Colonisation of the early ocean floor

Abstract

THE earliest well documented evidence of macroscopic life is provided by stromatolitic algae from sediments deposited in intertidal or shallow marine environments at least 2,700 m.y. ago¹. It is generally agreed that the earliest metazoan animals were soft-bodied forms which are rarely preserved but have left tracks, trails and burrows, collectively known as trace fossils. The earliest reliably identified trace fossil is a burrow system considered to have been produced by a worm-like organism, probably an annelid, found in shallow marine clastic sediments of the Grand Canyon Series (USA) suggested to be over 1,000 m.y. old². Vertical burrows have been found in the Buckingham Sandstone in North Australia, also a shallow water deposit dated radiometrically as more than 790 m.y. old², and in the Areyonga Formation, of similar age³. A more extensive collection of trace fossils has recently been recorded by Webby⁴ from the clastic shallow marine late Precambrian (about 600 m.y. old) Torrawangee Group of New South Wales (Australia) with three trace fossil genera positively identified: Planolites, Cochlichnus and Torrawanga. The late Precambrian Ediacara Beds of South Australia, also interpreted as shallow water clastic deposits, have yielded one positively identified trace fossil genus (Cochlichnus) and five other forms, all thought to have been produced by metazoa². The common factor in these occurrences is the shallow water environment. Most of the trace fossils are burrows of infaunal deposit feeders which inhabited offshore environments of relatively quiet water; but the record of vertical burrows, now most commonly found in the high energy intertidal zone, may indicate occupation of even the shallowest water niches. There is as yet no indication of colonisation of the deep oceans during Precambrian times.