Advanced crown-group Rossellidae (Porifera: Hexactinellida) resembling extant taxa from the Hirnantian (Late Ordovician) Anji Biota

Among living hexactinellids (glass sponges), the Rossellidae are one of the most distinctive species-rich families because of their unique macroscopic characters and, due to the resulting fossil record, are among the most useful for tracing the origins of hexactinellid diversification. Recent discoveries have extended the origin of the total group back to the Ordovician, but these fossils have, so far, all been identified either as stem-group representatives or as sponges with uncertain interpretation. New material described here from the Hirnantian (latest Ordovician) Anji Biota of Zhejiang, China includes demonstrable crown-group representatives of the family, which also appear morphologically similar to living genera and may be closely related to them. The new taxa are described as Crateromorpha? (Neopsacas?) macrospicula sp. nov., Pseudanoxycalyx verrucosus gen. et sp. nov., Eorosselloides antiquus gen. et sp. nov. and Archaeaphorme conica gen. et sp. nov. The similarity to modern forms implies extraordinary evolutionary stasis of at least some members of the modern deep-sea hexactinellid fauna since that time, and suggests that they had an even earlier origin and initial diversification. Possible examples of late Precambrian stem-group hexactinellids remain ambiguous, potentially implying very rapid evolutionary appearance and divergence of the class during the Cambrian or earlier Ordovician. A possible driver of this evolutionary rate change is the remarkable longevity of modern hexactinellids as an adaptation to cold-water environments; this potentially offers an explanation for discordances with molecular clock results, which have previously indicated a much later (late Palaeozoic) origin for total-group Rossellidae.