Maria McNamara wasn't looking for bone marrow when she scrutinized a set of amphibian fossils from 10-million-year-old lake sediments in north-eastern Spain. So the reaction was "just shock really" when she peered inside a tiny fractured bone and saw a red rind that looked like marrow--what may be the first fossilized example of this delicate tissue. The finding could suggest that marrow is more widely preserved than has been assumed. If so, such fossilized marrow could offer new clues about the physiology and lifestyle of extinct animals.
After spotting the tissue in September 2004, McNamara, a Ph.D. student in paleobiology at University College Dublin, closely examined many other specimens that had been collected in the 1950s from the same Spanish sulfur mine. Working with colleagues from Ireland, the United Kingdom, Spain, and the U.S., she found evidence of marrow in 10% of the 56 adult frog fossils and in one of 15 salamanders.
In the best preserved examples, a central blood vessel is surrounded by yellow, fatty marrow, which is wrapped in a red marrow. Besides being dehydrated and brittle, the tissue looks "very similar" to that of modern frogs, says McNamara, who describes the fossils in this month's issue of Geology. Her team is conducting geochemical tests to look for biomolecules such as fatty acids or amino acids to see how much original material might remain.
McNamara and her colleagues believe that the bone itself helps preserve the marrow by preventing bacteria from entering. She notes that marrow was extremely rare in 79 fossil tadpoles, whose bones weren't fully ossified when they died and were therefore more vulnerable to penetration by bacteria. Preserved marrow could reveal aspects of ancient physiology, such as whether an animal was starving or had hibernated, says McNamara.
So far, the evidence hasn't fully convinced paleontologist Mary Higby Schweitzer of North Carolina State University in Raleigh, who last year described blood vessels and other tissue from Tyrannosaur fossils (ScienceNOW, 24 March 2005). "Maybe it is marrow tissue, maybe not," she says. "It will be interesting to see what their further research shows."