Adaptive strategies and environmental significance of lingulid brachiopods across the late Permian extinction
Abstract Linguliform brachiopods are traditionally considered a conservative group which seems to pass through the late Permian extinction without any significant loss and even appear to thrive immediately after the extinction peak. In the Southern Alps, lingulids are very common in the post-extinction Mazzin Member (early Induan) of the Werfen Formation. Sparse occurrences are also known in the overlying Siusi and Gastropod Oolite members (late Induan and early Olenekian in age respectively). The recent discovery of well preserved specimens from a pre-extinction bed of the Bellerophon Formation (Changhsingian) has permitted a detailed comparative analysis, mostly based on the interior characters, preserved in the lingulid succession from across the extinction beds. The following effects on the lingulid populations have been analyzed: i) change in taxonomic assessment; ii) adaptive strategies during the surviving and recovery phases; and iii) environmental proxy connected with the killing mechanisms of the late Permian extinction. The pre-extinction individuals belong to Lingularia? cf. smirnovae Biernat and Emig, a species that is characterized by large-sized shells with a short lophophoral cavity. The post-extinction populations belong to different species and, probably, even to a different genus. The first post-extinction population (early Induan), with small-sized shells and long lophophoral cavity, has been referred to Lingularia yini (Peng and Shi). It records the most severe effects of the late Permian extinction on the marine ecosystems. The late Induan–Olenekian Lingularia borealis (Bittner), with large sized shells and long lophophoral cavity, appears during the first phase of the Triassic biotic recovery. The main adaptive strategies of Lingularia yini, in comparison with the Permian species, include: i) shell miniaturization; ii) increasing of the lophophoral cavity surface (respiratory surface); and iii) increasing of shell width/length ratio. These modifications are interpreted as adaptations towards warming and hypoxia, two main killing mechanisms of the marine biota. The recovery species Lingularia borealis maintains a large lophophoral cavity, indicating an adaptation towards predominant low oxygenated bottom marine waters. The appearance and the great abundance of Lingularia yini in the Mazzin Member (early Induan) represent a proxy of dysaerobic conditions, which determined the appearance of the second phase of the Lilliput biota, characterized by the definitive disappearance of the rhynchonelliform brachiopods and calcareous algae in the Southern Alps.