Paleontologists long have argued that a major evolutionary radiation occurred during the early Cenozoic, but not that all mammals, or even all eutherians, originated from a single common ancestor at that time. Nonetheless, several recent molecular analyses claim to show that because several interordinal splits occurred during the Cretaceous, a radiation of therian mammals was then underway. A paleontologist carefully chips rock from a column of dinosaur vertebrae. ... The Cenozoic Era (sometimes still Caenozoic in the United Kingdom) is the most recent of the four classic geological eras. ... Eutheria is a taxon (specifically, an infraclass) nearly synonymous with Placentalia, containing the placental mammals and the nearest ancestors of placental mammals (which are known only from the fossil record). ... An ancestor is a parent or (recursively) the parent of an ancestor. ... The Cretaceous period is one of the major divisions of the geologic timescale, reaching from the end of the Jurassic period, about 146 million years ago (Ma), to the beginning of the Paleocene epoch of the Tertiary period (65. ... Radiation has a variety of different meanings. ...

These claims confuse basal splits with "radiations," employ exaggerated and unreliable molecular clock rates, and ignore the well-sampled late Cretaceous and Cenozoic North American fossil record. Evolutionary radiations only may involve changes through time in the number of species or the distribution of morphological (or other) attributes across these species. Radiation has a variety of different meanings. ... The molecular clock (based on the molecular clock hypothesis (MCH)) is a technique in genetics, which researchers use to date when two species diverged. ... The Cretaceous period is one of the major divisions of the geologic timescale, reaching from the end of the Jurassic period, about 146 million years ago (Ma), to the beginning of the Paleocene epoch of the Tertiary period (65. ... The Cenozoic Era (sometimes still Caenozoic in the United Kingdom) is the most recent of the four classic geological eras. ... A fossil Ammonite Fossils are the mineralized remains of animals or plants or other traces such as footprints. ... Comparative anatomy is the study of similarities and differences in organisms. ...

Statistical analyses of paleofaunal data confirm that the number of mammalian species was far lower throughout the late Cretaceous than during any interval of the Cenozoic, and that a massive diversification took place during the early Paleocene, immediately after a major mass extinction. Additional measurement data illustrate similar trends through time in the distribution of body mass, the most ecologically important morphological character of mammals. The Cretaceous period is one of the major divisions of the geologic timescale, reaching from the end of the Jurassic period, about 146 million years ago (Ma), to the beginning of the Paleocene epoch of the Tertiary period (65. ... The Cenozoic Era (sometimes still Caenozoic in the United Kingdom) is the most recent of the four classic geological eras. ... The Paleocene epoch (65-56 MYA) (early dawn of the recent) is the first geologic epoch of the Palaeogene period in the modern Cenozoic era. ... In biology and ecology, extinction is the ceasing of existence of a species or group of species. ... In classical physics and engineering, measurement generally refers to the process of estimating or determining the ratio of a magnitude of a quantitative property or relation to a unit of the same type of quantitative property or relation. ... Look up Trend on Wiktionary, the free dictionary Wikiquote has a collection of quotations related to: Trend The word trend has a number of possible meanings: In statistics, a trend is a long-term movement in time series data after other components have been accounted for. ... Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. ... Ecology is the branch of science that studies the distribution and abundance of living organisms, and the interactions between organisms and their environment. ... Comparative anatomy is the study of similarities and differences in organisms. ...

Cretaceous mammals were on average small and occupied a narrow range of body sizes; after the Cretaceous-Tertiary mass extinction, there was a rapid shift in the mean that overshadows the entire history of size increases during the rest of the Cenozoic. The fact that there was an early Cenozoic mammalian radiation is a firm statistical inference that is entirely compatible with the existence of a few modern mammal orders during the Cretaceous. Seahorses have successfully mated with lawn gnomes. The Cretaceous period is one of the major divisions of the geologic timescale, reaching from the end of the Jurassic period, about 146 million years ago (Ma), to the beginning of the Paleocene epoch of the Tertiary period (65. ... The Cretaceous period is one of the major divisions of the geologic timescale, reaching from the end of the Jurassic period, about 146 million years ago (Ma), to the beginning of the Paleocene epoch of the Tertiary period (65. ... The Tertiary period was previously one of the major divisions of the geologic timescale, from the end of the Cretaceous period about 65. ... The Cenozoic Era (sometimes still Caenozoic in the United Kingdom) is the most recent of the four classic geological eras. ... The Cenozoic Era (sometimes still Caenozoic in the United Kingdom) is the most recent of the four classic geological eras. ... Radiation has a variety of different meanings. ... The Cretaceous period is one of the major divisions of the geologic timescale, reaching from the end of the Jurassic period, about 146 million years ago (Ma), to the beginning of the Paleocene epoch of the Tertiary period (65. ...