Relationships of the Cambrian Protomonaxonida (Porifera)

Abstract

The Protomonaxonida consist of a heterogeneous group of early fossil sponges traditionally assigned to the demosponges. However, an affinity to the hexactinellid-like Reticulosa has also been suggested, and their relationships are potentially critical to understanding the origins of the extant sponge classes. In this paper, the relationships of the protomonaxonid sponges to each other and to other sponge groups have been reassessed, using previously described specimens as well as new material from the Burgess Shale of Canada and the Hetang Biota of South China. The sponges fall into two coherent groups, one consisting of taxa with long, mostly sub-longitudinal spicules, and the other with complex arrays composed of tracts of minute (millimetre-scale) monaxons, which grade into aspiculate taxa such as the Vauxiidae. Previous ideas relating the Protomonaxonida to extant demosponge lineages are supported in the case of the second group, whereas the first group confirms the view of derivation from a hexactinellid-like ancestor. Whether the two groups were directly related or evolved monaxonid spiculation in parallel is currently uncertain.

New articulated protospongiid sponges from the early Cambrian Chengjiang biota

“…If true, the presence of hexactine-based spicules is not a synapomorphy differentiating hexactinellids from demosponges . Recently, the discovery of possible hexagonal axial canals in hexactine-based spicules of Cyathophycus and the presence of hexactine-based spicules in some members of the order Protomonaxonida (Botting et al, 2014a), which is traditionally regarded as demosponges (Finks et al, 2004), is consistent with the view that hexactine-based spicules may have evolved in stem-group siliceans and subsequently lost in crowngroup demosponges. This new view, however, needs to be tested through a comprehensive cladistic analysis.…”

“…Protospongiids are characterized by non-fused stauractine spicules, thus they are easily disarticulated upon death and degradation. The new fossils, along with articulated protospongiids previously reported from lower and middle Cambrian Lagerstätten (Walcott, 1920;Rigby, 1978Rigby, , 1983Rigby and Collins, 2004;Xiao et al, 2005;Rigby et al, 2010), help us to better understand the evolutionary significance of protospongiids (Mehl, 1991) and the early evolution of hexactinellids and siliceans (Botting and Butterfield, 2005;Botting et al, 2012Botting et al, , 2014a.…”

“…However, as discussed under the systematic paleontology, the family Protospongiidae currently includes genera with diverse morphologies, some with regularly arranged and ranked stauractines forming regularly nested quadrules, others with irregularly arranged and poorly ranked stauractines forming local sub-quadrules, and still others that may not have quadrules or stauractines (e.g., Quadrolaminiella and Saetaspongia). Thus, although the Protospongiidae has been traditionally classified in the class Hexactinellida (Finks et al, 2004;Carrera and Botting, 2008), it is possible that this family as currently defined may be paraphyletic and may include both hexactinellids and stemgroup siliceans, particularly if hexactine-based spicules are not a unique feature of hexactinellids (Botting and Butterfield, 2005;Botting and Zhang, 2013;Botting et al, 2014a). A more thorough systematic evaluation of this family may result in its breakup into several families.…”

An outline of the distribution and diversity of Porifera in the Ordovician Builth Inlier (Wales, UK)

“…In particular, faunas preserved through pyritisation that are now intensively weathered (such as the Chengjiang and Fezouata biotas; Gabbott et al, 2004;Van Roy et al, 2010) would not be expected to yield sufficient detail to describe species-level differences. In contrast, the Burgess Shale complex does preserve remarkable detail of at least the spicule arrangements, and this has led to the recognition of a diverse suite of apparently endemic species, especially in Hazelia (Rigby and Collins, 2004;Botting et al, 2013b). Many of these species are based at least partly on details of the arrangement of the fine spicules, which would be invisible if preserved as weathered pyrite.…”

“…In the latter group, the body wall thickness is a significant proportion of the diameter of the sponge, and individual spicules are generally much more robust; this group may include stemgroup hexactinellids and stem-group siliceans. The other groups used in this analysis are protomonaxonids (Group 1 of Botting et al, 2013b, but including the systematically uncertain Xylochos Botting, 2004), demosponges (crown-group; in this study consisting of 'keratosan' groups and anthaspidellid lithistids; Group 2 of Botting et al, 2013b), and heteractinids (presumed stem-group Calcarea and perhaps stem-group Porifera; Botting and Butterfield, 2005). In a few cases, relevant only to the total diversity discussed below, there are sponges that are not easily assignable to one of the above groups at this stage.…”