Abstract
The Eocene (∾48 Ma) Messel Pit in Germany is a UNESCO World Heritage Site because of its exceptionally preserved fossils, including vertebrates, invertebrates, and plants. Messel fossil vertebrates are typically characterized by their articulated state, and in some cases the skin, hair, feathers, scales and stomach contents are also preserved. Despite the exceptional macroscopic preservation of Messel fossil vertebrates, the microstructural aspect of these fossils has been poorly explored. In particular, soft tissue structures such as hair or feathers have not been chemically analyzed, nor have bone microstructures. I report here the preservation and recovery of osteocyte-like and blood vessel-like microstructures from the bone of Messel Pit specimens, including the turtles Allaeochelys crassesculpta and Neochelys franzeni, the crocodile Diplocynodon darwini, and the pangolin Eomanis krebsi. I used a Field Emission Scanning Electron Microscope (FESEM) and a Phenom ProX desktop scanning electron microscope (LOT-QuantumDesign) equipped with a thermionic CeB6 source and a high sensitivity multi-mode backscatter electron (BSE) for microscopical and elemental characterization of these bone microstructures. Osteocyte-like and blood vessel-like microstructures are constituted by a thin layer (∾50 nm thickness), external and internal mottled texture with slightly marked striations. Circular to linear marks are common on the external surface of the osteocyte-like microstructures and are interpreted as microbial troughs. Iron (Fe) is the most abundant element found in the osteocyte-like and blood vessel-like microstructures, but not in the bone matrix or collagen fibril-like microstructures. The occurrence of well-preserved soft-tissue elements (at least their physical form) establishes a promising background for future studies on preservation of biomolecules (proteins or DNA) in Messel Pit fossils.
Highlights
Osteocytes are the most abundant cells in the bone tissue of all vertebrates (Bonewald, 2011, references therein)
Osteocytes present a unique morphology because during development they become entrapped in the matrix they secrete; to obtain nutrients and eliminate waste, their cell membranes form extensions known as filopodia (Bonewald, 2011; Lin & Xu, 2011)
Osteocytes-like microstructures from the turtles Allaeochelys crassesculpta and Neochelys franzeni vary from semi-oval to elongated (Figs. 2A–2E), to very elongated (∼50 μm) (Figs. 2F–2H), almost all of them preserving second- and third-level ramifications of filopodia, and are yellow to orange in color
Summary
Osteocytes are the most abundant cells in the bone tissue of all vertebrates (Bonewald, 2011, references therein). Two studies described potential bacteria and other microorganisms in bone fragments, scales and eyes of fish, as well as in coprolites using TEM (Wuttke, 1983; Liebig, Westall & Schmitz, 1996). Whether this preservation could extend to the micro- or nano-scales is currently unknown
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