Francis Lethiers

Ostracods, organic matter and anoxic events associated with the Frasnian/Famennian boundary in the Schmidt quarry (Germany)

Abstract The distribution of ostracods across the Frasnian/Famennian boundary in the Schmidt quarry is documented and all the benthonic species figured. Eighty per cent of the Frasnian ostracods disappeared close to this boundary, and all myodocopid species (Entomozoacea and Cypridinacea) were exterminated. The recovery in the Famennian seems to have been slow and gradual, and twelve lazarus species have been identified. These benthonic ostracods probably survived the Late Frasnian anoxic event sheltered in shallower marine environments. A strong relationship exists in the Schmidt quarry between fluctuations of the oxygen content of the bottom waters as displayed by the ostracod distribution, and the total organic carbon contents measured by the Rock-Eval Pyrolysis. Two new species are erected: Ampuloides kellerwaldensis nov. sp. and Acratia badwildungensis nov. sp.

Ostracods Late Devonian mass extinction : the Schmidt quarry parastratotype (Kellerwald, Germany)

Abstract Sixty-eight ostracod species have been recognized (the benthonic one for the first time) close to the Frasnian/Famennian boundary in the Schmidt quarry parastratotype. In the Late Frasnian, ostracods belong to an assemblage of the Eifelian ecotype and to the Myodocopid ecotype. In the base of the Famennian, they belong exclusively to the Eifelian ecotype. The oxygen content of the bottom waters during the deposition of the Upper Kellwasser horizon was highly unstable whereas this horizon was known to be anoxic. The Late Devonian mass extinction, probably the second in order of magnitude during the Phanerozoic, took place in the last 5 cm of the Upper Kellwasser horizon. Twelve ostracod species recognized in the Schmidt quarry survived this exvent.

Ostracods from the Frasnian/Famennian transition beds in the Lijiaping section (Hunan, southern China)

Abstract Ostracods are highly abundant but poorly diversified in the Frasnian/Famennian (F/F) transition beds in the Lijiaping section of Hunan Province, southern China. Sixteen species have been recognized, and the fauna is distinctly dominated by Kloedenellacea, and by the genus Famenella 1953. This assemblage represents probably the first evidence from Hunan, of an ostracod fauna recovering from the Late Devonian mass extinction event.

Autopsie d'une extinction biologique. Un exemple: la crise de la limite frasnien-famennien (364 ma)

Abstract Without studying the causes of the F/F boundary mass extinction, the precise analysis of ostracod species shows evolutionary process only discernable at the global scale. About 75% of all neritic species disappeared in the Uppermost Frasnian in all studied regions of the world. Others survived, with some geographic changes, owing to littoral refuges (Lazarus eflect). Deep benthic ostracods seem almost untouched by this event. We show that new post-event species resulted from allopatric speciations by migration between neritic provinces or along continental slopes towards deeper environments. During the event surviving lineages show a continuous gradation from unscathed species to chronocline species, to phyletic subspeciations or speciations (= pseudo extinctions) and even to new genera. The durability of lineages is controlled by the migration of populations.

Ostracods Prove that the Frasnian/Famennian Boundary Mass Extinction was a Major and Abrupt Crisis

For the study of the Late Devonian mass extinction, probably the second largest during the Phanerozoic, more than 40,000 ostracods have been extracted from samples collected close to the Frasnian/Famennian boundary in principal reference sections all over the world. Their study shows that about 75% of all marine species disappeared abruptly close to this boundary.

The recovery of the ostracod fauna after the Late Devonian mass extinction : The Devils Gate Pass section example (Nevada, USA)

The recovery of the ostracod fauna is rapid after the Late Devonian mass extinction in the Devils Gate Pass section and fifty-nine species have been recognised in the Early Famennian. Forty-five appear for the first time, whereas fourteen others are known from the Late Frasnian (= lazarus species). The ostracod distribution in the Early Famennian at Devils Gate is greatly influenced by regional sedimentological factors principally induced by a drastic sea-level eustatic fall.

Les extensions stratigraphiques des espèces d'ostracodes sur les plates-formes dévoniennes: le concept des répartitions sigmoïdales

Abstract Some important notions concerning the processes of speciation are reviewed. According to studies of Devonian ostracods, the biostratigraphical range charts show that ranging species may take on the shape of a sigmoid, graphically speaking. The concept of the sigmoid-ranging species, from sedimentary platorm series, allow one to distinguish faunal phases of installation, stabilization and renewal during a transgressive-regressive cycle. In particular, this concept explains the differences in stratigraphic distribution between species; it agrees reasonably well with the punctuated equilibria model; it explains the frequency of the biostratigraphical zones; it fits with the notion of third-order cycles and integrates palaeobiogeographic mechanisms such as endemism and ubiquity of species. Some details of this concept are debated.

Micronewsomites et decoranewsomites, deux nouveaux genres d'ostracodes dévoniens

Resume Deux genres devoniens nouveaux appartenant a la famille Pachydomellidae sont fondes : Decoranewsomites (espece-type : Newsomites multicavus Rozhdestvenskaya, 1972 ) et Micronewsomites (espece-type : Newsomites natus Rozhdestvenskaya, 1972 ). La diagnose du genre Newsomites est emendee. Microcheilinella notabilis umbonata Polenova, 1960 , Newsomites notabilis kusnezkiensis Polenova, 1968 , et Newsomites notabilis arcticus Polenova, 1974 qui appartiennent au nouveau genre Micronewsomites, sont eleves au rang specifique.