Showing papers by "Anders Meibom published in 2011"

The ancient evolutionary origins of Scleractinia revealed by azooxanthellate corals.

Abstract: Scleractinian corals are currently a focus of major interest because of their ecological importance and the uncertain fate of coral reefs in the face of increasing anthropogenic pressure. Despite this, remarkably little is known about the evolutionary origins of corals. The Scleractinia suddenly appear in the fossil record about 240 Ma, but the range of morphological variation seen in these Middle Triassic fossils is comparable to that of modern scleractinians, implying much earlier origins that have so far remained elusive. A significant weakness in reconstruction(s) of early coral evolution is that deep-sea corals have been poorly represented in molecular phylogenetic analyses. By adding new data from a large and representative range of deep-water species to existing molecular datasets and applying a relaxed molecular clock, we show that two exclusively deep-sea families, the Gardineriidae and Micrabaciidae, diverged prior to the Complexa/Robusta coral split around 425 Ma, thereby pushing the evolutionary origin of scleractinian corals deep into the Paleozoic. The early divergence and distinctive morphologies of the extant gardineriid and micrabaciid corals suggest a link with Ordovician "scleractiniamorph" fossils that were previously assumed to represent extinct anthozoan skeletonized lineages. Therefore, scleractinian corals most likely evolved from Paleozoic soft-bodied ancestors. Modern shallow-water Scleractinia, which are dependent on symbionts, appear to have had several independent origins from solitary, non-symbiotic precursors. The Scleractinia have survived periods of massive climate change in the past, suggesting that as a lineage they may be less vulnerable to future changes than often assumed.

A unique skeletal microstructure of the deep-sea micrabaciid scleractinian corals

Abstract: Micrabaciids are solitary, exclusively azooxanthellate deep-sea corals belonging to one of the deepest-living (up to 5,000 m) scleractinian representatives. All modern micrabaciid taxa (genera: Letepsammia, Rhombopsammia, Stephanophyllia, Leptopenus) have a porous and often very fragile skeleton consisting of two main microstructural components known also from other scleractinians: rapid accretion deposits and thickening deposits. However, at the microstructural level, the skeletal organization of the micrabaciids is distinctly different from that of other scleractinians. Rapid accretion deposits consist of alternations of superimposed "microcrystalline" (micrometer-sized aggregates of nodular nanodomains) and fibrous zones. In contrast to all shallow-water and sympatric deep-water corals so far described, the thickening deposits of micrabaciids are composed of irregular meshwork of short (1-2 mu m) and extremely thin (ca. 100-300 am) fibers organized into small, chip-like bundles (ca. 1-2 mu m thick). Longer axes of fiber bundles are usually subparallel to the skeletal surfaces and oriented variably in this plane. The unique microstructural organization of the micrabaciid skeleton is consistent with their monophyletic status based on macromorphological and molecular data, and points to a diversity of organic matrix-mediated biomineralization strategies in Scleractinia. J. Morphol. 272:191-203, 2011. (C) 2010 Wiley-Liss, Inc.

53Mn-53Cr ages of Kaidun carbonates: 53Mn-53Cr ages of Kaidun carbonates

Abstract: – We report the 53Mn-53Cr systematics of three dolomite grains from two different CI1 clasts contained within the Kaidun meteorite breccia Three internal isochrones result in initial 53Mn/55Mn ratios of (42 ± 04) × 10−6, (46 ± 13) × 10−6, and (52 ± 11) × 10−6 These initial values are consistent with those measured for dolomite in the Orgueil CI1 chondrite (Hoppe et al 2007; Petitat et al 2009) but significantly lower than the initial ratio determined by Hutcheon et al (1999) from a combination of different carbonate types within various lithologies of the Kaidun meteorite We construct an accretion scenario for the Kaidun breccia by comparing the mineralogy and formation time scales of carbonates in the Kaidun CI1 lithologies to the analogous ones of the CI1 chondrite Orgueil In Orgueil, dolomite precipitation precedes the formation of the first bruennerite grains by a few million years (Hoppe et al 2007; Petitat et al 2009) As the CI1 clasts in Kaidun lack breunnerite grains, and considering that aqueous alteration occurred prior to reaccretion of the various clasts onto the Kaidun parent body (eg, MacPherson et al 2009), we hypothesize that after rapid accretion and early aqueous alteration occurring within the first approximately 4 Myr after solar system formation, impact disruption of several asteroids and their reassembly into the Kaidun parent asteroid was complete within an additional approximately 2 Myr This confirms that aqueous alteration, impact, and reaccretion of material in the asteroid belt were early processes that began contemporaneously with chondrule formation