I’ve been ruminating again about how our sloths might have died. Looking for fresh inspiration, I hiked down the Grant Wood trail here on the edge of Marion, IA to the Hughes peat bed, a site not unlike the ancient Tarkio Valley, where I recently learned a pair of bison–adult and juvenile– met their untimely ends 5,000 years ago.
No amount of logic or evidence can erase the image of unmatched ineptitude sloths have in the minds of the public and biologists alike. . . they are simply too different. Weirdness at this level prompted titters and contempt from the moment tree sloths were discovered. Buffon, naturalist to King Louis XV of France, suggested sloths were an experiment by The Creator to test the limits of life by piling one flaw upon another. . . one more and sloths could not exist at all (Martin et al., 1961). So it was with some surprise and no little delight that I read a recent paper by Vizcaíno (2009) that takes sloths’ greatest flaw–their lack of tooth enamel, and suggests it might in fact be their great evolutionary innovation.
Holmes and I spent a morning last week with Jonathan Thomas, a Ph.D. student in the UI Department of Anthropology using Geoscience’s scanning electron microscope (SEM) to check the sloth teeth for phytoliths. Jonathan is an archaeologist studying Neolithic Iberia, and is the “go-to” guy in Anthropology for SEM work. This was just the first step—a quick non-destructive screening to check the condition of the teeth and determine the feasibility of further analysis. Results exceeded all expectations and several apparent phytoliths were observed. We were joined by Meghann Mahoney from the UI Museum of Natural History.
Using a single bone to estimate the weight of an extinct animal, like we did last month, beats consulting the Psychic Hotline, but not by much. Scientists usually start with a weight-bearing bone like the femur. Unfortunately, fossil skeletons are usually incomplete and the bones fragmentary. When nothing else is available they look to tooth dimensions, jaw length, the thickness of a bone’s cortex, even the width of a joint (Scott, 1990). . . the equations are as numerous as the techniques for measuring the bones or the pieces thereof. (photo borrowed from)
We now have an official estimate for the weight of our adult sloth courtesy of Greg McDonald—2,829 pounds. We had assumed it was bigger than the average Megalonyx (2,400 lbs., McDonald, 2005) from the relative size of the teeth, but this confirms it and provides our first absolute number using a formal estimating technique based on the femur.
The dead of winter gives us the opportunity to catch up on our lab work. Holmes has been exploring a sample of clay we collected two years ago near the juveniles, and he found some clams inside! These are the first fossil mollusks we’ve seen in the clay. Jim Theler at the University of Wisconsin–La Crosse has identified them as members of the Sphaeriidae family which has two common genera Pisidium or pill clams and Sphaerium or fingernail clams.
Shipman (1981) states that only 1% of the terrestrial animals that die are preserved. This average however disguises the heavy bias in the fossil record in favor of species from lowland habitats (ponds, marshes, floodplains, etc.) where sediments accumulate and provide a protective blanket for the remains. Rapid burial is a critical factor improving the probability of preservation. Burial reduces physical weathering and reduces the opportunity for scavengers to damage and scatter the bones. Also, the more rapidly a specimen is covered, the poorer the environment for some destructive microbes. Most of the Earth’s continental surface is upland, but the preservation environment here is relatively poor.
Hamburger doesn’t fight back. A ground sloth wasn’t your average prey species–it would have been a challenge bringing down a healthy adult. Greg McDonald says an average full-grown Megalonyx weighed approximately 2400 pounds (McDonald, 2005). There’s some uncertainty about their weight, as you might imagine, given the paucity of complete skeletons, but let’s go with it. Besides, that’s probably a conservative estimate–our adult was a lot larger than average.