Crouching shells, hidden sponges: Unusual Late Ordovician cavities containing sponges Jino Park a , Jeong-Hyun Lee b , Jongsun Hong a , Suk-Joo Choh a, ⁎, Dong-Chan Lee c , Dong-Jin Lee d,e a Department of Earth and Environmental Sciences, Korea University, Seoul 02841, Republic of Korea b Department of Geology and Earth Environmental Sciences, Chungnam National University, Daejeon 34134, Republic of Korea c Department of Earth Science Education, Chungbuk National University, Cheongju 28644, Republic of Korea d Department of Earth and Environmental Sciences, Andong National University, Andong 36729, Republic of Korea e College of Earth Sciences, Jilin University, Changchun 130061, People's Republic of China abstract article info Article history: Received 30 August 2016 Received in revised form 2 November 2016 Accepted 4 November 2016 Available online 11 November 2016 Editor: Dr. B. Jones Marine cavities harbouring cryptic organisms have been ubiquitous throughout the Phanerozoic. However, our knowledge of early cryptic communities is as yet insufficient, and how metazoans began to utilize such habitats remains unknown. In this study, we document demosponge remains within intraskeletal cavities embedded in the micritic succession of a shallow carbonate platform in the Upper Ordovician (Katian) Xiazhen Formation of South China. Molluscs (gastropods, bivalves, and nautiloids) and corals (the solitary rugosan Tryplasma and colo- nial agetolitids) within the succession commonly contain patches of “spicular” demosponge remains (11%; n = 45/415), mainly occupying intraskeletal spaces with areas of 1–30 mm 2 in thin-section. Sponge occurrence varies according to sedimentary facies: within lime mudstone facies, sponges commonly occur both inside and outside intraskeletal cavities, suggesting that sponges would have inhabited and become preserved within any available space in this environment. In contrast, when other sessile organisms co-occur in wackestone to packstone facies, there are fewer sponge occurrences both inside and outside cavities, possibly due to competition in open habitats and/or their poorer preservation in such environments. Overall, this result suggests that sponges would have exploited cryptic habitats by normal expansion of the open-surface biota. In addition, compared with coeval reef and hardground crypts, the Xiazhen intraskeletal cryptic biota is monotonous in composition, suggesting “decoupled” occupation of cryptic habitats in different environments. © 2016 Elsevier B.V. All rights reserved. Keywords: Intraskeletal crypt Demosponge Micritic carbonate Late Ordovician South China 1. Introduction Marine cavities in reefs, borings, fissures, the undersides of shells and boulders, and within shells are shaded, semi-protected spaces that provide important marine habitats for many cryptic organisms (Kobluk, 1988b; Glynn and Enochs, 2011). In the geologic record, cryp- tic organisms are primarily identified by their in-place preservation, and they provide excellent opportunities for palaeoecological analysis (Kobluk, 1988b; Taylor and Wilson, 2003). Since the first appearance of cryptic metazoans in Ediacaran reefs (Wood and Curtis, 2015), cryptic organisms in the geologic record have displayed an overall trend of an increase in diversity and complexity as well as in habitat variety. The main groups of early Palaeozoic cryptic metazoans were sponges, bryo- zoans, and echinoderms, expanding in the middle to late Palaeozoic to also include tabulate and rugose corals, brachiopods, and foraminifera (Taylor and Wilson, 2003). It is interesting to note that early cryptic metazoans have been reported mainly from reef cavities (Kobluk and James, 1979; Kobluk, 1988a; Zhuravlev and Wood, 1995; Hong et al., 2014; Li et al., in press); it was not until the Late Ordovician that bryo- zoans and echinoderms invaded crypts associated with hardgrounds (Brett and Liddell, 1978). In modern shallow-marine environments, discarded shells are fre- quently utilized by various organisms, including hermit crabs, sipunculid worms, tanaids, fishes, octopuses, amphipods, and poly- chaetes (e.g., Williams and McDermott, 2004; Glynn and Enochs, 2011). However, fossil records of comparable organisms are scarce (cf. Lukeneder and Harzhauser, 2003; Luci and Cichowolski, 2014), and therefore how metazoans established themselves in these habitats is still an open question (Kobluk, 1988b; Zhuravlev and Wood, 1995; Taylor and Wilson, 2003). In this paper, we document unusual non- reef intraskeletal cavities (sensu Kobluk, 1988b) occupied by sponges from an Upper Ordovician shallow-subtidal micritic limestone succes- sion in South China, in order to provide evidence of how metazoans adapted to such cryptic spaces. Sedimentary Geology 347 (2017) 1–9 ⁎ Corresponding author. E-mail addresses: jinopark@korea.ac.kr (J. Park), jeonghyunlee@cnu.ac.kr (J.-H. Lee), bulgeun@korea.ac.kr (J. Hong), sjchoh@korea.ac.kr (S.-J. Choh), dclee@chungbuk.ac.kr (D.-C. Lee), djlee@andong.ac.kr (D.-J. Lee). http://dx.doi.org/10.1016/j.sedgeo.2016.11.003 0037-0738/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo