Affinity of RSF

The four round sponge fossils studied here have an elliptical shape and a diameter of 0.6 - 0.8 cm. No.42952 maintains the thickest carbonaceous remains but lacks spicules (Fig. 60a-c). Polygonal cracks are developed on the upper surface of the carbonaceous remains (Fig. 60b-c). By comparison, No.42436 (Fig. 60d-f) and No.42446 (Fig. 60g-i) have thinner carbonaceous films. Some small and faint marks, resembling moulds of spicules, are distributed on parts of the fossil surface, but they are not confirmatively of spicular origin (Fig. 60e, h-i). In addition, both No.42952 and No.42446 show features of double-layered carbon films (Fig. 60a, c, g & j), which indicate a former existence of spongocoel. No.42982 is intensively weathered and exhibits weakly preserved but authentic moulds of spicules with only scattered carbonaceous remains (Fig. 60k-l).

Wu (2004) interpreted RSF as sponge gemmules because they possess exceptionally recalcitrant organic matter and a round shape. However, gemmules of modern sponges are normally only 0.2 - 1 mm in diameter (Fell, 1993), much smaller than the RSF. If the amount of totipotent cells and nutrients in millimeter-sized gemmules are adequate for dormancy and later germination, then the production of centimeter-sized gemmules appears not to be economic. And as observed in modern examples, each sponge typically produces hundreds to thousands of gemmules which either attach to the substrate or fill in the sponge body (Fell, 1993). But little is known about the substratum for the gemmule-theory and even the largest sponge in the Chengjiang Biota (Quadrolaminiella; Hou et al., 2004) is not big enough to contain hundreds of centimeter-sized gemmules. Furthermore, fossil records of sponge gemmules were mostly reported from fresh water environments, except one possible example of Cretaceous age published recently (Petit &

Charbonnier, 2012, and references therein), and all of these fossils have only a size of about 1mm. In contrast to gemmules, buds of sponges have a size ranging from hundreds of micrometers to a few centimeters. They can be reinforced by collagen and spicules and exhibit a more or less spherical shape

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in some species (Fell, 1993). However, RSF contribute a large proportion to the Chengjiang sponge fossil record (12.48% of 2252 specimens studied by Wu, 2004), but to our knowledge no budding process has been directly observed in these normal sponge fossils.

Furthermore, some of the spicular RSF fit into the genera Triticispongia and Saetaspongia erected by Steiner et al. (1993) from another Cambrian fossil Lagerstätte in Sancha (Wu, 2004). While Saetaspongia (holotype about 3 x 4 cm large) was treated as normal sponge fossil, Triticispongia was originally interpreted as representing juvenile stages because their size hardly exceeds 10 mm. However, Rigby & Hou (1995) were apt to interpret T. diagonata as adult precursor of reticulosid sponges based on the well-organized skeletal structures and the absence of larger adult specimens in fossil collections of Chengjiang and Sancha. Later, larger specimens (with diameters of 2.5 - 3 cm) were discovered from Niutitang Formation in Songlin, Guizhou (Zhao et al., 2006). But, their giant sizes are more probably related to a different environment or evolutionary stage than representing the adult form of the analogues from Sancha and Chengjiang, because none of these three fossil sites contain both the big and small specimens at the same time. Furthermore, sorting during carcass transportation seems unlikely considering the same static depositional environment of the black shales in Sancha and Songlin and the different water dynamics between the background beds and event beds in Chengjiang Lagerstätten. For these reasons, we agree with the interpretation of Rigby & Hou (1995) on T. diagonata. Since also no reliable adult forms of other RSF are currently known to us, we further propose that at least a part of the RSF in Chengjiang Biota represent adult sponges which belong to various taxa but occupied similar ecological niches. In sponges, the exterior morphology is strongly affected by environment while the skeletal construction is controlled by gene expression. This explains the phenomenon that the RSF have at the same time a variety of skeleton structures but similar shapes and sizes and also occur in similar taphonomical facies. Nonetheless, this argument does not completely exclude the possibility that some of the RSF have gemmule or bud origin, especially for very small specimens.

In the light of the earlier taphonomical review, the continuous transition of RSF from explicit mineral skeletons to completely carbonaceous preservation

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resembles the reduction of spongin and/or collagen in the skeletal framework from keratose sponges to other demosponges and hexactinellids. In most keratose sponges, the whole skeletal framework consists of spongin; while in other demosponges the fibrillar collagen is still pervasive and often forms dense bands between spicules whereas in hexactinellids collagen forms a thin sheath wrapping the spicules but does not occur massively (Bergquist, 1978). This clearly underlines that the aspicular specimens with dense carbonaceous remains may represent a kind of primitive keratose sponge. On the other hand, the absence of the typical dendritic or anastomosing network of keratose sponges in the round sponge fossils may hinder this interpretation. However, this could be explained by consolidation of organic material during diagenesis.

The continuous carbonaceous films of No.42436, No.42446 and No.42952 could represent the end product of the diagenetic fusing of a fine network composed of dense spongin or collagen material, but not necessarily have to be the result of a continuous organic shall of the gemmule as suggested by Wu (2004). The polygonal fractures on the surface of No.42952 could also be due to diagenetic altering, since similar structures are also observed in the Wenghui fossils where dense organic carbon is preserved (Fig. 61e).

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Figure 61: Carbonaceous fossils from Xiaoyanxi Formation and Wenghui Biota. a the non-described sponge fossil from Early Cambrian Xiaoyanxi Formation. b-e the algal fossils from Ediacaran Wenghui Biota. e represents the white rectangle in d. white arrows in e point to polygonal cracks in the densely preserved carbon film. Number marks in a, b and c represent the location of Raman spectra sample spots.