Mass extinction was not a simple and quick process caused by a single factor. Some animals disappeared even before the end of the Cretaceous; others were already in clear decline, with their numbers and diversity decreasing, but many ceased to exist precisely at the K/T boundary. What were the actual patterns of mass extinction?
Many groups of animals survived the mass extinction
Survival of Crocodiles
The asteroid hypothesis cannot explain why only dinosaurs went extinct, while their closest evolutionary relatives, crocodiles, and birds (also archosaurs), continued to live and thrive as if nothing had happened.
All five families of Crocodyliforms survived the mass extinction and continued to exist into the Paleocene. Large terrestrial crocodilian predators, the Sebecidae, which reached lengths of up to 6 meters, existed from the late Cretaceous to the Miocene–Pliocene boundary. Yet proponents of the catastrophe claim that all animals weighing more than 25 kg went extinct. Another inconsistency.
It has come to the point where the survival of freshwater crocodiles is explained by the fact that their main food source was plant detritus, which entered the water in sufficient quantities even during the critical years of the catastrophe. This is blatant manipulation. Herbivorous crocodiles that appeared in the early Jurassic always constituted only a small group. They disappeared precisely in the Cretaceous-Paleogene extinction. Carnivorous species were the ones that managed to survive and evolve into the extremely dangerous predators of our era.
Even more fantastical is the suggestion that crocodiles fed on the carcasses of extinct animals. How long does a carcass remain in a state that a scavenger can still feed on it? Darkness covered the earth. “Photosynthesis ceased for almost 2 years after the impact.” And then what? Did crocodiles emerge from the rivers and wander through deserts and burned forests, collecting remnants of flesh? Did mammals evolve and populate the earth in those 2 years (instead of millions of years in the Paleocene) to provide food for crocodiles? One can endlessly create false explanations, but it is already clear that crocodiles and many other freshwater inhabitants survived the mass extinction, not because they fed on detritus.
Other Surviving Animals
Many groups of animals crossed the boundary between the Mesozoic and Cenozoic eras without noticing it. None of the deadly mechanisms of destruction killed them. Semiaquatic reptiles, the Choristoderes, successfully survived a series of catastrophic events and the “terrible asteroid winter.” They became extinct only in the Miocene (Evans et al., 2005). Turtles were also almost unaffected by the asteroid impact. All six families from the late Cretaceous period, including marine turtles, survived into the Paleogene and still have living representatives today (Novacek, 1999). More primitive lizards, highly specialized snakes, and other reptiles also survived.
By the Maastrichtian, only two families of pterosaurs remained. The asteroid may have influenced their extinction. But it is more likely that they disappeared due to competition with birds, which completely took over the skies.
Unlike dinosaurs, all major lines of mammals, including monotremes (egg-laying mammals), multituberculates, marsupials, and placentals, dryolestoideans, and gondwanatheres, survived the K/T extinction event, despite the loss and extinction of some species (Gelfo and Pascual 2001). Birds, fish, and insects survived and did not undergo significant changes.
Relatives of ammonites—nautiluses, other cephalopods such as octopuses and squids, as well as corals—also survived the mass extinction.
Large planktonic forms of foraminifera went extinct. At the same time, small cosmopolitan forms and benthic foraminifera underwent minor changes and survived the mass extinction event. This clearly indicates the inconsistency of the global ocean acidification hypothesis.
Plants Were Not Affected in Many Regions of the Earth
Marine phytoplankton suffered greatly, but terrestrial vegetation simply did not notice this catastrophe, even though the reduction in sunlight should have primarily killed photosynthesizing plants, not animals.
However, for example, when studying fossil pollen and spores from the late Cretaceous period in New Zealand, paleobiologist Kirk Johnson found no evidence of any “biotic upheaval.” Similar research conducted (Askin et al., 1994; Johnson, 1993) for Antarctica also did not record any radical changes in vegetation. Even for the benchmark Hell Creek Formation in eastern Montana (Stromberg et al. 1998), fossil pollen indicated only a gradual shift in the region’s flora “from more open to more closed and moist habitats.” In the north, in Canada, palynomorphs (Sweet et al., 1993) also indicated changes that were more gradual.
