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

photo courtesy of Thalia Sutton

Thanks to Thalia  for sending in these photos of the September excavation.  Lots more in Flickr.

Holmes,

We encountered three stratigraphic units (SU’s)–for convenience I’ll call these SU’s 1 through 3, in order of ascending age. All three are channel facies, meaning that they have sedimentary characteristics of having been deposited by swifter currents of water than were encountered in the blue clay where the sloth remains were found.

SU3 is a channel facies that is perhaps correlated with the sloth-bearing (slotheriferous?) blue clay. The blue clay represents a slackwater facies, where clayey sediments settled from suspension in a low energy environment with only very slight currents represented by micromorphologically visible laminations and oriented sand grains. In SU3, the blue-gray colored sediment matrix has sufficient very fine sand that it will not ribbon. There are also macroscopically-visible, discontinuous, laminations and lenses of fine to medium sand with rare pebbles. These materials were deposited near, and possibly within the stream channel.

 SU2 overlies SU3. It is a grayish brown silt loam that is finely laminated. In the south half of the island, SU2 has an abrupt, unconformable boundary with SU3, and appears to fill a small (ca. 1.5 wide by 30 cm deep) trough incised into SU3. This is perhaps an overflow channel scoured by floods and filled with more oxidized sediment that was probably reworked from a nearby, better drained sedimentary facies. The abrupt SU2-SU3 contact fades to the north in the profile, and in the north part of the island, has a conformable contact with SU3. This suggests that SU2 may be contemporaneous with the upper part of SU3. The SU2-SU3 contact trended northwest across the excavated surface, and seems to have been thicker in the west part of the excavated area of the island.

SU1 was encountered along the north side of the island. It is a laminated blue gray loam or very fine sandy loam. It is similar to SU3, but sandier, less consolidated, with more distinct laminations in the lower part. It yielded finely-divided flecks of bone and small bits of wood, and Euroamerican ceramics (undecorated whiteware). The latter suggests a historic channel fill deposit, perhaps of the 1917 excavated channel, or a post 1917 channel. At the south end of where we’d excavated, SU3 did have a very abrupt contact with what looked like SU2, which clearly indicates a historic age.

Thanks again,

Joe

I’ve long pondered the  improbability of 4 ft. of clay accumulating without a break–500 years (?) without a flood?! It’s comforting to find some evidence finally that they were indeed occurring. I expected a more dramatic signature–sand and rocks–but  Joe says the small differences seen here aren’t unusual for western Iowa at this time.  Flood water transports what’s available, and several thousand years of wind-borne deposits left a thick unstable blanket of loess in the region.  The hills literally melted away with the rain and spring thaws and for a long time the Tarkio was thick with loess sludge and not much else.   Thanks Joe for a vivid new picture of the Valley. . . . Dave

 

 

 

 

 

 

 

 

 

 

 

 

 

 The rain gods smiled on us all day Saturday, keeping thunderstorms west  of the site and downstream until nightfall.  No bones were found, but good fortune located the dig  in a section of the ancient Tarkio streambed unlike anything we’ve uncovered previously and the excavation produced a wealth of new information about the history of the watershed  thanks to the efforts of geoarchaeologist Joe Artz, Geospatial Program Director in the  Office of the State Archaeologist who assisted us on the dig.

A large crew of enthusiastic volunteers cleared the  target area quickly and then dug several deep trenches into our clay deposit for Joe to examine.  He used the walls like a professor with a blackboard tracing the evidence of the long and often violent history of erosion and deposition in the Tarkio Valley.

Slight color variations of the sediment and subtle differences in sand and clay content confirm a story that contrasts sharply with our long-held impression of placid water in the sloths’ valley.  The evidence points to multiple floods  cutting deeply into the  clay on many occasions.  

The Tarkio Valley has often been called  a  “hungry canyon” for its soil-devouring proclivities, but who would have guessed that history went back over 10,000 years!    More photos and Joe’s preliminary report tomorrow. . . Dave

Hey Holmes,

Acratocnus Ye was discovered in a sinkhole in Haiti in 1984, and subsequently at several other sites in the country.  “Ye,” pronounced “yeh,” is a Hatian Creole word meaning  “yesterday,” hence the common name: Yesterday’s Acratocnus. (MacPhee et al., 2000)  Radiocarbon dates show the Megalonychid sloths survived in Haiti until about 4,500 years ago when humans arrived on the island. (Steadman et al., 2005)

If Beth and Andy get DNA from the Acratocnus ye sample and fail with ours we can assume there’s probably no DNA in the ankle bone, but that doesn’t necessarily mean things are hopeless.  Conditions vary tremendously around the site and DNA preservation may have been favored in a different location.  We may have to explore trying some inexpensive bone screening tests to identify candidates for further analysis.

When you are doing a destructive test on an irreplaceable sample it’s comforting  to know  you have people like the staff at the McMaster Ancient DNA Centre  and the  Ancient DNA Lab at Penn State who are checking every step 3X before they proceed. . . . Dave

References

 MacPhee, R. D. E., White, Woods, C. A. 2000.  New Megalonychid sloths (Phyllophaga, Xenarthra) from the Quaternary of Hispaniola.  American Museum Novitates 3303:  1-32.

 Steadman, D. W., Martin, P. S., MacPhee, R. D. E., Jull, A. J. T., McDonald, H. G., Woods, C. A., Iturralde-Vinent, M., and Hodgins, G. W. L. 2005.  Asynchronous extinction of late Quaternary sloths on continents and islands.  Proceedings of the National Academy of Science 102: 11763-11768 

Then, as now, the challenge will be uncovering the biomolecular markers preserved inside and proving any positive findings derive from the sample and not mishandling.  So we study our sloths to better understand them and the effects of global climate change, and prepare for the future. . . and maybe to visit the stars. . . . Dave

Holmes,

I have the talus/ankle bone in the lab now… wow! That thing is like a rock.  I couldn’t ask for a better specimen!  I’m going to use a sterile dremel tool and a particular type of bit that generates less heat and produces shards of bone and not powder.

I’ll try to drill in about 1.5 inches, collect the powder, decalcify and then hit it with proK to kill any lingering proteins in there.  I’m going to make up fresh reagents this week.  I want everything to be right.  Will likely test for nuclear and mt DNA fragments next weekend.

It’s all about the specimens on your end, and you really came through with this one…. I won’t hurt it too bad, I promise.

Best,
AC

  Andy found all kinds of  DNA in the first samples we sent–horse, pig, cow, deer, human, etc.  No sloth DNA though.  The bones were small and broken–just fragments.  They had probably been soaking up rural Iowa DNA for years.  Now Andy has the thickest, densest, unbroken bone we have. The ICLIC people did a CT scan and it looks perfect inside too.

 Cloning DNA has become routine–the stuff of 7^th grade science classes, but contamination remains the biggest challenge of ancient DNA study. DNA is everywhere—people are constantly shedding it in dry skin cells, etc., not to mention what they carry around belonging to family members, pets, etc.,  and the microbes that live everywhere.  Chemical changes, water and warm temps begin breaking down DNA after death, while a host of decay organisms feed on the organic molecules and each other.  Retrieving verifiable ancient sequences from this rich DNA stew  after thousands of years is a major achievement.  Many an announcement of an ancient DNA discovery has been withdrawn when later analysis showed it merely to be the result of contamination.  For that reason special labs like the Ancient DNA Centre have been created with personnel trained in techniques designed to reduce the risk of errors.   No one has ever sequenced any part of the Megalonyx genome, but if anyone can do it, Andy can. . . . Dave