The KT Extinction event is the extinction event known for the fall of dinosaurs. The Cretaceous-Paleogene (K-Pg) extinction event or the Cretaceous-Tertiary extinction event is known as the KT extinction event. The KT extinction is characterized by the mass extinction of at least three quarters of the plant and animal species present during that time on Earth. It occurred about 66 million years ago. The species that survived the KT mass extinction included some ectothermic species like the sea turtles and crocodilians. Amongst the tetrapods anybody weighing more than 25 kgs were removed from the world map because of the extinction. The KT boundary extinction marked the end of the Cretaceous period and the beginning of the Paleogne period.
Introduction to KT Extinction
The KT mass extinction occurred at the end of the Cretaceous period which also marked the end of the Mesozoic era, which led to the extinction of many species most prominent of which were the dinosaurs. This event also marked the beginning of the Cenozoic Era which continues even today. The geological characteristics of this time period are defined by the thin layer of sediment, which is known as the K-Pg or KT boundary, containing high quantities of iridium that are found all over the world in both the marine and terrestrial rocks. Interesting fact about this is that the boundary clay which is covered in high levels of iridium is actually a more common component of the asteroids rather than Earth’s belt.
After some scientific research, it is now generally considered that the KT event was caused by the impact of a very big asteroid about 66 million years ago. It is said that the KT event asteroid size would have been about 10 - 15 km in width. The impact of such a large asteroid led to the destruction of the global environment. A lingering winter that followed the impact of the asteroid halted many important life cycle processes such as photosynthesis in plants and planktons. This disrupted the entire food chain and hence resulted in the extinction of many of the species. This impact hypothesis is known as the Alvarez hypothesis (as it was put forward by Luis Alvarez and his team of scientists). It gained wider acceptance with the discovery of the 180 km Chicxulub crater located in the Gulf of Mexico’s Yucatán peninsula. This discovery provided conclusive evidence that the thin layer known as the KT boundary extinction event geologic characteristic consisting of iridium containing sediments represented the debris from the asteroid impact.
The mass extinction of the various species can be attributed to the KT mass extinction as they occurred simultaneously following the asteroid impact, and hence can be said to have been caused by the asteroid. During the drilling of Chicxulub peak ring it was confirmed that the peak ring consists of granite that must have ejected from deep inside the earth within a few minutes of the asteroid impact. Some other factors that might have contributed to the extinction may have been the Deccan Traps and other volcanic eruptions, climate changes and changes in the sea level.
Almost three quarters of the species went extinct. Of the wide range of extinctions during the KT dinosaur extinction of non-avian category is the best known extinction event. Along with the KT line dinosaur other myriad organisms like the terrestrial organisms which include mammals, birds, lizards, insects, plants and also the pterosaurs. The KT boundary mass extinction was not limited to only the terrestrial species as mass extinction were also observed in oceanic organisms such as plesiosaurs, mosasaurs, teleost fish, sharks, mollusks and many other species of planktons. Although 75% of the species vanished in the KT Boundary extinction event, it also provided evolutionary opportunities to many species. Because of the dramatic changes in the conditions and the ample resource opportunities, many groups of living organisms underwent adaptive radiation i.e. a rapid increase in the divergence of existing species into new life forms. Many of the species grew within the ecosystems that had been disrupted. After the loss of life due to KT boundary mass extinction, mammals diversified during the Paleogene period and many life forms evolved such as horses, whales, bats and primates. The surviving avian dinosaur species diversified and radiated into the modern into all of the modern species of birds.
Brief Outlook: Evidences of KT Extinction
Most of the evidence relating to the period of KT or K-Pg Boundary and the extinction events have been found from the North American continent. It involves two major pieces of evidence - the dinosaur fossils and the evidence of plant remains.
Currently major information regarding the dinosaur extinction and the fossils surrounding it are found in the western region of North America. Fossil beds located at Hell Creek Formation of Montana, Judith River Formation in Montana, and the Dinosaur Park Formation in Alberta, provide information about the last 10 million years of Cretaceous helping in describing the change in the dinosaur species. The middle-late Campanian formations provide information about the diversity of the dinosaur species. The late Maastrichtian rocks at Hell Creek Formation show fossils that provide evidence of largest members of major dinosaur clades such as - Tyrannosaurus, Ankylosaurus, Triceratops, Pachycephalosaurus, and Torosaurus indicating that the food was more than enough right before the extinction.
Strong evidence comes from the rich and abundant pollen records before the extinction and the rise of the ferns post the extinction event. The sediments of the plant fossils such as the pollen grains and others around the extinction event are also indicative of the event taking place. For example, the geological layer with plant remains that belong to the time before the event shows pollen grains from angiosperms which is a higher class of plant life. The layer of sediments after the event show more and more fern spores. The availability of fern spores in large amounts show that ferns must have grown in abundance and they are the primary forms of vegetation that grows right after an extreme or blighting incident. They must have grown in the areas that had seen volcanic eruptions, and while studying the geologic layers it is found that there is a gradual increase in the plant remains from fern spores to pollen grains released by higher angiosperms.
