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.
So what big predators are known from Pleistocene Iowa? Wolves for sure. Dire wolves? Any sabertooth cats? What’s big enough to take out a full grown sloth?
Pete, There are no large ice-age carnivores curated in museums from Iowa except for the dire wolf. However, discoveries in contiguous states suggest that the short-faced bear, American lion, wolf, jaguar, scimitar cat, saber-tooth cat were present. The American cheetah has been recovered from western South Dakota but I left it off of the list because its known range seems to be to the west of that location. We probably should add paleoindians, documented in Iowa by artifacts and a skeleton in Minnesota, to this list. No evidence of butchering or tools have been recovered from the Tarkio Valley locality. The short-faced bear with its nine foot body probably could take on a sloth solo but the long, strong arm of the sloth certainly was a powerful weapon. Other carnivores, working in tandem, probably could wear a sloth, especially a juvenile, down. We can only speculate on defense mechanisms of a ground sloth.
The paucity of ice-age megamammals from Iowa is taphonomic. Large caves, rare in Iowa, are ideal for the accumulation and preservation of vertebrates. I have collected cave deposits for micromamals for years and never have been ‘skunked’ by a cave dig. Little caves will do for these. Large caves abound in Missouri and almost all of the above species are recorded from there. Many are in other surrounding states as well. Moreover, Missouri caves have been extensively investigated, most recently by paleontologists from the Illinois State Museum. Paleontologists interested in ice-age remains flock to caves, especially if they are handy. Thus, distribution maps of fossil mammals also illustrate the distribution of paleontologists or their favored collecting areas.
Google “FAUNMAP” and you’ll find several on-line maps showing the distribution of Ice Age mammal fossils being curated in the major museum collections (a limited sample, as Holmes notes). A particularly nice interactive version is at:
Thanks for the info -very interesting! Don mentioned in another post about putting up a sloth diagram with recovered bones shown. The presence or lack of certain bones might be useful in determining possible predator or scavenger ID. Short faced bears are though to have been scavenger/bone crushers, right? Something like that should be easy to see.
I remember working on the sloth pelvis a while back and finding tooth marks that matched closely with fox canines. At the time the idea was being kicked around that the reason the pelvis was so fragmented was that it may have been stepped on by mammoths. Any more thoughts on that?
Pete, I originally broached the idea that the crushed pelvis (and several other bones) could result from being stepped on by a mammoth or mastodon. I now believe the larger bones were broken up because of weathering. They did not get buried as rapidly as either the small or flat bones and, thus, were exposed to the elements for a much longer period of time. All but a few sloth specimens, less ankle and wrist elements, exhibit dry fracture. The pelvis was riddled with closely spaced fracture and probably collapsed largely under its own weight . A bump or two from almost any medium or large animal would speed things along. Dave found an article by Pat Shipman who observed African elephant behavior for a long period of time. Among other things, Shipman documented the demise, decay and breakup of dead African mammals around watering holes. She noted that when large animals encounter large bones, skulls especially she notes (or rocks), they step over or beside, not on, them. Apparently, they avoid them for the same reason that people avoid stepping on rocks if flat ground is available. No sense in risking a strain. I think that Dave put a ‘coup de grace’ on my mammoth theory.
I plan to have another look at the tooth marks on the pelvis Thanks for reminding me.
Holmes, don’t give up yet on the idea of mammoths dancing on the pelvis, and skull. Gary Haynes provides a slightly different perspective re. trampling. He agrees generally with Shipman about most elephants being careful where they tread, but notes exceptions. He says some seem more inclined than others to walk in a straight line, and once they chose a path, nothing deters them–large bones or entire carcasses. . . I knew someone like that in high school.
Haynes, G. 1988. Longitudinal studies of African elephant death and bone deposits. Journal of Archaeological Science 15: 131-157.