Precambrian Fossil Record Research Articles

Precambrian metazoans: the problems of preservation, systematics and evolution

The unique appearance of Vendian metazoans in the Precambrian fossil record is controlled not only by taphonomic and ecological factors, but also by the level of morphological and physiological organization of these animals. The peculiar nature of these factors means that the Vendian represents an important stage in metazoan evolution. This notion is supported by the Bauplan analysis of Vendian forms, many of which have a bodyplan (architectonics) that is most unusual in comparison with animals from later periods. This new information allows a revision of the systematics of Precambrian metazoans at a high taxonomic level. The new classes Cyclozoa, Inordozoa, and Trilobozoa are recognized among Vendian Coelenterata. The phylum Proarticulata, with classes Dipleurozoa and Vendiamorpha, represents the most primitive Vendian Bilateria. These, the oldest faunas on Earth, provide indication of the earliest modes of metazoan evolution that have not been part of the theoretical predictions of neontologists, and they also serve as a tool to check certain phylogenetic models.

NUCLEIC ACID ANALYSIS AND THE ENDOSYMBIOTIC HYPOTHESIS

SummaryIt is generally agreed that the first organisms were prokaryotic in nature. However, opinions differ as to the mechanism responsible for the origin of the eukaryotic cell – whether it arose by direct filiation from an ancestral blue‐green alga or by a series of symbioses.The timing of the emergence of photosynthesis in eukaryotes has largely been based on evidence from Precambrian palaeontology. For this reason attention is drawn to recent reassessments of the Precambrian fossil record, which have suggested that a number of structures previously considered to be remnants of eukaryotic nuclei and organelles may instead have been artifacts that formed in certain blue‐green algae during their lithification.Evidence regarding the mechanism responsible for the origin of chloroplasts may be sought in the molecular fossil record. Studies employing the techniques of DNA‐RNA hybridization and RNA sequencing (oligonucleotide cataloguing) have established the prokaryotic nature of chloroplast ribosomal RNAs. In addition, a close phylogenetic relationship between the blue‐green alga Aphanocapsa 6308 and the chloroplasts of Euglena gracilis has been indicated. By contrast, the sequence resemblance of prokaryotic and chloroplast 16 S rRNAs to eukaryotic 18 S rRNAs is revealed to be no greater than that expected between completely unrelated molecules. However, the size differences which exist between 16 S and 18 S rRNAs raise doubts as to whether these RNA species are in fact directly comparable. On the other hand, all known examples of mature 5 S rRNA are approximately 120 nucleotides in length, and thus this RNA would appear to bridge the evolutionary discontinuity between prokaryotes and eukaryotes in an almost unambiguous manner. The identification of 5 S rRNA in chloroplast ribosomes has therefore prompted a number of investigations designed to establish the prokaryotic or eukaryotic affinities of this RNA species. These have confirmed the prokaryotic nature of chloroplast 5 S rRNAs, and have indicated the sequence dissimilarity of these species to the cytoplasmic 5 S rRNAs of eukaryotes. However, it is pointed out that such results can only provide conclusive support for the endosymbiotic hypothesis, if rates of change in homologous RNAs have remained approximately constant in all lines.Combined evidence suggests that nucleotide substitutions in cytoplasmic ribosomal RNAs have occurred more frequently during the evolution of flowering plants, Gramineae in particular, than during the evolution of yeast or vertebrates. By contrast, chloroplast ribosomal RNAs are shown to be very strongly conserved. Moreover, considerable disparities are also noted in the relative amounts of change in prokaryotic and eukaryotic 5 S rRNAs. Although undoubtedly associated with their different architectures and functional roles, it is suggested that the primary explanation for this phenomenon lies in the fact that the cytoplasmic 5 S rRNAs of eukaryotes are not derived from that of the protoeukaryote host, but rather, from that of the protomitochondrion. The cytoplasmic j.8 rRNAs are now considered to be the extant descendants of the original protoeukaryote 5 S rRNA these species having increased in size in a similar manner to the other cytoplasmic ribosomal RNAs of eukaryotes.Because the rates of nucleotide substitution in homologous prokaryotic and eukaryotic ribosomal RNAs are non‐identical, consideration is given to derived character states. The late methylated sequence ‐Gm26Am26AC– has been identified in the 16 S rRNAs of many prokaryotic microorganisms and may possibly be universal in the 18 S rRNAs of eukaryotes. Although its presence has been indicated in the 18 S rRNA of Euglena gracilis, it is absent from the chloroplast 16 S rRNA of this protist, as well as from the 16 S rRNAs of Aphanocapsa 6308 and many other blue‐green algae. Direct filiation would seem unable to provide a satisfactory explanation for this phenomenon. As a result it ii included that chloroplasts, and most probably mitochondria also, are of endosymbiotic origin.

Precambrian Eukaryotic Organisms: A Reassessment of the Evidence

Comparison of partially degraded unialgal cultures of Chroococcus turgidus with coccoid microfossils from the Late Precambrian Bitter Springs formation, Australia, suggests that the Precambrian fossil record has been seriously misinterpreted. Use of degradational features as taxonomic characters has resulted in unrealistically high estimates of Precambrian algal diversity. There is at present no compelling evidence for the presence of eukaryotic microfossils in rocks from the Bitter Springs formation or any older sedimentary sequences.

Precambrian Paleobiology: Problems and Perspectives

In 1890, T. C. Chamberlin, the distinguished American geologist and educator, wrote a classic paper entitled method of multiple working The paper has been repeatedly reprinted in various forms (most recently in the May 7, 1965, issue of Science). On numerous occasions I have returned to this paper, struck by what I regarded initially as its rather curious central theme. In brief, Chamberlin a�gues that a scientist should take serious measures to avoid undue parental affection for a newly constructed theory (or paradigm, to use Kuhnian phraseology) ; that such affection too often leads the unwary to embrace concepts that are at best partial, and at worst erroneous, views of reality; and that to achieve the desired goal of objectivity, a scientist should erect a set of alternative working hypotheses that will serve to temper his enthusiasm for his favorite intellectual offspring. On first reading, this theme seemed flawed-how could a scientist generalize effectively, and how could he have the courage to put forth new but possibly radical and unpopular concepts unless he felt, and presumably felt deeply, that he really had discovered the Rosetta stone of his science? Would not such affection add to, rather than detract from, the workings of science? It seems to me now, however, that Chamberlin's message is well taken. To make progress in science, the acceptance of a given set of interpretations is no doubt required, but only up to a point ; the threshold of reasonability can be passed rapidly if one begins to press the data into a preconceived set of paradigmatic pigeonholes. It seems likely that this threshold has been surpassed in recent interpretations of the Precambrian fossil record. If this has in fact occurred, it is probably a function of the newness of the field. With few data available, it becomes relatively easy to construct uncomplicated, plausible, and seemingly correct hypotheses. Certainly, it could be a result of the style of evidence and argument necessitated hy historical science, where the data are always incomplete. Much of the evidence can be interpreted only in a qualitative, rather subjective, fashion, and proof' by analogy is commonplace. A careful reading of the l iterature suggests that