Thoracospongia sp.

Kouchinsky, Artem, Alexander, Ruaridh, Bengtson, Stefan, Bowyer, Fred, Clausen, Sébastien, Holmer, Lars E., Kolesnikov, Kirill A., Korovnikov, Igor V., Pavlov, Vladimir, Skovsted, Christian B., Ushatinskaya, Galina, Wood, Rachel & Zhuravlev, Andrey Y., 2022, Early-middle Cambrian stratigraphy and faunas from northern Siberia, Acta Palaeontologica Polonica 67 (2), pp. 341-464 : 437-439

Thoracospongia sp.

Fig. 70 .

Material.—Ten siliceous spicules, including figured SMNH Sp11409–11413, from sample 19/107, Yunkyulyabit-Yuryakh Formation, Khorbusuonka River, Siberia, Russia. Anomocarioides limbataeformis Zone, Mayan stage (correlated with the Drumian Stage).

Description.—Hexact follipinules with a modified obese axial ray rounded in cross-section. The ray is strongly inflated to form a straight or curved tear-shaped ellipsoid or a globule, 0.2–0.5 mm in diameter across the widest part and 0.4–0.5 mm long. The surface of this ray ornamented with multiple longitudinal undulating ribs bearing diminutive denticles. Ribs become more prominent distally and are spaced 15–30 µm apart. Other rays, incompletely preserved and probably broken off in two of the specimens ( Fig. 70C, D ) are slender, tapering, bearing an axial canal circular in cross-section. The four minute paratangential rays, 15–30 µm in diameter at their junction, are crossed at the same plane at 90° from each other. An additional ray, up to 50 µm in diameter is developed on the spicule distal end, perpendicularly to the paratangential rays.

Remarks.—Hexact follipinules identical in size and morphology are common in a more diverse and abundant assemblage of spicules described as Thoracospongia cf. follispiculata Mehl, 1996 , from the lower Mayan stage (Drumian Stage) Kuonamka Formation of the Anabar Uplift ( Kouchinsky et al. 2011), including pentactines with robust paratangential rays and stauractins with robust conical rays. The latter may partly represent pentactine follipinules with broken-off axial ray. Stauractines have not been however recognised in the material described herein that is different from the type species and from Thoracospongia cf. follispiculata . Thoracospongia lacrimiformis Peel, 2017b , from the Henson Gletscher Formation of North Greenland (Wuliuan Stage) differs by finer but more prominent ridges ( Peel 2017 b, 2019) and resembles “pinnular hexactines” from the upper part of Cambrian Stage 3 of the Altay Sayan Foldbelt ( Sugai et al. 2004). Unnamed obese pentactines from the Wilberns Formation of the Llano Uplift, Texas (Furongian Series) are smooth and possess more prominent paratangential rays ( Rigby 1975). In addition to Thoracospongia Mehl, 1996 , follipinules of broadly the same habit but with additional paratangential and secondary rays were described under the name Rigbyella Mostler and Mosleh-Yazdi, 1976 , from Iran. This generic name has been proved to be preoccupied and subsequently replaced by Rigbykia Özdikmen, 2009 . The set of Thoracospongia - type spicules described as Rigbyella sp. from the Miaolingian Burj Formation of Jordan includes both swollen and relatively slim varieties (Elicki 2011). In younger beds, acanthose hexactine and pentactine follipinnules occur in association with regular triaxon spicules in the Guzhangian Stage equivalent of Australia (Bengtson 1986), Cambrian Stage 10 of Baltica (Castellani et al. 2012) as well as in the Ordovician (Carrera and Maletz 2014; for further discussion see also Peel 2017b, 2018, 2019).

Despite drastic differences of local spicule assemblages, hexactinellid and demosponge macroscleres dominate and more species-specific microscleres are distinguished in rare cases. A few early Cambrian microscleres were described by Reitner and Mehl (1995) and Botting et al. (2015) in thin sections but none of them are convincing enough due to difficulty to distinguish microscleres from polycystine tests and broken macroscleres. Macroscleres in Cambrian spicule assemblages appear to have microsclere counterparts from the Furongian, Ordovician and later periods, such as Thoracospongia follipinule-like elements, Tanjaella equinate hexactines, Asterodesma aster-like sphaeroclones and many others ( Rigby 1975; Castellani et al. 2012; Carrera and Maletz 2014; Kruse and Reitner 2014). All these spicules are at least one order larger than their microsclere twins. Thus, either microsclere-bearing sponges originated from their macrosclere-bearing predecessors or Cambrian poriferans belonged mostly to extinct lineages being replaced during the latest Cambrian–Ordovician radiation. The finds of intermediate in size spicules in the middle Cambrian (Chen and Dong 2008), however, supports the suggestion of a gradual transition from sponges having larger skeletal elements to those with skeletons strengthened by microscleres. Bol’shaya Kuonamka River (see Kouchinsky et al. 2015a), Siberia, Russia. Dokidocyathus regularis Zone, Tommotian stage (correlated with the upper part of Cambrian Stage 2).

Description.—Pentactine megascleres with four paratangential rays diverging from their intersection at 150° with the axial ray and at 50–60° from each other. Rays with smooth surface, gently tapering and circular in cross-section.

Figures

Fig. 70. Spicules of hexactinellid Thoracospongia sp. from the middle Cambrian Yunkyulyabit-Yuryakh Formation, Khorbusuonka River, Siberia, Russia; sample 19/107. A–E. Follipinules with a modified obese axial ray, SMNH Sp11409–Sp11413, respectively. A1, lateral view on the axial ray; A2, four paratangential rays. Scale bar: 125 µm (A) and 250 µm (B–F).