About David

A guy with a thing for Ice Age sloths.

Walk Like a Sloth–lesson 1: the atlas

Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

compassGetting Oriented


The longer, deeper-cupped joints oriented up and down are anterior (skull end), the smaller, flatter more oval joints oriented more laterally are the posterior (tail end).  Now you just need to determine the back or dorsal surface from the ventral surface, to see which way the animal is facing.  That’s easily done by looking for the facet inside the vertebral canal on the posterior end.  That’s “down” or ventral and serves to accommodate a bony projection, the odontoid process of the next vertebra, the axis. The flattened wings projecting from the sides of the atlas are called the transverse processes.  Note they are much rougher on their dorsal (top) surface, serving to anchor the muscles used to help raise and turn the head.



SLOTH ATLAS, view from anterior or head end



HUMAN ATLAS, view from anterior or head end (Clipart courtesy FCIT 



SLOTH ATLAS, view from posterior






HUMAN ATLAS, view from posterior or tail end (Clipart courtesy FCIT) 

Key points

#1 The atlas is the first of seven (7) cervical or neck vertebrae. It is the only vertebra that has no centrum, the bony round weight-bearing block that is the largest portion of all other vertebrae.

#2 The atlas is basically just a large ring that attaches the skull to the backbone and gives animals the ability to nod their heads up and down by rolling the skull on the front end (nodding “yes,” hence the “yes joint”). The skull turns right and left by sliding around the second neck vertebra, the axis, on the back end (shaking “no,” hence the “no joint”).   Note the yes sockets are deeply concave (no lateral play in this joint) while the no sockets are relatively flat allowing sloths, like us, to turn their heads from side to side in a wide arc—there’s a much broader side-to-side motion than up and down. . . the better to watch for danger over your shoulder.

#3 There’s no room in the atlas for a centrum for it needs a large central hole to accommodate the spinal cord at its thickest point—where it exits the brain. [Compare this to the size of the spinal cord down at the back end.]  The large holes along each side of the atlas, snaking over and through the transverse processes, are called the transverse foramen—these canals are found only in cervical vertebrae. They protect the vertebral arteries that transport blood to the brain.  Two smaller foramina on each side allow passage of blood vessels into the nerve cord and the bone.greek atlas


 addedinfoAdditional information

The atlas is named for the Greek god Atlas who was tricked by Hercules to hold up the heavens.  He is commonly portrayed supporting the Earth on his shoulders.

Early observers were struck by the great length, width and thickness of the sloth’s  transverse processes–evidence of great muscular forces moving the head. (Owen, 1842)  Also note the roughness of the dorsal anterior surface of the atlas.  The ridges anchor one of the muscles that helps raise the head.  A sloth’s head, like ours, is heavy in proportion to the body so these muscles need to be large and strong.  Most of the head-lifting is done by the larger nuchal muscles running from the other cervical vertebrae, collar bones, ribs and shoulder blades to the back of the skull.
The broad rough surface on the dorsal (top) side of the transverse processes serves to anchor additional muscles for raising the head as well as some used to help turn the head which attach to the axis.   The muscles

image borrowed from Wikipedia

image borrowed from Wikipedia

used to lower or nod the head, the prevertebral muscles, are much smaller. Two of these attach to the underside of the transverse processes.   Most mammals let gravity do most of the work of lowering the head

(which is why our head drops when we nod off) so their prevertebral muscles are relatively small.

The sabre-toothed cat is a notable exception.  Their prevertebral muscles are much more developed to aid in stabbing.



Things to do

Move-your-head-like-a sloth.  Use the prototype to see how the joints of the atlas work to allow sloths toleidy1853 turn and move their heads “yes” and “no.” [Suggestion:  use your knuckles on a couple of fingers to represent the joint with the skull, occipital condyles.]

