New Articulated Protospongiid Sponges From the Camrbian Chengjiang Biota

New articulated protospongiid sponges from the early Cambrian Chengjiang biota

Authors:

Chen et al

Abstract:

Sponges are among the earliest diverging crown-group animals and widely regarded as the earliest biomineralizing animals. Indeed, unambiguous hexactine sponge spicules first occur in the lowermost Cambrian strata of the Fortunian Stage. Articulated sponge skeletons interpreted as hexactinellids and demosponges have been reported from Cambrian Stage 2–3 strata at multiple localities. Articulated sponge skeletons in the Chengjiang biota (Cambrian Stage 3), however, are dominated by forms interpreted as demosponges, despite the exceptional preservation in this biota. Here, we report new articulated sponge skeletons from the Chengjiang biota, including Paradiagoniella magna n. gen. n. sp. and P. xiaolantianensis n. gen. n. sp. The skeleton of both species consists of ranked stauractines and rare oxeas (straight or curved diactines), as well as hexactines in the former species. Their stauractines form irregular, nested, local sub-quadrules oriented obliquely to the sponge body axis. Sub-quadrules of different ranks are not in parallel arrangement. The two species are tentatively placed in the family Protospongiidae, which as currently defined may be a paraphyletic group including members of the total-group hexactinellids and perhaps stem-group siliceans. The phylogenetic placement of P. magna and P. xiaolantianensis is uncertain but, like many other protospongiids, they could be members of the total-group hexactinellids or stem-group siliceans. The diactines in the two species could be secondarily reduced hexactine-based spicules; alternatively, these two species may represent an evolutionary grade of stem-group siliceans with both diactines and hexactine-based spicules, the latter of which was lost in demosponges. A comprehensive cladistics analysis is needed to resolve the exact phylogenetic placement of the two species described here.