The Origin and Evolution of Mammals

The Synapsida are the 'mammal-like' reptiles and mammals, a group that diverged from a common ancestor shared with reptiles and birds about 340 million years. The fossil record of the synapsids is extraordinarily good, and documents the three phases of the history of the group, each one of which points to important evolutionary generalisations as well as relating an intrinsically fascinating story. The first stage leads from the origin of the group to the earliest mammals. The non-mammalian synapsids constituted the first radiation of fully terrestrial vertebrates, dominating the land long before the dinosaurs displaced them and took over that role. The fossil record illustrates the relationship between this radiation and the environmental conditions of the Permo-Triassic when it occurred. It also illustrates to a far greater degree than any other fossil record the origin of a major new taxon. The sequence of acquisition of mammalian structures and functions inferred from the fossils leads to an interpretation about the processes involved in the evolution of mammalian biological organisation. The second stage is the Mesozoic history of mammals. Throughout the Jurassic and Cretaceous Periods, mammals remained small, insectivorous or omnivorous animals living a nocturnal existence. They were abundant and diverse, but failed completely to evolve into any of the middle-sized and large-sized forms familiar amongst today's mammals. This is usually, though not completely satisfactorily, explained by competitive exclusion by dinosaurs. The third stage is the great Cenozoic radiation of mammals. From the moment the dinosaurs disappeared 65 million years ago, new kinds of mammals proceeded to evolve. Medium and large bodied herbivore and carnivore groups appeared early, and from then onwards a kaleidoscope of origins, flourishings, and extinctions of lineages of mammals took place. This great story interweaves changing climates, shifting continents, ecological opportunities, and the fulfilment of the adaptive potential of Mammalia. The latest molecular evidence that is having a huge impact on ideas about the timing and origins of the modern mammalian taxa is discussed along with the fossil evidence. The book reviews these three stages in turn, bringing up to date the palaeontological evidence and incorporating the molecular taxonomic data that has been rapidly accumulating over the last few years, and which is responsible for a series of exciting, radicalnew ideas about relationships amongst mammals and their inferred palaeo-biogeographic history.