Evolution in trilobites

Summary Trilobites with their 300 million years' existence as a class are considered in relation to the subtitle— the preservation of favoured races in the struggle for life —of Darwin's book On the Origin of Species (1859). Well-known evidence is recalled concerning minor steps in trilobite evolution. Graphs showing the number of subfamilies surviving each of the geological epochs (series) are used to show the distribution in time of the rises and falls of the class's evolutionary vigour. Major steps in evolution are discussed on the basis of modifications of pre-existing, or the appearance of new, exoskeletal structures in Lower and Middle Cambrian, Tremadoc and Ordovician evolutionary “bursts”. The longest-lived survivors in a super-family are not invariably the most generalized forms. The longest-lived super-family was the last to become extinct but like most of the other trilobite super-families, its origin is cryptogenetic. It is hoped that further exploration will diminish gaps in knowledge of trilobite phylogeny; this knowledge is still as elusive as is any classification acceptable above the category of superfamilies.