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.

The Grant Wood Trail starts on Hwy 13, just north of Squaw Creek Park and the Hwy. 100 intersection.  It was created in 1999 by members of the Linn County Trails Association, (http://www.linncountytrails.org/) and donated to the county in 2005.   The trail follows a route carved out by the defunct Chicago, Milwaukee, St. Paul and Pacific Railroad Company in 1872 on its way to Denver and places west.   It was Marion’s lifeline and road to prosperity for 100 years. The company abandoned the line in 1980, but telegraph poles and remnants of old sidings still peek out from the overgrowth.

The rolling hills that border the trail reminded residents of the paintings of a locally-born artist and time when more Iowans came from farms, but they speak to a geologist of a period more than 500,000 years ago when glaciers last visited these parts.  Erosion and the intense cold of the last ice advance softened the contours of the landscape (Prior, 1991) but left loess-covered hills that still compel nearly every first-time visitor to remark, “I thought Iowa was flatter.”

The railroad company planned to stay a while–sections of the old roadbed are paved with sparkling pink Sioux quartzite.  The rock is indestructible and has resisted erosion for 1.6 billion years where it is exposed in the NW corner of the state (Anderson, 1998). More diligent early road builders–train and auto—decided it was well worth the extra expense of hauling the crushed rock across the state for the maintenance costs it saved forever after.   Dorothy’s highway to Oz would have glistening pink if she had grown up a few hundred miles East.

Mammoth boulders line the fringes of the trail. Early observers called them “erratics,” recognizing their unusual character.  Iowa’s long stay at the bottom of the sea left it with a surfeit of sedimentary deposits, but these rocks are igneous and metamorphic—granite mostly—and don’t belong.   They were carried here from Canada by one of the early glacial advances and dumped.  They are the oldest rocks exposed in Iowa, aside from the meteorites that drop in from time to time.  Hikers walk passed them nowadays with hardly a glance, oblivious to their age and the unimaginable forces that brought them here.  Early settlers noticed–identifying Iowa’s geological resources was a priority.  Farming was too though, so they hitched up their teams and moved the smaller boulders into the farthest corners of these fields.  The biggest ones show signs that maybe dynamite helped with the moving.

Half way down the trail the land rises up on both sides as it cuts through a glacial paha—a Dakota Sioux word for hill (Anderson, 1998).  Hurricane-force winds sweeping off the last glacier blew sand and silt into massive dunes like this hill, mostly trending NW to SE.  Many an E. Iowa church, college or TV tower claims their peaks today, but the destiny of this paha lay in the calculations of a slick  RR accountant.  I wonder sometimes about railroad economics and why a company selects one path over another. . .  and their capacity–financial and technological–literally to move mountains.  The natural way forward for railroads is straight ahead, and in this instance they figured the fill they needed to bury the peat bog in their way lay just 1/4 mile further west . . . so onward!  Who needs a glacier when you have a railroad?  Did mining the peat before they buried it fit into their calculations?  Probably.

Scattered along both sides of the trail are traces of another ancient relic—remnants of Iowa’s tall-grass prairie.   Protection from rampaging plows and an occasional fire  sparked by a passing train were the only breaks these plants asked for to survive.  The tallest grass here is Spartina pectinata–prairie cord grass, or “ripgut” to those who know it especially well.  (Run your hand along the leaf-edge carefully.) Cord grass loves deep poorly-drained soil, and its appearance here tells me, as it told early settlers arriving by wagon, that soggy ground lies ahead and it would be wise to steer for high ground. The grass spreads  by extending its rhizomes and forms a thick gnarly mat which made the best sod houses (Madson, 1985).   Each clump is a clone–one individual self-renewing plant.  Spreading underground is common on the prairie–it’s difficult for seeds to get a toe-hold in the sod.  How long a clone can live is anyone’s guess–possibly hundreds of years. . . thousands isn’t out of the question (Steinger et al., 1996).    Sequoia trees tend to swagger, but this is Iowa, and our way is quiet dignity.

A sign overlooking the peat bed commemorates the discovery of a 5,000 year-old bison in 1969.  The skeleton was complete and articulated.  The animal may have been wandering through in the winter and broken through the ice.  Unable to extricate itself from the muck, it looked like it just hunkered down to die (Semken, pers. com.). The spring that keeps water flowing freely through the peat bed even in drought years when the nearby creek dries up (Hall, 1971) probably keeps areas of the ice thin–a death trap for large animals that misstep.  In 1988 the Cedar Valley Rocks and Minerals Society recovered a heifer and calf here lying side by side (Sonnleitner,  pers. com.) which apparently died the same way.  Could our sloths died like this?  Not mired down certainly–our bones aren’t articulated. . . but we’ve had other indications that winter may figure in the puzzle (also see Micozzi, 1986).

