Period· The columns of time

Ediacaran

635 Ma → 539 Ma

The Ediacaran, from 635 to 539 million years ago, was the first period added to the chart in 125 years, and it carries the appearance and then the disappearance of large soft-bodied organisms.

First period added to the chart in a hundred and twenty-five years, and the only Precambrian period whose base is defined by a section rather than a number. It carries the appearance and then the disappearance of large soft-bodied organisms.

What the evidence shows

Ratified in 2004, it was the first period added since the Ordovician in 1879, and the only one in the Precambrian whose base is a field section rather than a figure: the cap dolostone closing the Marinoan glaciation, in the Flinders Ranges of South Australia. There follows an assemblage of organisms difficult to classify, fronds anchored in the mud, quilted discs, ribbed ovals resting on the microbial mats they fed from, some more than a metre long. Fossil cholesterol extracted from Dickinsonia places that form among the animals. The ocean records at the same time the largest known carbon isotope excursion, of some twelve parts per thousand; in the last ten million years of the period the assemblage disappears, while the first burrowers and the first skeletons appear.

Bounds and conventions

Its base is a defined section, its top the base of the Cambrian at 538.8 million years: both limits are firm. What occurs between them is far less so: the cause of the Shuram excursion remains disputed between a real change in ocean chemistry and a late alteration of the rock, and it is not known whether the Ediacara biota became extinct or was displaced, although the survivors are mainly thin, broad forms, which points towards a fall in oxygen rather than competition.