Leptomitid sponges from the Cambrian (Stage 4) Balang Fauna of Guizhou, China

Abstract

New Cambrian sponge fossils can generate alternative interpretations of the phylogenetic and biogeographical history of early sponges. Recently, many sponge fossils have been discovered from the Stage 4 Balang Fauna of Guizhou, China; these sponges are dominated by protomonaxonids, some of which can provide reliable evidence for interpreting the above-mentioned history. The sponge fossils, having the basic skeletal architecture of leptomitids, are recognized here as Jianhella obconica nov. gen., nov. sp., and Leptomitus teretiusculus Chen, Hou et Lu, 1989. The new taxon has a large obconical shape; it is characterized by a dominantly longitudinal skeleton consisting of isolated longitudinal and oblique monaxons, and apparently lacks transverse spicules. The obconical body form and unique skeleton show that Jianhella nov. gen. clearly differs from previously described leptomitids and suggest that it may represent an intermediate morphology between leptomitids and basal protomonaxonids. The occurrence of Jianhella nov. gen. from a single deposit and its absence elsewhere may indicate that it was a monospecific, endemic genus, but may also suggest that it was a rare genus, so far exemplified by its occurrence in a single site with very few specimens. Additional specimens with an elongated tubular shape and a skeleton consisting of bundled longitudinal monaxons and isolated transverse spicules closely fit the description and illustrations of Leptomitus teretiusculus from the Cambrian biotas of South China. According to the spicule morphology, the degree of spicule bundling and the skeletal architecture, there likely exists a deep node separating Jianhella nov. gen. from the earlier ancestral members of Leptomitus. Fossil evidence, together with previous studies, confirms that Leptomitus generally had a relatively long stratigraphic range (Cambrian Stage 3 to Drumian) and had an effectively cosmopolitan distribution during the early and middle Cambrian.