A new Cambrian sphinctozoan sponge from North America, its relationship to archaeocyaths and the nature of early sphinctozoans

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Abstract
A new early Cambrian sponge of sphinctozoan organization named Polythalamia americana nov. gen. et sp. is described from Nevada and Alaska, U.S.A. P. americana nov. gen. et sp. shows little resemblance to other Cambrian reported sphinctozoans, but is similar to late Palaeozoic to early Mesozoic forms from Europe and Asia, e.g. Stylothalamia and Amblysiphonella. The polyphyletic origin of sphinctozoans based upon spicule criteria from younger examples is therefore corroborated by the varied morphology and clearly widespread palaeogeographic distribution of Cambrian forms, which were hitherto recognized only from Australia. P. americana nov. gen. et sp. possesses a relatively small, globular multi-chambered calcareous skeleton, with thin but densely perforate walls and a central retrosiphonate, perforate spongocoel. The skeleton is composed of an irregular microstructure of unknown, but probable calcitic, original mineralogy. There are no spicules or primary internal structures, but secondary vesicular filling tissue is present. The pore organization, however, it directly comparable to archaeocyaths, e.g. coscinocyathines, and this character is taken to be the only synapomorphy available for taxonomic and phylogenetic purposes. Indeed, P. americana nov. gen. et sp. is closely associated with a diverse archaeocyath assemblage and draws attention to morphological similarities between some archaeocyaths and sphinctozoans, thus questioning the criteria at present used to distinguish between these groups. Accordingly, a list of nomenclatural synonymies is given to aid comparison. Coscinocyathine archaeocyaths possess chambered juvenile stages and P. americana nov. gen. et sp. is suggested to represent an aspiculate lineage of sphinctozoan grade which may be derived from forms such as Clathricoscinus by neoteny. It appears that at least three clades of sphinctozoan-grade calcified sponges were present by end of early Cambrian time and their occurrence in association with island arcs of the palaeopacific rim is confirmed.