The Hughes peat bed is a bona fide ice age relic.  Hall (1971) dated the bottom layer to 11,880 years-old.  Snails and pollen suggest the area was a boreal spruce forest which gave way to oaks and elms by 9,300 years ago.  Ecologists would define this wetland as a spring fen—one of the rarest ecosystems in Iowa.  Pearson and Leoschke (1992) cataloged over 200 unique native plant species living in fens–10% of our native flora on less than 0.01% of the land. Fens truly are our emerald cities, and the Hughes peat bed is still a gem despite 150 years of peat extraction (White, 1868). More about the importance of peat lands in the past and in our future next time. . . . Dave

References

Anderson, W.I. 1998.  Iowa’s Geological Past:  three billion years of change.  University of Iowa Press, Iowa City, IA

Hall, S. A. 1971. Paleoecological interpretation of bison, mollusks and pollen from the Hughes peat bed, Linn County, Iowa.   Masters Thesis, Department of Geology, University of Iowa.

Madson, J.  1985.  Where the Sky Began.  Houghton Mifflin Company,  Boston MA

Pearson, J. A. and Leoschke, M. J. 1992.  Floristic composition and conservation status of fens in Iowa.  Journal of the Iowa Academy of Science 99: 41-52.

Prior, J. C. 1991. v Landforms of Iowa.  University of Iowa Press.  Iowa City, IA.

Steinger, T., Körner, C. and Schmid, B. 1996.  Long-term persistence in a changing climate:  DNA analysis suggests very old ages of clones of Alpine Carex curvula.  Oecologia 105: 94-99.

White, C. A.  1868.  First and Second Annual Report on the Geological Survey of the State of Iowa. Des Moines, IA.

Megalonyx upper caniniforms(L); molariforms (R)

Despite all evidence of their enduring success, the traditional view of sloths has long been that their simple ever-growing pegs of soft dentine imposed a serious constraint on their evolution. Even the scientists who acknowledge ground sloth achievements in South America discount the significance by pointing to their long isolation on the continent, supposedly inferior competition, and extinction after the Great American Biotic Interchange. They disregard sloths like Megalonyx and Paramylodon which successfully invaded the homeland of the allegedly superior mammals, dispersed widely, and competed successfully for millions of years, showing considerable adaptability in a wide range of habitats–faulty teeth and all (McDonald, 2005). Now Vizcaíno (2009) says scientists have overlooked some significant advantages of dentine teeth, and enamel isn’t something sloths lost but dumped to escape its limitations–opening the door for them to exploit niches unavailable to regular herbivores.

Abandoning enamel, Vizcaino says, and evolving ever-growing dentine teeth 60+ million years ago was the key innovation that allowed Xenarthrans to become abundant and widespread. The simple peg-like teeth that their earliest sloth ancestors evolved proved to be a remarkably flexible innovation. The enamel teeth of other herbivores wear out with age. Older animals have to adapt to the decline in their oral processing ability by altering what they eat and the way they eat it–selecting less fibrous leaves, chewing faster, spending more time chewing and eating greater quantities. They get less nutrition from their food because less of the interior of the cells is exposed to digestion by intestinal microbes and enzymes. The animals aren’t as well nourished so they suffer more stress due to disease, parasites and plant toxins. They spend more time in habitats providing less concealment and are more exposed to predators. Some herbivores simply starve to death because their teeth wear out, but most die for other reasons. Predation, parasites or disease may be the proximal cause of death, but theyare all the inevitable outcome of being born with enamel. Ever-renewing, self-sharpening teeth made of dentine, like sloths’, stay in optimal condition for life. [more about sloth teeth]

Were dentine teeth and their concomitant breeding advantages the evolutionary tools ground sloths used to conquer North America? An aging female Megalonyx with two healthy juveniles in tow, such as we may have in the Tarkio Valley would be compelling support for Vizcaíno’s theory. First, we need to prove our adult sloth was female, and then that the indications of arthritis we see come from old age and not an injury or a different disorder. . . ever-growing teeth aren’t much help when paleontologists need to determine a sloth’s age. If our adult was truly elderly you would expect more signs of the arthritis (McDonald, pers. com.). . . on the other hand, we shouldn’t expect zoo animal-age arthritis symptoms. . . life is hard, and relatively short, even in the Tarkio Valley. In any case, Vizcaíno presents an exciting new perspective. Maybe Buffon was right after all and sloths are a test. . . for us, to learn more about the wonders of life. . . . Dave

References:

Martin, P. S., Sabels, B. E. and Shutler, D. 1961. Rampart Cave coprolite and ecology of the Shasta ground sloth. American Journal of Science 259: 102-127.

