Organic phase (extracellular matrix, osteocyte, blood vessel) preservation in dinosaur bone: Chemical composition and hypotheses of preservation

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Bone as a hard tissue fossilizes well, providing the exceedingly rich fossil record of vertebrate evolution. Bone is a composite material of an inorganic phase (IP) of 70% by volume and an organic phase (OP) of 30% by volume. It has long been recognized that in addition to their shape, fossil bones also preserve their histological structure. Seen in a light microscope, the bone matrix is not affected much by fossilization because crystallite patterns are not altered diagenetically. The major changes during fossilization thus must take place at the nanostructural level.Recently it has been recognized that OP in bone such as osteocyte remains, blood vessels and sometimes extracellular matrix and cartilage can be liberated from well-mineralized fossil dinosaur bone by careful digestion with weak organic acids. The discovery of OP liberation led to a controversy over the nature of these pliable organic remains as original soft tissue preservation vs. biofilms produced by bone-degrading bacteria. Although the evidence now is in favor of original soft tissue, endogenous OP preservation requires independent confirmation because of its significance for the vertebrate fossil record. Two decades of acquisition form the basis of one of the world’s most extensive collections of histological samples of fossil bone in applicant Sander’s lab. Whereas most samples are well preserved, another set of samples is of particular interest for this project, however. These are samples that show a complex and unusual diagenetic history, often resulting in poor preservation of the histology. These specimens occur randomly throughout the sample base, independent of geologic age and sedimentary environment.Our proposal is a renewal of project A2 of the first funding period of the DFG research unit FOR 2685 “Fossilization”. Thus, the overarching aim of the proposed project remains the same, i.e., to understand the fossilization of bone. The aims for the second funding period are thus motivated by the four major results of the first funding period: 1. OP preservation is a standard feature of fossil permineralized bone. 2. OP preservation is independent of the geologic age of the sample. 3. OP preservation is independent of the depositional environment of the sample. 4. OP liberated from fossil permineralized bone is likely endogenous. In the second funding period, we particularly want to address three new hypotheses: H1: Strong diagenetic alteration destroys OP. H2: Strong diagenetic alteration reveals physicochemical boundary conditions of bone mineralization. H3: Heme is preserved in deep time in fossil bone. To address these hypotheses as well as the earlier ones, we will use state-of-the-art analytical methods from organic chemistry (for the OP and heme) and mineralogy (for the IP and diagenetic phases, especially in the poorly preserved samples).