Stephen Kershaw, Qijian Li

Hypercalcified sponges are poriferans with a calcareous skeleton secreted on and in the soft tissue. Living examples and fossils of some such sponges in Mesozoic and Cenozoic strata contain sponge spicules and can be classified within modern poriferan groups of the Classes Demospongiae and Calcarea, which are otherwise almost entirely soft-bodied. However, other fossil forms, largely Palaeozoic archaeocyaths, stromatoporoids and chaetetids, lack spicules, so their classification relies on the calcareous skeleton alone. Because of these discrepancies, although the hypercalcified skeleton is useful for low-level taxonomy in fossils, it has no proven phyletic value, so the systematic position of non-spiculate forms is problematic. Thus the hypercalcified skeleton has for many years been considered a grade of organisation of the skeleton, and the terms archaeocyath-grade, stromatoporoid-grade, chaetetid-grade, sphinctozoan-grade and inozoan-grade are applicable. Nevertheless, archaeocyaths have been separated as a class, by sponge researchers. Two older classification terms are redundant: sclerosponges (for hypercalcified sponges in general) and pharetronids (for a subgroup divided into sphinctozoans and inozoans). Pharetronids are polyphyletic within the Demospongiae and Calcarea.
Hypercalcified sponges history began with archaeocyaths (Early Cambrian). Prominence of stromatoporoid-grade in the mid-Palaeozoic, and chaetetid-grade in the Carboniferous, was followed by a sparse record in both groups for much of the Permian while sphinctozoan- and inozoan-grades expanded. The Mesozoic has a good record of sphinctozoans, inozoans, stromatoporoids and chaetetids up to the end-Cretaceous. Cenozoic forms are uncommon but 19 genera of modern-day demosponges and calcarean sponges encompass all five grades, versus the total modern sponge diversity of 680 genera. Hypercalcification is diverse in modern sponges, involving aragonite, high-Mg & low-Mg calcite; ancient groups reflect this range in their variation of preservation (including widespread diagenetic alteration) that makes understanding of hypercalcification mechanisms problematic.
Presence of hypercalcified sponges from Early Cambrian to modern times, with short breaks associated with extinction events, demonstrate hypercalcification was an iterative evolutionary feature. This means that, for example, the stromatoporoid-grade, which appeared in the Early to Mid-Ordovician, continued through history to the modern representatives, albeit with taxa turnover through time. Stromatoporoids are traditionally viewed as becoming extinct at the end-Devonian Hangenberg event, but because this is a grade, rather than a proven phyletic group, discussion of the extinction of stromatoporoids as a group has little meaning; it is more appropriate to consider that certain sponge taxa, possessing stromatoporoid-grade skeletons, became extinct. Rare stromatoporoid-grade taxa in the Lower Carboniferous support such a view. Although their polyphyletic nature was recognized for Mesozoic and Cenozoic forms, the 2015 Treatise on hypercalcified sponges treats stromatoporoids and archaeocyaths as distinct groups. However, hypercalcified sponges are a complex of sponge taxa that just happened to hypercalcify, occurring across the classes of Demosponge and Calcarea. Thus fossil hypercalcified sponges may be considered as being an integrated system of hypercalcification, alongside the evolutionary history of the phylum Porifera, to aid understanding of their changes in time.