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 that a significant transition occurred between the Mesozoic and Cenozoic eras at least since the 1820s. Around this time dinosaur fossils were first being described in the scientific literature. Nevertheless, so few dinosaurs were known that the significance of their passing 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might age, deteriorate, and die like individual animals became popular starting in the late 19th century, but were superseded by the Neo-Darwinian synthesis.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. (en)