Valospongia sonorensis, Beresi & Botting & Palafox & Buitrón Sánchez, 2017

Beresi, Matilde Sylvia, Botting, Joseph P., Palafox, Juan J. & Buitrón Sánchez, Blanca E., 2017, New reticulosan sponges from the middle Cambrian of Sonora, Mexico, Acta Palaeontologica Polonica 62 (4), pp. 691-703 : 696-698

Valospongia sonorensis sp. nov.

Fig. 5A .

Etymology: After the state of Sonora, México, were the specimens were collected.

Type material: Holotype: IANIGLA-PI 3094a/b, a partial specimen showing well-preserved outer skeletal layer with mounds, and spiculation of inner layer. Paratype: IANIGLA-PI 3095, fragment with part of the spicular net, from the type locality; found in the central part of the section, approximately at 40 m stratigraphically above the base of the formation.

Type locality: Cerro El Mogallón quarry, near Arivechi, 237.5 km East of Hermosillo , Sonora, México .

Type horizon: Ptychagnostus atavus Biozone , Series 3, middle Cambrian.

Material.— Type material and IANIGLA-PI 3119, fragment with part of skeleton from the type locality.

Diagnosis.— Medium to large Valospongia with prominent subcircular mounds spaced slightly less than their diameter apart, and numerous small, rounded parietal gaps in the intervening body wall; primary skeleton composed of irregular network of spicular bundles formed by parallel, adjacent rays of large stauractines (and possibly diactines); irregular meshwork of smaller stauractines forms secondary skeleton, outlining the parietal gaps; mounds covered by additional fine skeletal reticulation of indistinct spicules, dense at edge of mounds and more open at summit.

Description.— Material is preserved as oxidised pyrite replacement of spicules, and also mineralized traces of soft tissue over substantial parts of the specimens. No complete material available, but fragments imply a large, broad sponge body ( Fig. 5A ); holotype 61 mm maximum wide and over 86.5 mm tall; counterpart 56 mm at widest and over 84 mm tall. Oscular margin probably not preserved, although one edge is concavely curved, and may represent oscular rim; no distinct marginalia or prostalia visible.

Specimen shows all the skeletal layers compressed onto each other.Outer skeletal wall with low,subcircular mounds, mostly 3–4 mm in diameter (total range 1–6.5 mm), evenly spaced but irregularly distributed over the entire preserved surface. The mounds are perforated at their crests and appear to represent parietal gaps through the thin sponge wall, possibly representing lateral oscula by analogy with extant euplectellid (and other) hexactinellids. These mounds are now filled by fine argillaceous matrix.

Some mounds are covered with a thin, irregular mesh ( Fig. 5A 8) of tiny stauractines up to 0.25 mm long, irregularly organised but with some indication of a consistent, orthogonal arrangement. The fine skeletal net extends across the mounds on both counterparts. The outer layer of the skeleton between the mounds is preserved as a thin sheet of oxidised iron minerals showing the original extent of soft tissues, containing tiny spicules, which outlines numerous small parietal gaps. Spicules in this layer could correspond to autodermalia, covering the exterior of the sponge including the lateral surface of the mounds. The best-preserved regions in the holotype show an apparently regular grid with minute spicules of 0.2 mm long, oriented diagonally relative to the larger spicules, and spaces consistently around 0.1 mm across, between spicules that appear to be simple stauractines in semi-regular arrangement surrounding and overlying the mounds. Also, a dense felt of small, fine spicules (mainly irregularly oriented stauractines with ray length up to 1 mm) outlines close-packed, circular to oval spaces that form a dense, disordered array. The spicules are arranged to form an irregular polygonal meshwork, with gaps up to maximum of 0.5–1 mm across. The skeletal areas between gaps are normally 0.5 mm wide at narrowest.

Primary, presumed choanosomal, skeletal layer (or possibly layers according to Rigby 1983; Fink and Rigby 2004; Botting and Muir 2014) composed of continuous strands formed by overlapping spicule rays, and forming an irregular, pseudo-orthogonal and pseudo-diagonal reticulation. Spicules may be entirely stauractines, but diactines are also probably present in the largest strands; hexactines cannot be excluded, but no evidence for distal or gastral rays observed. Primary strands or fibres typically separated by 5–8 mm. The largest distinct spicules forming the bundles have coarse, straight and simple rays up tp 30–40 mm long and with basal ray diameter of approximately 0.3–0.6 mm. Some rays appear curved and sinuous, including strand-like structures up to 37 mm long and 0.3–0.4 mm in diameter, but it is unclear whether these represent single spicule rays or bundles composed of a few straight rays that overlap in an arc. Spaces between primary strands are subdivided by similar strands composed of smaller spicules, ultimately resulting in square to irregular spaces of typically 7–13 mm.

