Fossil hypercalcified sponges; types, relationships and geological history

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 also been separated as a class, by sponge researchers, creating a quandary about their taxonomic status. Two older classification terms are redundant: sclerosponges (previously a class of all hypercalcified sponges) and pharetronids (previously a subgroup now divided into sphinctozoans and inozoans). Pharetronids are polyphyletic within the Demospongiae and Calcarea. Hypercalcified sponges’ history began with archaeocyaths (early-mid Cambrian). Then, 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 and 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, demonstrates that hypercalcification was an iterative evolutionary feature. For example, the stromatoporoid-grade appeared in Early to Mid-Ordovician and continued through geological history to modern representatives, albeit with taxa turnover through time. Stromatoporoids are traditionally viewed as becoming extinct at the end-Devonian Hangenberg event, but because they form 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 Lower Carboniferous strata 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. Modern hypercalcified sponges are sponge taxa that just happened to hypercalcify. Thus fossil hypercalcified sponges may be best considered as a system of hypercalcification across a complex and varied skeletal morphospace within Classes Demospongiae and Calcarea, alongside the evolutionary history of the phylum Porifera, to aid understanding of their changes in time. • We review the range of hypercalcified sponges through geological time, to make a comprehensive survey of this mixed group of sponge organisms, and potential sponge organisms. • As far as we are aware, there are no similar publications available, so this document intends to provide an overview of this complex subject, to make an accessible paper for specialists and non-specialists whose research materials may include such organisms. • We explain that hypercalcified sponges are found as a series of forms called grades of organisation, that are found across the traditional taxonomic groups of Demosponges and Calcarean sponges, and that the hypercalcified sponges are polyphyletic. • We explain that archaeocyaths are the first major group of fossil hypercalcified sponges with a wide diversity of genera, but they became extinct before the end of the Cambrian Period. Then, in Early Ordovician, hypercalcified sponges are dominated by lithistids and calathids followed by appearance and dominance of stromatoporoids through to the end-Devonian, after which stromatoporoids become very rare. The chaetetids, which occur from Silurian through to modern times, became abundant in the Carboniferous. Then the sphinctozoans and inozoans were very common in Permian and Triassic, but were affected by the end-Permian extinction. Mesozoic hypercalcified sponges include stromatoporoids, chaetetids, sphinctozoans and inozoans, but are rare after the end-Cretaceous extinction, yet there is a small group of living representatives of all four of these types. Archaeocyaths may be represented in one modern genus. • Overall we conclude that hypercalcified sponges are best viewed as a complex of hypercalcification from early Cambrian to modern; the process of hypercalcification survived all mass extinctions and evolved iteratively in different sponge groups through geological time.