In 2004 a new period was added to the geological record. This is the Ediacaran period, which covers the timespan 635-542 million years ago. The period begins with a change from an icehouse to greenhouse state, and ends with the diversification of modern, skeletonized animals at the base of the Cambrian period (Chapter 16). During this time period, the first multi-celled animals appeared in the fossil record. Some of these animals appear to have no living relatives, and belong to an extinct group of metazoans, the Ediacaran fauna. These are the most diverse and common animal remains. Much more infrequent are the first traces of modern multicellular animals, both bilaterian and prebilaterian arms of the family tree.
The timescale for Ediacaran events has been difficult to determine because fossils of any kind are rare, relative dating is controversial, and it is unclear if the many glacial events of the late Precambrian were all of global extent and whether their correlation one with another is correct. The best suggestion at present is summarized in Fig. 15.6. A series of major glaciations culminated in a global catastrophe, with icebergs in equatorial waters. It has been suggested that the ocean froze completely, producing a "Snowball Earth", and causing productivity to collapse. It is at least clear that the Earth became an extremely challenging environment for life. This huge glaciation ended abruptly at 635 million years ago and is marked worldwide by a limestone which is the product of the abrupt release of organic carbon into the world ocean, which stimulated primary productivity. This limestone defines the base of the Ediacaran period.
The first animal remains are 600 million years old, and come from southwest China, from rocks found just above the cap carbonate. They are tiny embryos, 200-400 |lm in diameter, that exhibit the early developmental stages of both bilaterian and prebilaterian metazoans, probably the remains of small worms and jellyfish. As yet, this find is unique and of extreme importance in understanding the early evolution of animals. The only other clear evidence of bilaterians in the Ediacaran period comes at 565 million years ago, when the trails of gut-bearing organisms are found.
The most dramatic fossils of the Ediacaran are much younger than the Chinese embryos, and have been difficult to classify. They are found in rocks mainly 570-542 million years of age, with over 100 species being identified. They evolved from simple, round forms into a wide range of shapes, some of which appear to have lived in specific environments. Although soft bodied, they are successfully preserved in course sandstones that were deposited in high energy environments. They are the Ediacaran fauna.
Two divergent views are held about the classification of these organisms. At one extreme, it is suggested that they represent early examples of modern groups, especially soft corals and jellyfish. At the other extreme, it is suggested that they comprise a kingdom of their own, called the Vendobionta, and bear little relationship to modern metazoans.
Structurally, three groups of organisms seem to be present in the fauna: (i) those with radial symmetry, which may be related to jellyfish or corals; (ii) those with bilateral symmetry, possibly related to more advanced organisms such as worms or arthropods; and (iii) those with an unusual symmetry that may not be represented by living groups. The significance of this classification based on symmetry is that modern high level classification relies partly on this feature (Fig. 15.6). Thus jellyfish and their relatives are regarded as having a long, separate evolutionary history to other metazoans because they have a different type of symmetry in their body plan. The same principles can be applied to Ediacaran animals. Ifthree different types of symmetry really are present in these assemblages, they
Fossils or events
must represent the end products of a considerable evolution of multicelled animals.
An example of the radial group of species is Cyclomedusa, an organism with concentric rings, possibly a jellyfish. An example of the bilateral group is Spriggina. Tribrachidium shows an unusual, in this case threefold, symmetry. Dickinsonia is an organism that can be interpreted as either a member of a modern group or something totally different. It superficially resembles a polychaete worm, although it may be more similar to a coral polyp. However, close examination of its segments suggests that they interfingered with one another and may have had a support function, like the stitching of a quilt. This condition is seen in no living organism. Profoundly unusual symmetries such as this may imply that some of the Ediacaran organisms should be classified in a major group of their own, separate from all living metazoans.
Fossils of Ediacaran-like organisms are found in the late Ediacaran, around 549 million years ago, with calcareous skeletons. Organisms such as Cloudina from the Nama group in Namibia represent the earliest biomineralized organisms, and are found 7 million years before the Precambrian-Cambrian boundary.
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