Jongsun Hong

Tales from the crypt: early adaptation of cryptobiontic sessile metazoans

ABSTRACT Cryptic ecosystems of modern and ancient reefs contain substantial amounts of biodiversity. It remains uncertain, however, when and how metazoans adapted to such space. Early Cambrian reef systems witnessed the rise and fall of the earliest known cryptic sessile metazoans. Subsequent Middle Cambrian to Early Ordovician microbial-dominated reefs were generally devoid of true frame-building metazoans, as well as cryptic sessile fauna. The Early Ordovician microbial–siliceous sponge patch reefs of the Dumugol Formation, Korea represent one of the oldest in situ spiculate sponge-bearing cryptic communities exploiting intraskeletal cryptic environments. Less than half of these small millimeter- to centimeter-scale crypts contain low-diversity sessile cryptic assemblages of spiculate sponges and microbialites. The cryptic sponges that attach to the walls of the cavities or on top of internal sediments do not show any skeletal distortion at their contacts with host organisms. The spiculate sponges occur both in open spaces as well as in crypts, and are interpreted to be pioneers of intraskeletal crypts after the death of the cavity-providing organisms. The behavior of occupying transient cryptic habitats is interpreted as an incipient stage of sessile metazoan adaptation to a cryptic space by an opportunistic member of the epibenthic community. This resulted in the extension of the open surface community into crypts which occurred far in advance of the eventual establishment of obligate cryptic forms. The present study provides a critical link for establishing the origin and evolutionary history of early cryptic sessile metazoan adaptation.

Early Ordovician reefs from the Taebaek Group, Korea: constituents, types, and geological implications

The Early Ordovician (early to middle Floian) bioherms of the Dumugol Formation, Korea, are compiled and their paleoenvironmental and paleogeographic implications are discussed. These reefs are mostly made up of microbialite (stromatolite and thrombolite) and lithistid sponge Archaeoscyphia, with subordinate “receptaculitid” calathids. Three types of reefs are identified based on biotic association and texture: 1) lithistid sponge-microbialite, 2) microbialite (thrombolite) with minor lithistid sponge, and 3) lithistid sponge-microbialite-calathid. The first and third type reefs are surrounded by intraclastic-skeletal packstone to grainstone and overlain by lime mudstone, whereas the second type reefs are surrounded and overlain by bioturbated wackestone and nodulebearing shale. These relationships appear to reflect varying depositional conditions during development of the reefs. The constituents of the Dumugol reefs are roughly comparable to coeval structures of Laurentia and South China with the exception of the absence of incorporated sessile organisms (i.e., Lichenaria, Pulchrilamina, and bryozoan) and delayed arrival (more than 10 myr) of calathids in the Sino-Korean Craton. This temporal disparity of biotic appearance is probably related to differential dispersal rates and patterns of sessile organisms which are largely controlled by the relative position of landmasses, epicontinental seas and major oceans. Further discovery and study of the Early Ordovician reefs from the Sino-Korean Craton will provide crucial information for understanding migration pathways of sessile organisms and paleogeographic reconstruction of the western margin of Gondwana in the Early Paleozoic.

Cambrian Stem-group Cnidarians with a New Species from the Cambrian Series 3 of the Taebaeksan Basin, Korea

Five species, Lipopora lissa Jell and Jell, 1976, Lipopora daseia Jell and Jell, 1976, Tretocylichne perplexa Engelbretsen, 1993 from Australia, Cambroctoconus orientalis Park, Woo, Lee, Lee, Lee, Han and Chough, 2011 from China, and Cambroctoconus kyrgyzstanicus Peel, 2014 from Kyrgyzstan, belonging to the Cambrian stem-group cnidarians have been documented in the fossil record. Cambroctoconus coreaensis sp. nov., interpreted here as a stem-group cnidarian, from the Seokgaejae section in the Daegi Formation, Taebaek Group (Cambrian Series 3), Taebaeksan Basin, central-eastern Korean Peninsula, has a slender cup-shaped skeleton. A cladistic analysis produced 21 most parsimonious trees, which invariably placed the six stem-group cnidarians below the crown-group, but their relationships within the stem-group are unresolved. Nine out of the 21 trees suggest a monophyletic relationship for the Cambrian stem-group cnidarians, whereas in six other trees a monophyly of Cambroctoconus and Tretocylichne appeared as the sister-group to the crown-group cnidarians with Lipopora at the most basal branch. This result may reflect the fact that crown-group cnidarians evolved in the Precambrian, and suggests that the diversity of stem-group cnidarians was a result of an independent radiation in the Cambrian.

Early labechiid stromatoporoids of the Yeongheung Formation (Middle Ordovician), Yeongwol Group, mideastern Korean Peninsula: Part I. Environmental distribution

This study examines depositional facies and environmental significance of early labechiid stromatoporoids in the Yeongheung Formation (late Middle Ordovician), Yeongwol Group in the mideastern part of the Korean Peninsula. The formation is composed of five depositional facies: lime mudstone to wackestone (LM/W), peloidal, intraclastic and bioclastic packstone to grainstone (P/G), laminated dolomitic lime mudstone (LMdl), algal laminite (La), and oolitic packstone to grainstone (P/Go). Three types of shallowing upward cycles in the succession are identified, including subtidal (LM/W–P/G), subtidal to peritidal (LM/W–P/G–LMdl–La), and peritidal cycles (LMdl–La). The stromatoporoids occur exclusively in the packstone to grainstone (P/G) facies of the subtidal cycle. These early labechiids inhabited on a grainy substrate under moderate- to high-energy conditions, conforming to the previous notion that stromatoporoids primarily occupied level-bottom or reefs, surrounded by grainy calcareous sediments during the Middle Ordovician.

Thrombolite reefs with archaeocyaths from the Xiannüdong Formation (Cambrian Series 2), Sichuan, China: implications for early Paleozoic bioconstruction

The early Cambrian incorporation of organisms with calcareous skeletons into microbial reefs initiated the metazoan bioconstructions of the Phanerozoic. Microbial reefs containing archaeocyaths from the middle early Cambrian Xiannüdong Formation of the South China Block are investigated. The Xiannüdong thrombolitic frameworks are composed primarily of Girvanella clumps and crusts, micritic clumps, and subordinate Epiphyton bundles. Amalgamated microbial frameworks contain sparse and rare (<5%) archaeocyaths, with irregular archaeocyaths dominating the regular archaeocyaths by 6:1, and enclosed by Girvanella and other microbial elements. These Xiannüdong thrombolitic reefs are broadly similar to other lower Cambrian thrombolitic reefs containing archaeocyaths, developed around shoals and lagoons. Similar thrombolitic reefs of the middle Cambrian from the Sino- Korean Block and Australia show reduced and increased contributions of Girvanella and Epiphyton, respectively, and the incorporation of lithistid and heteractinide sponges instead of archaeocyaths. These data suggest that the late early Cambrian decline of the archaeocyaths and their nearly instantaneous replacement by other metazoans allowed the continuance of microbial reefs with rare metazoans until the late Middle Ordovician shift in reef construction to metazoan reefs.