Shell morphology and structure of Upper Permian and Lower Triassic lingulids and their significance in the Mesozoic-Cenozoic phylogeny of Lingulidae (Brachiopoda)

The systematics of lingulids was mainly based, in the past, on the outer shell morphology (e.g., outline, inflation and ornamentation) and the late Paleozoic – Cenozoic species were mostly referred to the extant genus Lingula. More recently, on the basis of the internal characters (e.g., position and shape of muscle scars, pedicle nerve grooves, mantle canal system), new late Paleozoic – Mesozoic genera have been proposed. Besides, authors highlighted the importance of shell structures in the reconstruction of lingulid phylogeny. The generic re-assessment of extinct species is therefore necessary. The lingulids are considered a “disaster taxon”, which flourished immediately after mass extinction events, within strongly impoverished marine benthic communities dominated by few holdovers. The lingulids are abundant in the lowermost Triassic beds, which record the most severe effects of the end-Permian extinction on marine ecosystems. They are frequent in shallow marine formations located both in the Tethyan and Panthalassa margins. Despite their great abundance and geographical diffusion, recent systematic studies are very few and only based on the internal characters, disregarding shell structures. These latters are very useful to distinguish the extant genera Lingula and Glottidia, respectively characterized by spheroidal and rod-like (virgose) and trellised (baculate) structures. The analysed material was collected in Upper Permian and Lower Triassic beds of Tethyan and Panthalassa margins from Southern Alps, Spitsbergen and Wyoming. The studied valves show frequently a spectacular preservation of the internal morphological characters, which allowed us a careful reconstruction of the inner shell surface. They show similarities with Lingularia, an extinguished genus, which would be characterized by the baculate structure and representing, for some authors, the ancestor of both extant genera. However, all the shell fragments observed under SEM have a virgose structure. These data provide further knowledge to the discussion regarding the phylogeny of modern lingulids.