Kui Yan

The Ordovician Acritarch Dactylofusa velifera Cocchio 1982: A Biostratigraphical and Palaeogeographical Index Species

Dactylofusa velifera Cocchio 1982 is an easily recognisable, fusiform Ordovician acritarch species with a characteristic membrane. The taxonomy of this taxon is revised based on a literature review and on the investigation of new fossil assemblages from sections in South China. Morphological and biometric studies show that a subdivision into three varieties is justified. The taxonomical rank of Dactylofusa velifera var. brevis Albani 1989 is changed and its diagnosis is emended; Dactylofusa velifera var. velifera is the automatically created autonym. An additional new subspecific taxon is erected: Dactylofusa velifera var. sinensis var. nov. The stratigraphical and palaeogeographical occurrences of Dactylofusa velifera are reviewed, indicating that the species was widely distributed around the peri-Gondwanan margin during the Early Ordovician and early Middle Ordovician. Dactylofusa velifera is of biostratigraphical importance because its First Appearance Datum (FAD) can be used to indicate the late Tremadocian. Palaeobiogeographically, its distribution generally corresponds to that of the Early Ordovician ‘messaoudensis-trifidum’ acritarch assemblage in the peri-Gondwanan region.

A review of the Ordovician acritarch genus Barakella Cramer & Díez 1977

ABSTRACT The Ordovician acritarch genus Barakella Cramer & Díez 1977 and two species, B. felix and B. fortunata, bearing characteristic anastomosing filamentous elements at one pole were first described from the subsurface of the Kasba Tadla Basin, Morocco. Subsequently, another species was described from the South Chinese Ordovician and was later attributed to the genus as Barakella rara. New material from the type locality in Morocco and additional sections in South China are used for a review of the genus Barakella. Based on this review, including statistical analysis, Barakella rara is considered a junior synonym of Barakella felix. Barakella is a useful taxon for biostratigraphical applications in the Ordovician. Its first appearance datum (FAD) lies in the early Floian (Early Ordovician, time slices 2a–2b) in both South China and Wales. Except for a doubtful record from Sweden, all records of Barakella are from the peri-Gondwana margin, making the genus an indicator of the peri-Gondwana palaeoprovince. The appearance of Barakella in South China indicates that Barakella possibly originated in shallow-water nearshore environments, and later spread to offshore environments.

The palaeobiogeographical spread of the acritarch Veryhachium in the Early and Middle Ordovician and its impact on biostratigraphical applications

The genus Veryhachium Deunff, 1954, is one of the most frequently documented acritarch genera, being recorded from the Early Ordovician to the Neogene. Detailed investigations show that Veryhachium species first appeared near the South Pole in the earliest part of the Tremadocian (Early Ordovician). The genus was present at high palaeolatitudes (generally>60° S) on the Gondwanan margin during the Tremadocian before spreading to lower palaeolatitudes on the Gondwanan margin and other palaeocontinents (Avalonia and Baltica) during the Floian. It became cosmopolitan in the Middle and Late Ordovician. Although useful for distinguishing Ordovician from Cambrian strata, the diachronous first appearance data of Veryhachium morphotypes mean that they should be used with caution for long-distance correlation.

An Early–Middle Ordovician acritarch and prasinophyte assemblage from the Meitan Formation in Qijiang, Chongqing, South China

Acritarchs and prasinophytes representing 39 species in 26 genera are identified from the Early–Middle Ordovician Meitan Formation in the Guanyinqiao section, Qijiang, Chongqing in South China. This palynomorph assemblage is dominated by the genera Polygonium (35.2%), Striatotheca (15.2%), Stelliferidium (12.5%) and Leiosphaeridia (10.2%) showing similarities to assemblages from deflexus–suecicus graptolite biozone (Meitan Formation, upper Floian–lower Dapingian) in the Honghuayuan section and can be correlated to the contemporaneous global acritarch assemblages. The composition of the acritarch assemblage from the Meitan Formation of the Guanyinqiao section implies an inner-shelf depositional environment for the studied successions.