The most obvious fossils in Precambrian sediments are structures, called stromatolites, built up in layers by blue-green algae, but many microfossils have also been reported; some of these latter are rods and spheres which were perhaps formed by algae or bacteria. Traces of animals are absent until relatively late Precambrian times.

Stromatolites appear very early in the geological record and continue to the present day. They are laminated calcareous structures built up from fine sedimentary material (usually calcium carbonate) accumulating on blue-green algae (see Fig. 39). Modern filamentous algae trap fine-grained sediment particles with sticky mucosal coverings coating minute algal strands. Sediment which has been trapped in this way is then bound by further growth of the algae. The oldest stromatolites may be around 3,350 million years old (Muir and Grant, 1976); from this time onwards they are the dominant fossils in the Precambrian and they have been used as a basis for correlation (Walter, 1972; Hoffman, 1974). The oldest known water-laid sediments are about 3,800 million years in age, so some primitive forms of life appear to have existed not very long (geologically speaking) after the earliest direct record of water on the earth's surface.

The evidence from a sedimentary basin about 1,800 million years old, in Northern Canada, suggests that stromatolites had a greater depth range in the Precambrian than in later times; they appear to have lived in deep water as well as in shallow water (Hoffman, 1974). Some individual stromatolite colonies are very large: over 10m high. Today, stromatolites are restricted to very shallow water, and to some hypersaline (very salty) lagoons. This restriction is due to the action of grazing animals, like some gastropods, which can destroy the laminations faster than the blue-green algae can form them, and to destruction by burrowers such as bivalves and worms. The algae can therefore only flourish today where these destructive animals are absent (Garrett, 1970); thus at present they are found mainly in hypersaline lagoons — they have a greater tolerance of salinity than most animals — and on tidal flats exposed for long periods, where grazing animals are absent and burrowers are few. These observations help to explain why stromatolites were more abundant in the Precambrian (before the grazers and burrowers evolved) than at later times.

Plants do not necessarily need free oxygen, but animals do: the two universal necessities for animal life are water and oxygen. Early evidence of water is seen in the oldest sediments (3,800 million years old) which were deposited by water currents, but it is probable that free oxygen was not produced until much later in the Earth's history. After the time when plants first developed (about 3,400 million years ago), there was probably a very long period before the amount of free oxygen in the atmosphere and dissolved in seawater reached a level which could support the first primitive animals (Cloud, 1976). The oldest red (oxidized) sediments are about 1,800 million years old, and occur in many parts of the world.

Apart from a few possible burrows, perhaps made by annelid worms, the first reliable records of fossil animals occur in late Precambrian rocks, younger than about 700 million years. The Pound Quartzite of Ediacara in South Australia has yielded soft-bodied coelenterates, annelid worms and some other animals of uncertain affinities (Glaessner and Wade, 1966); many of them were possibly free-floating (Cloud, 1968). Elements of the Edia-cara fauna have been recorded in south-west Africa (in beds considered to be of Lower Cambrian age), in England (in Charnian beds of about 680 million years), in Siberia (670 million years) and in south-east Newfoundland. The fauna thus appears to span a long period of time (680 to 570 million years), but there are some doubts about the exact age of many of the beds in which it occurs (Glaessner, 1971).

It is not known when animals first developed. They might have had a long history prior to the development of the Ediacara fauna, or they might have developed rapidly shortly before this fauna appeared. There is also uncertainty about the exact relations between the Ediacara fauna and those animals present in the Early Cambrian. Were the varied Lower Cambrian faunas all descended from the Ediacara coelenterates and annelids? Or were they descended from soft-bodied ancestors which developed slowly over a long period of time before the Ediacara animals developed? In the absence of any direct evidence, we can only speculate. Cloud (1968, 1976) summarizes most of the relevant facts and concludes that some rapid late Precambrian adaptive radiation in animals is the best guess in the light of our present knowledge.

The Appearance of Animals with Hard Parts

Many geologists have postulated that some critical geochemical change or some physical event on the Earth may have been responsible for the appearance of animals with hard parts at the base of the Cambrian. However, there is no evidence of any significant differences between most Cambrian sediments and those that were laid down at the end of the Precambrian. Nor can the late Pre-cambrian glaciations be held directly responsible, for it now appears that these ice ages occurred at slightly different times on different continents, so that there may not ever have been a single major glacial event.

A biological change seems more probable. If we study the trilobites, archaeocyathids, brachiopods, sponges and molluscs which appear at or near the base of the Lower Cambrian, we can observe that they all have one feature in common: the hard parts are external. Though the skeletons clearly support the animal and act as attachments for muscles, the most important feature which they all possess in common is that they protect the animals. Very few Early Cambrian shelled animals were burrowers or active swimmers. The variety of forms present in the Early Cambrian suggests that the immediate soft-bodied ancestors must have been quite diverse. Though there may have been a late Precambrian evolutionary radiation, this variety was greatly enhanced when animals with hard parts appeared. The hard part could perhaps have been developed in response to the evolution of the first carnivores (Hutch-inson, 1961), the only change required at the start of the Cambrian being for some scavenging worm to start eating animals that were still alive instead of those that were already dead. All the other developments could have stemmed from this change. In this connection, it is relevant to note that animal groups (like the grapto-lites, fish and corals) where the skeleton is not for external protection, do not appear until after the Lower Cambrian.

Stanley (1976) considered that the known Cambrian faunas could all be descended from the animals represented in the Edia-cara fauna. He pointed out that, during the first part of Early Cambrian time (the Tommotian), the only groups additional to those in the Ediacara fauna were: hexactinellid sponges, archaeo-cyathids, inarticulate brachiopods, gastropods, hyolithids, and a few problematical groups. The remaining Early Cambrian taxa (including the trilobites and the articulate brachiopods) appeared in the later parts of the Early Cambrian.

Stanley concluded that two basic factors could have been responsible for the widespread development of hard parts in these different groups: first, protection from predators; and second, the increase in efficiency and performance brought about by the development of hard parts, which allowed many taxa to undergo adaptive radiation and relatively rapid diversification. The hard parts act as body supports and as muscle attachments, and it is difficult to conceive of a trilobite or a brachiopod functioning without their hard parts. Their soft-bodied ancestors must have been very different types of animals.

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