HOW CAN WE MAKE EVOLUTIONARY SENSE OF THE EDIACARA BIOTA?

Correct interpretation of Precambrian fossil record is fundamental to a proper understanding of evolution of the biosphere. It is vexing, therefore, that the biological relations of many undoubted Precambrian fossils are proving difficult to resolve. A major problem here concerns the recognition of true versus false homologies between living and fossil groups. And in no group is this problem more clearly illustrated than with the Ediacara biota (about 580-543 Ma).

Over the last fifty years, two main view have held sway with regard to the nature of the enigmatic Ediacara biota: that it contains the ancestors of major animal phyla that radiated at the base of the Cambrian, notably of cnidarians (like Charnia sp.) and bilaterians (like Dickinsonia sp.). Or that it has almost no relationship with these later Cambrian phyla.

We here use our new laser scanning and automontage technology to examine details of the holotype of Charnia masoni and the Goldring lectotypes of Dickinsonia costata, both of the Ediacara biota. It will be argued that Charnia masoni cannot be related to the modern cnidarian group, the sea pens, with which it has for so long been compared, because they have opposite growth polarities. Recent evolutionary studies also show that seapens are a highly derived group of actively burrowing cnidarians that are likely to have evolved later than the Palaeozoic.

Likewise, we argue that Dickinsonia costata cannot be admitted into the Bilateria because it shows sigificant differences in growth. In our study, it is unlikely to be a close relative of the polychaete annelids with which it, and its fossil relatives, have often been compared.

There is, therefore, a need to avoid the traditional practice of translating Phanerozoic animal phyla back into the Ediacaran. Studies of growth and development arguably provide an important tool kit for the better understanding of enigmatic fossil groups, especially at key calibration points in the evolution of the biosphere.