 & methane emissions, and widespread ignorance or apathy about any of these things (Bradshaw, et al. 2021). The conditions being created on the Earth as you read this have been compared to and understood in the context of past mass extinctions, especially in light of recent revelations: for example, it has been demonstrated that ocean anoxia, acidification, and the formation of "dead zones" is occurring in parts of the ocean today (Gobler & Baumann, 2016; Mancini, et al. 2024).

Victims - Though the extent of calculated losses has been contentious and disputed (MacLeod, 2015), recent surveys show without a doubt that a significant defaunation and extinction of lineages is occurring across all biological lineages and that we are in the beginning pains of a mass extinction event (Cowie, et al. 2025; Ceballos & Ehrlich, 2018). A 2023 analysis estimated that ~1.97 million species could become threatened with extinction due to various human activities (Hochkirch, et al. 2023), while the contributions from anthropogenic (human-induced) climate change could further accelerate extinctions with a excess of 1.5°C global average temperatures (Urban, 2024). In the oceans, in freshwater systems, and on terrestrial habitats, life is being impacted. Surveys estimate that 40.7% of amphibians, 21.1% of non-avian reptiles, 13.6% of birds, and 25.4% of mammals are threatened with extinction (Cox, et al. 2022). >40% of insects - usually outliers in past extinction events - are at risk (Sánchez-Bayo & Wyckhuys, 2019). 26% of freshwater fishes & 30% of freshwater crayfish, shrimp, and crabs are at risk (Sayer, et al. 2025). 12.7% of marine fishes are at risk (Loiseau, et al. 2024), with sharks, rays, and large bony fishes especially depleted. 44% of coral species are at risk (IUCN, 2024) and complex reef ecosystems are vulnerable to total collapse. Among mollusks, it is estimated that between 7.5-13% of described snails and clams have been wiped out since 1500 (Régnier, et al. 2017); land snails & freshwater bivalves have been especially vulnerable. Roughly 39% of vascular plants (angiosperms, gymnosperms, ferns, & lycopods) are at risk (Lughadha, et al. 2020). Overall, it has been estimated that ~30% of known species have either been killed off or threatened with extinction within the last 500 years (Isbell, et al. 2022).

Survivors - A 2023 study reported that "49% and 3% of species currently remain stable or are increasing, respectively" and that this survivability shows "a tendency to expand towards temperate climates" (Finn, et al. 2023). That said, the sheer scale of biological destruction and its multifacited human causes will almost certainly have uncalculated consequences and affect these numbers. We must recognize one key fact: this is the only extinction event in Earth's history whose definite cause (humanity) is also its solution. The very same power that has allowed humans to change the surface of the planet is the very same that can end the devastation and safeguard biodiversity (including our own wellbeing). We need only choose to do so.

Closing Thoughts

This has been a worthwhile exercise, and I have learned a lot in researching and preparing this summary of extinction events, and I have tried to be comprehensive in scope (however imperfect I might have been).

I have gathered a few key lessons in outlining the history of extinctions, and here's what I've taken to heart:

Plankton and marine invertebrates have been the backbone of understanding extinction events. In fact, their very nature as common, easily-preserved fossils is what allowed geologists and paleontologists to create the time scale of the Earth, whose major divisions have been marked by the loss or appearance of specific invertebrate fossils. They are particularly vulnerable to extinctions, and many major lineages have succumbed to death, thus altering the foundations of oceanic food webs.

Plate tectonics have been one of the major driving forces of extinction events. While the hypothesis of a "cyclicity of mass extinctions" due to cosmological phenomena has been heavily debated, there is a clear correlation between the movements of the continents and extinction events. Volcanic eruptions are usually the result of tectonic activities, and it is through them that significant ocean anoxia and climatic warming/cooling occur. As well, the closing or opening of isthmi due to continental drift have helped regulate the positions of oceanic currents. All these phenomena have been involved in the majority of the Earth's extinction events. Bolide impacts, for all their popularity as research subjects, have only truly been confirmed as harbingers of extinction for the End Cretaceous Event.

Life is resiliant, but not all-powerful. Though our fellow organisms have survived all the previously described events, it is clear that today's sample is the exception: >99% of all the species that have ever lived are now extinct. The threats to their wellbeing have been many and without warning, and the loss of even a few species or clades has upset whole ecosystems. This is all the more reason why we must do everything we can to preserve and cherish the life that still exists on the Earth. The Holocene Mass Extinction has been particularly severe and all-encompasing, and it is a fact that diversity enhances survivability in the long-term. The Great Dying took millions of years for life to fully recover, and we're basically playing out this extinction event on a significantly shorter timescale, with no clear guarantee of the outcome; we are truly living in unprecedented times. If we want to have a future worth living, we must embrace this understanding of past extinction events and rebuild our connections with life on Earth.

Special thanks to @albertonykus & @otussketching for productive conversations on the nature of extinction events.

Book Citations

Michael J. Benton. Extinctions (Thames & Hudson, 2023)

Peter Brannen. The Ends of the World (Ecco, HarperCollins, 2017)

Norman MacLeod. The Great Extinctions (Firefly Books, 2015)

Paper Citations

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Madelaine Böhme, 2003. The Miocene Climatic Optimum: evidence from ectothermic vertebrates of Central Europe (Palaeo)

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Corey J. A. Bradshaw, et al. 2021. Underestimating the Challenges of Avoiding a Ghastly Future (Frontiers)

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Robert H. Cowie, et al. 2025. Denying that we may be experiencing the start of the Sixth Mass Extinction paves the way for it to happen (Trends in Ecology & Evolution)

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Yong Du, et al. 2022. A massive magmatic degassing event drove the Late Smithian Thermal Maximum and Smithian–Spathian boundary mass extinction (Global and Planetary Change)

Catherine Finn, et al. 2023. More losers than winners: investigating Anthropocene defaunation through the diversity of population trends (Biological Reviews)

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IUCN Red List Press Release, 2024. Over 40% of coral species face extinction (IUCN)

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Adam R. Lewis, et al. 2008. Mid-Miocene cooling and the extinction of tundra in continental Antarctica (PNAS)

Nicolas Loiseau, et al. 2024. Inferring the extinction risk of marine fish to inform global conservation priorities (PLOS Biology)

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Maria Rose Petrizzo, et al. 2022. Biotic and Paleoceanographic Changes Across the Late Cretaceous Oceanic Anoxic Event 2 in the Southern High Latitudes (IODP Sites U1513 and U1516, SE Indian Ocean) (Paleoceanography and Paleoclimatology)

Catalina Pimiento, et al. 2017. The Pliocene marine megafauna extinction and its impact on functional diversity (Nature Ecology and Evolution)

Claire Régnier, et al. 2017. Measuring the Sixth Extinction: what do mollusks tell us? (The Nautilus)

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Douglas S. Robertson, et al. 2013. K-Pg extinction: Reevaluation of the heat-fire hypothesis (JGR Biogeosciences)

Francisco Sánchez-Bayo & Kris A.G. Wyckhuys, 2019. Worldwide decline of the entomofauna: A review of its drivers (Biological Conservation)

Catherine A. Sayer, et al. 2025. One-quarter of freshwater fauna threatened with extinction (Nature)

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Zhen Xu, et al. 2025. Early Triassic super-greenhouse climate driven by vegetation collapse (Nature Communications)

Mark. C. Urban, 2024. Climate change extinctions (Science)

Alessandro Zanazzi, et al. 2007. Large temperature drop across the Eocene–Oligocene transition in central North America (Nature)