A possible explanation for this is that the effect of atmospheric dusting was stronger at lower latitudes and closer to the impact site, such as in North America. Plants unaccustomed to seasonal changes in the Sun’s position were particularly hard hit. Plants at higher latitudes, accustomed to seasonal reductions in light, were able to survive, as were the animals that fed on them.
The gradual mass extinction
Extinction of Marine Organisms
The role of cephalopod mollusks began to decline after the Cenomanian-Turonian event (91.5 ± 8.6 million years ago). This was long before the end of the Cretaceous. The number of ammonite families decreased, and belemnites almost disappeared, giving way to squids and fish. Large bony and cartilaginous fish preyed upon cephalopods, and marine reptiles topped the food pyramid. Up to a quarter of cartilaginous fish genera perished, while bony fish were almost unaffected. About 27% of marine vertebrates went extinct. On land, families of megalosaurs and stegosaurs disappeared during this time.
Ichthyosaurs had already gone extinct in the early Cretaceous period, and only rare and declining species of plesiosaurs remained. Mosasaurs dominated the seas, reaching their peak in the late Maastrichtian. However, for example, in the Gulf of Mexico, their diversity (as well as that of other large vertebrates) sharply declined several million years before the asteroid impact.
Planktonic organisms went extinct over a period of 10,000 years (Perch-Neilsen et al., 1982). Various marine benthic inhabitants and filter feeders went extinct, but benthic predators and detritivores suffered little. The nature of the extinction of many marine groups shows a gradual decline throughout the late Cretaceous (Kauffman, 1984).
Mass Extinction Began Before the End of the Cretaceous
A—Late Cretaceous non-bird dinosaur finds (asterisks). B—record of non-bird dinosaurs at the K/T boundary (asterisks) and new sites (question marks) possibly preserving a similar record. Shaded areas represent marine environments covering continental plates. (After Archibald 1996.)
The consequences of the asteroid impact should have been not only large-scale but also abrupt. However, all asteroid hypotheses (impact theories), including astronomical ones, do not match the supposed duration of the mass extinction. Many groups of animals began to die out long before the end of the Cretaceous. There is also evidence of the existence of Paleogene (i.e., post-Cretaceous) dinosaurs, mosasaurs, and other animals considered extinct at the Cretaceous-Paleogene boundary.
Available data indicate that dinosaurs began to go extinct even before the asteroid impact (“Extinction-The Asteroid-impact,” 2016). Sloan et al. (1986) argued that the disappearance of dinosaurs likely spanned a period of at least seven million years.
To date, researchers have found very few dinosaur remains from even a few thousand years before the collision. Mass extinction, it seems, occurred gradually and affected some species more than others.
A study published in the journal Nature Communications provides new evidence of a global decline in dinosaur groups, driven by ecological factors, long before the asteroid impact.
In the last 300,000 years of the Cretaceous period in Montana, the mammal community (Protungulatum), which is characteristic of the Paleocene, increased. As their numbers grew, the number of dinosaurs decreased until they disappeared completely (Van Valen and Sloan, 1977; Sloan et al., 1986).
Decline in Dinosaur Diversity
Dr. Fabien Condamine and colleagues studied the dynamics of speciation and extinction of six key dinosaur families throughout the Cretaceous period. They found that their diversity began to decline as early as 76 million years ago. “Their rates of extinction rose, and in some cases, the rate of origin of new species dropped off.”
Some particularly successful dinosaur families, such as hadrosaurs, may have displaced several other herbivores. This led to a reduction in the diversity of these dinosaurs.
Dr. Condamine and his colleagues used Bayesian modeling techniques to account for various uncertainties, such as incomplete fossil records, uncertainties in fossil dating, and uncertainties regarding evolutionary models.
“In all cases, we found evidence for the decline prior to the bolide impact,” said Dr. Guillaume Guinot, a researcher at the Institut des Sciences de l’Evolution de Montpellier and CNRS.
The last ten million years of the Mesozoic saw a gradual decrease in the number and biodiversity of dinosaurs (Sullivan, 1998).
Dinosaur Faunas of the Late Cretaceous
Comparison of the Campanian Dinosaur Park Formation (42 valid species) with the late Maastrichtian Hell Creek Formation (25–33 valid species) shows a high species richness in the earlier formation.
