Mammals are ubiquitous in today’s world. Over 5,000 species, from hamsters to humans, are distributed across the globe. But how did mammals become so successful? This question gets to the heart of a much wider mystery: how major groups of plants and animals rise up over evolutionary time. How do their evolutionary radiations unfold: when do they happen, how quickly, and what drives them? Answering these questions will help us better understand how major groups become successful over evolutionary time and how biodiversity is affected by large, infrequent events like mass extinctions. It will also give unique insight into the early stages of our own evolutionary story.
Two main handicaps have held back generations of researchers. First, tackling the mystery of the mammal radiation requires multiple lines of evidence that transcend traditional research boundaries. Second, we still know very little about those mammals that flourished during the ~10 million years after the end-Cretaceous extinction (the early Paleogene). They are often ignored, because they mostly belong to ‘archaic’ groups, with uncertain relationships to both Cretaceous and modern mammals. How to place these ‘archaic’ species—some 200+ species of incredible anatomical, dietary, and body size diversity— on the family tree of mammals is one of the great unsolved problems in palaeontology.
A wealth of new fossils (including specimens collected over the past decade by our team) and new multidisciplinary analytical techniques together provide an unprecedented opportunity to untangle the biology and phylogeny of these critical early Paleogene species, and then to use that information to better understand how mammals ascended to dominance and what role the end-Cretaceous extinction played in this story. In doing so, our team integrated, for the first time, a wealth of data on the anatomy, genetics, ages, genealogy, and body sizes of early mammals and their modern relatives, providing the most detailed look yet at a major evolutionary radiation in the fossil record.
Detailed Objectives:
1) What are the genealogical relationships of Paleogene mammals: how are they related to each other and to their Cretaceous forebears and living mammals? We tested whether some/all ‘archaic’ Paleogene species are early members of major living mammal groups, failed experiments in mammalian evolution that did not produce any descendants, and/or linked to Cretaceous species that lived alongside the dinosaurs.
2) When did placental mammals and major subgroups originate? We tested the time component of the three main hypotheses for placental diversification—explosive, long-fuse, and short-fuse—and determine which best fits the available data, thus establishing whether placentals and major subgroups originated alongside, or after the extinction of, dinosaurs.
3) What effect did the end-Cretaceous extinction have on mammalian evolution? We tested the rate component of the three main hypotheses for placental diversification, and determine whether there were changes in biodiversity across the Cretaceous-Paleogene boundary, whether the extinction preferentially wiped out certain types of mammals, and how mammals emerged from the extinction and radiated to become the diverse animals we know today.
