In November, 2005 I was invited to identify the jawbone of an extinct Ice Age animal recovered from a gravel bar in the Missisippi River. A quick look confirmed that Danny West, of Greenville MS, had found the dentary of a giant ground sloth. The fossil was in an excellent state of preservation with two complete molars and the canine intact. The Wests have an extensive collection of Pleistocene vertebrate fossils from our area, Northwest Mississippi, and I have borrowed several of their finds to make molds and casts.
About this time I was made aware of some experiments being undertaken at Louisiana State University in the relatively new science of paleophytology. This is the study of phytoliths (literally "plant stones") which are microscopic mineral bodies, usually silicates, produced naturally by some plants. Being minerals they do not decay like organic plant matter, and being somewhat unique as to the species producing them, these particles are useful in archaeology and paleontology. By analyzing the soil around habitats and middens of primitive dwelling sites, researchers can identify the different phytolith producing plants these people utilized. Paleontologists are able to collect phytolith-bearing material from the calculus (plaque) of fossil teeth. This is helpful in determining the diets of these extinct animals.
Since phytoliths had already been harvested from the calculus of mastodon and bison teeth, the possibility of retrieving some from the sloth mandible was discussed. Ground sloths had never before been investigated in this manner. Calculus normally collects around the gum line of herbivores. Examining this unwashed specimen I found it had absolutely smooth enamel on the sides of all the teeth. There was no fossilized plaque at all.
However after cleaning the jaw to prep it for moldmaking, I noticed a thick layer of compacted plant particulate in the center of the occlusal surface of the canine tooth. Megalonyx had a wedge-shaped upper canine which fit into a linear groove in the bottom canine. It was in this groove that the organic material had accumulated. I enclosed the jawbone in a plastic bag to prevent cross contamination and began to scrape the material from the surface of the tooth.
Since this matter had taken most of the sloth's lifetime to accumulate, I decided to remove it in layers, assuming that layered samples might be indicative of dietary preferences at different stages of life. I collected five sets of scrapings, cleaning the surface after each one. I stored them separately, photographing the altered area at each stage with a 5.1 megapixel digital camera on superfine setting and 2592 X 1944 pixel aspect ratio.
As an amateur I tend to over-document so, for reference, I removed some matrix which had accreted itself to the empty alveolus wall in two different places and documented this with the camera. Matrix can be useful in determining the geologic strata of a fossil's original location. Believing the matrix would prove to be loess, I collected a loess sample from a fresh exposure in Warren County MS for comparison.
I also took several macro-photographs of the scrapings. The samples and the Warren County contribution were sealed in vials, labeled and sent to Dr. John H. Wrenn, Ph.D., Associate Professor of Geology and Geophysics at L.S.U. Extracting phytoliths from fossil calculus involves soaking in hydrochloric acid and Schulze solution to dissolve organic matter. Samples must be centrifuged with zinc bromide to remove the silicates from suspension. Heavy liquid flotation and distilled water washes complete the process. Dessicated samples are mounted on slides and viewed with polarizing petrographic microscopes to distinguish phytoliths from grains of quartz. Though "plant stones" were described by Darwin, phytolithology did not evolve appreciably as a science until the 1970's.
The scrapings were indeed amorphous vegetable matter, but the final results of the tests were unfortunately not overwhelming; some fossil pollen grains and an unidentified phytolith, but negative results produced by these tests are not always without merit. The animal may have preferred immature leaves and twigs, which do not contain significant numbers of these mineral bodies. Fruits and nuts in our temperate zone contain no appreciable amount of silica at all. Grasses are the best known phytolith producers. Perhaps the ground sloth was not a grazer. Since they were descended from tree dwelling sloths, their diet could have consisted entirely of woody dicots which would not have contributed significantly to phytolith presence in the calculus. More samples from different sloth fossils are needed to help understand whether the absence of phytoliths is also indicative of this animals diet.