A range of data supports a Campanian-Maastrichtian decline in the richness of ornithischians (Barrett et al., 2009; Upchurch et al., 2011) and theropods (Barrett et al., 2009), but not sauropodomorphs. Because only one genus, Alamosaurus, is known from the Late Cretaceous of North America. The diversity of ornithischians remains low throughout the late Maastrichtian.
Let’s consider the two most well-known dinosaur faunas of the Late Cretaceous of the Western Interior Seaway. From the Judith River fauna (Campanian, 79–75.3) to the Hell Creek fauna (Maastrichtian, 72.1–66), the absolute species diversity of dinosaurs decreased by 40% (from 32 to 19). The most significant decline was in large, likely herding ceratopsids (from five to two) and hadrosaurids (from seven to two). The decline mainly affected the more common ornithischians rather than the much rarer saurischians (Archibald 1996).
People usually raise two objections against this data. First, the Judith River fauna is better studied; hence, more species were found. This is not true. The most studied is actually the Hell Creek Formation fauna (Sheehan et al., 1991; Pearson et al., 2002; Fastovsky & Sheehan, 2005). This is evidenced by the number of publications dedicated to it compared to those on the Judith River fauna.
Second, the Judith River fauna is abnormally rich, hence more finds. This seems more like an attempt to wriggle out of an uncomfortable position. Abnormally rich? Are there any proofs? No? Then one could just as well claim that the Hell Creek fauna is abnormally rich and that dinosaurs had already gone extinct elsewhere by that time. Such a statement is mere verbal juggling and not worthy of serious consideration.
Herbivorous Dinosaurs Extinct First
The lack of diversity among large herbivorous dinosaurs, which predators preyed upon, weakened the dinosaur food chain. Non-avian dinosaurs became vulnerable to changes, and the collapse of the food chain destroyed the dinosaur kingdom species by species.
Herbivorous species tend to disappear first, making late dinosaur communities unstable and prone to collapse if environmental conditions worsened. The overall climate became cooler, complicating life for dinosaurs dependent on warm temperatures. Long-lived dinosaur species were more prone to extinction, reflecting their difficulty in adapting to changing planetary conditions. The decline in herbivorous animals made ecosystems unstable.1
S. Brusatte from the University of Edinburgh states in one of his articles that there was no decline in dinosaur diversity at the end of the Cretaceous period. However, he provides data showing that this did not apply to all dinosaurs. Hadrosaurs and ceratopsians, two key groups of large herbivores in these ecosystems, experienced a decline. The disappearance of these forms directly affected the stability of terrestrial ecosystems, making their communities more vulnerable to cascading extinctions.2
Research by paleobiologist J. Mitchell and his colleagues from the University of Chicago showed that the decline in hadrosaur and ceratopsian diversity, and more importantly, the decline in community diversity (where the same species began to dominate), caused cascading transformations in trophic chains. These changes reduced the resilience of ecosystems to external impacts. Consequently, the impact of a celestial body (or powerful volcanic eruptions) occurred during a period of significant ecosystem restructuring, which amplified its effect.
Evidence of Declining Species Diversity in Hell Creek
The vast volume of exposed fossil-bearing sedimentary layers in the Hell Creek Formation of late Cretaceous North America, which evidently had favorable conditions for fossil preservation, explains the significant number of finds. Researchers have not found similar conditions for the end of the Cretaceous elsewhere in the world, making comparisons impossible. However, even here, the youngest dinosaur found in this section died about a thousand years before the meteorite impact. This also supports the idea of the asynchronous extinction of animals in different geographic regions.
It is also worth noting the rarity of dinosaur fossils in the upper 3 meters of the Hell Creek Formation. This may indicate that the dinosaur population had already declined before the impact of the bolide. However, ceratopsid fossils found in the argillite layers within the “gap,” about 15 cm below the K/Pg boundary, indicate that they had not yet gone extinct.
Sheehan and others (1991) used statistical analysis to argue that the composition of the Hell Creek dinosaur fauna was unchanged at the end of the Cretaceous. Hurlbert and Archibald (1995) demonstrated that this analysis had flaws in its application, preventing any reliable conclusions from being drawn. It’s important to note that Sheehan et al. (1991) studied fossils mostly identified at the family level but drew conclusions about species numbers.
