Morpho-anatomical features of lingulid brachiopods from the Late Ordovician Pingliang Formation and their evolutionary and ecological implications

The Brachiopoda were one of the most successful groups of marine invertebrates during the Palaeozoic Era, exhibiting exceptional diversity and abundance. Linguloid brachiopods (superfamily Linguloidea), initially characterized by an epibenthic mode of life in the Cambrian, underwent substantial diversification during the Ordovician, including the emergence of infaunal forms. This interval marks the peak of their morphospace occupation and ecological diversification. To gain a deeper understanding of the ecological evolution of linguloid brachiopods, a comprehensive investigation of their morphological and anatomical evolution is essential. Here, we report a linguloid brachiopod species from the Upper Ordovician (Sandbian) Pingliang Formation at the Xilinggou section, southern Ordos Basin, North China Platform. Using geometric morphometrics, morphospace occupation was visualized for this species and Anomaloglossa porca, the latter of which was the most recently documented linguloid brachiopod in this section. Results indicate that the two species occupy different areas of morphospace, and the new linguloid brachiopod is assigned to Pseudolingula quadrata, which represents the first report of Pseudolingulidae from the North China Platform. Pseudolingula quadrata exhibits a clearly defined visceral area and preserves complete impressions of the musculature and mantle canal system. The absence of dorsal vascula media within the mantle canals indicates a close similarity to extant infaunal taxa. Based on geometric morphometric analysis and anatomical examination of linguloid brachiopods from the Cambrian to the present, our results not only reinforce the establishment of Pseudolingulidae at the family level, but also demonstrate significant modifications in shell shape and mantle canals that indicate their transition from an epifaunal to an infaunal lifestyle, providing critical insights into their evolutionary trajectory and ecological evolution.