Szabó János szerk.: Fragmenta Mineralogica Et Palaentologica 24-25. 2007. (Budapest, 2007)
period, both worldwide and in the TR. The two minor diversity peaks in the Bajocian and Tithonian were apparendy related to rejuvenation of the tectonic movements along fault scarps. The above interpretation largely follows that in VÖRÖS (1993), but here we mention a further factor what might have contributed to the growth of brachiopod communities. A hypothesis, that cold seeps may have supported certain Mesozoic brachiopod communities, was put forward by SANDY (1995). Oversaturated, dense brines are inherent parts of some carbonate platform margins (PAI'LL & NEUMANN, 1987, PAULL et al. 1991, 1992). These brines (rich e. g. in methane) flow outwards and discharge at the feet of the escarpments. Methane is metabolized by bacteria and this carbon source is used by dense communities of benthic organisms (CAMPBELL Sc BOTTJRR 1995). The possible activity of submarine cold seeps might be related to rejuvenation of tectonic movements and might carrv nutrients to the starving environment and by this means might support chemosynthetically based communities. In the last years we tried to obtain stable isotopic (8 18 0 and 8 13 C) evidence from the TR to support this hypothesis but hitherto the data are not conclusive (Table 1). A remarkable temporal gradient, the dominance of the smooth forms toward the end of the Jurassic, is observed in the brachiopod faunas of the TR. As it is shown already by the specimens figured on Plates I to III, even among the Rhynchonellida, the ribbed species disappear gradually in the Middle Jurassic (Bajocian) and are absent in the Late Jurassic (Tithonian) faunas. This is partly due to the gradual deepening of the sea floor of the area of the TR, and, at the same time this reflects a global change in brachiopod history. According to VÖRÖS (2005) the smooth, mainly sulcate brachiopods flourished continuously in the bathyal zones as well adapted communities in the Mesozoic. Here they may have survived the crises and, under appropriate palaeogeographj cal situations, may have recolonized the shallow bathyal or even sublittoral regions of the western margins of the ocean basins (V ÖRÖS 2005). The Mediterranean microcontinent system (where the territory 7 of the TR belonged to) at the western part of the Jurassic Tethys might have been such an intermittendy invaded region. These kind of migration events seem to be manifested in the Bajocian and Tithonian brachiopod diversity maxima in the TR. In the latter case, the dominant Pygopidae give an excellent example of brachiopods specially adapted to deeper marine environment with ven 7 limited food supply (AUER 1965, VOGEL 1966, MICHALIK 1996). Figure 6 — Temporal changes in the species diversity of brachiopod orders and suborders in the Jurassic of the Transdanubian Range. — A: Athyridida.
