New early Cambrian sponges of the Siberian platform and the origins of spiculate crown‐group demosponges

Abstract

Sponges are among the most common and diverse Cambrian fossils. However, the origin of the crown groups of the extant sponge classes constitutes the principal problem in the understanding of the evolution of these lower metazoans. New intact spiculate sponges from the lower Cambrian Stage 3–Stage 4 Sinsk Lagerstätte of the Siberian Platform enable a better understanding of the early evolution of crown‐group demosponges. The skeletons of Neomenispongia plexa and N. diazoma gen. et sp. nov. consist mostly of simple oxeas, which are organized in relatively regular tufts that are additionally strengthened by sigmoidal spicules. The C‐shaped elements of N. diazoma are megascleres in their size range but have sigmoidal shapes similar to sigma microscleres of extant demosponges; the sigmoidal spicules of N. plexa fully accord with microscleres and are the smallest spicules in known Cambrian demosponges. Together with an unnamed early Cambrian demosponge from the Sirius Passet biota (Greenland) and middle Cambrian Ulospongiella from the Burgess Shale (Canada), the new species represent the earliest heteroscleromorph demosponges and indicate an evolutionary origin of microscleres from megascleres. The thin, homogenous skeleton of Keithospongos loricatus gen. et sp. nov. is built of small, spirally arranged oxeas corresponding to the skeletal structure of the primitive Hazeliidae, which have been interpreted as the ancestral skeletal organization of demosponges. These new sponges therefore provide a link from extant spiculate demosponge groups to their more familiar Cambrian ancestors.