 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; August 2006; v. 34; no. 8; p. 641–644; doi: 10.1130/G22526.1; 2 figures. 641 High-fidelity organic preservation of bone marrow in ca. 10 Ma amphibians Maria E. McNamara Patrick J. Orr    School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland Stuart L. Kearns Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK Luis Alcala ´ Fundacio ´ n Conjunto Paleontolo ´gico de Teruel-Dino ´polis, Avenida Sagunto s/n, 44002 Teruel, Arago ´n, Spain Pere Anado ´n Consejo Superior de Investigaciones Cientı ´ficas, Institut de Ciencies de la Terra ‘‘Jaume Almera’’, Lluı ´s Sole ´i Sabarı ´s s/n, 08028 Barcelona, Spain Enrique Pen ˜ alver-Molla ´ Division of Invertebrate Zoology, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024-5192, USA ABSTRACT Bone marrow in ca. 10 Ma frogs and salamanders from the Miocene of Libros, Spain, represents the first fossilized example of this extremely decay-prone tissue. The bone mar- row, preserved in three dimensions as an organic residue, retains the original texture and red and yellow color of hematopoietic and fatty marrow, respectively; moldic osteoclasts and vascular structures are also present. We attribute exceptional preservation of the fossilized bone marrow to cryptic preservation: the bones of the amphibians formed pro- tective microenvironments, and inhibited microbial infiltration. Specimens in which bone marrow is preserved vary in their completeness and articulation and in the extent to which the body outline is preserved as a thin film of organically preserved bacteria. Cryptic preservation of these labile tissues is thus to a large extent independent of, and cannot be predicted by, the taphonomic history of the remainder of the specimen. Keywords: taphonomy, organic preservation, bone marrow, Miocene, frogs, Spain. Figure 1. Map showing location of Barrio de las Minas near Libros, Teruel. Inset— position of Teruel province within Spain. INTRODUCTION Blood and bone marrow are the most labile, decay-prone tissues in the vertebrate body (Custer, 1974), and therefore almost invariably degrade rapidly during the initial stages of de- cay. Most purported examples of fossilized red blood cells have proved controversial, and some have been reinterpreted subsequently as artifacts, e.g., pyrite framboids (Martill and Unwin, 1997; however, see Higby Schweitzer and Horner, 1999). Furthermore, high-fidelity preservation of extremely labile tissues almost always results from their replication in early diagenetic authigenic minerals (Briggs, 2003; Martill, 1988; Voigt, 1939). The sole excep- tions are the unmineralized blood vessels and cell-like microstructures, possibly osteocytes, recovered by Schweitzer et al. (2005a, 2005b) after dissolving fragments of dinosaur bone; it was suggested that preservation of these struc- tures may have resulted from ‘‘some kind of unknown geochemical replacement process’’ (Schweitzer et al., 2005a, p. 1955). Here we report the first example of fossilized bone marrow (in ca. 10 Ma amphibians from north- eastern Spain) and provide a model of how extremely labile tissues such as these can be preserved with histological fidelity as three- dimensional organic residues (the terms ‘‘or- ganic’’ and ‘‘carbonaceous’’ do not imply that the original biomolecular composition is un- altered, in whole or in part). Such tissues pro- vide insights into the physiology of ancient organisms (Schweitzer et al., 2005a) and are potential targets for the recovery of biomole- cules. Therefore, an understanding of the taph- onomic processes responsible, specifically the environmental context and precise geochemi- cal conditions under which such preservation occurs, is crucial to realizing the paleobiolog- ical potential of these organic remains. GEOLOGICAL BACKGROUND The Libros Basin forms the southern part of the Teruel graben in northeastern Spain; its in- fill is early Miocene to Pliocene age, varies from 300 to 500 m in thickness, and includes an early late Miocene (Vallesian, 11.2–8.7 Ma) gypsum-dominated 120-m-thick lacus- trine sequence (Libros Gypsum; Ortı ´ et al., 2003). Thermally immature organic-rich lam- inated mudstones (oil shales: total organic car- bon, 1%–2.6% [de las Heras et al., 2003]) of the deep-water Libros Gypsum Unit crop out in the Barrio de las Minas near Libros (Fig. 1); these mudstones host exceptionally pre- served fauna and flora that include salaman- ders, frogs (both adults and larvae), birds, snakes, insects, arachnids, and leaves (Nava ´s, 1922). Adult and larval specimens of the frog Rana pueyoi and adult specimens of the sal- amander Triton sp. occur as articulated skel- etons enclosed in a thin, dark brown, carbo- naceous bacterial biofilm that defines part, or the entire outline, of their soft tissues. In ad- dition, patches of dermal collagen fibers, mor- phologically identical to those in extant frogs, are replicated in calcium phosphate (Fig. 2A). METHODS Fossilized Bone Marrow Samples of bone marrow were extracted us- ing sterile tweezers; no mechanical separation from the bone or dehydration was necessary. The solid, brittle samples were mounted on aluminum stubs for scanning electron micros- copy (SEM) without further preparation. For SEM, the sample illustrated (Figs. 2D– 2F) was gold coated and examined with a Hi- tachi S-3500N variable pressure microscope equipped with an EDAX energy dispersive X- ray spectrometer (EDS) running Genesis soft- ware. Observations were made at an acceler- ating voltage of 15 kV, with acquisition times of 60 s for EDS spectra. For transmission electron microscopy (TEM), samples were washed in ethanol 3 times, each for 30 min. The sample of fossil bone marrow was then impregnated with TAAB EM resin under vacuum in the follow- ing resin:ethanol mixtures, each for 2 h: 10%,