First report of sponge traces from Korea: modern and fossil Entobia on carbonate rocks and shells

Abstract

Diverse spherical structures were detected on the surfaces of fossil and modern carbonate rocks and shells, including Precambrian stromatolite of Socheong Island, Tertiary oysters of the Chunbuk Conglomerate, Quaternary scallop fossils of the Seoguipo Formation, and modern bivalve shells of Dokdo. They are characteristic traces of sponge animals (i.e., Family Clionidae) who actively penetrate carbonate minerals, and two ichno-species were identified on the basis of network of internal structures: Entobia megastoma and Entobiaovula. Internal structures were observed under scanning electron microscope (SEM) using embedding techniques. Most holes (< 2 mm in diameter) are randomly distributed on rock and shell surfaces, and linearly distributed holes are also often observed. The surface holes are connected to vertical tubules (< 1 mm in both diameter and length). At the end of the vertical tubules, large balls (ca. 1.5 mm in diameter) or irregularly flat chambers (bigger than balls) are generally developed, from which Entobia traces grow finally forming complicated Entobia networks. The Entobia balls produce horizontal tunnels connected tothe next balls. Spine-like processes are sometimes developed on the surfaces of horizontal tunnels. Surfaces of both Entobia balls and tunnels are decorated by bumpy and raspberry-like structures, which are typical features of carbonate minerals carved by modern sponge animals.he Seoguipo Formation, and modern bivalve shells of Dokdo. They are characteristic traces of sponge animals (i.e., Family Clionidae) who actively penetrate carbonate minerals, and two ichno-species were identified on the basis of network of internal structures: Entobia megastoma and Entobiaovula. Internal structures were observed under scanning electron microscope (SEM) using embedding techniques. Most holes (< 2 mm in diameter) are randomly distributed on rock and shell surfaces, and linearly distributed holes are also often observed. The surface holes are connected to vertical tubules (< 1 mm in both diameter and length). At the end of the vertical tubules, large balls (ca. 1.5 mm in diameter) or irregularly flat chambers (bigger than balls) are generally developed, from which Entobia traces grow finally forming complicated Entobia networks. The Entobia balls produce horizontal tunnels connected tothe next balls. Spine-like processes are sometimes developed on the surfaces of horizontal tunnels. Surfaces of both Entobia balls and tunnels are decorated by bumpy and raspberry-like structures, which are typical features of carbonate minerals carved by modern sponge animals.