Radiation

The major subgroups of mammals - dogs, cats, primates, ungulates - all originated relatively suddenly in a burst of evolution during the early Paleocene; once established, the number of groups remained constant to the present. The pattern of increase is called radiation and its exact form can differ between groups. Radiation in the mammals is illustrated by the manatee opposite whose lineage moved back from the land to the sea.

What factors explain the timing and pattern of a groups' rise and fall?

Why, for example, did the mammals remain a minor group for about 150 million years after their origin and then radiate rapidly 50 - 60 million years ago?

The obvious answer is that until the Paleocene the ecological niches later to be occupied by mammals were already filled by dinosaurs: the mammals radiated as the ecological replacements for the dinosaurs.

The rise and fall of a higher taxon has often been connected with the fall of one set of ecologically equivalent species and the later rise of another set.

Why should one higher taxon replace another higher taxon of ecologically similar species?