Aretz et al_2011.pdf - ORBi - Université de Liège

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Kölner Forum Geol. Paläont., 19 (2011)
M. ARETZ, S. DELCULÉE, J. DENAYER & E. POTY (Eds.)
Abstracts, 11th Symposium on Fossil Cnidaria and Sponges, Liège, August 19-29, 2011
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Deep insight into deep-water corals: unique microstructure of
micrabaciids
Katarzyna JANISZEWSKA 1 , Jaroslaw STOLARSKI 1 , Marcelo KITAHARA 2 & Stephen D.
CAIRNS 3
1Institute of Paleobiology, Warsaw, Poland; k.janiszewska@twarda.pan.pl, stolacy@twarda.pan.pl
2James Cook University, Townsville, Australia
3Department of Invertebrate Zoology, National Museum of Natural History, Smithsonian Institution,
Washington, D.C., USA
The skeleton of scleractinian corals consists of two microstructural regions: (i) rapid accretion deposits
(“centres of calcification”, “early mineralization zone”) and (ii) thickening deposits (“fibers”, “stereome”).
Traditionally, size and arrangement of “centers of calcification” were used as main diagnostic characters of
the high-rank scleractinian taxa (WELLS 1956). However, combined molecular and microstructural data
revealed that equally valuable skeletal characters that supports molecular groupings are thickening
deposits. For example, Acroporidae, the most speciose extant scleractinian family have thickening deposits
formed as shingle-like units. Longer axes of fibers are aligned within individual bundles and are parallel to
the skeletal surface whereas Pocilloporidae representatives have dome-shaped bundles of fibers
perpendicular to the surface of the skeleton, showing a microtuberculate pattern on the surface. Thickening
deposits in Flabellidae form thin scale-like bundles of fibers arranged quasi-parallel to the septal plane. All
scleractinian taxa mentioned above are characterized by distinct types of thickening deposits (Fig. 1; see
also GAUTRET et al. 2000; STOLARSKI 2003; NOTHDURFT & WEBB 2007), and were recovered as monophyletic
in molecular studies. Herein, based on skeletal analysis of the enigmatic group of deep-water micrabaciid
corals, we provide further evidence supporting a role of thickening deposits in phylogenetic
interpretations.
Micrabaciidae are solitary, azooxanthellate corals living today in the deepest parts of the ocean (up to
5000 m, CAIRNS 1989). They are known since the Early Cretaceous and attributed to five genera:
Letepsammia 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). Modern
and fossil species have porous, delicate skeleton and unique "bifurcation" of septa and costae. In contrast to
other scleractinians, thickening deposits of micrabaciids, regardless of the geographic and bathymetric
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 small, chip-like bundles forming an irregular criss-cross pattern on the
skeletal surface. Also, unlike other scleractinians, the growth layers in micrabaciid thickening skeleton are
not well discernible.
The herein described microstructural pattern of thickening deposits is unique among corals and
supports the monophyletic status of micrabaciids (KITAHARA et al. 2010). In some geochemical proposals,
formation of the fibrous part of the skeleton is considered to be analogous to inorganic precipitation of
CaCO3. In contrast to this interpretation, the consistent occurrence of distinct microstructural patterns of
thickening deposits in different scleractinian clades (acroporids, pocilloporids, flabellids, micrabaciids),
favours the organic matrix-mediated model of coral mineralization. According to it, these are
macromolecules that form under a tight genetic control, and which are responsible for nucleation, spatial
delineation and organization of basic microstructural units.
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