Quartz sand particles  have tiny cracks and imperfections in their crystal structure that trap electrons emitted by radioactive elements in the surrounding sediment.  The longer they are buried the more electrons they trap.  Sunlight resets the “clock” so keeping the sample dark is essential.  Accuracy is about plus or minus 10-15%.  Expect a date in a couple of months. . . . Dave

Many thanks to the  Cedar Valley Rocks & Minerals Society for sponsoring the event, Steve Struckman, UI College of Engineering Prototyping Center for expediting fabrication of the prototypes, Tony Smith at the Hobby Corner for his skillful paint-job, and graduate assistant Youbing Yin, Iowa Comprehensive Lung Imaging Center for fixing the last-minute kinks in the STL files.  

The radius and ulna prototypes provoked no little amazement.  Stop by the museum for a lesson on how sloths moved their arms and to examine the other prototypes.  Sloth on! 

Extraction on the tooth proved to be a labor intensive process to even recover a milligram. The sample, NOSAMS # 81815 labeled 10 RSF C14 005, which was adequate by weight, yielded only 1.7 micromoles of C02. When asked if sacrifice of a whole tooth would produce a date, Bob replied that we needed to recover at least 50 micromoles. This would require at least 150 extractions, would be a few months work and still probably would not generate sufficient CO2 for our purposes. He concluded,  “I think that it is not going to be a datable specimen.”

Earlier, we had submitted a bone sample and a dental sample from the adult to the Keck Carbon Cycle Accelerator Mass Spectrometry facility at the University of California-Irvine. They could not recover enough collagen to date the specimen either. After attempts by two world class facilities, we are convinced that it is not possible to directly date sloth remains with current technology. Alternatives for direct dating are under consideration.  Pollen and seeds are in the matrix and offer radiocarbon alternatives. There is also the potential to date the sediments above and below the sloth-bearing matrix. 

 Exciting as they are, the Tarkio Valley sloths continue to be analytically evasive. 

Sloth on. Holmes

Two fused caudal vertebrae (left) with normal vertebra (right)

two adjacent vertebrae fused as the wound healed making movement in the joint impossible.   Did  another animal step on it accidentally? Was it wounded by a predator?  Injured in a fight with another sloth?  There’s no way of knowing.   If sloths used their tails like the third leg of a stool, i.e. to lean back on when they stood up, as some scientists suggest, our sloth was probably in pain every time she did.  Ground sloths used their tails in different ways–in Peru they flapped them while they swam grazing on seaweed on the bottom of the ocean. (Muizon and McDonald, 1995)

When giant sloths were first discovered some scientists pointed to their powerful tails as evidence they had probably lived in very large trees and used them to hang from branches like monkeys. (Lund, 1838) Owen debunked this by pointing out the natural curve in the way the caudal vertebrae articulate–down and back, not up and forward as a prehensile tail must.  He noted too the size and direction of the vertical spines and transverse processes, and other anatomical features indicating ground sloths’ tails were fully as long and as powerful as their legs–even kangaroos don’t have tails as thick and strong in proportion to their length. (Owen, 1842)

This perspective of sloth tails as third legs was inspirational.  When the first dinosaur was discovered in America in 1858, Joseph Leidy, a professor of anatomy at the University of Pennsylvania, named the reptile Hadrosaurus, and suggested it probably browsed upright like a sloth based on the disproportionate size of its legs and tail vs. its fore-limbs.  This was a striking contrast to the clumsy iguana-like postures British scientists had given dinosaurs four years earlier when they made their initial public appearance in the life-size models constructed for the Crystal Palace Park exhibition.  It’s a perspective that inspires dinosaur researchers still today. (Borsuk-Bialynicka, 1977)

Glyptodon research hints at another intriguing possibility for the tail.  Like ground sloths, these large heavily armored South American cousins of the armadillo are herbivorous and became extinct at the end of the ice ages.  The largest measured ten feet long and weighed  more than two tons (i.e. sloth range). (Fariña, 1995)  Calculations show glyptodons could support their entire weight on one leg–that is, like ground sloths, they could walk bipedally. (ibid.)  Fariña has also found other indicators of surprising Glyoptodon athleticism.  He believes bipedalism was a necessity for breeding, and perhaps also handy for fighting or defense. Healed fractures are common in their carapaces and have been attributed to intraspecific fights (Ferigolo, 1992).  Fariña suggests a quick pivot on one leg and a sweep of their hind ends could turn glyptodons’ heavy armored tails into lethal weapons, somewhat like the behavior proposed for Stegosaurus and Ankylosaurus.