McDonald, H. G. 2005. Paleoecology of extinxt xenarthrans and the great American Biotic Interchange. Bulletin of the Florida Museum of Natural History 45: 313-333.

Meritt, D. A. 2008. Xenarthrans of the Paraguayan Chaco. In The biology of the Xenarthra, S. F. Vizcaino and W. J. Loughry (eds.). University Press of Florida. pp. 294-299.

Nowak, R. M. 1991. Walker’s mammals of the world. Vol.1. John Hopkins University Press, Baltimore

Vizcaíno, S. F. 2009 The teeth of “toothless”: novelties and key innovations in the evolution of xenarthrans (mammalia, Xenarthra). Paleobiology 35: 343-366.

Many plants absorb silica from the ground water and redeposit it in their cell walls and intercellular spaces. Phytoliths, from the Greeek phyto (plant) and lithos (stone), are rock hard and highly abrasive.   They provide structural support for plants and also serve to discourage herbivores and other attackers.

 Like concrete poured into a form, phytoliths take on the shape of their surrounding cells, varying with plant species and the type of tissue–bark, stem, fruit, etc.  They can be used to help identify their host plants long after all the organic material around them has decayed. Archaeologists use phytoliths to track the domestication of corn and other crops.   Paleoecologists look for them in deposits to help reconstruct ancient ecosystems. 

Phytoliths become embedded in teeth by chewing, as we found aplenty (left).   Bacteria can also cement them into tooth crevices. Extraction and identification of the phytoliths buried in a couple of our sloth’s teeth could provide some valuable insight into their diets, or at least the plants they ingested accidentally while munching on something nearby. That will entail scraping off the top layer of the tooth, dissolving the dentine and separating the plant-stones from the sand and grit to get accurate measurements and  an all-around view.  It promises to be a fruitful area for further study.  Thanks to Jonathan for the lesson. . . . Dave

In theory, femurs are simply columns supporting a TBD weight, subject to forces of tension and compression that engineers have understood for 2,000 years. Distal limb bones share the load in uncertain ways and can mislead–cranial measurements even more. Measurements directly related to the bone’s weight-bearing capacity (e.g. cross-section) promise the best results, but how do you measure something as irregular as a sloth femur? Femur length brings a much higher margin of error (Scott, 1990).

Our formula, like many others, is based on correlations derived from ungulates, for which there is a wide sample, but is that baseline relevant to ground sloths? The standard estimating error is 42%, applied to other ungulates. In other words, if our sloth were a moose, and we knew for sure ground sloths were strictly quadrupedal, didn’t have large weight-bearing tails, and their mass was distributed like the average mammal (i.e. 60% on the forefeet and 40% on hind feet, Alexander, 1985), then we could be 95% sure our sloth weighed between 453 and 5,205 pounds.

A baseline drawn from living Xenarthrans is hopeless for narrowing the estimate. We’re trying to cross a chasm of million years of separate evolution, significant disparities in habitat and life-style, and a huge size difference. . . . We might average estimates derived from other bones, but given the scarcity of fossils, the chance of finding that same combination of bones at another site is virtually nil. Comparing different animals using different bones would only increase the expected error. For all of its flaws, Greg’s methodology is still the best option. (sloth photo borrowed from)

Why bother when the potential for error is obviously so great? Paleoecologists can draw a wide range of fundamental conclusions from an estimate of an animal’s mass including metabolic rate, food intake, foraging time, forage quality and retention time, home range size, social patterns, population density, gestation period, litter size, life span, etc. (Peters, 1983; Schmidt-Nielsen, 1984). These are especially insightful for megaherbivores (mass > 1 ton), like ground sloths, with their special challenges and opportunities (Owen-Smith, 1988).

Until we invent a time machine there’s no way of determining how accurate the 2,829 pound-estimate is for our adult. Greg’s number feels right, but its greatest value may be in comparing this specimen to other Megalonyx specimens, and not in the absolute number. The best answer to the query, “How much did it (they) weigh?” may be, “about as much as a small elephant.” Disappointingly imprecise, I know, but as clear a picture as we have . . . . Dave

References

Alexander, R. M. 1985. Mechanics of posture and gait of some large dinosaurs. Zoological Journal of the Linnean Society 83: 1-25.

