Taxonomic revision of the genus Ichthyosarcolites Demarest, 1812, and description of a new canaliculate rudist from the Cenomanian of Slovenia: Oryxia sulcata gen. et sp. nov. (Bivalvia, Hippuritida)

摘要

Ichthyosarcoliteswas amongst the first rudist genera to be described. Nineteen species have been assigned to this genus since its introduction in 1812, all from shallow-marine tropical carbonates of Albian and Cenomanian age (mid-Cretaceous). Almost all nominal species suffer from vague original descriptions, and some forms were inaccurately assigned to the genus. Several species were defined on the basis of the number of flanges along the shell, such asIchthyosarcolites rotundus,I.?monocarinatus,I.?triangularis,I.?bicarinatus,I.?tricarinatus,I.?alatusandI.?polycarinatus. An analysis of the relative position of the flanges on the shell by hierarchical clustering helps with taxon definition. Two species with a single flange are here recognised, one with a dorsal flange (I.?monocarinatus), the other with a ventral one (I.?triangularis). There is no consistency in flange distribution on the shells whatever their number is, i.e., two or more, and homology of individual flanges cannot be demonstrated. Morphometric analysis finds a link between the shape of the general cavity and flange number and distribution. Out of the nineteen species named in the literature, only four can be retained after our thorough morphological analyses, namelyIchthyosarcolites rotundus(defined by a circular shell that lacks flanges),I.?triangularis(defined by a single dorsal flange),I.?monocarinatus(defined by a single ventral flange) andI.?bicarinatus(defined by two flanges or more). Flanges have very variable morphological features amongI.?bicarinatus. In addition, we here erectOryxia sulcatanov. gen. et sp., a new form based on specimens previously identified asI.?bicarinatus,I.?poljakiandI.?tricarinatus. This new taxon is defined by the presence of a ligament groove, this in contrast toIchthyosarcoliteswhich never shows any trace of ligament.Oryxianov. gen. may be the sister group of a clade of canaliculated forms that lost contact between the ligament support and the shell exterior and inc