The Silurian fishes from north-western Hunan, China are characterised by the earliest known galeaspids
Dayongaspis Pan and Zeng, 1985 and Konoceraspis Pan, 1992, and the earliest known antiarch Shimenolepis
Wang J.-Q., 1991, as well as rich sinacanth fin spines. Shimenolepis from Lixian County in north-western
Hunan, which was dated as the Telychian (late Llandovery), has long been regarded as the oldest representative
of the placoderms in the world. As such, in addition to eastern Yunnan and the Lower Yangtze Region,
north-western Hunan represents another important area in South China that yields important fossil material
for the research of early vertebrates and related stratigraphy. Here we summarise the Silurian fishes known in
north-western Hunan so far, and classify them into three vertebrate assemblages (i.e., the Wentang, Maoshan,
and Yangtze assemblages). Based on the updated Silurian vertebrate and stratigraphic databases, the Silurian
fish-bearing strata in north-western Hunan can be subdivided into the Rongxi, Huixingshao, and Xiaoxi formations
in ascending chronological order, which can be correlated with the Lower Red Beds, the Upper Red Beds,
and the Ludlow Red Beds in South China, respectively. A new look at the Silurian strata in Lixian suggests that
the age of Shimenolepis is late Ludlow rather than late Llandovery as previously suggested. The research on
Silurian fishes and biostratigraphy in north-western Hunan not only provides morphological data of early vertebrates,
but also offers new palaeoichthyological evidence for the subdivision, correlation, and age assignment of
the Silurian marine red beds in South China. The establishment of a related high-precision Silurian stratigraphic
framework in north-western Hunan will help to elucidate the temporal and spatial distribution of Silurian fossil
fishes, deepen the understanding of the evolution of early vertebrates, and unravel the coevolution between
Silurian vertebrates and the palaeoenvironment.
The mid-Ludfordian pronounced, positive carbon isotope excursion (CIE), coincident with the Lau/kozlowskii extinction event, has been widely studied so far in shallow-water, carbonate successions, whereas its deep-water record remains insufficiently known. The aim of this research is to reconstruct the sedimentary environments and the palaeoredox conditions in the axial part of the Baltic-Podolian Basin during the event. For these purposes, the Pasłęk IG-1 core section has been examined using microfacies analysis, framboid pyrite diameter and carbon isotope measurements. The prelude to the event records an increased influx of detrital dolomite interpreted as eolian dust, coupled with a pronounced decrease in the diameter of the pyrite framboids, indicating persistent euxinic conditions across the event. The event climax is recorded as the Reda Member and consists of calcisiltites, composed of calcite microcrystals (‘sparoids’), which are interpreted as suspensoids induced by phytoplankton blooms in the hipersaturation conditions present in the epipelagic layer of the basin. Both the prelude and climax facies show lamination, interpreted as having resulted from periodical settling of marine snow, combined with hydraulic sorting within a ‘benthic flocculent layer’, which additionally may be responsible for a low organic matter preservation rate due to methanogenic decomposition. Contrary to the observed basinward CIE decline in the benthic carbonates in the basin, the Reda Member records an extremely positive CIE (up to 8.25‰). Given the pelagic origin of the sparoids, the CIE seems to record surface-water carbon isotope ratios. This points to a large carbon isotope gradient and kinetic fractionation between surface and bottom waters during the mid-Ludfordian event in a strongly stratified basin. The Reda facies-isotope anomaly is regarded as undoubtedly globally triggered, but amplified by the stratified and euxinic conditions in the partly isolated, Baltic-Podolian basin. Hence, the common interpretation of the basin record as representative for the global ocean needs to be treated with great caution.