Keratose-like Sponges in Lower Paleozoic Successions and Their Implications for Paleoecology and Carbonate Sedimentology

LEE Jeong-Hyun1& HONG Jongsun2

1 Department of Geology and Earth Environmental Sciences, Chungnam National University, Daejeon 34134, Republic of Korea; jeonghyunlee@cnu.ac.kr;

2 Department of Geology, Kangwon National University, Chuncheon-si, Gangwon-do 24341, Republic of Korea; jhong@kangwon.ac.kr.

1 Introduction

Fossil sponges, the most basic multicellular animals that have existed since the late Precambrian, are generally preserved as scattered spicules (e.g., spiculites) or calcified remains (e.g., lithistid sponges). Sponges were stratigraphically, geographically and environmentally widespread, and ecologically important throughout the Earth's history. However, it is hard to estimate their paleoecological importance because soft-bodied remains of sponges are only rarely preserved.

Keratose demosponges are those whose skeletons consist of fibers of spongin and lack spicules, and include the phylogenetic groups Dictyoceratida and Dendroceratida (Luo& Reitner, 2014). Because of their non-spicular nature, their fossil record is very limited. Recent studies (within the last 5 years) have reported remains of keratose-like sponges from carbonate rocks, which have been commonly overlooked and not well understood (Table 1). These remains are characterized by anastomosing fibrous fabrics and automicritic clumps filling interspaces. It has been suggested that these fibrous fabrics preserved within carbonates represent remains of keratose sponges (Luo & Reitner, 2014), though this needs to be confirmed by further detailed taxonomic studies. Therefore, the term ‘keratose-like sponge' will be used in this manuscript.

Keratose-like sponges are hard to recognize in the rock record, because they are not distinctive in outcrop and on slabs, and can only be easily identified under the microscope (Lee et al., 2014). However, the increasing number of reports of these keratose-like sponges suggests that they would have been important during geological history. In this manuscript, we summarize the reported occurrences of keratose-like sponges from lower Paleozoic successions (Table 1).

2 Examples of keratose-like sponges in early Paleozoic successions

The earliest known example of keratose-like sponges was found in the Tommotian of Siberia. Luo (2015) summarized known occurrences of keratose-like sponges in her thesis, and introduced this example, in which keratose-like sponges dwelled within cryptic cavities of archaeocyath-microbial reefs. This material is not well described and therefore needs further study.

During the middle Cambrian, keratose-like sponges were locally found as patches within Epiphyton-dominated microbialites of the middle Cambrian Zhangxia Formation, Shandong, China (Adachi et al., 2015; Lee et al., 2016b) and the coeval Daegi Formation, Taebaek, Korea (Hong et al., 2012; Hong et al., 2016). In contrast, keratose-like sponges were common components of mid-Cambrian maceriate reefs in western North China (Wuhai, Inner Mongolia, China) (Lee et al., 2016c), as detailed in the next paragraph.

Late Cambrian (Furongian) keratose-like sponges mainly occur in maze-like maceriate reefs that were previously described as microbialites. Lee et al. (2014) reported abundant keratose-like sponges from the maceriate ‘microbialite' of the Furongian Chaomidian Formation, Shandong, China. Within these reefs, sponges occupy up to half of the reef volume. They are also found among ‘stromatolite' layers of the Chaomidian Formation in Shandong as well as Beijing (Chen et al., 2014). Similarly, keratose-like sponges have also been reported from maceriate reefs and stromatolites of Utah, USA (Coulson &Brand, 2016). On the basis of the temporally restricted but paleogeographically widespread occurrences of maceriate reefs throughout the Furongian and Early Ordovician, Lee et al. (2014, 2015) suggested that the sponges would have been responsible for the reef macrofabrics during this time interval. In addition, small patches of keratose-like sponges are found within spongocoels of lithistid sponges in Furongian sponge-microbial reefs of Nevada, USA (Lee et al., in prep.).

Early Ordovician keratose-like sponges have mainly been reported from cryptic interspaces of lithistid sponge-microbial reefs. Hong et al. (2014) detailed occurrences of keratose-like sponges within cryptic interspaces of lithistid sponge-microbial reefs from the Dumugol Formation, Taebaek, Korea. A follow-up study by Hong et al. (2015) on the same reefs reported variously functioning keratose-like sponges as encrusters and rare framework-builders within the reefs as well as possible sediment stabilizers occupying intergranular pores of the reef-flank conglomerate. In addition, possible keratose-like sponges are found within spongocoels of Early Ordovician lithistid sponges of Hubei (Liu et al., 1997, figs. 6.1-6.3) and Anhui (Adachi et al., 2009, fig. 5D), China. Keratose sponges also occur within‘stromatolites' of the Lower Ordovician Mungok Formation, Yeongwol, Korea, where they constitute a significant portion of the reefs (Yang & Lee, in prep.).

Two examples of the Middle Ordovician keratose-like sponges have been reported.Lee et al. (2016a) described bivalve-sponge-microbial boundstone from the Pingliang Formation, Shaanxi, China, where sponges encrusted on or dwelled within bivalves. The Duwibong Formation, Taebaek, Korea, yields stromatoporoid-bryozoan reefs, where keratose-like sponges dwelled between the framework and within bored spaces (Lee et al., 2016b; Hong et al., in submission).

Late Ordovician occurrences of keratose-like sponges are detailed from the Xiazhen Formation of South China.Keratose-like sponges enclose coralomorph(Rhabdotetradium) tubes and principally constructed reefs (Kwon et al., 2012), although these sponges appear to be subordinate elements of coral-stromatoporoid bioherms in the same succession (Park et al., 2015). A follow-up study of the same area reported similar sponge remains within a 50-m-thick micritic limestone succession, in which undisturbed ‘spicule networks' comprise approximately 13% of the sedimentary rock (Park et al., 2015). These sponges also occur within skeletal cavities of micritic successions (Park et al., 2017).

3 Summary

Although still preliminary, this summary shows that keratose-like sponges had important paleoecological and sedimentological roles within carbonate environments. Keratose-like sponges were important, at least locally, within early Paleozoic reefs: they were common cryptic dwellers in many reefs, and also important encrusters within ‘stromatolites' as well as dwellers in reefs containing framework-building archaeocyaths and lithistid sponges. They could have formed frameworks within maceriate ‘microbialites', possibly generating in these characteristic, widespread reefs throughout the Furongian and Early Ordovician. Similarly, these sponges were primarily responsible for Late Ordovician skeletal reef construction devoid of stromatoporoids and corals, all of which suggest that early Paleozoic keratose-like sponges had an opportunistic nature. Such ecologic behavior offers insights into the concealed processes of early Paleozoic reefs, analogous to those of siliceous sponges in Recent coral reefs and sponge mounds (Bell, 2008). Occurrences of keratose-like sponges constituting major portions of Late Ordovician non-reefal micritic limestones are noteworthy, because they could have been important sediment producers during the early Paleozoic. Reports of similar sponges from Devonian and Triassic reefs (Luo & Reitner, 2016) as well as other geologic time intervals (Luo, 2015) suggest that such sponges would have been common in geologic history: further detailed study is required.

Acknowledgements This study was supported by grants from the National Research Foundation of Korea to JHL (No. 2016R1C1B1012104) and to JH (No. 2017R1C1B1007344). We wish to thank colleagues including S. J. Choh (Korea Univ.), D. J. Lee (Andong National Univ.), J. Park (Korea Univ.) and many others who helped us to identify and conduct research on keratose-like sponges. This is a contribution to the IGCP Project 653.

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