Owen-Smith, R. N. 1988. Megaherbivores: The influence of very large body size on ecology. Cambridge University Press, Cambridge.

Peters, R.H. 1983. The ecological implications of body size. Cambridge University Press, Cambridge.

Scott, K. 1990. Postcranial dimensions of ungulates as predictors of body mass. In Body Size in Mammalian Paleobiology: Estimation and Biological Implications. J. Damuth and B.J. McFadden (eds.). Cambridge University Press, Cambridge.

Schmidt-Nielsen, K. 1984. Scaling: Why is animal size so important. Cambridge University Press, Cambridge.

The folks at ICLIC who produced the CT scans that we posted last month showed us more of their amazing capabilities and produced a Quicktime video of the femur  that allows viewers to turn the bone and see it from all sides. Thanks again to Dr. Eric Hoffman, Jered Sieren, and Youbing Yin at ICLIC and Jason Bertram, UI Museum of Natural History webmaster and Informatics major, School of Library and Information Science,  who formatted and compressed this file for the blog. More bone-movies will be coming soon.   

femur movie  [file size= 2MB]  use your mouse or arrow keys to turn

This is the left femur.   The large ball at the top connects with the hip, of course, and the knee is at the bottom. Turn the bone and notice the shaft isn’t round like the bones of most other mammals.  That’s normal for sloths, not the result of burial or some mishap. The shape may be an adaptation to a tripodal stance using the muscular tail as a brace to stand upright (Fariña,  Vizcaíno and Bargo, 1998), or it may just be phylogenetic, i.e. an ancestral  hold-over (ibid.).  The rugosity or rough texture of the surface is typical for ground sloths too, greatly increasing the attachment points for the huge leg muscles.

Notice the epiphyseal ring that partially encircles the head. That’s a growth plane for this end of the bone.  The fact that it hasn’t fused completely yet indicates our sloth was still growing. If the sloth were younger this ring would be more pronounced and circle the entire head. Different bones/ends stop growing at different times.  That’s one way scientists can determine an animal’s age–it may be the only way for adult sloths given their ever-growing teeth.  Recovering an assortment of bones from three sloths of different ages is the reason Greg calls the Tarkio Valley site a Rosetta Stone for understanding Megalonyx development (pers. comm.).

The head of the femur points upward at about 35° putting the legs closer together and under the center of gravity (McDonald, 1977).    That’s an important clue for scientists trying to determine how Megalonyx moved and whether it walked on four legs, or two. The head angles forward at about 45° matching the backward-pointing direction of the hip socket (ibid.), giving sloths a wide, knees-apart stance. Don’t tell John Wayne he stood like a sloth. . . those are fighting words pardner.

For the weight estimate Greg used the formula log₁₀ mass = 3.4855 x log₁₀ femur length (in centimeters) -2.9112 (formula F1 in  Fariña et al., 1998 and Scott, 1990).  The length of the femur and body mass of armadillos, sloth relatives, scale reasonably close to those of the average mammal (Fariña and Vizcaino, 1997), and femurs have been recovered from a fair number of ground sloths.  Still, estimating the mass of an extinct animal from a single bone is an extreme act of faith, especially when it’s a ground sloth.   This specific formula, like many allometric equations, is derived from ungulates, and one applies it to other taxa with some peril, especially Xenarthrans which are defined by their unique bone structure.

Still scientists persist because an animal’s mass can tell us so much about how it lived.  Focusing on one element and one unambiguous measurement reduces the errors that would come from using different bones and techniques with their corresponding formulas. Greg’s approach is simple and straight-forward and provides a consistent baseline for comparing different species of sloths and identifying trends in size over time.  The formula has been adopted by sloth scientists here and in South America, and   it’s the basis for the 3,070 pound estimate that we cited for our Paramylodon (McDonald, 2005).  Next time:  Shooting in the dark:  the fine print of the weight estimate . . . . Dave

References

Fariña, R.A., Vizcaíno, S.F. 1997.  Allometry of the bones of living and extinct armadillos (Xenarthra, Dasypoda).  Zeitschrift für Saugetierekunde 62: 65-70.

Fariña, R.A., Vizcaíno, S.F. and Bargo, M. S. 1998.  Body mass estimations in Lujanian (Late Pleistocene-Early Holocene of South America) mammal megafauna.  Mastozoología Neotropical 5: 87-108.

