Adaptive specialization of a unique sponge body from the Cambrian Qingjiang biota

Evolution and diversity of biomineralized columnar architecture in early Cambrian phosphatic-shelled brachiopods

Biologically-controlled mineralization producing organic-inorganic composites (hard skeletons) by metazoan biomineralizers has been an evolutionary innovation since the earliest Cambrian. Among them, linguliform brachiopods are one of the key invertebrates that secrete calcium phosphate minerals to build their shells. One of the most distinct shell structures is the organo-phosphatic cylindrical column exclusive to phosphatic-shelled brachiopods, including both crown and stem groups. However, the complexity, diversity, and biomineralization processes of these microscopic columns are far from clear in brachiopod ancestors. Here, exquisitely well-preserved columnar shell ultrastructures are reported for the first time in the earliest eoobolids Latusobolus xiaoyangbaensis gen. et sp. nov. and Eoobolus acutulus sp. nov. from the Cambrian Series 2 Shuijingtuo Formation of South China. The hierarchical shell architectures, epithelial cell moulds, and the shape and size of cylindrical columns are scrutinised in these new species. Their calcium phosphate-based biomineralized shells are mainly composed of stacked sandwich columnar units. The secretion and construction of the stacked sandwich model of columnar architecture, which played a significant role in the evolution of linguliforms, is highly biologically controlled and organic-matrix mediated. Furthermore, a continuous transformation of anatomic features resulting from the growth of diverse columnar shells is revealed between Eoobolidae, Lingulellotretidae, and Acrotretida, shedding new light on the evolutionary growth and adaptive innovation of biomineralized columnar architecture among early phosphatic-shelled brachiopods during the Cambrian explosion.

Adaptive specialization of a unique sponge body from the Cambrian Qingjiang biota

Sponge fossils from the Cambrian black shales have attracted attention from both palaeontologists and geochemists for many years in terms of their high diversity, beautiful preservation and perplexing adaptation to inhospitable living environments. However, the body shape of these sponges, which contributes to deciphering adaptive evolution, has not been scrutinized. New complete specimens of the hexactinellid sponge Sanshapentella tentoriformis sp. nov. from the Qingjiang biota (black shale of the Cambrian Stage 3 Shuijingtuo Formation, ca 518 Ma) allow recognition of a unique dendriform body characterized by a columnar trunk with multiple conical high peaks and distinctive quadripod-shaped dermal spicules that frame each high peak. The body shape of this new sponge along with other early Cambrian hexactinellids, is classified into three morpho-groups that reflect different levels of adaptivity to the environment. The cylindrical and ovoid bodies generally adapted to a large spectrum of environments; however, the dendriform body of S. tentoriformis was restricted to the relatively deep-water, oxygen-deficient environment. From a hindsight view, the unique body shape represents a consequence of adaptation that helps maintain an effective use of oxygen and a low energy cost in hypoxic conditions.