19 research outputs found
Dimorphism and evolution of Albarracinites (Ammonoidea, Lower Bajocian) from the Iberian Range (Spain)
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
Coralline algal facies and their palaeoenvironments in the Late Eocene of Northern Italy (Calcare di Nago, Trento)
The Calcare di Nago is a carbonate unit of Middle-
Late Eocene (Bartonian and Priabonian) age which is
well exposed at the north-eastern end of Lake Garda, on
the western margin of the Lessini Shelf (Southern Alps).
This unit is highly fossiliferous as far as the coralline red
algae and large foraminifera are concerned. Corals, bryozoans,
echinoderms, and molluscs are also present. The
present study deals with the relationships among the
coralline taxa, the coralline growth-forms, and their
facies development in the Priabonian part of the type section of the Calcare di Nago. The taxonomic investigation
led to the identification of 15 coralline red algal species
belonging to 7 non-geniculate and 2 geniculate genera. One
species of Peyssonneliacean (red alga) and one of Halimedacean
(green alga) were also recognized. The quantitative and
qualitative analyses based on coralline red algae and large
foraminifera enabled five facies to be distinguished: Algal
crust-branch rudstone, AlgallDiscocyclina packstone, Coralalgal
boundstone, Rhodolith mound wacke/packstone, and
Rhodolith pavement. According to the coralline assemblages,
coralline growth-forms, and large foraminiferal associations,
the five facies reflect solid and soft substrate
types. Some of these facies are dominated by in situ rhodoliths,
others by reworked algal debris. In the architecture of an
interpreted prograding carbonate ramp, shallow water facies
are dominated by members of the subfamily Mastophoroideae,
while deeper water facies are dominated by those of
the subfamily Melobesioideae and family Sporolithaceae.
There is a significant increase both in size and in constructional
voids of the rhodoliths with depth. A concomitant
decrease in algal species diversity with depth has been also
recognized. Large Discocyclina assemblages are localized
across the inner and mid ramp boundary. Pellatispira and
Biplanispira are present only in the uppermost mid-ramp.
Nummulites, Assilina, and Spiroclypeus are dominant together
with small orthophragminids both in the mid- and
uppermost outer ramp facies