Echinoderms: Fossil Record

Abstract

The statement that the echinoderms evolved from bilaterally symmetrical ancestors occurs in many textbooks of invertebrate zoology, but the only evidence that bilaterally symmetrical adult echinoderms might have existed comes from modern echinoderm larvae, and the theme of much of this book is that this evidence is open to an entirely different interpretation. Those who claim that bilaterally symmetrical adult echinodrms must have existed at one time will probably suggest that such animals were soft-bodied and left no fossils or that the relevant fossils have yet to be found. The earliest known fossil echinoderms, however, were not bilaterally symmetrical and gave no hint that they might have been derived from bilateral forms. Some of the earliest known fossils of the group are shown in Figure 12.1, and only some of them were pentaradial. The curious Helicoplacus (Fig. 12.1b) was a triradial form with its body spirally twisted, and Paul and Smith (1984) have suggested that all the pentaradial echinoderms evolved from ancestors like this.