Major environmental, climate and sealevel
changes occurred in the Western Tethyan Realm during the
late Sinemurian to early Pliensbachian time interval. Here, we
examine how these changes affected the taxonomic composition
of radiolarian fauna. Radiolarian assemblages were collected
on Mount Rettenstein (Northern Calcareous Alps) from
a siliceous limestone and marl succession, deposited in a
well-oxygenated basin a few hundred metres in depth on the
continental shelf at the western edge of the Neotethys Ocean.
Radiolarian research was complemented with elemental and
isotope geochemistry on bulk carbonate samples. The siliceous
microfaunas below and above the stage boundary consist of
more than 80% sponge spicules and less than 20% radiolarians,
with a strong predominance of the Order Spumellaria.
The Nassellaria to Spumellaria abundance ratio ranges from
1:5 to 1:3. At the Sinemurian–Pliensbachian transition, a significant
drop in diversity occurred, accompanied by a
substantial change in relative abundances of radiolarian taxa.
The most severely affected groups were surface-dwelling radiolarians
(Angulobrachiidae, Hagiastridae, Pantanelliidae;
mostly Gorgansium, Poulpidae and Ultranaporidae), which
almost or completely disappeared. In contrast, Archaeocenosphaera,
Praeconocaryomma, Zhamoidellum and Lantus
became abundant and were apparently the most resistant to
environmental stress. The changes in radiolarian assemblages
were local and probably induced by the end-Sinemurian sealevel
drop that transformed the area into a semi-enclosed basin
with restricted ocean circulation. The exchange of water
masses and thus radiolarian faunas with the open sea was
reduced and their productivity may have been lowered by the
lower inflow of fertile waters from the ocean