Since the 19th century, a significant amount of research has been conducted on the Cretaceous–Paleogene extinction event, the mass extinction that ended the dinosaur-dominated Mesozoic Era and set the stage for the Age of Mammals, or Cenozoic Era. A chronology of this research is presented here. Paleontologists have recognized since at least the 1820s that a significant transition occurred between the Mesozoic and Cenozoic eras. Around this time dinosaur fossils were first being described in the scientific literature. Nevertheless, so few dinosaurs were known that the significance of their extinction went unrecognized, and little scientific effort was exerted toward finding an explanation. As more and more different kinds of dinosaurs were discovered, their extinction and replacement by mammals was recognized as significant but dismissed with little examination as a natural consequence of the mammals' supposed innate superiority. Consequently, paleontologist Michael J. Benton has called the years up to 1920 as the "Nonquestion Phase" of Cretaceous–Paleogene extinction research. Ideas that evolution might proceed along pre-ordained patterns or that evolutionary lineages might age, deteriorate, and die like individual animals became popular starting in the late 19th century, but were superseded by the Neo-Darwinian synthesis. The aftermath of this transition brought renewed interest to the extinction at the end of the Cretaceous. Paleontologists began dabbling in the subject, proposing environmental changes during the Cretaceous like mountain-building, dropping temperatures or volcanic eruptions as explanation for the extinction of the dinosaurs. Nevertheless, much of the research occurring during this period lacked rigor, evidential support or depended on tenuous assumptions. Michael J. Benton called the years between 1920 and 1970 the "Dilettante Phase" of Cretaceous–Paleogene extinction research. In 1970, paleontologists began studying the Cretaceous–Paleogene extinction in a detailed, rigorous way. Benton considered this to be the beginning of the "Professional Phase" of Cretaceous–Paleogene extinction research. Early in this phase, the pace of the extinctions and the potential role of the Deccan Traps volcanism in India were major subjects of interest. In 1980, father and son duo Luis and Walter Alvarez reported anomalously high levels of the platinum group metal iridium from the K–Pg boundary, but because iridium is rare in Earth's crust they argued that an asteroid impact was needed to account for it. This suggestion set off a bitter controversy. Evidence for an impact continued to mount, like the discovery of shocked quartz at the K–Pg boundary. In 1991, Alan Hildebrand and William Boynton reported the Chicxulub crater in the Yucatan Peninsula of Mexico as a probable impact site. While the controversy continued, the accumulating evidence gradually began to sway the scientific community toward the Alvarez hypothesis. In 2010, an international panel of researchers concluded that impact best explained the extinction event and that Chicxulub was indeed the resulting crater. Because the estimated date of the object's impact and the Cretaceous–Paleogene boundary (K–Pg boundary) coincide, there is now a scientific consensus that this impact was the Cretaceous–Paleogene extinction event which caused the death of most of the planet's non-avian dinosaurs and many other species. The impactor's crater is just over 177 kilometers in diameter, making it the second largest known impact crater on Earth.
19th century
1820s
1825 Georges Cuvier recognized that significant changes to Earth's biota occurred between the Mesozoic and the Cenozoic eras. Because the most familiar and distinctive Mesozoic lifeforms known at the time were marine, he speculated that life had not yet fully conquered the land. He attributed the end-Cretaceous mass extinction to a catastrophic drop in sea levels that destroyed the habitats of the era's characteristic fauna. He concluded that the mammals of the Cenozoic represented Earth's first truly terrestrial fauna.
1830s
1831 Gideon Mantell recognized dinosaurs as evidence for reptilian dominance over the land in addition to the dominion over the sea held by ichthyosaurs and plesiosaurs. He therefore declared the Mesozoic era to be the "Age of Reptiles". Distinguishing the Mesozoic "Age of Reptiles" from the Cenozoic "Age of Mammals" highlighted the differences these two eras of geologic time.
1840s
1842 Sir Richard Owen proposed that the major reptile groups of the Mesozoic were driven extinct as the oxygen contents of Earth's atmosphere rose to levels better suited for birds and mammals.
1850s
1854 Charles Darwin published On the Origin of Species. He regarded the extinction of most taxonomic groups as occurring gradually through the piecemeal loss of member species. However, he considered the extinction of the ammonites at the end of the Mesozoic to have been "wonderfully sudden".
1880s
1882 Othniel Charles Marsh interpreted the extinction of the dinosaurs as a gradual decline over the course of the Cretaceous.
1890s
1898 Arthur Smith Woodward also advocated that the dinosaurs gradually declined into extinction late in the Mesozoic.
20th century
1900s
1905 Frederic Brewster Loomis argued that the plates adorning the backs of stegosaurs were maladaptive traits that sapped their vigor and signaled their impending extinction. Similar arguments would later be extended to the extinction of the dinosaurs overall by Woodward in 1910.
1910s
1910 Woodward gave an address to the British Association for the Advancement of Science in which he declared the cause of the extinction of the dinosaurs to be "racial senility"- the idea that evolutionary lineages had finite lifespans the way individual organisms do and also exhibit age-related deterioration over time as well. Woodward argued that traits like large size, spiny coverings and lack of teeth seen in some later dinosaurs were signs that the group was approaching its inevitable end.
1917 Franz Nopcsa suggested that dinosaurs may have developed overactive pituitary glands that led them to become pathologically gigantic in an evolutionary parallel to acromegaly in modern humans. He also suggested that a "[d]iminution of sexual activity" may have played a role in their demise.
1920s
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