What Ended the Dinosaurs
Extinction is the flip side of evolution. Just as every organism that lives will die, every species that evolves will eventually go extinct. In some cases, it is subtle, as when one species is more or less replaced by a descendent species. In other cases, it is due to competition with other species, in which a species proves less able to respond; perhaps it reproduces more slowly or cannot exploit a food source as well. One very visible way this can happen is when a species arrives from elsewhere (on its own, or in recent history transported by humans) and spreads rapidly, crowding out local species or using methods of predation local species have not been exposed to. Island ecosystems are very vulnerable to this. One kind of extinction you might not think about happens to species living with each other. Many organisms live on, inside, or otherwise closely interact with other organisms, and if one species in such a relationship goes extinct, others that relied on it are also at risk. Think of the gut bacteria that must have lived inside of a Triceratops, or mites that were adapted to live on Tyrannosaurus. Another common way for extinction to occur is due to changes in the ecosystem that a species cannot respond to quickly enough. This can be gradual, like rising seas flooding coastal plains, or a warming climate pushing cold-adapted species to higher elevations until they run out of land. It can also be almost instantaneous, like an asteroid impact, or a volcanic eruption destroying a tropical island.
Extinctions have constantly occurred throughout geologic time. The rate varies, but we can say that there is a certain predictable “background rate” of extinction. Occasionally we see events in time when the extinction rate is much greater than we’d expect, which we can tell from the kinds of fossils found before and after the event. These events are mass extinctions and are due to causes or combinations of causes that are too disruptive for organisms to adapt. For example, the extinction at the end of the Cretaceous is famously attributed to an asteroid impact. The mass extinctions that closed the Permian and Triassic are thought to have occurred due to enormous volcanic eruptions venting gases into the air and causing rapid global warming and changes to the oceans. Because organisms aren’t adapted for very rare global events, survival is more a matter of luck than some innate “superiority”. Dinosaurs survived the mass extinction at the end of the Triassic, but that doesn’t mean they were superior to the animals that went extinct or somehow “deserved” to succeed. Similarly, they went extinct at the end of the Cretaceous (except for a few groups of birds), but that doesn’t mean that mammals are superior. Human modifications of Earth systems are causing steep decline in populations of many species during the current geologic age through pollution, habitat destruction, and climate change which could lead to a sixth mass extinction.
The Triassic Period ended with a major extinction event. Many species of plants, marine invertebrates, and vertebrates died out. This extinction is perhaps best known for eliminating many groups of terrestrial vertebrates, leaving many niches open that were soon filled by dinosaurs. The most likely primary cause of this extinction is massive volcanic activity along the future Atlantic coasts of Africa, Europe, North America, and South America, set in motion by the beginning of the breakup of the supercontinent Pangea. As these continents began to separate, immense amounts of lava were erupted. These eruptions released gases such as carbon dioxide into the atmosphere, upsetting the world’s climate and ocean chemistry.
In the oceans, many groups suffered declines, and some were lost entirely, such as some groups of coiled-shelled squid-like ammonoids. Two groups that had existed for hundreds of millions of years, the conulariids (small coral-like animals that grew four-sided triangular coverings out of mineralized rods) and conodonts (small eel-like cousins of vertebrates), disappeared as a result of Late Triassic changes. On land, numerous groups went extinct. They included metoposaur amphibians and almost all of their close relatives; aetosaurs; phytosaurs; many other groups of reptiles; and dicynodont mammal relatives.
The Jurassic Period began in a world marked by the extinction event at the end of the Triassic, but is not known for any mass extinctions of similar extent. In the oceans, there was a smaller extinction approximately 183 million years ago that is thought to have been the result of large volcanic eruptions. It reduced many groups of marine invertebrates. At the end of the Jurassic, many of the groups that had risen to dominance underwent a transition. Some groups, such as long-tailed pterosaurs, terrestrial crocodile relatives, diplodocid sauropods, and plated stegosaurs, went into decline, with other animals taking their places in the Early Cretaceous (short-tailed pterosaurs over long-tailed pterosaurs, other sauropods over diplodocids, ankylosaurs over stegosaurs, etc.). The extent of the change is not entirely clear, though. The rock record for the first part of the Cretaceous is not as good as the Late Jurassic, with nothing to compare to famous Late Jurassic fossil-producing areas such as the Morrison Formation in North America, Solnhofen in Europe, Tendaguru in Africa, and the Tiaojishan Formation in Asia.
The Cretaceous ended with perhaps the most famous mass-extinction event of all, but there were other extinctions of note during the period. There were two minor mass-extinctions during the middle Cretaceous. The later of the two, at around 94 million years ago, is notable for the extinction of the ichthyosaurs. In general, the Cretaceous was a time of significant turnover as continents separated, inland seas grew and then shrank, and flowering plants came on the scene. For example, many dinosaur communities “streamlined” with fewer groups holding more diversity. Most groups of sauropods went extinct during the middle Cretaceous, to be replaced by titanosaurs. Stegosaurs went extinct, but ankylosaurs persisted. Medium and large carnivorous theropods, previously diverse, dwindled to tyrannosaurs in Asia and North America, and abelisaurs in the other continents. There was not one event, though, but a series of changes over time.
