Decumbispongia yuani, Botting & Muir & Li & Lin, 2013

Botting, Joseph P., Muir, Lucy A., Li, Xiang-Feng & Lin, Jih-Pai, 2013, An enigmatic, possibly chemosymbiotic, hexactinellid sponge from the early Cambrian of South China, Acta Palaeontologica Polonica 58 (3), pp. 641-649 : 642-644

Decumbispongia yuani sp. nov.

Fig. 1 .

Etymology: After Yuan Xunlai, in appreciation of his discovery and previous work on the Hetang Formation sponges.

Type material: Holotype, NIGP154188 , near−complete specimen showing some preservation of spicule impressions on the external mould . Paratypes: two semi−complete specimens ( NIGP154189–154190 ) from the type locality, including one partial specimen with clear spicule impressions, and one partial specimen preserved mostly flattened. A probable juvenile is present on the same slab as the holotype .

Type locality: Hongda Quarry near Lantian, Anhui (29 ° 55' N; 118 ° 05'E), South China .

Type horizon: Hetang Formation, early Cambrian (Series 2).

Material.— One semi−complete specimen, NIGP154191 , from Xidi Brick Pit excavated in the Hetang Formation of Anhui (29 ° 52' N; 118 ° 03' E) . One further specimen ( NIGP 154192 ) from the equivalent Niutiang Formation of Guizhou .

Diagnosis.—As for genus.

Description.—Broadly C−shaped sponge with rounded (originally circular) cross−section, in various degrees of compression. The confirmed specimens are consistently curved until they approach half−rings. Largest complete specimen is 52 mm across, with maximum body diameter 14 mm (holotype 47 mm with body diameter 13 mm), although one partial specimen appears be substantially larger (approximately 70 mm, width 22 mm). The only specimen from the Hetang Formation smaller than the holotype is a slightly curved juvenile, 7 mm long and 1.5 mm wide; although it has not been possible to confirm whether it represents the same species, the body proportions are consistent with the other specimens. The Niuitang Formation specimen is slightly more slender, at 8 mm wide and 38 mm across. This very small dataset displays an extremely strong quadratic correlation (R 2 = 0.995; Fig. 2 ) and a slightly less significant linear regression (R 2 = 0.963). With the limited dataset it is not possible to judge between these scenarios, particularly given the small uncertainties regarding the diameter of two specimens. However, it is clear that there is a general increase of width with overall size. There also appears to be an increasing curvature with growth stage, from nearly straight juveniles to C−shaped mature adults, but further specimens are necessary to test the validity of this pattern.

Specimens are in most cases preserved at least partially in three dimensions, with a thick weathered mineral deposit obscuring surface detail and implying more substantial early pyritisation (or other mineralisation) than it is seen in other sponges in the deposit. Two specimens show completely three−dimensional preservation, two are largely flattened, and one is effectively flat with a slightly distorted outline. Original cross section appears to have been approximately circular. The cylindrical body is weakly ribbed in some specimens, with six or seven slight constrictions and expansions along the length of the holotype. One termination tapers slightly more than the other, and is also slightly more curved in the available material. By comparison with known hexactinellids, and to maintain some curvature at all growth stages (which may have been important for inferring stability in life position, discussed below), the tapering end is assumed to be proximal. Despite the three−dimensionality, terminations do not show any evidence for an osculum or other apical modification.

Dermal spicules are weakly visible on the surface of the holotype, and are locally well preserved on a small (width 16 mm) fragment (NIGP154191; Fig. 1B ), and also on the juvenile. All spicules are hexactine−based, with distal rays reduced to small projections that are most pronounced in the largest spicules, and paratangential rays simple, curved to match the surface of the sponge body but not otherwise distorted except for minor deflection. Proximal rays cannot be confirmed with available material. Maximum observed ray length is 6 mm, with basal ray diameter up to 0.3 mm. Spicules are arranged orthogonally or diagonally to the sponge axis, with orthogonal spicules dominant in specimen

http://dx.doi.org/10.4202/app.2011.0140

NIGP154191 and diagonal spicules dominant in the small area of spicules in the holotype. Diagonal spicules are normally oriented at close to 45 °, but a few are at apparently random angles to the sponge axis. Spicule rays are often locally adjacent and almost bundled, with in some cases ray centres also clustered, unlike in the standard quadruled array of reticulosans. Arrangement of small spicules is unclear, although traces of them are seen in NIGP154190, dominantly orthogonal to the body axis.

