The absence of animal fossils in sediments older than about 570 Myr has two possible explanations. Either the absence is genuine, or the organisms concerned were cryptic and too small to be easily fossilized. The former view corresponds with a literal reading of the fossil record, and as such is naive. Animals almost certainly had a history prior to 570 Myr, but this in turn provokes two further questions. Was the time of origination relatively shallow, say 700 Myr, or deep, in excess of 1000 Myr? Evidence from molecular 'clocks' is used to argue the latter, notwithstanding evidence of highly variable substitution rates. Second, did the first animals have any similarity to the extant meiofauna or planktotrophic larvae, both of which are widely cited as plausible models for envisaging pre-Ediacaran forms. Both, however, suffer from difficulties, and a more plausible strategy may be to consider the fungal-metazoan connections rather than superficially appealing analogies drawn from more recent faunas.
The Cambrian explosion, about 544 million years ago, marks the sudden appearance of animal skeletons and carapaces in the geological record. All of a sudden, a wide range of animal phyla discovered how to produce highly functional biominerals with complex shapes and sufficient strength to support locomotion and to provide shelter. We now know that it was not a single discovery, but a cluster of many: the fossil and DNA records show that prior to the Cambrian, the different animal stocks were already in existence (Ayala et al., 1998; Balavoine and Adoutte, 1998), but that they carried no minerals. How could the complex calcifying machinery emerge in so many stocks independently? To answer to that question, we have to hypothesize that the functional components of the calcifying machinery were already present before the cambrian radiation, but that these components were used for other functions than mineralization. Thus, during the Precambrian/Cambrian transition, the animals only had to orchestrate these available functions so that well-organized biominerals could emerge (Westbroek and Marin, 1998). Our hypothesis is based on the following observations:- the skeletal matrix proteins that regulate the growth of CaCO3 crystals show striking similarities with mucus substances (Marin et al., 1996). In the heavily supersaturated oceans of the late Precambrian, mucus substances secreted by the "naked" Ediacara fauna may have represented a protective layer against the spontaneous precipitation of CaCO3 nuclei on their epithelial tissues. At the Precambrian/Cambrian transition, the same inhibiting mucus may have been re-used for a new function, to keep crystallization in check. These findings are now being more firmly established by the cloning of an entirely new gene which encodes a "mucin-like" protein of the skeletal matrix.- Lopez and co-workers (Atlan et al., 1997) discovered that the bivalvian mother-of-pearl can induce the synthesis of bone tissues by osteoblasts (the bone-forming cells). This remarkable result strongly suggests that the nacre contains one or more signal-molecules, like BMPs (Bone Morphogenetic Proteins), capable of activating the osteoblasts. These molecules are probably part of a very ancient regulatory system, which antedates the metazoan radiation. During the Cambrian explosion, this system may have been slightly modified and adapted, for driving the process of mineralization. These two aspects of animal calcification (inhibition of mineralization by skeletal matrix and regulation of mineralization by upstream signal-molecules) are, at present, investigated with the techniques of molecular genetics.
Atlan G, Balmain N, Berland S, Vidal B & Lopez E, C. R. Acad. Sci. Paris, 320, 253-258, (1997).
Ayala FJ, Rzhetsky A & Ayala FJ, Proc. natl. Acad. Sci. USA, 95, 606-611, (1998).
Balavoine G & Adoutte A, Science, 280, 397-398, (1998).
Marin F, Smith M, Isa Y, Muyzer G & Westbroek P, Proc. natl. Acad. Sci. USA, 93, 1554-1559, (1996).
Westbroek P & Marin F, Nature, 392, 861-862, (1998).
The affinities of a considerable part of the earliest skeletal fossils are problematical, but the investigation of their microstructures may be useful for understanding of biomineralization mechanisms in early metazoans and helpful for their taxonomy. The skeletons of the Early Cambrian anabaritids, hyoliths, and molluscs increased accretionary by marginal secretion of new growth lamellae or sclerites. The basal elements are aragonitic fibers. Biomineralization process was in its inception and did not produce as many variations as in later forms. Therefore it seems unlikely that similar microstructures within the groups derived convergently, but reflect a common origin and may be usefully combined with data on general morphology.
