Three-dimensionally preserved soft tissues and calcareous hexactins in a Silurian sponge: implications for early sponge evolution

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.

The sponge pump as a morphological character in the fossil record

“…560 Ma (Shore et al 2021). If these findings are corroborated with further observations, the lack of undisputable sponge fossils during this interval (800–560 Ma) is puzzling, unless the environment was not conducive to the preservation of soft tissue and assuming that early sponges lacked a mineral skeleton (Antcliffe et al 2014; but see Nadhira et al 2019). On the other hand, molecular phylogenies suggest a deep Cryogenian divergence for crown group Porifera, implying that spiculate sponges should be present in the late Neoproterozoic (Cryogenian–Ediacaran); but so far none fits the criteria proposed by Antcliffe et al (2014), who argue that the earliest sponge spicules do not appear until well into the Cambrian at 535 Ma.…”

Hexactins in the ‘protomonaxonid’ sponge Choiaella and proposal of Ascospongiae (class nov.) asa formal replacement for the Protomonaxonida

“…The only reason to place the ascosponges into the Hexa ctinellida would be the presence of hexactins, but these are also known among fossils in heteractinid calca reans such as Eiffelia (Botting & Butterfield 2005) and in the entirely calcitic reticulosan-like sponge Carduispongia (Nadhira et al 2019) that shows calcarean-like soft tissue structure. Hexactins are also present in the proposed stemgroup demosponges Cyathophycus loydelli Botting, 2004 andConciliospongia (Botting et al 2017).…”

“…Among the potential early-branching groups, the calcarean lineage is represented by heteractinids (Botting & Butterfield 2015) and shows hexactins but almost no indication of diactins. The body form of early heteractinids such as Eiffelia is globose rather than conical, and no additional similarities are seen with the calcareous ʻreticulosanʼ Carduispongia that may represent a different part of the calcarean stem group (Nadhira et al 2019). Similarities to late stem-group Silicea are also relatively few, as these are best represented by the reticulosans like Cyathophycus Botting et al 2017) that show a regularly reticulate primary skeleton with additional internal layers.…”

Newly discovered morphology of the Silurian sea spider Haliestes and its implications

“…They may have been used to cling onto surfaces, such as the body of sponges that are abundant in the Herefordshire fauna (Nadhira et al . 2019; Siveter et al . 2020, fig.…”