McDonald, H. G.  1977.  Description of the osteology of the extinct gravigrade Edentate Megalonyx with observations on its ontogeny, phylogeny and functional anatomy. Unpublished M.S. thesis, University of Florida.

McDonald, H. G. 2005.  Paleoecology of extinct Xenarthrans and the Great American Biotic Interchange.  Bulletin of the Florida Museum of Natural History 45: 313-333.

Scott,  K. 1990.  Postcranial dimensions of ungulates as predictors of body mass.  In Body Size in Mammalian Paleobiology:  Estimation and Biological Implications.  J. Damuth and B.J. McFadden (eds.).  Cambridge University Press, Cambridge.

In this part of the world fingernail clams are found in all kinds of freshwater—ponds, lakes, rivers and creeks of all sizes; in silt, mud, sand and gravel. Sphaeriidae bury themselves in the bottom and don’t move around much, unlike snails, so they generally serve as good indicators of the habitat (Herrington and Taylor, 1958). Some are highly sensitive to their environment and make excellent environmental barometers of water quality and other environmental conditions (Kilgour and Mackie, 1991).  If we can identify the species that will establish  its habitat requirements and help define some specific conditions  at the site when they were alive (e.g.water chemistry, hydrology, sediments, etc.).  It’s a particularly difficult family to classify though–we’ll need a specialist.   There are more  fingernail clams in our sample—that could be useful too.  Funk and Reckendorfer (2008) report a correlation  between the shell variation within a population to the temporal and spatial variability of the environment, with stable homogenous environments showing less variability. Stay tuned, we could have a lot of fun with these!

 Holmes also uncovered some large mussels—Unio sized in the sample. They are highly degraded though and very fragile.  Identifying them will also be difficult.  If we can though it will help us to further delimit the water and bottom conditions when they were alive

There are a couple of exciting conclusions we can draw from the presence of these bivalves:  1) The clams grew here and weren’t carried in by the current. They are buried in the same silty clay we find everywhere else at the site indicating the velocity of the water was very slow—not fast enough to transport these far, even with their sail-like shapes—especially the big shells.  2) The fingernail clams are full-grow, they are not the juveniles of  fresh water mussels (naiads). They generally live 1-3 years.   Were the sloths wading through a soggy wetland or in the shallows of a lake when they met their end?   Could this area dry out every summer and the clams still survive?  How long can clams “hold their breaths?”Answers to these questions will take us a giant step toward defining another dimension of the sloths’ habitat. . . . Dave 

References

Funk, A. and Reckendorfer, W. 2008.  Environmental heterogeneity and morphological variability in Pisidium subtruncatum (Sphaeriidae, Bivalvia). International Review of Hydrobiology 93: 188-199.

Herrington, H.B. and Taylor, D.W. 1958.  Pliocene and Pleistocene Sphaeriidae (Pelecypoda) from the central United States.  Occasional Papers No. 596, University of Michigan Museum of Zoology,  29 pp.

Kilgour, B.W. amd Mackie, G.L. 1991. Relationship between demographic features of a pill clam (Pisidium casertanum) and environmental variables. Journal of the North American Benthological Society 10: 68-80.

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

Most of what we know about upland residents, like Megalonyx, comes from their lowland visits.  The probability of their preservation is directly correlated to the frequency and duration of their lowland sojourns. Some animals visit daily to drink. Others are only present seasonally (e.g. maternity herds) or annually (during migration). We should ask what brought our sloths to water.  When we find the fossils of an upland animal here we often wind up interpreting the environment in which the animal died, not the environment in which it lived. We will certainly learn a lot about where the sloths died . . . the challenge will be figuring out how they lived.  

Holmes

 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. 

Best estimates put the weight of adult dire wolves at about 150 lbs. (Lange, 2002).  Sabertooth cats weighed in at 660 lbs. (Barton et al., 2002.   In other words, these two formidable predators weighed just 6% and 25% of the average Megalonyx.  Putting that in a human perspective, that’s a predator of 10 lbs. and 45 lbs. respectively — house cat and dalmation-sized, assuming an adult human weight of 175 pounds–nothing to lose any sleep over.

There’s some debate about whether the sabertooth was a solitary or social hunter, but one thing is sure—any predator that hunted Megalonyx needed lots of help.  That’s important because Haynes says packs of predators eat in highly predictable ways.  Next post:  Haynes’s signs of predation.   Dave