Siliceous spicules in a vauxiid sponge (Demospongia) from the Kaili Biota(Cambrian Stage 5), Guizhou, South China

A new sponge from the Marjum Formation of Utah documents the Cambrian origin of the hexactinellid body plan

Modern poriferans are classified into four classes-Calcarea, Demospongiae, Hexactinellida and Homoscleromorpha-the recognition of which in fossil specimens almost exclusively relies on spicule morphology and arrangement. Early fossil representatives of the phylum Porifera are morphologically diverse, and many of them problematically display characteristics that are incompatible with the classification scheme developed for modern taxa. Critically, hexactine spicules-a diagnostic feature of hexactinellids among modern taxa-are found in various Cambrian and Ordovician taxa that cannot be accommodated within the hexactinellid body plan. Here we describe a new poriferan from the Drumian Marjum Formation of Utah, Polygoniella turrelli gen. et sp. nov., which exhibits a unique combination of complex anatomical features for a Cambrian form, including a syconoid-like organization, a thick body wall, and a multi-layered hexactin-based skeleton. The hexactinellid-like body wall architecture of this new species supports a Cambrian origin of the hexactinellid body plan and provides valuable insights into character evolution in early glass sponges.

Evolution of the main skeleton-forming genes in sponges (phylum Porifera) with special focus on the marine Haplosclerida (class Demospongiae)

The skeletons of sponges (Phylum Porifera) are comprised of collagen, often embedded with small siliceous structures (spicules) arranged in various forms to provide strength and flexibility. The main proteins responsible for the formation of the spicules in demosponges are the silicateins, which are related to the cathepsins L of other animals. While the silicatein active site, necessary for the formation of biosilica crystals, is characterized by the amino acids SHN, different variants of the silicatein genes have been found, some that retain SHN at the active site and some that don't. As part of an effort to further understand skeleton formation in marine sponges of the order Haplosclerida, a search for all silicatein variants were made in Irish species representing the main clades of this large sponge group. For this task, transcriptomes were sequenced and de novo assembled from Haliclona oculata, H. simulans and H. indistincta. Silicatein genes were identified from these and all available genomes and transcriptomes from Porifera. These were analysed along with all complete silicateins from GenBank. Silicateins were only found in species belonging to the class Demospongiae but excluding Keratosa and Verongimorpha and there was significant duplication and diversity of these genes. Silicateins showing SHN at the active site were polyphyletic. Indeed silicatein sequences were divided into six major clades (CHNI, CHNII, CHNIII, SHNI, SHNII and C/SQN). In those clades where haplosclerids were well represented the silicatein phylogeny reflected previous ribosomal and mitochondrial topologies. The most basal silicatein clade (CHNI) contained sequences only from marine haplosclerids and freshwater sponges while one silicatein from H. indistincta was more related to cathepsins L (outgroup) than to the overall silicatein clade indicating the presence of an old silicatein or an intermediary form. This data could suggest that marine haplosclerids were one of the first groups of extant demosponges to acquire silicatein genes. Furthermore, we suggest that the paucity of spicule types in this group may be due to their single copy of SHNI variants, and the lack of a silintaphin gene.

Discovery of missing link between demosponges and hexactinellids confirms palaeontological model of sponge evolution

The two major extant groups of siliceous sponges, Demospongiae and Hexactinellida, are generally regarded as sister groups forming the clade Silicea, although the nature of their last common ancestor is uncertain. The fossil record contains a diverse range of basal demosponges that appear to have evolved from hexactine-bearing reticulosan ancestors, although a compelling morphological intermediate has not previously been discovered. Here we describe a new species of fossil sponge, Conciliospongia anjiensis gen. et sp. nov., from the Late Ordovician (~444 Ma) Anji Biota of South China. This species has a reticulate, tufted skeleton of minute monaxon spicules, characteristic of the fossil demosponge family Hazeliidae and modern heteroscleromorphs, with hexactine spicules and a globose body form inherited from reticulosan ancestors. This transitional morphology had previously been hypothesized in palaeontological studies. This morphological intermediate between two extant classes further confirms siliceous sponge monophyly and demosponge–hexactinellid spicule homology, and supports the primitive, stem-silicean interpretation of simpler-structured fossil reticulosans.

Siliceous spicules in a vauxiid sponge (Demospongia) from the Kaili Biota(Cambrian Stage 5), Guizhou, South China

Fossils of the sponge Angulosuspongia sinensis from calcareous mudstones of the middle and upper part of the Kaili Formation (Cambrian Stage 5) in the Jianhe area of Guizhou province, South China, exhibit an apparently reticulate pattern, characteristic of the Vauxiidae. Energy Dispersive X-Ray Spectrometry (EDS) and Raman spectroscopy analysis indicate the presence of silica in the skeletal elements of these fossils, suggesting that this taxon possessed a skeleton comprised of spicules. This is the first confirmation of siliceous skeletal elements in fossils of the family Vauxiidae, and it lends support to the hypothesis that some early demosponges possessed biomineralized siliceous skeletons, which were subsequently lost and replaced by spongin later in the evolutionary history of this lineage. The new materials provide critical insight into the phylogeny and evolution of biomineralization in the Demosopongiae.