Anabaritids were possibly three-ray symmetrical cnidarians. Their thecae likely consisted of aragonitic fibers arranged in spherulitic sectors. In Jacutiochrea tristicha, Tiksitheca licis and Cambrotubulus conicus the fibers were finely replaced by celestite (SrSO4) with more or less significant amount of barite (BaSO4) before burial into sediment or rather early in diagenesis. Identical fibrous fabric was replicated by phosphate on the internal molds of Anabarites modestus, A. signatus, A. tricarinatus, and Tiksitheca licis.
Hyolitha is recently regarded as a separate phylum close to molluscs and sipunculans or a group within one of these. Their exoskeleton likely consisted of two basic components: a network of organic filaments, and aragonitic fibers arranged in accordance with them. Crossed-lamellar microstructure typical of molluscs and attributed to hyoliths is reinterpreted. The thecae of hyoliths are built of 1-2 layers of mineralized fiber bundles. In the outer layer the bundles are directed longitudinally and inclined towards the apex. Each bundle contains a pore channel. Transverse bundles of the inner layer run around the theca, but may branch and produce an orthogonal network. Pores between them were possibly connected to the pore system of the outer layer. In the opercula fibrous bundles and pore channels were inclined towards the center.
In shells of mollusc-like organisms the outer layer likely consisted of low aragonitic prisms, flattened spherulites or sclerites. The inner layer was composed of aragonitic fibers that may be arranged in lamellae. Crossed-lamellar microstructure existed in early molluscs. A phosphatized layer of fibers was observed under an outer scleritic layer of Purella. A fibrous layer was situated below an outer prismatic or spherulitic in Aldanella. A laterally compressed monoplacophoran Anabarella plana and the earliest rostroconch Watsonella had identical shell microstructures (prismatic and crossed-lamellar). This supports a position of Anabarella as an evolutionary link between monoplacophorans and rostroconchs (ancestors of bivalvians).
The fact that also early animals underwent embryological development should have come as no surprise. The recent discoveries of fossilized embryos in Cambrian (Zhang & Pratt, 1994; Bengtson & Yue, 1997) and Neoproterozoic (Xiao et al., 1998) rocks thus in themselves taught us more about taphonomy and fossilization than about biology. Nonetheless, the availability of fossil embryos promises to extend considerably our understanding of early animal evolution. There are, however, methodological obstacles to be overcome before palaeoembryology can contribute significantly to biological knowledge. The first obstacle concerns the recognition of embryos as such. Initial experiences suggest that fossil embryos may not be uncommon but have simply gone unrecognized. This calls for education and for more appropriate micropalaeontological techniques. A second obstacle concerns the problem of connecting the different developmental stages with each other. Embryos are simple in structure and may undergo several total reorganizations during their development, and the morphological similarity with the larvae, juveniles, or adults may be weak. As fossil embryos cannot be raised in petri dishes, this calls for very careful analyses of large samples to ascertain which stages belong to the same organism. In fact, only one of the Proterozoic-Cambrian embryos found to date has been safely matched with an adult form. A third obstacle lies in the fact that fossilized embryos often have only their surfaces well preserved - it may thus be difficult to figure out the exact configuration of blastomeres. In rare cases, however, phosphatization may have replicated internal structures as well, and it is important to work out non-destructive methods to study such material. With these obstacles overcome, fossil embryos may help to give fundamental insights into questions such as the reproductive strategies of extinct animals, the respective roles of planktotrophy and lecithotrophy in the evolution of animals, the evolution of the full life cycle in animals over evolutionary time, and the dates of origination of animal lineages.
Bengtson, S & Yue Zhao, Science, 277, 1645-1648, (1997).
Xiao, S, Zhang, Y & Knoll, A, Nature, 391, 553-558, (1998).
Zhang, X & Pratt, B, Science, 266, 637-639, (1994).
