The problem of the Precambrian-Cambrian boundary

In the last few years the problem of the lower boundary of the Cambrian has been dealt with in dozens of works which considered not only the history of the question but all the current material available. It is a subject discussed at many national and international symposia. Although great progress has been achieved in drawing together the points of view on the position of the boundary (compare the results of the symposia held in Paris in 1958 with that in 1974), the range of different opinions still remains wide, particularly with regard to the zonal scales (for example, in Siberia, from the base of the Nemakit-Daldyn horizon up to the base of the beds with Fallotaspis). First, therefore, we give consideration to those questions which often constitute a stumbling block. It seems necessary to make a reservation that some of the difficulties arising may undoubtedly be referred to as purely psychological. Considerable progress in selecting approaches to the solution of the PrecambrianCambrian boundary problem was stimulated by a rejection of the principle of using abiotic historical geological methods (such as palaeoclimatology, tectonics and the like) and a firm preference being given to biostratigraphy as the discipline which justified itself in the solution of Phanerozoic problems. However, even such a strictly definite approach has not saved us from a considerable divergence of views: first, the biostratigraphic data themselves are still not used uniformly (for example appraisal of the first appearance of various forms); secondly, a tendency is still strong to consider the trilobites as the 'orthostratigraphic' group of the Cambrian for solving this problem; thirdly, on the one hand, a skeletalized fauna is made a fetish, on the other hand, this notion is interpreted quite differently. It is the usage of the notion 'skeletalized fauna' that constitutes one of the most complex questions. Though a mutual understanding of different points of view is not necessarily a key to the solution of the problem, it could certainly be a means of eliminating seemingly important disagreements. In fact, some investigators use the term 'skeletalized fauna' for any hard form of tubular or shell-like dwelling-place as well as for spicules of sponges and tests or shells of protistids. If we followed this interpretation, we could naturally confirm the existence of' skeletalized fauna' in the Vendian and the Judoma complex, or at least in their upper parts. Those who reject the tubes of polychaetes and analogous forms as a physiologically based hard shell skeleton, point to the absence or extreme scarcity of normal skeletalized forms of invertebrates in the Vendian. But it is remarkable that the investigators of either group cite the same lists of fossils in the upper Vendian bed (Yudomian). For these upper Vendian beds, including the Nemakit-Daldyn horizon (Manykai as termed by other authors), both groups of researchers listed usually sabelliditids, angustiocreids (Anabarites) and very peculiar Protohertzina. Thus, in actual fact we have no contrasting faunal groups here, the point is that some consider the most abundant tubular organisms as a specific group comparable with other skeletalized organisms of the Phanerozoic. A lot of confusion was caused by the notion' sudden appearance of the skeletalized fauna'. The supporters of the view that different groups of organisms developed their capacity to build skeletons gradually point to the later appearance, for example, of trilobites and some other organisms, forgetting that it was those same investigators who drew attention to the known' suddenness' of the origin of the true skeletalized fauna who demonstrated both the late occurrence of trilobites and the early appearance of tubular organisms (Anabarites,