Abstract
Sponges (Porifera), as one of the earliest-branching animal phyla, are crucial for understanding early metazoan phylogeny. Recent studies of Lower Palaeozoic sponges have revealed a variety of character states and combinations unknown in extant taxa, challenging our views of early sponge morphology. The Herefordshire Konservat–Lagerstätte yields an abundant, diverse sponge fauna with three-dimensional preservation of spicules and soft tissue. Carduispongia pedicula gen. et sp. nov. possesses a single layer of hexactine spicules arranged in a regular orthogonal network. This spicule type and arrangement is characteristic of the reticulosans, which have traditionally been interpreted as early members of the extant siliceous Class Hexactinellida. However, the unusual preservation of the spicules of C. pedicula reveals an originally calcareous composition, which would be diagnostic of the living Class Calcarea. The soft tissue architecture closely resembles the complex sylleibid or leuconid structure seen in some modern calcareans and homoscleromorphs. This combination of features strongly supports a skeletal continuum between primitive calcareans and hexactinellid siliceans, indicating that the last common ancestor of Porifera was a spiculate, solitary, vasiform animal with a thin skeletal wall.
Highlights
The origin of the sponges (Porifera) is inextricably bound up with the origin of the Metazoa
This is common in other calcitic Herefordshire fossils, for example, ostracods and brachiopods [26,27], where soft tissue and biomineralized calcite structures have merged as part of the preservation process
The new evidence from the new Herefordshire sponge supports the hypothesis that hexactine spicules are plesiomorphic for Porifera [16], by confirming that they occur in the absence of siliceous biomineralization
Summary
The origin of the sponges (Porifera) is inextricably bound up with the origin of the Metazoa They are traditionally considered to be the earliest-branching metazoans, but an alternative hypothesis holds that they may be secondarily simplified from organisms of ctenophore- or cnidarian-grade [1]. Hexactinellida have hexactine siliceous spicules with a square axial filament/canal and additional, distinct microscleres. Siliceous spicules are secreted onto the axial filament, whereas calcareous spicules are secreted inside an organic sheath surrounding the spicule (see Uriz [6] for a review). Intermediates between these states are unknown in extant taxa; without fossil evidence their evolutionary pathways would be unresolved, including the question of whether spicules are homologous between classes
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