California fills the gap

In California, sedimentation continued from late Proterozoic times right through into the beginning of the Palaeozoic Era and its initial period, the Cambrian. Altogether some three kilometres of Neoproterozoic sediments (ranging in age from 1.0-0.542 billion years ago) accumulated in shallow marine seas, depositing sands, silts and carbonates with the occasional invasion of land-derived river-borne sediment from the east and southeast. Most interesting of all are strata known as the Kingston Peak Formation, which includes sandstones, volcanic rocks and limestones with different kinds of conglomerates, including a diamictite. Overall, the strata result from the rifting apart of Rodinia and crustal stretching around 700 and 600 million years ago.

In Death Valley National Park, near Saratoga Spring, diamictite conglomerates can be seen to contain a variety of pebbles of different size, shape and composition, all 'floating'; that is, surrounded by finer-grained sediment. These characteristics are very different from those of a typical beach conglomerate. Instead, they reflect deposition from sediment-laden and land-derived ice as it floats out to sea and melts. Similar diamictites are found in the nearby Ibex Hills. Limestones typical of wave- and current-agitated shallow, warm waters occur below and between diamictite horizons. The sequence is capped by another kind of limestone known as the Noonday Dolomite. This was also deposited on a shallow marine continental shelf margin, but in quiet waters with thin rhythmic layering thought to have been produced by the growth of algal mats over the sediment surface.

Measurement of carbon isotope values through the limestones shows positive values below, between and immediately above the diamictites. The only negative values are found higher up in the Noonday Dolomite, repeating the distribution seen in similar rocks in Namibia and Australia. So these late Proterozoic deposits repeat the pattern that has been found worldwide at this level and that is seen as evidence for an extensive if not necessarily global ice age at this time.

The uppermost, postglacial sequence of Proterozoic strata in Death Valley is called the Stirling Quartzite. It was from here that some of the oldest fossil shells in the world were found in the 1960s and 1970s. They are therefore of considerable interest.

For around 200 years one of the base lines of historical palaeontology has been the argument that the fossil record of shelled organisms begins with a 'bang' or explosion in diversity at the beginning of Cambrian times. Hundreds if not thousands of fossil collectors around the world have spent vast amounts of effort examining earliest Cambrian-age strata and the immediately older sedimentary rocks below. All seemed to confirm the view that shelly fossils first appeared in the earliest Cambrian-age strata. But in the late 1950s, the discovery of some tiny tubular and mineralised fossils in late Proterozoic strata in Brazil showed that the story is more complex and more interesting.

A sharp eye is needed to spot the fossils, as well as sufficient knowledge and inquisitiveness to give them a second glance. At first, they are not very impressive, just tiny, gently curved tubes a few millimetres wide and a centimetre or two long. Looking more closely, the tubes turn out to have thin walls, partly stiffened with carbonate mineral deposits. The tubes expand upwards and have growth lines on the outside, which are developed into circular flanges in some species. Just in terms of their form, they look quite like the tubes of a group of living marine 'worms' called the pogonophorans, but we have no soft-part evidence to support such an affinity. Indeed, there are reports that some Chinese fossil tubes of this general kind also have a simple branching pattern. If this is correct, then they are not pogonophorans but might rather have a link with the sea anemones, a group of primitive animals that later gave rise to the corals.

At first, the Brazilian specimens were thought to be a kind of calcareous alga. Then more were found in Namibia, southern Africa, and called Cloudina after Preston Cloud, an American pioneer of Precambrian palaeontology. Further finds in South America led to the realisation that they are not calcareous algae but the fossilised living tubes of a distinct group of extinct animals that together are now referred to as cloudinids. They have a global distribution in late Proterozoic strata and in some localities they are so common that they built reef-

Preston Ercelle Cloud, Jr., 1912-91, American palaeontologist, after the navy trained at George Washington University and Yale, joined the United States Geological Survey in 1941, professor at Yale (from 1946-8), returned to the survey until 1961 when he moved to a succession of university posts in California. Pioneered the study of Precambrian microfossils and his book, Cosmos, Earth, and Man, was published in 1978.

like mounds on the seabed. In Namibia, the reef mounds also contain another slightly skeletonized small organism called Namacalathus. This has a tubular stalk leading to a globular structure with six side openings, giving a hexagonal symmetry and a single, slightly larger opening at the top. So the pre-Cambrian evolution of mineralised skeletons was not a singularity restricted to the cloudinids, but extended to at least one other kind of organism.

So far, no shells of typical early Cambrian organisms have been found associated with the cloudinids and their 'friends'. Despite the more ancient and precocious development of skeletonisation, the idea that the main biological revolution occurred at the base of the Cambrian may still be correct. Nevertheless, late Proterozoic strata contain another palaeontological surprise that was not recognised until the 1950s - the so-called Ediacarans.

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