Interpretation and identification of Proterozoic trace fossils present unexpected and possibly unique difficulties. Siliciclastic sediments of the Terminal Proterozoic preserve elongate tubular organisms and structures of probable algal origin, many of which are very similar to trace fossils. Such enigmatic structures include Palaeopascichnus and Yelovichnus, previously thought to be trace fossils in the form of tight meanders. The record of undoubted trace fossils is meagre and is dominated by simple horizontal structures formed close to the sediment surface. None of the many reports of trace fossils in sediments older than about 600 Ma is uncontroversial, and the first unequivocal trace fossils post-date the Varanger Ice age, which ended about 590 Ma ago. Probably the oldest trace fossils are younger than 570 Ma. We tentatively suggest that three steps of increased diversity and complexity of Terminal Proterozoic trace fossils can be recognized. The earliest undoubted trace fossils are unbranched horizontal forms broadly of a Planolites-type. A second step is marked by the appearance of loosely meandering patterns of unbranched traces such as Helminthopsis and Gordia. In sections younger than about 550 Ma a further but modest increase in diversity occurs. This involves the appearance of simple branching burrow systems; traces with bilobed and three-lobed lower surfaces, rare treptichnids, and also radular marks. Despite this increase in diversity the depth of bioturbation remains, with very rare exceptions, shallow (<1 cm) and the ichnofabric virtually unrecognizable in polished sections. A more distinctive ichnofabric first appears in the Cambrian with the appearance of Treptichnus pedum, other treptichnids, and large Psammichnites-type burrows. Radular marks and complex burrows of Treptichnus-type probably provide the strongest direct evidence for soft-bodied bilaterian animals pre-dating the Cambrian. There are, however, no trace fossils with imprints of appendages. The increase in trace fossil diversity through the Terminal Proterozoic then escalated into the Cambrian, so that by the end of the Cambrian most major types of invertebrate trace fossils had appeared. Taphonomic conditions for the preservation of very shallow infaunal activity would have been particularly favourable in the Proterozoic because of the limited bioturbation, and sediment binding by extensive microbial/algal mats. Under these conditions traces formed directly on the sediment surface may have been more readily fossilized, for example by blanketing from a muddy tempestite. Trace fossils as small as 0.1 mm in diameter are found in the Terminal Proterozoic. The absence of any sign of benthic activity by animals (or protoctists) before about 570 Ma provides a framework which needs to be compared and contrasted with results from molecular studies. Together these data will throw new light on the timing of appearance and organization of the earliest animals.
The increasing knowledge on Vendian palaeontology leads to evaluate the Cambrian radiation as a transitional rather than "explosive" biotic episode in the history of life. At the present, processes as biomineralization are known to occur prior to the Cambrian times, and some metazoan groups such as mollusks and poriferans seem to have a late Neoproterozoic record. Though it has an objective basis, the notion of a sudden biotic turnover at the beginning of the Cambrian Period is magnified to a certain extent by the sedimentary hiatuses spanning many of the Vendian-Cambrian successions around the world. Important hiatuses are present in such successions from northern and southern Spain, but not in the central part of the country, where the Vendian/Cambrian transition is recorded without important breaks. The western part of the northern side of the Valdelacasa anticline (Toledo Mountains) is one of the most suitable areas to study this transition in central Spain. Here, fine siliciclastics of the Río Huso Group crop out providing a fairly rich record of ichnofossils, small shelly fossils and trilobites. The stratigraphic succession is ca. 2,000 m thick and includes a lower unit of greenish shales, a middle unit including black, micro laminated shales, phosphate beds and conglomerates, and an upper unit of greenish shales, very fine sandstones and calcareous sandstones.
Cambrian-diagnostic trace fossils appear from the base of the lower unit, including Phycodes pedum, Monomorphichnus lineatus and small specimens of Psammichnites ichnosp. The middle unit contains shelly fossils, namely Cloudina co-existing with anabaritids. The upper unit has recently provided new finds: a lower assemblage consisting of small shelly fossils (aff. Aldanella and hyolithids) and trilobites (aff. Hupetina), and an upper assemblage with more diverse small shelly fossils (aff. Aldanella, aff. Rhombocorniculum, circothecids and allathecids). Trace fossils in the upper unit are of a bigger size and include feeding burrows of several patterns, such as Dactyloidites ichnosp., Treptichnus bifurcus and big specimens of Psammichnites gigas. This succession is overlain by sandstones and shales of the Azorejo Formation containing Rusophycus ichnosp.
According to this fossil record, the age of the succession ranges from the lowermost Cambrian to the late Tommotian.