A range of measurements for spicules of Valospongia sonorensis sp. nov. are provided in Table 2.

Remarks.— The structure of Valospongia is complex and disputed, with implications for systematic placement and relationships with other early reticulosans. The main components of its skeletal architecture are summarised here:

(i) Dermalia: a dense surface layer with small (1 mm) circular spaces over the entire surface, perhaps also with distinct, semi-regular lattice of tiny stauractines as autodermalia; these also cover the summits of low mounds.

(ii) Parenchymal/choanosomal skeleton: consisting predominantly of continuous inner layer or layers of semiquadruled, semi-regular stauractines (and/or diactines and hexactines) combining to form irregularly orthogonal and diagonal elements. The long rays of stauractines form the primary skeletal support, with greatly elongated overlapping rays outlining roughly rectangular or triangular regions, subdivided by finer skeletal strands. The largest strands, probably composed at least partly of long diactines, are irregularly orientated as part of the primary skeletal structure.

Only two species of Valospongia have been previously described: the type species V. gigantis Rigby, 1983 from the middle Cambrian of Utah, and V. bufo Botting and Muir, 2014 from the late Tremadocian of North Wales. Valospongia gigantis Rigby, 1983 was assigned to Hydnodictydae Rigby, 1971 and was characterized as a thin-walled sponge sculptured with low mounds and fine-scale dermal spiculation, both over and outside the mound surfaces, together with prominent skeletal strands that were also emphasized by Finks and Rigby (2004). These features are also observed in the new material from Sonora, leaving no doubt as to the generic assignment.

The main difference between the new species and V. gigantis Rigby, 1983 is in the presence of numerous small rounded gaps between the spicule strands. As the type material of both preserves evidence of soft tissue, the difference is obvious, but in purely skeletal remains the distinction may be obscure. In that case, skeletal proportions and the wider separation of the mounds are perhaps the best discriminating characters. The mounds and the inter-mound areas of the new species appear to be covered by a reticulate meshwork of tiny stauractines, suggesting distinct autodermalia; this has not been preserved in the type species, however, so its taxonomic significance remains unclear.

The other known species, V. bufo Botting and Muir, 2014 from the late Tremadocian of Wales, is known from numerous specimens that are all far smaller than the type species or V. sonorensis sp. nov. It also has smaller and more close-packed mounds than in the Mexican species, and its inner layer is composed of dense felt of small, fine hexactines or derivatives in parallel, perhaps partly quadruled array, which is very different to the well developed skeletal bundles in the other species.

A possible Valospongia was described by Wu et al. (2014) as V. cf. gigantis , but the features of this partial sponge are very unclear. There appear to be three-dimensional mounds projecting from the body wall, but the spicules are weakly preserved. As this is the only record of the genus from China, however, it should be treated with some caution.

Stratigraphic and geographic range.—The range is restricted to the middle Cambrian El Mogallón Formation, Arivechi, central Sonora, Mexico.

Figures

Fig. 5. Reticulosan sponge Valospongia sonorensis sp. nov. from the middle Cambrian El Mogallón Formation at Cerro El Mogallón, Sonora, Mexico. A. Holotype IANIGLA-PI 3094a/b, overall view of part and counterpart of specimen. A1, view of inner layer of the holotype; A2, detail showing base of mounds and largest spicules in semi-quadruled array; A3, view of outer layer; A4, part of the skeleton, showing semi-regular outer spicules, mounds and parietal gaps of inner layer; A5, detail of mound covered in parts by reticulate mesh, visible in places; A6, close-up of round recrystallised mound; A7, detail of a mound base with dermalia visible in places, and parietal gaps of inner layer; A8, view of part of parenchymal/choanosomal skeleton with largest spicules and smaller spicules distributed at parietal gap margins; A9, bundles composed of the largest distinct spicules, with coarse, straight and simple rays. B. Paratype IANIGLA-PI 3095; B1, overall view of the fragment; B2, detail showing largest spicules of primary reticulation and smaller spicules at parietal gap margins. C. IANIGLA-PI 3119, fragment of V. sonorensis with spiculation of the outer wall obscured by pyritisation at the margin of subcircular parietal gaps.