Ratcliffespongia arivechensis, 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 : 694-696

Ratcliffespongia arivechensis sp. nov.

Fig. 4 .

Etymology: From Arivechi, town in the state of Sonora, México, where the specimen was collected.

Holotype: UNAM 953 , a very well-preserved fragment of body wall showing both skeletal details and parietal gaps outlined by ranked stauractine spicules.

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

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

Diagnosis.— Ratcliffespongia with irregular array of subcircular parietal gaps, and smaller gaps in largest spaces between major gaps; skeleton of stauractines (perhaps also hexactines/pentactines) with some rays aligned to produce arcuate bundles; rays straight or very slightly curved; smaller stauractine spicules sometimes with one ray deflected.

Description.— Only a single fragment available, 18 × 19 mm ( Fig. 4A ) without base or osculum. Thin-walled, with a single skeletal layer pierced by prominent, large, circular to elliptical parietal gaps, partly outlined by irregularly- oriented, ranked stauractines of four size orders in non-parallel arrangement. The parietal gaps are separated by skeletal regions with width slightly less than their diameter, on average, and are arranged without any regular pattern. Largest circular gaps are 2.0– 3.4 mm wide, with distance between them typically around 1–2 mm (see Table 1). Additional small circular openings, normally 0.4–1.0 mm across, present between right-angled rays of the largest stauractines in areas between the large gaps.

Skeleton ( Fig. 4A, B ) composed possibly of simple stauractines rather than hexactines, as there is no trace of distal or gastral rays in any spicules. At least four ranks of spicules evident (see Table 1). Largest stauractines may be locally aligned, but alignment is not constant across the fragment. Smaller stauractines in non-parallel arrangement occupy areas between parietal gaps. Individual spicules with rays straight and simple, not obviously tapering visibly. Firstorder spicules with rays 2.4–2.9 mm long and with basal ray diameter of 0.20–0.34 mm. Some larger-order spicules have closely-spaced, aligned rays, combining to form continuous, straight or sinuous strands or bundles across the preserved fragment, extending at least 20 mm. Strands are not parallel to each other, and show no clear geometric relationship.

Second-order, intermediate-sized spicules have rays 1.6–1.8 mm long and are 0.08–0.12 mm in basal diameter. Third-order spicules have rays 1.2–1.3 mm long and 0.06–0.07 mm wide. Fourth-order (smallest) spicules with rays 0.60–1.0 mm long and 0.03–0.05 mm in basal diameter.

Commonly three of the rays of smaller spicules are nearly at right angles, but the fourth is often at an odd angle, apparently to accommodate other spicules and the margins of small parietal gaps. Spicules replaced by pyrite, secondarily oxidised to ferroan oxides (goethite/limonite), with the body wall tinted by iron oxides that mark the extent of soft tissues and outline parietal gaps; the entire sponge fragment is surrounded by a pale oxidation halo.

No dermal or gastral layer preserved. No evidence for basalia or marginalia is seen in the specimen.

Remarks.— The typical structure of the family Hintzespongiidae is confusing, as it has been described by different authors ( Rigby and Church 1990: 331; Finks and Rigby 2004; Wu et al. 2005) as thin-walled, conico-cylindrical with one- or two-layered wall; if a double-layered wall, then one is a thin inner layer with prominent oval to circular parietal gaps surrounded by irregularly oriented stauractines, and the other an outer, regularly reticulate layer similar to Protospongia . and with possible rhabdodiactine prostalia. As discussed at the genus level, the second wall layer of Ratcliffespongia is an error introduced by misinterpretation of one of the species assigned to it, and the type species defines the single-layered skeletal architecture that characterises the genus.

The well-preserved fragment of R. arivechensis sp. nov. most closely resembles the type species of Ratcliffespongia , rather than Hintzespongia (formerly Ratcliffespongia ) wheeleri , and shows very similar skeletal architecture. The most significant difference is the presence of arcuate strands composed of adjacent spicule rays, in a similar manner to that seen (and much more strongly developed) in Valospongia (see below). Ratcliffespongia multiforamina Wu, Yang, Jannussen, Steiner,Yang, and Zhu, 2005 , from the Huangboling Formation of China, also conforms well with the type species of Ratcliffespongia , but shows few, if any, deflected rays, and includes pentactine or hexactines as well as stauracts. It is the only species of Ratcliffespongia known from virtually complete fossils, and was a broadly conical sponge; this may represent the body shape of the genus as a whole, but that cannot as yet be assessed. The early Cambrian R. freuchenensis Botting and Peel, 2016 , from the Sirius Passet Biota, north Greenland, is known from fragments of a stauract-based skeleton that has smaller parietal gaps than R. arivechensis ; most spicules are significantly smaller than in the current material, and more densely arranged, and the parietal gaps are separated by skeletal regions roughly as wide as their diameter. Many of these differences could be ontogenetic, but R. freuchenensis also shows strongly deflected rays in most of the larger spicules, rather than such deflections in some of the smaller spicules only. The two species are clearly closely related, but are certainly distinct. Liu et al. (2012: fig. 3) described as Protospongia ? sp. a sponge from the top of the middle Cambrian Mantou Formation, Henan Province, China, which shows parietal gaps surrounded by an irregular array of hexactine-based spicules, some with strongly deflected rays. Although proximal and/or distal rays are present in some spicules, this specimen is probably assignable to Ratcliffespongia or a related lineage, and is very similar to R. freuchenenis .

Other parietal gap-bearing taxa from the Burgess Shale-type faunas include Stephenospongia Rigby, 1986 , assigned to the Hintzespongiidae ( Finks and Rigby 2004) , which is also known from fragments only. This is a thin-walled sponge that can be distinguished by its elongate parietal gaps, the presence of numerous hexactines, arrangement of spicules to form dense tracts between the closely-spaced gaps, and by the large size of the spicules relative to the parietal gaps. The only non-Laurentian Cambrian sponge recorded as Hintzespongia ( Yang et al. 2010) appears to show no evidence of a secondary spicule layer, and is likely to represent instead Ratcliffespongia or a related genus.

Stratigraphic and geographic range.— Recognised from the middle Cambrian at the Cerro El Mogallón, Arivechi, central Sonora .

Figures

Fig. 4. Holotype UNAM 953 of the reticulosan sponge Ratcliffespongia arivechensis sp. nov. from the middle Cambrian El Mogallón Formation at Cerro El Mogallón, Sonora, Mexico. A. Skeletal layer with larger circular to elliptical parietal gaps. B. Detail showing reticulation of single skeletal layer. C. Enlargement showing that larger-order spicules are locally oriented with closely-spaced, aligned rays, combining to form straight or sinuous bundles.