The earliest ostracods: the geological evidence

Palaeobiodiversity and Palaeoenvironments 04/2008; 88(1):11-21. DOI: 10.1007/BF03043974

ABSTRACT The oldest assumed ostracods appear in the fossil record from the TremadocianPaltodus deltifer conodont Biozone. Although geographically widespread these early ostracods have no obvious Cambrian antecedents. Their first
appearance at ca. 485 Ma contrasts with molecular evidence that suggests a much earlier (latest Proterozoic or Cambrian) origin
for ostracods. Some Cambrian bivalved arthropods such asAltajanella andVojbokalina, conventionally referred to the Bradoriida, have carapace morphologies that resemble Ordovician palaeocopid ostracods, though
such a relationship is unproven without soft part anatomy. Evidence from preserved soft anatomy demonstrates that Bradoriida,
such asKunmingella, and Phosphatocopida, essentially the Cambrian ‘ostracod’ record of traditional usage, belong outside the Eucrustacea. Early
Ordovician ostracods appeared first in shallow marine, oxygenated environments on shelf margins, in a similar setting to other
elements of the ‘Paleozoic fauna’. Their biodiversity was low (3 named genera and ca. 12 species), though some taxa such asNanopsis andEopilla achieved widespread dispersal between major Ordovician palaeocontinents. As bradoriids were largely extinct by the Late Cambrian,
ostracods do not appear to have directly competed with them for shallow marine environments. The rapid colonisation of these
settings by ostracods may have been facilitated by the available ecospace vacated by Bradoriida.

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    ABSTRACT: In marine environments Ostracoda and Foraminifera have been very successful invaders. During the Phanerozoic they colonised the majority of shallow, marginal to deep water, fully marine habitats. Both groups had developed physiological adaptations which pre-adapted them to the invasion of new marine habitats. They adopted a broad range of feeding strategies and reproduction modes. The production of resting stages and brood care may also have contributed to them being efficient invaders. They are also both highly tolerant to variations in salinity. The first invasions of non-marine habitats by ostracods appear to have taken place at the turn of the Devonian and Carboniferous. It is estimated that there had been between 9 and 12 independent invasions of fresh waters by the ostracods. In contrast Foraminifera are typically marine organisms, and only a few species of agglutinated and organic-walled Foraminifera are to be found in brackish and freshwater environments. Agglutinated species build their test using ambient components but are not commonly regarded as calcifying organisms. An impact of salinity on foraminiferal calcification has been observed in several studies. It seems that Foraminifera are incapable of constructing a fully calcified test in low salinity regimes; they use sea water not only as a source of ions to construct shell, but also as a biomineralisation solution. Thus, the success of ostracods in invading freshwater habitats can be attributed to their development of a more effective mechanism of calcification in low mineralisation waters. The core question of this study is to examine possible causes for the differences in success between the two taxa.
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    ABSTRACT: Despite the well-established phylogeny and good fossil record of branchiopods, a consistent macro-evolutionary timescale for the group remains elusive. This study focuses on the early branchiopod divergence dates where fossil record is extremely fragmentary or missing. On the basis of a large genomic dataset and carefully evaluated fossil calibration points, we assess the quality of the branchiopod fossil record by calibrating the tree against well-established first occurrences, providing paleontological estimates of divergence times and completeness of their fossil record. The maximum age constraints were set using a quantitative approach of Marshall (2008). We tested the alternative placements of Yicaris and Wujicaris in the referred arthropod tree via the likelihood checkpoints method. Divergence dates were calculated using Bayesian relaxed molecular clock and penalized likelihood methods. Our results show that the stem group of Branchiopoda is rooted in the late Neoproterozoic (563 ± 7 Ma); the crown-Branchiopoda diverged during middle Cambrian to Early Ordovician (478 ∼ 512 Ma), likely representing the origin of the freshwater biota; the Phyllopoda clade diverged during Ordovician (448 ∼ 480 Ma) and Diplostraca during Late Ordovician to early Silurian (430 ∼ 457 Ma). By evaluating the congruence between the observed times of appearance of clade in the fossil record and the results derived from molecular data, we found that the uncorrelated rate model gave more congruent results for shallower divergence events whereas the auto-correlated rate model gives more congruent results for deeper events.
    Palaeoworld 02/2015; DOI:10.1016/j.palwor.2015.02.003
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    ABSTRACT: The 1st International Symposium on Ostracoda (ISO) was held in Naples (1963). The philosophy behind this symposium and the logical outcome of what is now known as the International Research Group on Ostracoda (IRGO) is here reviewed, namely ostracodology over the last 50 years is sociologically analysed. Three different and important historic moments for the scientific achievements of this domain are recognised. The first one, between about 1963-1983, is related to applied research for the oil industry as well as to the great interest in the better description of the marine environment by both zoologists and palaeontologists. Another important aspect during this period was the work by researchers dealing with Palaeozoic ostracods, who had their own discussion group, IRGPO. Gradually, the merger of this latter group with those dealing with post-Palaeozoic ostracods at various meetings improved communication between the two groups of specialists. A second period was approximately delineated between 1983 and 2003. During this time-slice, more emphasis was addressed to environmental research with topics such as the study of global events and long-term climate change. Ostracodologists profited also from the research “politics” within national and international programmes. Large international research teams emerged using new research methods. During the third period (2003-2013), communication and collaborative research reached a global dimension. Amongst the topics of research we cite the reconstruction of palaeoclimate using transfer functions, the building of large datasets of ostracod distributions for regional and intercontinental studies, and the implementation of actions that should lead to taxonomic harmonisation. Projects within which molecular biological techniques are routinely used, combined with sophisticated morphological information, expanded now in their importance. The documentation of the ostracod description improved through new techniques to visualise morphological details, which stimulated also communication between ostracodologists. Efforts of making available ostracod information through newsletters and electronic media are evoked.
    Palaeogeography Palaeoclimatology Palaeoecology 08/2014; 419. DOI:10.1016/j.palaeo.2014.08.016 · 2.75 Impact Factor


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May 26, 2014