Brachiopod palaeoecology on a Thethyan Jurassic Seamount (Pliensbachian, Bakony Mountains, Hungary)

Abstract

The environmental distribution of brachiopods in the Pliensbachian of the Bakony area (Hungary) was studied on the basis of a very rich fauna (more than 100 species and 6000 specimens) collected from 31 localities. The analysis was carried out using an “empirical approach”, i.e. the discussion started from the ancient environment. A depositional model for the Pliensbachian of the Bakony area was outlined and five lithologies and corresponding depositional environments were differentiated: (A) Red, massive, manganiferous limestones — seamount top; (B) Hierlatz-type limestones — seamount slope and foot; (C) Crinoidal limestones — basin margin; (D) Spiculitic cherty limestones — transition to basin interior; (E) Ammonitico rosso limestones — basin interior. Taphonomic studies of the different fossil assemblages revealed that strong ecological mixing must be taken into account in the seamount slope and basin margin environments. Environmental information obtained from fossils associated with brachiopods strengthened the view that the five depositional environments outlined in the depositional model correspond to five general palaeoenvironments. The study of environmental distribution of brachiopods led to the following conclusions: (1) The seamount top is characterized by a low-diversity fauna where, besides the dominant terebratulids, strophomenids play a significant role. A characteristic taxon is the genus Koninckodonta. (2) In the very rich fauna of the seamount slope the orders Terebratulida and Rhynchonellida play an equally important role. (3) The basin areas are characterized by a low-diversity fauna dominated by large terebratulids. Characteristic taxa are Hesperithyris renierii and Lychnothyris rotzoana. (4) Short-looped terebratulids were better adapted to unfavourable conditions than the rest of the brachiopod fauna. (5) Sulcate and “axiniform” morphological types do not show any definite relationship with environment or depth.