Dimorphism and evolution of Albarracinites (Ammonoidea, Lower Bajocian) from the Iberian Range (Spain)

Abstract

Several tens of specimens of Lower Bajocian Albarracinites (type species A. albarraciniensis Fernandez-Lopez, 1985),
including microconchs and macroconchs from the Iberian Range, have been studied. This ammonite genus ranges in the
Iberian Range from at least the Ovale Zone to the uppermost Laeviuscula Zone of the Lower Bajocian (Middle Jurassic). The
macroconch counterpart is thought to be a group of stephanoceratids previously attributed to Mollistephanus, Riccardiceras
and other new forms described in this paper. Two chronologically successive species of Albarracinites have been identified:
A. albarraciniensis and A. submediterraneus sp. nov. The evolution of the Albarracinites lineage represents a hypermorphic
peramorphocline starting from depressed, small and slender serpenticones of A. westermanni, to larger planorbicones with
more cadiconic phragmocones and body chamber of subcircular cross section belonging to A. submediterraneus sp. nov.,
through A. albarraciniensis Fernandez-Lopez. In contrast, Mollistephanus planulatus (Buckman), M. cockroadensis Chandler
& Dietze and M. mollis Buckman represent a peramorphocline by acceleration, producing adults of similar size but more
compressed and with increasing ontogenic variation of shell ornament. Albarracinites and Mollistephanus subsequently
developed two opposite peramorphoclines or gradational series of morphological changes undergoing greater development
and ontogenic variation. These two genera show diverse palaeobiogeographical distributions too. Albarracinites is rarely
recorded in the Mediterranean and Submediterranean from the Discites to the Laeviuscula Zone, whereas Mollistephanus is
more common in north-western Europe and other biochoremas of the western Tethys from the Discites Zone to the Sauzei
Zone. Albarracinites seems to be the earliest stephanoceratid lineage in western Tethys, branching off from the otoitid
Riccardiceras by proterogenetic change and resulting in paedomorphosis at the Aalenian/Bajocian boundary.