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The ecological significance of solitary coral and bivalve epibionts on Lower Cretaceous (Valangianian-Aptian) ammonoids from the Italian Dolomites

Lukeneder A.

Acta Geologica Polonica

2008

Vol. 58, no. 4

425-436

Lower Cretaceous deposits of the Puez section in the Dolomites (northern Italy) yielded a rich ammonoid fauna (28 genera, n = 424) showing unique epifaunal encrustations by the ahermatypic solitary scleractinian ?Cycloseris LAMARCK, 1801. The coral encrusted only the outer shell surfaces of the ammonoids; the inner surface remained unaffected. Such a Cretaceous community and the relationship between the two fossil groups are described for the first time. The shells of dead ammonoids sank to the sea bottom and became colonized by the coral larvae, as documented by the location of the epibionts on only one side of the shells. The coral was fixed to the ammonoid shell for its entire life. Only the "sediment free" upper side of the ammonoid shells could be inhabited by epibionts such as corals and serpulids. The encrustation of ammonoid shells by the bivalve Placunopsis represents a different situation in that both sides of the ammonoid shells were affected, pointing to encrustation of floating ammonoids. This long-term infestation in the water column contrasts with coral settlement on the sea-floor. Ammonoid specimens encrusted by Placunopsis never exhibit encrustation by corals. The ammonoid-coral relationship from the Dolomites is documented from the Valanginian to Aptian interval. Examples of coral epibionts on ammonoids and other fossil groups throughout the geological column are briefly reviewed.