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In the geologic record, the K-Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium, which is rare in the Earth's crust but abundant in asteroids.
It is generally believed that the K-Pg extinction was triggered by a massive comet/asteroid impact and its catastrophic effects on the global environment, including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis.[5] The impact hypothesis was bolstered by the discovery of the 180-kilometre-wide (112 mi) Chicxulub crater in the Gulf of Mexico in the late 1970s,[6] which provided conclusive evidence that the K–Pg boundary clay represented debris from an asteroid impact.[7] The fact that the extinctions occurred at the same time as the impact provides strong evidence that the K–Pg extinction was caused by the asteroid.[7] However, some scientists maintain the extinction was caused or exacerbated by other factors, such as volcanic eruptions,[8] climate change, and/or sea level change.
A wide range of species perished in the K–Pg extinction. The most well-known victims are the non-avian dinosaurs. However, the extinction also hit other terrestrial organisms, including mammals, pterosaurs, birds,[9] lizards,[10] insects,[11] and plants.[12] In the oceans, the K–Pg extinction devastated the giant marine lizards (Mosasauridae), plesiosaurs, fish,[13] sharks, mollusks (especially ammonites) and many species of plankton. It is estimated that 75% or more of all species on Earth vanished.[14] Yet the devastation caused by the extinction also provided evolutionary opportunities. In the wake of the extinction, many groups underwent remarkable adaptive radiations — a sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches resulting from the event. Mammals in particular diversified in the Paleogene,[15] producing new forms such as horses, whales, bats, and primates. Birds,[16] fish[17] and perhaps lizards[10] also radiated.
Extinction patterns
The blue graph shows the apparent percentage (not the absolute number) of marine animal genera
becoming extinct during any given time interval. It does not represent
all marine species, just those that are readily fossilized. The labels
of the "Big Five" extinction events are clickable hyperlinks; see Extinction event for more details. (source and image info)
The K–Pg extinction was a global event. The event appears to have hit all continents at the same time. Dinosaurs, for example, are known from the Maastrichtian of North America, Europe, Asia, Africa, South America and Antarctica,[19] but are unknown from the Cenozoic anywhere in the world. Similarly, fossil pollen show devastation of the plant communities in areas as far flung as New Mexico, Alaska, China, and New Zealand.[12] The event also affected all seas and oceans. Widespread groups such as mosasaurs and ammonites disappeared around the world. Furthermore, the extinctions occurred at the same time on land and in the sea.
The fossil record shows that the tempo of the K–Pg extinction was extremely rapid, occurring on a scale of thousands of years or less. In some cases, it is possible to study fossils on a very fine scale - centimeter-by-centimeter - through the K–Pg rocks. Examples include marine microfossils, such as calcareous nanoplankton and foraminifera, and terrestrial plant pollen. Here, the fossils show that the ecosystem remained relatively stable up to the K–Pg boundary, at which point many species suddenly vanish. For groups with a poorer fossil record, such as dinosaurs, fossils are unlikely to be preserved just below the K–Pg boundary. For example, only a few dozen Tyrannosaurus skeletons are known, and so the odds of finding one a few centimeters below the boundary are low. This effect, called the Signor-Lipps effect, causes many species appear to vanish before the K–Pg boundary, creating the illusion of gradual extinction. Nevertheless, improved sampling shows that groups once thought to undergo a slow decline, such as dinosaurs, actually disappear suddenly near the K–Pg boundary. Reworking—when fossils are eroded from older rocks and deposited into younger rocks—can also make extinction appear gradual. For example, in the Bug Creek Anthills beds in Montana, dinosaur fossils occur alongside mammals from the earliest Paleocene, which created the illusion that dinosaurs dwindled as mammals radiated. Reworked fossils are recognized because they tend to be rare and are often damaged by the reworking.
The patterns are critical to understanding the cause of the extinctions. The fact that the extinction is severe, global, and rapid suggests that the extinctions result from a severe, global, and rapid environmental disturbance- an environmental catastrophe. In the 1970s and 1980s, this led scientists to seriously consider catastrophic mechanisms such as supernovas, volcanic eruptions, and asteroids, and sparked new interest in catastrophism in geology and paleontology.
The extinction was also highly selective. Some groups were relatively unaffected, others were devastated, and some were eliminated entirely. Many species of alligator, turtle, and salamander survived, for example. Mammals, birds, and lizards suffered high rates of extinction. Non-avian dinosaurs and pterosaurs were wiped out entirely.
Even though the boundary event was severe, there was significant variability in the rate of extinction between and within different clades. Species that depended on photosynthesis declined or became extinct as atmospheric particles blocked sunlight and reduced the solar energy reaching the Earth's surface. This plant extinction caused a major reshuffling of the dominant plant groups.[20] Photosynthesizing organisms, including phytoplankton and land plants, formed the foundation of the food chain in the late Cretaceous as they do today. Evidence suggests that herbivorous animals died out when the plants they depended on for food became scarce. Consequently, top predators such as Tyrannosaurus rex also perished.
Coccolithophorids and molluscs (including ammonites, rudists, freshwater snails and mussels), and those organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought that ammonites were the principal food of mosasaurs, a group of giant marine reptiles that became extinct at the boundary.[21]
Omnivores, insectivores and carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. At the end of the Cretaceous there seems to have been no purely herbivorous or carnivorous mammals. Mammals and birds that survived the extinction fed on insects, worms, and snails, which in turn fed on dead plant and animal matter. Scientists hypothesize that these organisms survived the collapse of plant-based food chains because they fed on detritus (non-living organic material).[22][23][24]
In stream communities few animal groups became extinct because stream communities rely less directly on food from living plants and more on detritus that washes in from land, buffering them from extinction.[25] Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the water column than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton while animals living on or in the ocean floor feed on detritus or can switch to detritus feeding.[22]
The largest air-breathing survivors of the event, crocodyliforms and champsosaurs, were semi-aquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms or fragments of organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous.[23]
After the K–Pg extinction event, biodiversity required substantial time to recover, despite the existence of abundant vacant ecological niches.[22]
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