On the origin and palaeobiogeographic spread of Oxfordian ammonite genus Passendorferia and Tethyan subfamily Passendorferiinae Melendez

Oxfordian ammonite group Passendorferiinae (known as "Mediterranean Perisphinctids") form a lateral divergent branch of the main stem of Perisphinctidae, originating probably from Late Callovian Grossouvriinae (Alligaticeras) and spreading mainly in the Mediterranean (Tethyan) Province during the Early, Middle and Late Oxfordian. This morphologically homogeneous group is characterized by the evolute serpenticone coiling with sub-circular to subquadrate whorl section. Ribbing "simple" radial, is formed by mostly single and symmetrically bifurcate ribs, turning into thicker, straight, "columnar" ribs, slightly elevated on the ventral margin in adult stage of macroconchs. All these features make them strongly homoeomorphic with Tethyan Kimmeridgian Nebrodites, with which they have been often mistaken. The phyletic link might be represented by the genus Geyssantia Melendez, from Late Oxfordian Planula Chronozone. Microconchs, 80 to 120 mm Dm are isocostate, densely ribbed, lappeted and, show typically the development of parabolic nodes at the final part of adult phragmocone. Macroconchs 240 to over 320 mm Dm, slightly variocostate, show thick uniform columnar ribs in adult stage. Two main genera are so far recognized, evolving independently during Early and Middle Oxfordian: Passendorferia Brochwicz Lewiński, known from Lower Oxfordian early Cordatum Chronozone to Upper Oxfordian, Bimammatum-(?)Hauffianum Chronozone, characterized by extremely evolute serpenticone inner whorls with subcircular whorl section leading to subquadrate middle and outer whorls, and Sequeirosia Melendez, known mainly from the Middle Oxfordian characterized by more involute serpenticone coiling with subquadrate whorl section from early inner whorls. Passendorferiinae colonised and spread mainly on the southern margin of Tethys, being autochthonous in epioceanic carbonate platforms of western Tethys, from Hungary to southern Alps, Sicily, North Africa or Betic ranges, and reaching occasionally the external deeper areas of shallow epicontinental platforms at the northern margin of Tethys. This separate biogeographic distribution with respect to Perisphinctinae probably reflects a progressive extensional block differentiation of western Tethys at the Callovian-Oxfordian boundary and at the onset of Middle Oxfordian Transversarium Chronozone. Their rapid evolution provides them a biostratigraphic value similar to that of Perisphinctinae. At the turn of Middle-Late Oxfordian they gave rise to early Ataxioceratinae (Orthosphinctes), which would replace the Perisphinctinae and colonise the marginal epicontinental blocks of northern Tethys.