Combining soft-bodied and three-dimensional fossils to reveal evolutionary modifications in early lingulellotretid brachiopods.

Living lingulide brachiopods are traditionally recognised as representatives of evolutionary conservatism, showing little change in general-morphology from their Cambrian ancestors. However, less attention has been given to their anatomical and ontogenetic modifications since their initial appearance. Among these, lingulellotretids are unique, characterized by their typical elongate pedicle foramen and large pseudointerarea. This study describes exquisitely preserved soft-tissue and phosphatic shells of Lingulellotreta from Cambrian Series 2 deposits in China and Kazakhstan. Biomineralized novelties in Lingulellotreta, including elongate pseudointerarea forming a pouch-like visceral cavity and columnar shell architecture, probably were evolutionarily modified from the unmineralized tubular ancestor Yuganotheca during the Cambrian Explosion. Lingulellotretids, however, faced extinction in the Early Ordovician, exemplifying a short-lived evolutionary experiment with a tubular body form in early brachiopods. Since the early Cambrian, lingulide brachiopods have exhibited a long-term evolutionary trend marked by the reduction of pseudointerarea, reflecting a convergence toward a more efficient body plan that ultimately became dominant in later lineages. The intensification of skeletal defences and the increasing demands of filter feeding within benthic communities likely drove these evolutionary modifications and ecological adjustments, culminating in the development of the distinctive, persistent tongue-shaped body of linguloid brachiopods during the Great Ordovician Biodiversification Event.