Decline in the Rate of Species Emergence
The decrease in diversity is not due to an increase in extinction rates but rather to a decline in the rate of emergence of new families.
Calculating the extinction and speciation rates for each time interval separately allowed the conclusion: “The decline of dinosaurs lasted tens of millions of years before their final extinction” (Sakamoto M. et al., 2016).3
Bristol paleontologists (including Michael Benton) published the results of their research (2016), which confirmed a long-term decline among all dinosaurs and in all 3 dinosaur subclades: Ornithischia, Sauropodomorpha, and Theropoda, where the rate of speciation slowed over time and was eventually exceeded by the rate of extinction tens of millions of years before the K/Pg boundary.
Analysis of Dinosaur Eggshells
Research on over 1000 dinosaur eggshell samples in central China revealed only three types of eggshells in deposits from 68 to 66 million years ago (just before the K/T boundary), indicating consistently low dinosaur biodiversity during this period. This low diversity persisted in central China for two million years before the mass extinction, suggesting that dinosaur populations were likely declining globally before their extinction.4
Decline in Marine and Terrestrial Biodiversity
Ammonites went extinct 100,000 years before the end of the Cretaceous. Of the 28 ammonite species, 6 became extinct significantly earlier, 3 to 10 either disappeared before the end (possibly due to major regression) or survived almost to the K/T boundary. Only 12 species survived to the Cretaceous-Tertiary boundary (Marshall and Ward, 1996). The transition of ammonites to heteromorphic forms before extinction indicates some instability in this group long before the K/Pg boundary.
63% of bivalve mollusks became extinct during the last ten million years of the Cretaceous period (Raup and Jablonski 1993). Rudists apparently went extinct somewhat earlier than this boundary.
Inoceramoidea (another group of Cretaceous bivalve mollusks) asynchronously went extinct about a million years before the end of the Cretaceous period, in the middle Maastrichtian (MacLeod et al. 1996). By the time of the K-Pg boundary and the possible asteroid impact, 66% of foraminifera species had already disappeared.
The described ten-centimeter zone within the boundary layers at Zumaya (Spain) just below the Cretaceous-Tertiary boundary shows that the extinction of fauna was already massive among both nanoplankton and globotruncanids. Before the asteroid impact! (Percival and Fischer).
Asynchronous Mass Extinction Processes in Different Places
Additionally, dinosaurs disappeared around the Cretaceous-Paleogene boundary at different times in various geographic regions. For example, researchers found the last dinosaur eggs in younger deposits in China than in France. In Canada, they disappeared 150,000 years before the Paleogene. Mosasaurs in the same area existed right up to the boundary.
Long before the end of the Cretaceous, extinction processes began for other Mesozoic reptiles: ichthyosaurs and pterosaurs. Overall, the process of replacing Mesozoic organisms with Cenozoic ones was not rapid or instantaneous, but gradual. Estimates of the extinction period vary and range from several thousand years to 2 million years. It is too long a period for a single catastrophic event. Groups of new animals and plants with a Cenozoic appearance began to emerge tens of millions of years before the end of the Mesozoic. For example, pollinating insects, angiosperms, lizards, snakes, and mammals. It is clear that it is impossible to explain all stages of this transition with a single cosmic catastrophe at the very end of the Cretaceous.
Did Paleogene dinosaurs exist?
The hypotheses presented below lack both well-argued refutations and sufficient evidence. These hypotheses are worth considering, but not treat as established fact.
There are individual finds from China and North America dating back 64.5 million years. Their dates are not undisputed and are the subject of scientific discussions. It is possible that rare, isolated populations of dinosaurs survived for some time in the Paleogene. The most reliable specimen is a hadrosaur femur from New Mexico.
Dinosaurs from Ojo Alamo and Animas
The Ojo Alamo (“Poplar Eye” in Spanish) and Animas formations, which date back to the very beginning of the Paleogene (early Paleocene) at the junction of Colorado and New Mexico in North America, contain the remains of duck-billed hadrosaurs.
