Where the Diceras dwells… : The hippuritid fauna of the ‘Kelheimer Diceras-Kalk’

The Late Kimmeridgian ‘Kelheimer Diceras-Kalk’ (now Kelheim Member, Frankenalb Formation) preserves one of the richest Late Jurassic invertebrate faunas of Europe, which was monographed by Munich palaeontologists in the late 19th century. New fossil collections from a quarry at Saal an der Donau include numerous epidiceratid and a few diceratid rudists – but not a single Diceras, which, as it turns out, erroneously lent its name to the stratum. Instead, another small diceratid species, Valletia auris, occurs together with three species of Epidiceratidae and the requieniid Hypelasma salevensis. Epidiceratidae comprise the two ‘classic’ species Epidiceras carinatum (previously described as Diceras bavaricum) and Plesiodiceras muensteri, and one new genus and species, which is so far only known from Saal. The large Epidiceras and the moderately-sized Plesiodiceras are widespread and common in Late Jurassic reefal-lagoonal carbonates, and well-known from numerous Tethyan localities. The small Valletia and Hypelasma, however, were probably often overlooked or considered as juvenile diceratids/epidiceratids. At species level, it is astonishing that Epidiceras carinatum, which was first described from the early Tithonian of Sicily, makes its only other reported appearance in the Kelheim area in Bavaria. The most unexpected result, however, is the occurrence of a new genus of Epidiceratidae with a rather peculiar morphology: the commissure is lunate in outline; two strong, highly elevated, crest-like carinae raise from the ends of the crescent, and due to its regular curl, the attached valve attains a woodscrew-like shape, while the shorter free valve only gets a slight backward twist. The hippuritids occur within a highly diverse community dominated by corals, brachiopods, bivalves and gastropods, and the new genus, as well as the two species newly recorded from the area once more testify the important role of Late Jurassic reefal environments as a cradle of speciation.