The juvenile specimen on the holotype slab also preserves a few large spicules, with ray length approaching 1 mm; these are oriented both orthogonally and diagonally to the axis, and the longest rays curve substantially in order to follow the sponge surface. In each sponge that preserves spicules they are seen intermittently, and mainly in the central part of the sponge, but they appear to be consistent in arrangement over the preserved regions. There is no indication of modified spicules in the basal or apical regions, and no basalia have been observed.

Remarks.—The preservation of the sponge as three−dimensional bodies, in contrast to all other taxa in the known fauna, implies that it was originally a solid construction rather than a thin wall. This is supported by the extensive (weathered) mineralisation of the interior and lack of osculum. It is not known whether the interior of the sponge was spiculate; the observable spicules are typical of reticulosan wall spicules, but could also constitute a distinct dermal layer. The unusual arrangement of the spicules, particularly the sometimes−overlapping spicule centres and near−bundled rays, would be distinctive even if the sponge consisted only of a thin skeletal wall, preserved entirely flattened.

The first specimens were not originally recognised as sponges due to difficulties in interpreting the preservation. Where the specimens were completely mineralised in three dimensions, the weathered material constituting the interior of the body shows dominantly transverse cracking due to weathering−related volume changes, often infilled by further mineralisation. In the specimen from the Niutitang Formation, a tectonic fabric has resulted in a reticulate crack network whose orientation is independent of the fossil. Both these textures can be superficially similar to spicules, but the massive pyritisation and subsequent oxidation has in most cases destroyed genuine spiculation. The sponge nature of the fossil was recognised only following the discovery of two specimens that had undergone more extreme weathering, leading to removal of the material from the inside of the sponge body, and a mould of the exterior. Even in these cases, spicules are unclear over large areas of the surface, suggesting that massive pyritisation of the soft tissue subsumed surface spiculation into a homogeneous deposit.

An alternative interpretation that must be considered is of a different organism that has agglutinated detrital spicules onto its exterior wall. This, however, is ruled out by the regular curvature of the spicules matching that of the wall, and by the spicule arrangement ( Fig. 1B 2 ). A dominantly orthogonal array with other spicules positioned at around 45 ° is typical of reticulosans (e.g., Carrera and Ortega 2009) but difficult to understand for an agglutinating organism in which particles would be expected to be either highly regular in orientation, or disorganised. The sparse distribution is also unlikely for an agglutinated array, whose purpose would presumably be for defence. Finally, the range of spicule sizes present is typical of reticulosan hexactinellids ( Botting 2003), but would be highly unusual in an agglutinating organism as the incorporated particles are normally size−selected. There is therefore no doubt that Decumbispongia yuani gen. et sp. nov. is a sponge.

Geographic and stratigraphic distribution.— Decumbispongia yuani is currently known from black shales of the Hetang and Niutitang formations of Anhui and Guizhou provinces, South China, of early Cambrian (Series 2) age.

Figures

Fig. 1. Hexactinellid−like sponge Decumbispongia yuani gen. et sp. nov. from the Hetang Formation near Lantian, Anhui (A–D) and Niutitang Formation at Danzhai, Guizhou (E). A. Paratype NIGP154189, a flattened, near−complete specimen preserved as black iron minerals. B. Paratype NIGP154190. Fragment with relief showing spicule arrangement preserved at surface, consisting of irregularly clustered, orthogonal and diagonal hexactine−based spicules (B1). Camera lucida drawing of B1, clarifying poorly preserved spicule array (dashed line acts as reference for the most prominent spicule) (B2). C. Holotype NIGP154188, near−complete specimen showing annulations, tapering distal(?) end and weakly preserved spicules (arrowed). D. Probable juvenile specimen on slab with NIGP154188, showing straighter growth form and well−developed spicules (arrowed). E. NIGP154192.

Fig. 2. Plot of maximum sponge diameter (measured across most distant two points on outer surface) versus maximum width (in millimetres) for the available specimens, with intercept set to 0 and a quadratic correlation plotted with regression coefficient R2 = 0.997. Largest two specimens are incomplete, and measurements are approximate; assume errors in d of up to 5 mm for these two only.