Paleomagnetic and independently palynological analyses confirm the Cenozoic dating of the find. This is the right femur of a hadrosaur, 1.3 meters long. Direct dating of the bone showed that its age is 64.8 ± 0.9 million years. This means, according to James E. Fassett of the U.S. Geological Survey, that dinosaurs still existed in the southern United States 500,000 years after the end of the Cretaceous period. Geomagnetic analysis reveals that water flow did not transport the fossils, and they did not get reburied in Paleogene deposits hundreds of thousands or millions of years after the Cretaceous catastrophe. The surface of the bone has no signs of transportation. Pollen and fossilized leaves indicate that this rock belongs to the Paleogene.
Above. Partially excavated bone assemblage before jacketing and removal. At the bottom. A drawing of the hadrosaur skeleton shows the positions of bones (shaded) identified in the bone assemblage in photograph above. The length of the skeleton is 13.5 m. The figure is from Fassett et al. (2002).
The most significant dinosaur bone assemblage, definitively found in situ and discovered in the Ojo Alamo Sandstone in the San Juan Basin, contains 34 skeletal elements of a single hadrosaur. These bones belonged to an animal that lived and died in the Ojo Alamo Sandstone in the early Paleocene. Hunt and Lucas described this bone assemblage in 1991.
Isotopic analysis of dinosaur fossils shows that the content of various uranium and rare earth element isotopes is exactly the same as in other early Paleocene deposits, not Cretaceous.
Reeside (1924) also reports finds of dinosaur bones (specifically, ceratopsian dinosaur fossils) in the early Paleocene Animas Formation in the northern part of the San Juan Basin.
Paleontologists from the University of Alberta in Canada, led by Larry Heaman, conducted a preliminary geochronological analysis of the femur of Alamosaurus sanjuanensis, found in the Ojo Alamo Sandstone in the southern part of the San Juan Basin. They concluded that its age is, according to various estimates, only 64–65.1 million years. This means that this species of herbivorous giants could have lived for at least several hundred thousand years after the mass extinction.
The Ojo Alamo Sandstones are only 3000 km from the Chicxulub crater, which should have quickly wiped out dinosaurs in this area.
Other Paleogene Findings
Some evidence suggests that some dinosaurs (theropods and others) existed in India for some time at the beginning of the Paleogene after their extinction elsewhere.
Other evidence of Cenozoic dinosaurs includes dinosaur remains from Hell Creek, found at a height of 1.3 m above the Cretaceous-Paleogene boundary. This corresponds to a span of 40,000 years.
Researchers discovered fossils of the Mosasauridae family in the Hornerstown Formation in the eastern United States in early Paleocene deposits. However, the possibility of reworking from earlier layers is plausible.
Discovered in China’s Fangou Formation in 1995, the fossils of Qinornis paleocenica date back 61 million years. This makes Qinornis the first and only proven non-avian ornithurine (bird-tailed) to survive into the Paleocene. This is a true relic of the dinosaur era.
An extremely unlikely event—that the undisputed remains of several non-avian dinosaurs will be discovered later than the K/T boundary—can still happen. However, even in this case, the existing data still convincingly indicate that the ultimate cause of the extinction of non-avian dinosaurs occurred at or before the K/T boundary. Some organisms can persist in isolated refuges even after the vast majority of their species has disappeared. However, this, accordingly, rules out an instantaneous catastrophe as the cause of extinction.
Rapid Ecosystem Recovery after Mass Extinction
Quick Return of Life to the Chicxulub Crater
In 2016, researchers conducted underwater drilling in the central part of the Chicxulub crater (Gulf of Mexico). It was part of the International Ocean Discovery Program and International Continental Drilling Program. The study of a 76-centimeter layer of sediment, formed immediately after the impact, showed that life (in the form of foraminifera and small crawling and burrowing bottom-dwelling animals) returned to the crater very quickly—possibly within just a few years. Clear traces of crawling and burrowing are already present in the upper 20 cm of the transitional layer. The entire layer, including the upper part with traces of crawling, formed in less than six years.
Thus, the discovered traces of crawling and burrowing indicate that some bottom-dwelling life was thriving in the crater just a few years after the impact. The traces appeared while the sediment was still very soft, which means they formed during or immediately after the transitional layer formed.
Proximity to the impact site did not delay recovery, and therefore, there were no impact-related ecological barriers to the resumption of a highly productive ecosystem.