In consequence, the Valdelacasa anticline seems to be one of the best areas in western Europe to make detailed palaeontological studies at the Vendian-Cambrian transition.
Phytoplanktonic protists, represented mostly by organic-walled cysts and/or envelopes of algae and probably of other unknown phyla and referred to the informal group of acritarchs, are extremely abundant in the Neoproterozoic and Cambrian marine successions. The well-established and relatively short durations of discrete taxa together with their cosmopolitan distribution make them ideal for studies of microbiotic diversity trends. Compilation of the occurrences throughout the Neoproterozoic and Cambrian shows patterns in speciation, decline in diversity and extinction of phytoplankton that suggest significant fluctuations in the taxonomic composition and biodiversity of primary producers. As evidenced by well-defined acritarch assemblages, the early Neoproterozoic plankters remained in stasis for short periods of time, collapsing down to low diversity assemblages. During the Ediacarian times, morphologically diverse acritarchs and cyanobacteria flourished and then subsequently declined during the Kotlinian or Yudomian times. The rise of new disparate assemblages during the Cambrian was essential for early metazoan diversification due to their role in generating nutrients at the basic link in the food web (primary producers) for the evolving consumers. Speciation and extinction patterns and major taxonomic turnovers recorded in acritarchs are paralleled with only a short time lapse by radiations of Cambrian metazoans, i.e. first shelly meatazoans, the Chengjiang, the Sirius Passet and the Burgess Shale faunas. Cambrian acritarch assemblages evolved over realtively short time spans, diversifying from intial low-diversity residual populations after gradual diversity declines.
The so-called Vase shaped microfossils1, exhibiting a small size (around 100 m) and distinctive morphology, are known from Neoproterozoic sediments around the world. Their restricted stratigraphic range, and the consistency of both their morphology and size in a wealth of samples, suggest that they form a natural grouping. Of particular interest is their anterior-posterior differentiation, the oldest to be seen in any fossil. The biological affinities of the group are unclear, but several morphological analogues exist in extant organisms, including the loricas of some heterotrophic protist such as tintinnids or thecamoebas, and the reproductive structures (sporangium, gametangium) of fungi, algae and other protoctists. Their resemblance to the Palaeozoic fossil group Chitinozoa has also been widely discussed in the literature. Most detailed observations of vase-shaped microfossil morphology are based on specimens preserved as organic-walled vesicles that can be isolated from the sediment by standard palynological techniques. The vase-shaped microfossils reported here derive from phosphatic nodules of the Late Proterozoic (Late Riphean to Early Vendian) Visingsö Beds in Southern Sweden. The fossils are preserved as phosphatic moulds with no trace of organic matter remaining. They are exceptionally well preserved, suggesting that phosphatization took place at an early stage of diagenesis when the walls of the microfossils were still intact. These Visingsö vase-shaped microfossils reveal internal anatomical features that complement existing knowledge of this problematic group and which constrain the range of their possible biological affinities.
Extremely well preserved acritarchs from the Lower Cambrian Lükati Formation in Estonia were studied. Surface sculptures and opening structures of acritarch vesicle walls were observed using the transmitted light and the scanning electron microscopic (SEM) techniques. Besides traditional observations of the outer sculpture, the application of the SEM allowed to obtain images illustrating the inner wall surface of microfossils from the genera Skiagia, Globosphaeridium and Archaeodiscina. The structure of a plug which separates processes of Skiagia compressa from the central body is recorded. The ultrastructural study is performed using the transmission electron microscopy (TEM). Single or double layered wall structures are observed in different acritarch genera. Widely distributed pores, which penetrate the upper part of the wall, are recorded only in Tasmanites. The single layered wall of Archaeodiscina embraces a hollow rounded cavity which probably corresponds to the star-shaped thickening of the vesicle previously observed using the transmitted light microscopy and the SEM. The system of radially arranged channels penetrating the vesicle wall has not been observed in the present material. The TEM study of processes in acanthomorphic acritarchs supports previous descriptions based on observations made by other microscopic techniques. Long hollow processes with funnel-like tips and solid thorn-shaped processes are recorded respectively in Skiagia and Globosphaeridium. The wall thickness varies from 200 nm to 1.5 µm in various acritarch genera.
