Bivalve reefs from the Upper Triassic of Iran

Abstract

In the Upper Triassic Nayband Formation of east-central Iran, bivalves repeatedly form small patch reefs
on a mid to outer mixed carbonate-siliciclastic ramp in close stratigraphic neighbourhood to coral and coralsponge
reefs. In contrast to other Triassic-Jurassic bivalve-dominated patch reefs, the bivalve reefs of the
Nayband Formation are characterized by a comparatively high diversity of framebuilding taxa. These include
taxa from three different families, i.e., the ostreids Umbostrea emamii, U. iranica and U.? aff. parasiticum,
the prospondylids Newaagia stocklini and Persia monstrosa, and the plicatulids Eoplicatula parvadehensis
and Pseudoplacunopsis asymmetrica. The bivalve reef constructors may have had a competitive advantage
over coral and calcareous sponges in environments characterized by a higher degree of turbidity and/or higher
nutrient contents.