Biotic control of (low) biodiversity in rudist bivalve reefs – an example from the Tithonian of Austria

It is generally regarded that rudist bivalve reefs harbour a comparably low biodiversity of associated biota. However, while rudist associations themselves have been intensely studied, the interaction of these bivalves with other organisms has been less frequently addressed. Rigorous assessment of biodiversity in rudist reefs is generally lacking. Extending from an example from the Tithonian (Late Jurassic) Ernstbrunn Limestone from the Alpine-Carpathian transition area in Austria the present study tries to assess the – low – biodiversity in rudist reefs, and investigate into its causes. Vertical sections of a large block of reefal Ernstbrunn Limestone have been polished and scanned for rudist Heterodiceras, Epidiceras) and non-rudist fossils, employing investigation under UV-light. Additionally, (semi-)quantitative collections of reworked Tithonian fossils from karst fissures are evaluated. The observed low biodiversity in the epidiceratid rudist reefs contrasts with a generally high biodiversity in the lagoonal facies of the Ernstbrunn Limestone. The study of in situ reefs and thin section analysis show that, apart from congeners, epidiceratid rudist shells are neither overgrown nor significantly incrusted by other organisms during life. The case study is supplemented by a review of the literature on associations of rudists and some of their potential forerunners. Since environmental factors can usually be ruled out, the impression is gained that, as a kind of an ecologic autapomorphy, rudists actively diminished biodiversity in their immediate surrounding. Because bivalves hardly have mechanical abilities of self cleaning and defence, it is proposed that rudists were capable of producing effective biochemical deterrents.