A long-term problem of interpretation of the pre-Hercynian cycle in the Western Carpathians is insufficient or completely lacking biostratigraphical datings. At present, the fossil remnants (condontoid elements, tests of primitive single cell foraminifers and peculiar chitinous shields) were extracted from lydites of the Gelnica Group. The conodonts are represented by suberectiform elements with smooth surface or conical elements with cusp and dentate posterior blade. Their systematic appurtenance may be preliminarily specified to the genera Panderolus, Proconodontus, Cordylodus and Prooneotodus? which are the simplest forms of distacoid conodonts already described from the Cambrian, but mainly from the Ordovician. Most numerous from extraced fossils are single cell spherical tests (sometimes even monoassociations). The tests have thin microgranular, from the inner side even hyaline walls with large inner chamber and oval apertural opening. According to the morphostructure they very likely belong to foraminifers of the family Saccaminidae BRADY. Some of them may be identified directly with the species of Psammosphaera cava MOREMAN, Psammosphaera micrograna EISENACK, Amphitremoida tubulosa EISENACK, Weikkoella sphaerica SUMMERSON, Hemisphaerammina bradyi LOEBLICH & TAPPAN, etc. The foraminifers Psammosphaeridae are already known from the Cambrian, particularly rich association they, however, occurred in the Ordovician and Silurian. Silurian associations of psammosphers are, however, also associated by more developed forms of foraminifers (Ammodiscidae, Trochamminidae, Tolypamminidae), which are missing in lydites of the Gemericum. In insoluble residues relatively frequently segmented chitinous shields appear. The functional morphology and chitinous composition of shields indicate their affinity to embryonal forms of arthropods (protaspids). Such analogies mainly result from the shield-like character of tests, the presence of distinct dorsal ring with articulation and frilled side membranes resembling pleural lobes. In lydites of the Gemericum the protaspid larvae are just chitinous cuticles without exoskeletons. Such chitinous "shirts" could be derived from exuvia as development of protaspid larvae passed through several stages of moulting. Chitinous microfossils with affinity to protaspid larvae may also indicate connection with the dominance of arthropods (mainly trilobites) in the Late Cambrian and Ordovician.
Investigation of present iron precipitation by bacteria can lead to a better understanding of the origin of ancient iron formations. In anoxic sediments, Fe2+ is abundant available and can be oxidized by bacteria to ferric iron without participation of molecular oxygen. These phototrophic bacteria isolated by Ehrenreich et al. (1994) and Heising et al. (1998) use ferrous iron as an electron donor for their anoxygenic photosynthesis. In bicarbonate-buffered environments, the redox potential of Fe2+ /Fe3+ is strongly determined by the solubilities of the iron species FeII-carbonate and FeIII-hydroxide. Under these conditions the redox potential is low enough that Fe2+ can serve electrons for photosynthesis according to:
Fe2+ -> 1 Fe3+ + 1 e-
(at approx. +200 mV)
However, there is a problem for bacteria gaining their electrons from iron:
-low substrate solubility (siderite, FeII-phosphate, FeS)-insoluble product (Fe(OH)3, goethite, haematite etc.)
Growth experiments in deep agar cultures have shown that an iron oxidation occurs with FeCO3, FeSO4 as substrates. No oxidation was observed on FeII3 (PO4)2 (vivianite), FeS and pyrite. In the ultrafiltrated supernatant of cultures there was found an increased solubility of iron. Ferrous and ferric iron concentrations were about 100-fold higher than the calculated values. These observations led to the search for an iron chelator. Liquid chromatography mass spectroscopy of the supernatant showed a compound with mass 151, which was not identified yet.
As the anoxygenic photosynthesis probably evolved before oxygenic photosynthesis (Schidlowski 1979), this group of bacteria may is responsible for ancient iron deposits. Now we have the means to compare recent precipitates with precambrian Fe-mineral deposits potentially formed by these bacteria.
Ehrenreich A, Widdel F, Appl Environ Microbiol, 60, 4517-4526, (1994).
Heising S, Schink B, Microbiology, 144, 2263-2269, (1998).
Schidlowski M, Orig Life, 9, 299-311, (1979).
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