A new sponge genus from the Chengjiang biota with an intriguing combination of skeletal characters

The development of fossil sponge systematics is hindered by factors such as their low anatomical complexity and the low preservation potential of many characters used in the definition of extant sponge clades. Here we describe Calliospongia chunchengia gen. et sp. nov., a new sponge from the Lower Cambrian Chengjiang biota, which possesses a unique combination of characters. The skeleton of C. chunchengia is similar to those of leptomitid protomonaxonids in being composed of large spicules arranged longitudinally. However, it is distinct enough to belong in its own genus and even question a leptomitid affinity, because, unlike the members of this group, which usually have two categories of diactine spicules, its skeleton is entirely composed of triactines. The new taxon shares the presence of large megascleres with demosponges, hexactinellids and the ascosponges, suggesting an affinity to the total group of Silicea. It is also reminiscent of the hexactinellid-like reticulosans, showing a potential connection between two major types of Early Palaeozoic sponge body plans. A more accurate definition of the Cambro-Ordovician groups and the creation of a total evidence framework that integrates extinct and extant sponge morphology into the phylogenetic scenario derived from phylogenomics seem to be essential steps for the improvement of our understanding of early sponge evolution.