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Orbirhynchia nadiae, a new rhynchonellide brachiopod species from the Lower Aptian of the Gargano Promontory (southern Italy): shell structure, stratigraphy, palaeoecology and taphonomy

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Abstract:

Stratigraphic and palaeontological analyses of the late Early Aptian margin of the Apulia carbonate platform exposed in the Gargano Promontory (northern Apulia, southern Italy) have been carried out. A monospecific brachiopod assemblage has been recovered as a primary shell accumulation within the lowermost drowning succession capping the Apulia platform margin at the onset of the “Selli” Oceanic Anoxic Event 1a. A new rhynchonellide brachiopod species, Orbirhynchia nadiae, is introduced. The new species and the associated assemblage (abundant cyanobacteria, echinoids, crinoids, chaetetid sponges, encrusting foraminifera, and ostracods) suggest that fertile, nutrient enriched, possibly alkaline waters developed as a consequence of the “Selli” palaeoceanographic event which supported r-mode generalist, suspension feeders to thrive. Orbirhynchia nadiae is distinguished from other species of the same genus by its medium size, facultative asymmetry of the anterior commissure, numerous fine ribs and pointed umbo. The orientation and shape of the dental plates and crural base projections are shown to vary within Orbirhynchia Pettitt. Orbirhynchia is a Cretaceous rhynchonellide genus with more than 20 described species available at present. It shows large intrageneric morphologic variation, and the variation within particular species may also be noteworthy as in this case. The shell microstructure of the new species was investigated using both scanning electron microscopy and acetate peels. Orbirhynchia nadiae reveals large fibres of the secondary shell layer, isometric subquadrate, rhomboidal to polygonal in cross sections. As such it confirms the recent hypothesis of a correlation between shell microstructure and the type of crura in post-Palaeozoic rhynchonellides. The falciform crura of the new species are associated with coarse fibrous shell microstructure. Orbirhynchia nadiae from the late Early Aptian of Southern Italy is one of the earliest valid records of the genus Orbirhynchia, thus extending its geographic distribution.
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