Latitudinal variation of brachiopod ornamentation in the Jurassic faunas from the western Tethys and its possible relation to a predation gradient

Abstract

In modern seas shell ornamentation becomes stronger towards the equator as a response to the increasing predation and a similar relationship was recorded in the case of mid-Palaeozoic brachiopods. It is expected that similar trends and relationship existed in the Jurassic, in the time of the Mesozoic Marine Revolution. The paper describes the latitudinal variation of brachiopod ornamentation in the western part of the Jurassic Tethys and neighbouring epicontinental areas and discusses their relationships to a possible predation gradient. The database included 29 localities (Pliensbachian, Bajocian, Callovian, and Tithonian/Volgian) covering the western Tethyan area from the palaeolatitudes 45° N to 10° S. In the Jurassic western Tethys, the brachiopod ornament was apparently the strongest in the high palaeolatitudes (45° N); decreased in the mid-latitudes and reached high values again near the equator. Considering the anti-predatory function of ornament, and other alternative hypotheses, the present study suggests that in the Jurassic the predation gradient was partly opposite to that of the Palaeozoic. The distribution of two main types of ornament showed opposite trends: spinosity was confined to higher palaeolatitudes whereas ribbing was stronger near the equator. This bipolarity is tentatively interpreted as adaptations to different types of predation; ribbing was a possible tool against durophages, whereas spinosity is supposed to hinder drilling predators.