Could sloths have wielded their tails in a similar fashion to whack enemies?  As appealing as the image of Ninja sloths with tails of steel may be,  Greg McDonald tells me not to get my hopes up.  He hasn’t seen a similar wound in any other Megalonyx (pers. comm.), which probably means they aren’t practicing tail sweeps or similar WWF-style moves on each other or predators.  Moreover, their bone morphology suggests very limited athleticism.

Sloth bones are solid—not like tree limbs, but filled with a network of spars and braces called cancellous bone, more so than any other living animal. (De Toledo, 1996) It’s a design for resisting compressive force, i.e. weight. The long bones of most animals are hollow—an optimal design to counter the stress of bending and acceleration. Sloths, like elephants, walked stress-free, with straight legs–an ambling even gait with short even strides and minimum acceleration.   Elephants don’t gallop, but they still manage to cover a lot of ground walking quickly, when they want . . . sloths too . . . when they wanted.   But there may be a lot more to the cancellous structure of sloth bones.

Osteoporosis researchers tell us the arrangement of spaces and buttresses inside bones makes a big difference in how loads and stresses are transmitted.  (Oxnard, 1990) What looks to be a random pattern may not be at all.   CT scans, high-speed computers and advanced mathematics may one day reveal the exact forces that produce the bony arrangement. . .  until then all we can do is speculate, and again elephants offer some intriguing possibilities.

Next time:  Sloth songs.  Dave

References

Borsuk-Bialynicka, M.  1977.  A new Camarasaurid Opisthocoellicaudia skarzynskii gen. n., sp. n. from the Upper Cretaceous of Mongolia.  Palaeontologia Polonica 37: 5-64.

Fariña, R.A. 1995.  Limb bone strength and habits in large glyptodonts.  Lethaia 28: 189-196.

Ferigolo, J. 1992.  Nonhuman vertebrate paleopathology of some Brazilian Pleistocene mammals.  Paleoepidemiologia e Paleopatologica:  estdos multidisciplinares.  A. J. G. Araújo and L. F. Ferreira (eds.)  Escola Nacional de Saúde Pública, Rio de Janeiro.  Pp. 213-234.

Lund, P. W.  1838.  View of the Fauna of Brazil, prior to the Last Geological Revolution. Kjöbenhavn.

De Muizon, C. and H. G. McDonald, H. G. . 1995.  An aquatic sloth from the Pliocene of Peru.  Nature 375:  224-227.

Owen, R. 1842.  Description of the Skeleton of an Extinct Giant Sloth, Mylodon robustus, Owen:  with observations on the osteology, natural affinities, and probable habits of the Megatheroid quadrupeds in general. John Van Voorst, Publ.  London.

Oxnard, C. E. 1990.  From giant ground sloths to human osteoporosis.  Proceedings of the Australasian Society of Human Biology 3: 75-96.

De Toledo, P. M. 1996.  Locomotory Patterns Within the Pleistocene Sloths.  Ph.D. thesis Department of Geological Sciences, University of Colorado.

The first 3″ core was taken upstream from the site on the southeast bluff overlooking the Tarkio, about 10 meters downstream from the northeast corner of the Athen’s property. This well produced a 40 foot core which was augmented by a five-foot, auger sample into underlying pre-Illinoian till. After completion, the rig was moved to the northwest bank of the Tarkio on the old Tiemann property (now Gary Peregrine’s farm).  We started drilling about 15 feet from the north face of the sloth excavation pit, but the effort ended prematurely when the core barrel became blocked by a rock. A second core was initiated about 15 feet downstream from the first. This core extended to 35 feet and bottomed out in a sand deposit that kept refilling the hole, stopping deeper exploration. This core undoubtedly penetrated the sloth-bearing level which lies about 24 feet below the surface of the field.

 Harold will describe and interpret the cores for his master’s thesis at UCM. It is believed that the Athen core sampled the valley wall of the Tarkio and that the Peregrine core penetrated valley fill deposits of the DeForest Formation. It is  not clear yet if the sloth-bearing deposits are associated with the older wall deposits or the younger fill deposits. Age differences are substantial.

Before we left, Bob Athen produced his latest bone discovery.  According to Greg McDonald, it appears to be the proximal end of a sloth metapodial. It is not complete though and Greg currently has the specimen to study for a potential match.  The specimen came from the creek bed in  the excavation area.

Holmes

Tests on the Megalonyx are more hopeful– Robert Feranec, Curator of Vertebrate Paleontology, New York State Museum,  managed  to extract a small amount of collagen from a molar  while he was assisting Alex Bryk, Penn State,  with a stable isotope analysis.  Bob had to do 7 separate extractions, but he got plenty.  To his eye, the collagen “looks fine. ” It weighed about 2.5mg. That’s not a lot, but enough. An AMS radiocarbon date should be available from Woods Hole  next month. . . . Dave