Chuaria and Precambrian amoebae

Overall, the 1700 m (5000 ft) or so of Chuar strata repeat an alternating pattern of shale and carbonate strata in 'bundles' some hundreds of metres thick. The stromatolite horizons occur sporadically within the carbonate strata. In between are thick sequences of interlayered shales (laminated muds) and thin sandstones. The latter are often covered with ripple marks and sometimes mudcracks, showing that the original sediment surfaces were occasionally exposed to the air and dried out. Within the shales Walcott spotted some tiny black specks, which aroused his curiosity.

Like any fossil hunter, he well knew that many interesting fossils are very small, especially those that occur first in the rock record. To look for them, you have get down on your hands and knees and put your nose to the rock. Walcott was lucky: some of these black specks were up to 5 mm wide, huge by micropaleontologists' standards. They appeared as round or oval discs made of some carbonaceous material, which must have puzzled Walcott considerably. But at least they boosted his confidence so that he could write in 1891 'that the life in the [Precambrian] seas was large and varied there can be little, if any doubt ... It is only a question of search and favourable conditions to discover it.' Eventually, in 1899, he formally named his Chuar fossils Chuaria circularis and described them as a kind of primitive shellfish called a brachiopod (or lamp-shell).

Brachiopods look like molluscan clams, but belong to a separate and different group of sea-dwelling shellfish. Although most brachiopods have calcareous shells like clams, there are small, primitive forms known from Cambrian strata that had organic shells, some of which were indeed circular in plan. Walcott thought that this was where his Chuaria circularis belonged. But even these apparently convincing Precambrian fossils did not make much of a stir. Walcott was wrong in thinking that his little Precambrian discs were brachiopods, but was right in thinking that they were organic in origin.

The true nature of Chuaria was not realised for nearly 100 years, not until 1985, when it was redescribed as the flattened and carbonised remains of an unusually large and spherical unicelled algal cyst, called an acritarch. Chuaria has now been found at many levels within the Chuar strata where it is generally microscopic (ranging in size from 5 mm down to 70 microns; a micron is a thousandth of a millimetre or four millionths of an inch) and occurs with other kinds of microscopic acritarchs. We now know that similar microfossils occur in late Proterozoic (Neoproterozoic) strata around the world dating from around 800-700 million years ago.

Again, it was chemical preparation of shales that revealed these microfossils. Shales are essentially muds and largely composed clay minerals that are remarkably resistant to chemical attack, which is partly why they are so good for making ceramics. Often the organic component of shales is very dilute and so the clay minerals have to be got rid of by drastic chemical treatment in order to concentrate enough organic material for observation under the microscope.

When muds such as these are many hundreds of millions of years old and have been buried beneath many kilometres of younger strata, they are lithified into hard, almost slaty shales. The only way to get rid of the clay minerals is to physically break the shale into small pieces in a rock crusher, then treat them with a very nasty chemical called hydrofluoric acid. This is highly dangerous, as it is colourless and has no odour but gives off a highly toxic gas and can dissolve flesh and bone quite readily. Plastic or wax is about the only material that can safely contain it. Even so, it may take several treatments and gentle heating (in special fume cupboards) to remove the clays, leaving a black sludge, if there was any organic material in the rock to begin with.

The 'organics' have to be oxidised using another cocktail of dangerous chemicals, then washed clean and permanently preserved in some liquid such as glycerine. If there is any fossil material left intact, it can be observed with a high-power microscope. Surprisingly, despite the very rough treatment, some microfossils are amazingly well preserved. So much so that some biologists have in the past refused to believe that certain Precambrian material is fossil at all, but rather think that it is modern contamination. Such contamination can indeed happen, but nowadays preparation techniques are rigorous enough to avoid such mistakes.

Even more interesting than the acritarchs are some strange, flask-shaped microfossils discovered in the 1970s. This time the microfossils were recovered by less drastic preparation techniques. The fossils occur in carbonate nodules -oval-shaped limestones that develop within the original mud sediment before it is flattened into shale. As a result, any fossil material within the nodules is protected from flattening. Another bonus is that the carbonate of the limestone can easily be dissolved by less noxious acids such as acetic or dilute hydrochloric acid. Nodules from one shale horizon yielded astonishing numbers of tiny (37-170 microns long) vase-shaped microfossils.

Using this extraction technique, Harvard student Susannah Porter recovered many thousands of specimens. At least 10,000 specimens were present in a sugar-cube-sized piece of nodule. Made of organic carbon, the teardrop-shaped flasks have an opening at one end and Porter was able to show that the vases were made by a kind of amoeboid unicell (Melanocyrillium) that lives within minute shells of their own making. It now turns out that such protists were common in late Proterozoic strata and their increasing abundance through Earth Time tells us that life in the oceans was becoming more diverse and complex by 750 million years ago.

Algae and cyanobacteria were the main micro-organisms that used light energy from the sun to photosynthesise organic compounds essential for their existence. Such organisms are called primary producers and form the base of the food chain for all other organisms. The evolution of the amoebae marks the next step in the development of the food chain, as they fed on the algae and bacteria. Susannah Porter also found that some of the amoebae shells have holes bored in them showing that they, in turn, were attacked and eaten by some as yet unknown predators. This is the oldest fossil record of such animals in Earth Time and records the formation of this important step in the evolution of life. Perhaps it also records the beginning of the arms race. Luckily, the event can be dated accurately to around 750 million years ago by the radiometric analysis of minerals in a volcanic ash that covers the shale and nodule layer in the Canyon.

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