Another evidence is also available from the mass extinction event. The vanishing of the planktons, an integral part of marine life, abruptly and right at the KT boundary mass extinction. Similarly the extinction of ammonite genera near the K-Pg boundary also is a sample of the effects on marine life. Although it began gradually, during most of the late Cretaceous period, there may have been an abrupt vanishing of them. Many of the processes that were ongoing during the late Cretaceous period suddenly ended abruptly due to the mass extinction. Also, in such processes, the temperature of the Earth increased by three to four degrees rapidly about 65.4 - 65.2 million years ago. Along with the increase in the temperature of the Earth, the temperature of water decreased and that caused many drastic changes and disruption in the diversity of marine life.
There is a scientific consensus that the impact of the asteroid on the surface led to a megatsunami. This megatsunami made up of the colossal waves because the impact was about 100 m high, because the area where the impact occurred was near a shallow water area of the Caribbean Sea and Gulf of Mexico. The form of the thick sediment layer deposited by the waves especially near the La Popa basin in northeastern Mexico and also some graded sand deposits in the Chicxulub Crater. Sedimentary rock fossil deposits from the KT impact carried the remnants of mangrove type ecosystems which is an evidence of the impact water moving back and forth in the region of Gulf of Mexico and also the remaining fish fossils in the shallow water that were not removed or displaced by the scavengers.
The Duration of KT Extinction Event
The time period for which the effects of the extinction event lasted is debatable. There are various theories indicating different time durations from a few years to a few thousand years. The issue arises from the fact that the fossil records indicate different theories. Some of the fossil records show an abrupt end and indicate the possibility of a rapid extinction whereas most of the extinct species may have vanished gradually long after the recent fossil was found. There are continuous fossil-rock beds that are attributed to covering a time period from several million years before the KT extinction event to several million years after the extinction. On the other hand, sedimentation rates and the thickness of the layer at the time of the KT event from three sites show that the extinction was rapid and lasted for less than a thousand years. For example at the site of Denver basin in Colorado, an increasing amount of fern fossils show times lasting from 1000 to less than 71,000 years after the deposition of sediment layer from the KT extinction event. From the site mammalian fossils that were recovered are dated from approximately 1,85,000 years and not more than 5,70,000 years which indicate that there had been rapid rates of extinction of biotic forms of life and the starting recovery in the Basin during the redevelopment of life in the area.
Pattern of Extinctions
The KT extinction event encompassed all the continents that were present at that time. One of the most prominent species that were affected, the non-avian dinosaurs, whose fossils are found all over the world in the present day continents of North America, Europe, Asia, Africa, South America and Antarctica during the Maastrichtian time period. Since the beginning of the Cenozoic Era there has been no sign of these species anywhere in the world. Similarly the fossils of plant remains such as the pollens which tell about the devastation in plant life are seen as far as New Mexico, Alaska, China and New Zealand.
Although the extinction of the species was severe there was variation in-between and within the different clades. There were a lot of debris particles in the atmosphere all over the world due to the impact and the volcanic eruptions across the planet. Because of that the species that had depended on sunlight for photosynthesis went extinct as the atmospheric particles blacked the sunlight and hence the solar energy reaching the ground declined severely. Also, due to such a rapid extinction there was a major reshuffling amongst the dominant groups of plants. As many of the species vanished, with the recovering vegetation being available in plenty the survival chances for omnivores, insectivores and carrion-eaters increased. Any of the mammals that depended on a single source of food be it either the herbivores or the carnivores did not survive. Most of the mammals and the birds that survived fed on the insects, worms, and snails who in turn fed on dead organic matter from plants and animals.
Some of the species that survived the event belonged to the stream ecosystem. This is because such species rely less on living plants but more on the food obtained in the form of detritus i.e. dead organic plant matter that are washed in from the land. Although complex similar such patterns of survival are also common to the oceanic species. Amongst those species extinction was faced severely by the ones living in the water column of oceans as compared to the ones living either on or inside the sea/ocean bed or floor. Organisms that live in the water column are mostly dependent on the primary sources consisting of living phytoplanktons whereas the ones living on the seafloor are more dependent on the detritus. Molluscs such as ammonites, freshwater snails, mussels, etc along with coccolithophorids, and the organisms that depended on these organisms for food sources suffered heavy loss and extinction. This is illustrated by the understanding that ammonites on whom the mosasaurs were dependent along with a group of marine reptiles faced extinction because of the vanishing of ammonites.
An interesting example of survival is by the crocodilian life forms and related species. The crocodyliforms were amongst the largest breathing survivors of the time along with champsosaurus. These organisms were semi-aquatic and were dependent on dead organic matter for food and nutrition. It is this quality that is still seen in modern crocodilians. The modern species of crocodilians live their life as scavengers and have the ability to survive without food. The young ones of the species are small and grow slowly while depending largely on the invertebrates and dead organisms for food at least in the initial years of their life cycle. These characteristics are the ones that helped them to survive at the end of the Cretaceous period during the extinction event.
Even then it is noteworthy that the redevelopment and recovery of the biodiversity and increase in the population of many species took significant time. An artist’s interpretation of the possible impact by the asteroid is given in the image below:
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Another image by an artist, shows the depiction of the life of Thescelosaurus shortly after the KT mass extinction event. All the organisms belonging to these species die because of starvation.
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