RT Journal Article
T1 Dimorphism and evolution of Albarracinites (Ammonoidea, Lower Bajocian)from the Iberian Range (Spain)
A1 Fernández López, Sixto Rafael
AB 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 theIberian Range from at least the Ovale Zone to the uppermost Laeviuscula Zone of the Lower Bajocian (Middle Jurassic). Themacroconch counterpart is thought to be a group of stephanoceratids previously attributed to Mollistephanus, Riccardicerasand 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 hypermorphicperamorphocline starting from depressed, small and slender serpenticones of A. westermanni, to larger planorbicones withmore 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 morecompressed and with increasing ontogenic variation of shell ornament. Albarracinites and Mollistephanus subsequentlydeveloped two opposite peramorphoclines or gradational series of morphological changes undergoing greater developmentand ontogenic variation. These two genera show diverse palaeobiogeographical distributions too. Albarracinites is rarelyrecorded in the Mediterranean and Submediterranean from the Discites to the Laeviuscula Zone, whereas Mollistephanus ismore common in north-western Europe and other biochoremas of the western Tethys from the Discites Zone to the SauzeiZone. Albarracinites seems to be the earliest stephanoceratid lineage in western Tethys, branching off from the otoitidRiccardiceras by proterogenetic change and resulting in paedomorphosis at the Aalenian/Bajocian boundary.
PB The Trustees of the Natural History Museum,
SN 1477-2019
YR 2013
FD 2013-12
LK https://hdl.handle.net/20.500.14352/33458
UL https://hdl.handle.net/20.500.14352/33458
LA eng
DS Docta Complutense
RD 27 abr 2024