The obtained results do not support the hypothesis that the meteorite poisoned the surrounding waters or otherwise delayed the recovery of ecosystems in the immediate vicinity of the epicenter. The community of planktonic organisms living in the water column above the crater after the catastrophe was quite healthy and highly productive (Lowery et al., 2018).5
Species Survival and Fossil Evidence
Gerta Keller from Princeton University and Thierry Addate from the University of Lausanne (Switzerland) conducted research at numerous sites in Mexico, focusing particularly on a 30-foot layer lying directly below the iridium layer and deposited over 300,000 years.
By analyzing fossil evidence in this small section, they counted 52 species directly below the iridium layer. Then they counted the number of species above it. The result was the same: 52. “Not a single species disappeared due to the Chicxulub impact,” Keller asserts. She also points out that the deposits above the iridium layer do not display any signs of displacement or mixing from earthquakes or tsunamis. Instead, they accumulated gradually over a very long period of time.
Isn’t it ironic—the terrible asteroid wiped out dinosaurs worldwide but couldn’t destroy a single species right at its impact site?
Absence of Mass Dinosaur Burials
If the extinction of dinosaurs, according to the “single impact” theory, was caused by the fall of a giant asteroid, then huge accumulations of dinosaur remains dating to the Cretaceous-Paleogene boundary should be found worldwide, specifically in the iridium-rich sediment layer. Researchers have not discovered any mass dinosaur graves from this period anywhere. There are no reports of even a single dinosaur fossil found directly in the iridium layer.
It is evident that in a catastrophe, dinosaurs, like other animals, would have died quickly. Their mass remains should be found directly in the impact layer, which has global distribution, i.e., in the clay formed from dust settling from the atmosphere or lying directly on it. However, so far, there have been no finds of dinosaur remains in this layer at all.
There are many dinosaur graves formed as a result of natural disasters—floods, volcanic eruptions, and the like. For example, catastrophic volcanic eruptions have left mass graves of animals, including terrestrial dinosaurs—the Jehol Biota graves of exceptional preservation.
But there are no traces of the “largest cataclysm.” The puzzle doesn’t fit.
In other words, the meteorite hypothesis of mass extinction remains unproven until researchers find mass dinosaur graves directly in the impact layer, despite any modeling of the effects of the meteorite impact.
What are the actual causes of dinosaur extinction?
There are many hypotheses about the extinction of dinosaurs. However, there are two main arguments in favor of the idea that the K/T extinction was the result of multiple causes rather than just one. First, there are no uniform patterns of extinction and survival at the K/T boundary. The large phylogenetic and ecological differences in survival strongly suggest that one cause is not enough. However, together, volcanism, marine regression, and asteroid impacts can explain the differentiated survival patterns. Second, some events proposed as the sole cause of the K/T extinctions are either not associated with other mass extinctions or have occurred at other times without causing mass extinction.
For example, large asteroid craters do not coincide with any mass extinctions. But all five major mass extinctions correlate with global marine regressions (Hallam and Wignall, 1997). And the extinctions at the end of the Jurassic and the end of the Cretaceous are associated with the largest marine regressions since the beginning of the Triassic period about 250 million years ago (Smith et al., 1994).
Complex Causes of Mass Extinction
What then caused the disappearance of the dinosaurs? Obviously, it was a whole complex of reasons that acted gradually and led to a change in biota. These include:
- Changes in ocean currents due to continental drift;
- Some climatic changes;
- The disappearance of shallow marine basins due to a sharp drop in sea level in the Maastrichtian age;
- Increased volcanic activity and the release of ash and carbon dioxide during eruptions;
- Evolutionary problems of some groups of organisms;
- And a number of other internal processes in the Earth’s biosphere.
It is clear that during this period, the environment underwent radical changes, and many species had to compete for scarce resources. Each of these causes may have been necessary for these extinctions, but none was sufficient to cause the clearly differentiated pattern of extinctions.
Many vertebrate species quickly perished due to the loss of a greater number of plant species and the overall reduction in biomass. This was due to the negative impact on ecosystems on land, which were already under significant stress, exacerbated by worsening environmental conditions. The last of the large herbivorous non-avian dinosaurs disappeared. Shortly after, the remaining predatory non-avian dinosaurs followed. Larger species disappeared first. In some parts of the world, large reptiles may have lingered for a while, but eventually, they too vanished.
