Why is it that only 6 Megalonyx skeletons of any consequence have ever been discovered and no direct adult-juvenile association at all, but we’re lousy with sloths? What’s so unique about the site or the circumstances surrounding our sloths’ deaths? Much progress has been made in understanding the processes of fossilization and decomposition, but the essential mystery remains–why do just a few thousandths of 1% of all bones become fossilized, absent embalming and burial? (Gill-King, 1997) How did we beat the odds?
External conditions (i.e. soil chemistry, microbes, etc.) are generally believed to determine the fate of bones, but Bell et al. (1996) suggest internal factors are more important. The bodies of all animals contain a host of microorganisms in their guts ready to escape after the death of their host and use its vascular and lymphatic systems to invade the major body organs. The microbes could be getting into the bone marrow by the Haversian canals that maintain the bones in life. The actions of predators, and their manner of killing–specifically, disemboweling prey and disrupting the microbial escape routes may be critical for explaining bone preservation. I love the irony–fall to predators but live forever as a fossil. Cool.
If you want to become a fossil, getting eaten by wolves may be your ticket. Avoid hyenas–they’ll eat you bones and all. Lions and tigers will do a lot of damage to your bones too. A pack of wolves is perfect. Haynes (1988) says they view internal organs as among the choicest bits and consume them first–bye bye gut microbes. Better yet, wolves rarely leave any tooth marks on the bones of a kill–hello immortality.
Shipman (1981) says predation is overemphasized as a contributor to the fossil record, accounting for less than10% of the mortality of the average prey species, but we have evidence that our toddler survived at least one attack. Maybe our sloths weren’t so lucky the last time. Micozzi (1986) adds some intriguing support.
Forensic scientists have long studied the process of decomposition to estimate the time of death. The test carcasses are often temporarily stored in a freezer. Micozzi wondered if freezing and thawing were affecting the results and compared the decomposition of fresh rats to ones frozen and thawed. He found freezing killed the gut microbes that normally started the process of putrefaction. Decomposition proceeded from the outside in, driven mostly by aerobic decay (i.e. oxygen-using microbes). The fresh, non-frozen rats decomposed from the inside-out, driven by the internal anaerobic bacteria (living without oxygen). Micozzi couldn’t follow his experiment long enough to determine if the defrosted rats were more likely to be fossilized, but clearly they were on a different chemical/microbial pathway.
Trueman and Martill (2002) suggest gut bacteria may be the initial attackers of bone–pioneers opening virgin bone to settlement by waves of followers, and anything that disrupted them–e.g. gutting by a predator, butchering or freezing halts the process of bone decay. Once started, they believe decay proceeds to complete destruction. Only those bones that escape attack by internal bacteria, or have them halted in their tracks by a rapid physical or chemical change, survive to become fossils.
So how can we tell if predators killed our sloths? . . . Dave
Bell, LS, Skinner, MF, Jones, SJ . 1996. The speed of post mortem change to the human skeleton and its taphonomic significance, Forensic Science International 82: 129-140.
Gill-King, H. 1997. Chemical and ultrastructural aspects of decomposition. In Forensic Taphonomy: The Postmortem fate of human remains, WD Haglund and MH Sorg eds. pp. 93-108.
Haynes G. 1988. Prey bones and predators: potential information from analysis of bone sites. Ossa: 7: 75-97.
Micozzi, MS. 1986. Experimental study of postmortem change under field conditions: effects of freezing, thawing, and mechanical injury Journal of Forensic Sciences 31: 953-961.
Shipman, P. 1981. Life History of a Fossil: An Introduction to Taphonomy and Paleoecology, Harvard University Press.
Trueman, CN and DM Martill, DM. 2002. The long-term survival of bone: The role of bioerosion. Archaeometry 44: 371-382.