_________________________________________________________________________________________________________ Kölner Forum Geol. P<strong>al</strong>äont., 19 (2011) M. ARETZ, S. DELCULÉE, J. DENAYER & E. POTY (Eds.) Abstracts, 11th Symposium on Fossil Cnidaria and Sponges, <strong>Liège</strong>, August 19-29, 2011 _________________________________________________________________________________________________________ 58 Fig. 1: Section of the “Hasenstein” 20 km north of Graz exhibiting fossiliferous bed<strong>de</strong>d limestones passing into a massive stromatoporoid-cor<strong>al</strong> reef at its top.
_________________________________________________________________________________________________________ Kölner Forum Geol. P<strong>al</strong>äont., 19 (2011) M. ARETZ, S. DELCULÉE, J. DENAYER & E. POTY (Eds.) Abstracts, 11th Symposium on Fossil Cnidaria and Sponges, <strong>Liège</strong>, August 19-29, 2011 _________________________________________________________________________________________________________ Deep insight into <strong>de</strong>ep-water cor<strong>al</strong>s: unique microstructure of micrabaciids Katarzyna JANISZEWSKA 1 , Jaroslaw STOLARSKI 1 , Marcelo KITAHARA 2 & Stephen D. CAIRNS 3 1Institute of P<strong>al</strong>eobiology, Warsaw, Poland; k.janiszewska@twarda.pan.pl, stolacy@twarda.pan.pl 2James Cook University, Townsville, Austr<strong>al</strong>ia 3Department of Invertebrate Zoology, Nation<strong>al</strong> Museum of Natur<strong>al</strong> History, Smithsonian Institution, Washington, D.C., USA The skel<strong>et</strong>on of scleractinian cor<strong>al</strong>s consists of two microstructur<strong>al</strong> regions: (i) rapid accr<strong>et</strong>ion <strong>de</strong>posits (“centres of c<strong>al</strong>cification”, “early miner<strong>al</strong>ization zone”) and (ii) thickening <strong>de</strong>posits (“fibers”, “stereome”). Tradition<strong>al</strong>ly, size and arrangement of “centers of c<strong>al</strong>cification” were used as main diagnostic characters of the high-rank scleractinian taxa (WELLS 1956). However, combined molecular and microstructur<strong>al</strong> data reve<strong>al</strong>ed that equ<strong>al</strong>ly v<strong>al</strong>uable skel<strong>et</strong><strong>al</strong> characters that supports molecular groupings are thickening <strong>de</strong>posits. For example, Acroporidae, the most speciose extant scleractinian family have thickening <strong>de</strong>posits formed as shingle-like units. Longer axes of fibers are <strong>al</strong>igned within individu<strong>al</strong> bundles and are par<strong>al</strong>lel to the skel<strong>et</strong><strong>al</strong> surface whereas Pocilloporidae representatives have dome-shaped bundles of fibers perpendicular to the surface of the skel<strong>et</strong>on, showing a microtuberculate pattern on the surface. Thickening <strong>de</strong>posits in Flabellidae form thin sc<strong>al</strong>e-like bundles of fibers arranged quasi-par<strong>al</strong>lel to the sept<strong>al</strong> plane. All scleractinian taxa mentioned above are characterized by distinct types of thickening <strong>de</strong>posits (Fig. 1; see <strong>al</strong>so GAUTRET <strong>et</strong> <strong>al</strong>. 2000; STOLARSKI 2003; NOTHDURFT & WEBB 2007), and were recovered as monophyl<strong>et</strong>ic in molecular studies. Herein, based on skel<strong>et</strong><strong>al</strong> an<strong>al</strong>ysis of the enigmatic group of <strong>de</strong>ep-water micrabaciid cor<strong>al</strong>s, we provi<strong>de</strong> further evi<strong>de</strong>nce supporting a role of thickening <strong>de</strong>posits in phylogen<strong>et</strong>ic interpr<strong>et</strong>ations. Micrabaciidae are solitary, azooxanthellate cor<strong>al</strong>s living today in the <strong>de</strong>epest parts of the ocean (up to 5000 m, CAIRNS 1989). They are known since the Early Cr<strong>et</strong>aceous and attributed to five genera: L<strong>et</strong>epsammia YABE & EGUCHI 1932, Leptopenus MOSELEY 1881, Stephanophyllia MICHELIN 1841, Rhombopsammia OWENS 1986 and Micrabacia MILNE-EDWARDS 1849 (the latter known only from the fossil record). Mo<strong>de</strong>rn and fossil species have porous, <strong>de</strong>licate skel<strong>et</strong>on and unique "bifurcation" of septa and costae. In contrast to other scleractinians, thickening <strong>de</strong>posits of micrabaciids, regardless of the geographic and bathym<strong>et</strong>ric origin of samples, are composed of irregular meshwork of extremely thin (ca. 100-300 nm) and short (1-2 µm) fibres that are organized into sm<strong>al</strong>l, chip-like bundles forming an irregular criss-cross pattern on the skel<strong>et</strong><strong>al</strong> surface. Also, unlike other scleractinians, the growth layers in micrabaciid thickening skel<strong>et</strong>on are not well discernible. The herein <strong>de</strong>scribed microstructur<strong>al</strong> pattern of thickening <strong>de</strong>posits is unique among cor<strong>al</strong>s and supports the monophyl<strong>et</strong>ic status of micrabaciids (KITAHARA <strong>et</strong> <strong>al</strong>. 2010). In some geochemic<strong>al</strong> propos<strong>al</strong>s, formation of the fibrous part of the skel<strong>et</strong>on is consi<strong>de</strong>red to be an<strong>al</strong>ogous to inorganic precipitation of CaCO3. In contrast to this interpr<strong>et</strong>ation, the consistent occurrence of distinct microstructur<strong>al</strong> patterns of thickening <strong>de</strong>posits in different scleractinian cla<strong>de</strong>s (acroporids, pocilloporids, flabellids, micrabaciids), favours the organic matrix-mediated mo<strong>de</strong>l of cor<strong>al</strong> miner<strong>al</strong>ization. According to it, these are macromolecules that form un<strong>de</strong>r a tight gen<strong>et</strong>ic control, and which are responsible for nucleation, spati<strong>al</strong> <strong>de</strong>lineation and organization of basic microstructur<strong>al</strong> units. 59
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