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The Viséan–Serpukhovian boundary is poorly defined in South China, hampering regional and global stratigraphical correlations. The foraminiferal and conodont distribution of the Baping Formation in the carbonate-slope Danlu section permits the recognition of an interval from the middle Viséan to the uppermost Serpukhovian in a continuous succession. The base of the Serpukhovian in Danlu is recognized by the first occurrences of Janischewskina delicata, Howchinia subplana and questionable ‘Millerella’ tortula. At a slightly younger level, the conodont Lochriea ziegleri is first recorded. A calibration on the first occurrence of L. ziegleri in different basins at a global scale has been revised compared to auxiliary markers within the ammonoids and foraminifers. The late occurrence of L. ziegleri in the Danlu section also supports a lack of synchronicity in the global first occurrence of this taxon. This study calls for the recognition of a new base for the Serpukhovian under a far better correlation between different zonal schemes and fossil groups.
Brachiopod taxonomy is based on descriptions of shell morphology and key characters, but diagenesis generally modifies or erases some of them, hindering brachiopod identification. Brachiopods that are taxonomically related usually present shells with similar appearance but can differ in size (i.e., Rhynchonellata). Some aspects of morphology – for example the angular measurement of the curvature of the shell or details of shell microstructure – could aid taxonomic identification. Gigantoproductids, which lack a robust taxonomy, have the largest shells among brachiopods and are ideal for this kind of study because of their gigantic size and morphological variability. Furthermore, they have a great abundance and worldwide distribution during the mid-Carboniferous. More than 700 samples have been collected from Sierra Morena (Spain), Montagne Noire (France) and Adarouch (Morocco) identifying up to six gigantoproductid genera: Globosoproductus, Semiplanus, Kansuella?, Latiproductus, Gigantoproductus and Datangia. Microstructural features from 170 thin sections belonging to gigantoproductid ventral valves have been studied, and six crystal morphologies have been distinguished within the tertiary layer: subhorizontal, imbricated, crenulated, acicular, short and long columnar morphologies. Moreover, 23 complete shells from all genera have been selected to investigate shell size and curvature. Results from this study emphasise that shell size, curvature and crystal shape are taxa-related. Finally, a remarkable morphological change in the gigantoproductid populations from the western Palaeo-Tethys occurred during the Viséan–Serpukhovian, from thin-shelled genera with subhorizontal morphology (Viséan) to thick-shelled genera with a tertiary layer consisting of long columnar crystals (Serpukhovian). This study proves that microstructure, maximum thickness and shell spiral characterisation are robust characters when applied to gigantoproductid taxonomy, but also have great potential in other brachiopod groups.
A new foraminiferal subzone (Cf5α or MFZ12α) in between the classical foraminiferal zonal biozones is characterized by the first occurrence of Archaediscus at concavus stage, primitive species of Pojarkovella, as well as the first Endothyranopsis s.s. This interval is represented in England, France and Morocco (in the western Palaeotethys) and in South China, and more widely in Iran (in the eastern Palaeotethys), where it is partly similar to the MFZ11B subzone defined by previous authors. The position of this new biozone within the Livian or Holkerian substages suggests that it has to be considered as part of the middle Viséan substage. We therefore propose the abandonment of the notation MFZ11B, which includes lower Viséan rocks, and the subdivision of the middle Viséan zones MFZ12 and Cf5 into two subzones MFZ12α or Cf5α, and MFZ12β or Cf5β, with the latter subzone containing the classical Livian–Holkerian foraminiferal guides Pojarkovella nibelis and Koskinotextularia. Furthermore, the lower Viséan MFZ11 zone can be subdivided in most Palaeotethyan basins into three subzones: a lower MFZ11α subzone (characterized by the first occurrence of Uralodiscus rotundus, as well as most species of Glomodiscus); a middle MFZ11β subzone (characterized by the first occurrence of Archaediscus at involutus stage and Conilidiscus); and an upper MFZ11γ subzone (characterized by the first occurrence of Nodosarchaediscus, Consobrinellopsis and Lituotubella).
An extensive study of the microstructure, nanostructrure and crystallographic properties of six taxa belonging to four different genera of Devonian and Carboniferous Syringoporicae showing dense phaceloid (Pleurosiphonella), pseudocerioid (Neomultithecopora) and cerioid growth patterns (Roemeria and Roemeripora) has been done in order to disclose the similarities and differences in the growth processes at the biomineral scale and understand the growth processes that provide organisms with an evolutionary advantage to colonize different habitats. All the skeletons have similarities regarding the biocrystallization process, showing that the Syringoporicae skeletons are a product of matrix-mediated biocrystallization. Micro- and nanotextural features are common in all of the skeletons studied, showing that they were composed of hierarchical structures. All studied taxa possess a complex nanostructure composed of co-oriented rounded nanocrystals with different sizes and morphologies, depending on the taxon. The identified microstructures include granules, lamellae, fibres and hyaline elements. The crystallographic techniques demonstrate that all of them except the hyaline elements are biogenic in origin. Granules could be aborted fibres during the growth of two corallites in contact. On the other hand, the study of the biomineral properties suggests that the skeleton structure is a reflection of the genetic code. The median lamina was formed by the joint crystallization of both polyps at the same time. The variation in the internal structural organization (phaceloid, pseudocerioid or cerioid) was conditioned by the environment (stressful situations or feeding strategies); on the contrary, the final structure is controlled by genetics and their crystallographic properties are characteristic for each internal structural organization.
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