The end of the Cretaceous is the second largest mass-extinction, behind only the extinction at the end of the Permian. Although there is some discussion about certain groups being on their way out near the end of the Cretaceous, or perhaps even going extinct some hundreds of thousands or tens of thousands of years before the end, this kind of thing is hard to tell with the level of accuracy provided by the rocks. More importantly, we know that these groups lasted to practically 66 million years ago, the end of the Cretaceous, and then we don’t see any good evidence for them in the following Paleocene Epoch. (You might know this extinction as the K–T extinction, for the “K” of Kreide, the German word for “Chalk” and “Cretaceous” and the “T” of “Tertiary”. Geologists and paleontologists are moving away from using the Tertiary Period, and often now use K–Pg, with “Pg” for “Paleogene”.)
The end-Cretaceous extinction struck down many groups, although it particularly affected land-dwelling animals larger than a few pounds in weight. Aside from a few groups of birds, all dinosaurs went extinct. This includes not only the groups that are classically seen as dinosaurs, but also many early birds, including all those that had teeth or prominent hand claws. Pterosaurs also went extinct. Smaller land vertebrates, and those that could shelter in burrows or the water, were not as strongly affected. Catastrophic extinctions also occurred in the oceans, wiping out abundant and significant groups such as the ammonites, belemnites, giant inoceramid bivalves, reef-forming rudist clams, mosasaurs and plesiosaurs. Almost all colonial corals also went extinct.
The cause or causes of the end-Cretaceous extinction have long been controversial. Many proposed explanations fail because they focus on one part of the extinction (particularly the extinction of dinosaurs) while ignoring other parts. Just to complicate things, the end of the Cretaceous had several events that could easily cause some level of extinction, or amplify the effects of other events. These include an enormous outpouring of lava in western India (the Deccan Traps), the loss of shallow marine habitats as sea level decreased and continental seas drained away, and climate change resulting from this and other changes in geography. However, most scientists now accept that the best explanation is the impact of an asteroid probably 6 to 10 miles across. This asteroid struck just offshore of what is now the Yucatan Peninsula, producing what is called the Chicxulub crater. Although the crater itself is buried, its structure, more than 90 miles across, is apparent below the surface. It also left a variety of other physical clues. In the area of the crater itself there is “shocked quartz”, quartz crystals that have been disrupted by a physical shock. Where we have rocks that span the end of the Cretaceous and beginning of the Paleogene, we can find a thin layer of clay featuring an abnormal concentration of the heavy chemical element iridium, which is rarely found at Earth’s surface but can be found in asteroids. This bed also includes minerals that have been melted, ejected, and re-cooled as glassy objects (tektites and tiny spheres known as impact spherulites).
The impact would have released far more energy than any human-made explosion, immediately creating a megatsunami in the Gulf of Mexico and scattering a portion of crust into the atmosphere. As the ejected material returned into the atmosphere, it heated greatly, raising air temperatures to oven-like levels. Organisms within this fallout zone would have been killed unless protected. Lingering dust in the atmosphere would have cut off sunlight, striking at the base of food webs on land and in water. Plants would have died or gone dormant until the atmosphere cleared, which in turn would have led to the death of herbivores and the carnivores that fed on them. Only animals with more flexible lifestyles would have survived. To make things worse, the impact would have vaporized marine rocks containing carbonate- and sulfate-rich rocks, altering the composition of the atmosphere, producing widespread acid rain, and acidifying the oceans. This last part is important because if water is too acidic, animals with shells made of calcium carbonate cannot grow shells. With phytoplankton cut off from the sun and many organisms unable to form shells, marine ecosystems would have collapsed. It would have been years before the atmosphere cleared, and much longer, perhaps millions of years, for the surviving organisms to repopulate and diversify into the niches left behind by the extinct organisms of the Mesozoic.
Site Index and Credits
Age of Dinosaurs (2021)
Text by Justin Tweet (AGI). Contributors: Vincent Santucci (GRD), Adam Marsh (PEFO), ReBecca Hunt-Foster (DINO), Don Corrick (BIBE). Project Lead / Web Development, Jim Wood (GRD).
Tweet, J.S. and V.L. Santucci. 2018. An Inventory of Non-Avian Dinosaurs from National Park Service Areas. in Lucas, S.G. and Sullivan, R.M., (eds.), Fossil Record 6. New Mexico Museum of Natural History and Science Bulletin 79: 703-730. https://irma.nps.gov/DataStore/Reference/Profile/2257153
Santucci, V.L., A. Marsh, W. Parker, D. Chure, and D. Corrick, 2018. “Age of Reptiles”: Uncovering the Mesozoic Fossil Record in three Intermountain national parks. IMR Crossroads. Spring 2018, p. 4-11. https://irma.nps.gov/DataStore/Reference/Profile/2253529
Last updated: July 8, 2022