Decrease in Dinosaur Species Diversity
As any group of organisms evolves, new species constantly emerge while old ones disappear. This is because extinction is a natural part of the evolutionary process. Statistical analysis of fossil finds shows that dinosaurs were going extinct over several million years before the end of the Cretaceous period. The decrease in dinosaur species diversity occurred at a relatively constant rate in the late Cretaceous. However, the emergence of new species soon failed to compensate for the disappearance of existing ones.
In Canada, 75 million years ago, there were 35 species of dinosaurs; 68 million years ago—25 species; and 65 million years ago (the end of the Cretaceous)—only 6 species. The last 7 million years of the Cretaceous period (the Maastrichtian age) saw a widespread reduction in the diversity of dinosaurs and other archosaurs. The previous species went extinct at the same rate as before, but no new ones appeared to replace them. And this continued until the complete disappearance of the group.
Conclusion
Dinosaurs went extinct—this is a fact.
An asteroid hit Earth—this is a fact.
The Deccan Traps erupted around the same time—this is a fact.
Everything else is our interpretation and modeling of events to understand what happened.6 Even the iridium layer at the K/T boundary appears to have different origins at different locations.
Researchers studied the broader context of mass extinction, including half a million years before and after the K/T boundary. They based their studies on fossil clusters, climate changes, and sea level changes. All of them show significant changes preceding the K/T boundary (see review in Keller, 2001).
At the end of the Maastrichtian, the last seven genera of dinosaurs became extinct, but the rate of extinction did not stand out in any way. Dinosaurs had fewer offspring and aged faster—typical ecological responses to declining ecosystem productivity. E.g., less energy exchange and thus less food, and increased environmental mortality risk (including a higher likelihood of dying at any age).
Analysis of trace diversity shows that by this time, mammals had already constituted a large share of the diversity among terrestrial vertebrates, rather than dinosaurs. (Bone material does not allow for such a complete picture).
The volcanism of the Deccan Traps added stress to already vulnerable global ecosystems. Perhaps it wasn’t enough to cause a mass extinction on its own, but it was at least strong enough to make life a little more vulnerable to the effects of the asteroid impact that completed the job. After the meteorite impact 66 million years ago, the affected species were unable to recover and eventually disappeared from the face of the Earth.
Mass Extinction Cannot Have a Single Cause
Paleontologists have long argued that the fossil record does not support a single cause for mass extinction and, therefore, scenarios involving multiple events, including strong volcanism, rapid climate changes, and sea level changes (Archibald, 1996; Keller, 1996; MacLeod et al., 1997) and more than one impact (Keller et al., 1997; Keller et al., 2002; Keller, 2001), are preferable.
Many people want to find a single “main” cause, common to all extinctions. This would be very convenient (we love it when everything is simple and neat), but the world is not arranged that way.
It is unlikely that the simplest explanations are suitable for the inherently very complex phenomena of biosphere changes. Neither the Chicxulub asteroid nor the Deccan Traps are single-factor causes of the mass extinction at the K/T boundary. A significant change in the Earth’s biosphere could have resulted only from a complex of very complicated, multi-stage processes, generating a whole network of feedback loops (both positive and negative), cumulative effects, and the like. This includes the aforementioned “catastrophic” and climatic events and many other paleoecological phenomena acting together (after Kyrill Eskov).
- F.L. Condamine et al. 2021. Dinosaur biodiversity declined well before the asteroid impact, influenced by ecological and environmental pressures. ↩︎
- Stephen L. Brusatte et al., 2014. The extinction of the dinosaurs ↩︎
- Sakamoto M., et al., 2016. Dinosaurs in decline tens of millions of years before their final extinction. ↩︎
- Fei Han et al. 2022. Low dinosaur biodiversity in central China 2 million years prior to the end-Cretaceous mass extinction. ↩︎
- Lowery et al., 2018. Rapid recovery of life at ground zero of the end-Cretaceous mass extinction. ↩︎
- Archibald, J. D., and David E. Fastovsky. “Dinosaur extinction.” The Dinosauria: Second Edition (2004). ↩︎
