Monospecific mass occurrence of a new species of the Early Jurassic genus Arzonellina (Brachiopoda) at Fenyveskút (Bakony Mountains, Hungary)

Abstract

A peculiar slab of brachiopod coquina was found at Fenyveskút locality (Lókút, Bakony Mountains, Hungary). The current investigation demonstrated that it was a mass occurrence of monospecific brachiopods which belong to the kingenoid genus Arzonellina Sulser, 2005, recently described from Switzerland. Detailed investigations of the external and internal morphology (the latter by serial sections) of the specimens proved that they represent a new species: Arzonellina bogicae n. sp. This new species is introduced, described and illustrated here in details. The age of the brachiopod coquina and the new species is considered Sinemurian on the basis of circumstantial evidence from the locality Fenyveskút, where the lithologically very similar, Sinemurian Hierlatz Limestone is frequent. The previously documented occurrences of Arzonellina in Switzerland and Montenegro are also Sinemurian in age. For better understanding the sedimentary history of the slab of brachiopod coquina with Arzonellina, the Jurassic megabreccia at the locality Fenyveskút is re-described here. The lithology and fossils of the major components (blocks) and the matrix are documented in detail, and illustrated with thin section photomicrograps. Detailed study of the sediments that accumulated in an internal open space (vug) of the formerly lithified (cemented) brachiopod coquina revealed that the piece of the Arzonellina coquina was incorporated into the megabreccia in the Bajocian. The monospecific mass accumulation of brachiopods gave a hint to an association with hydrocarbon seeps (“cold seeps”). However stable isotopic results from the Fenyveskút locality do not show any signatures that would indicate the influence of that special environment.