Revision of the genus mariania (Echinoidea) with the description of two new species from the miocene of Italy

Abstract

New material from the Miocene of Italy allows revision of the spatangoid genus Mariania Airaghi, 1901 and proposal of an emended diagnosis. Particularly characteristic, previously overlooked features of the genus include the presence of well-developed phyllodes made up from short, almost equidimensional plates in oral ambulacra II, III and IV. Unlike in other Spatangoidea, where the adoral plates rapidly become elongated towards the margin, they stay short in Mariania and are not constricted halfway between the peristome and the margin. In addition, most species of Mariania possess a characteristic domal profile with steep sides and lack a raised keel in aboral interambulacrum 5. Their petals are wide, open distally and extend almost to the margin. The plastron is not indented behind the episternal plates and the labral plate extends to the second ambulacral plates. Fascioles are missing in all specimens examined. The combination of these morphological features enable the separation of Mariania from the genera Macropneustes, Hypsopatagus and Spatangus, to which members of the genus have been assigned by previous authors. Cladistic analysis carried out to unravel the uncertain systematic position of Mariania failed to find well-supported relationships, but firmly places Mariania within the Brissidina. Most previous family attributions could be, however, ruled out. Based on the available data a placement within Spatangoidea seems most likely, where it takes up an intermediate position between maretiids, loveniids and spatangids. Three different species are identified within the studied sample: Mariania marmorae, the type species of the genus; M. stefaninii sp. nov. from the late Burdigalian-early Langhian of northern Italy; M. comaschicariae sp. nov. from the Burdigalian of Sardinia. These new species are distinguished from M. marmorae by their lower tests, shorter labral plates and shorter petals. Mariania comaschicariae sp. nov. differs from M. stefaninii sp. nov. by its lower test, more anterior apical disc and less numerous plates in the oral anterior paired ambulacra. Test morphology and parent rock sedimentology suggest that Mariania was an epifaunal echinoid, which lived in inner shelf environments, characterized by soft bottoms and a tropical climate.