In life, the vertebral column is wrapped in a tough elastic covering called intervertebral substance which acts like a spring holding the vertebrae together and bringing them back into alignment as you turn and move. Springing saves energy–15%, or more, so animals don’t have to use as much muscle power to hold their spines in position. (Hildebrand, 1985)  But the atlas lacks the intervertebral substance—it is attached to the head and the axis with only ordinary cartilage joints and ligaments, allowing the atlas and skull to turn in any direction without resistance.  Imagine how much more work it would be to turn your head if the atlas was wrapped in the tough elastic that covers the other vertebrae.  [Try turning your head as though it were held by a giant rubber band (with straining sound effects of course!.  Now let it spring back to the center (“boingggg.”)]

atlas flexm
Flex Your Muscles
The human atlas offers little leverage for moving the head, so most of the work is done by larger and more superficial groups of muscles attached to the torso.  However, the deep small muscles serve an essential function (besides ensuring your head doesn’t fall off)—they constantly relay information to the brain about the position of your head and neck.  This so called proprioceptive function may be more important than their contracting ability. (Aiello and Dean, 1990).  Close your eyes.  Move your head.  You don’t need your eyes open to sense when your head is level.  Your inner ears are helping too.


thinkaboutThings to think about

Why are atlases and skulls so often missing at fossil sites? As a consequence of the lack of intervertebral “elastic” covering the atlas, the head is only loosely attached to the rest of the body.  After death, the heavy skull and atlas are often the first bones to become separated from the skeleton, especially in water.  In contrast, the rest of the vertebrae are often the last bones to separate or disarticulateFinding the adult sloth’s skull at the fossil site is strong evidence the animal decayed where it died, and probably lived there–the sloths weren’t swept there by a flood, for example.  Having some confidence the animals lived where they were found means the other fossils (e.g. seeds, pollen, turtle bones, etc.) found with the bones are evidence of the sloths’ habitat and wider ecosystem.  What could these other fossils tells us? [Answer: nearby geography, plants, climate, season, etc.]


futureFuture research

Scientists are using morphometrics, a science measuring subtle differences in the shapes of bones in different species, to study the atlas and help identify different forms of locomotion. In theory, the head of a quadruped hangs from its neck requiring more robust muscular support and atlas bone structure than an animal with an upright posture (e.g. a biped, like humans) where the head is simply balanced on top. (Manfreda et al., 2006)



Atlases are different for every species of mammal, but all have a similar and unmistakable shape because they all serve the same basic purpose—allowing the skull to turn freely.   Whether the similarity between humans and sloths is related to a common posture and mode of locomotion, or an adaptation to something else is not known and the answer awaits further research.  By itself the atlas can’t tell scientists if Megalonyx is bipedal, but the similarities with the human atlas are striking.


To learn more about or to borrow the University of Iowa Museum of Natural History Geo-2-Go Discovery Trunks  call or contact the museum.



Aiello, L. and Dean, C. 1990.  Introduction to Human Evolutionary Anatomy. Academic Press Limited.  San Diego, CA

Educational Technology Clearinghouse, Florida Center for Instructional Technology, College of Education, University of Southern Florida http://etc.usf.edu/clipart/

Hildebrand, M. 1985.  Walking and running.  In Functional Vertebrate Morphology.  M. Hildebrand, D. M. Bramble, K. F. Liem and D. B. Wake (eds.) Harvard University Press,  Cambridge, MA.

Manfreda, E., Mitteroecker, P., Bookstein, F. L., and Schaefer, K. 2006. Functional morphology of the first cervical vertebra in humans and nonhuman primates.  The Anatomical Record Part B:  New Anat.): 2898: 184-194.



SEM phytolith screening

mounting2 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.

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Preliminary stratigraphic report

Joe Artz  sent us the following report concerning the clay layers we observed Saturday.  A 10,000 B.C .  or early 20th century flood shortly after the stream was straightened (ca. 1917-1923) would explain why we didn’t find any bones in what appeared to be such promising ground.

joe ripples466Holmes,

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.

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Site prep a success

Everything is going according to plan.  The water level is about as  low as we’ve ever seen it.    Will cleared a lens shaped island in the middle of the creek Friday–approximately  30 ft. long and 15 ft. wide.  Work went fast, we’re out from behind the sand bags and didn’t have the usual 15 inches of muck to tend with.   We’ve dug up part of this area before but sections were covered by the berm and look promising.  Bob Athen is optimistic.  30% chance of rain later today.   Photos next week. . . . Dave

DNA tests update

We received an urgent inquiry from Andy last week asking if we had used shellac on the ankle bone that we sent him. . . he was having problems dissolving the sample.  Standard procedure after cleaning and grinding up the bone  is soaking the fragments in ethylene diamine tetra acetic acid (EDTA) which dissolves the calcium and frees the DNA inside.   We assured Andy we hadn’t used shellac or anything else, anticipating his tests and not wanting to introduce a  foreign chemical that could interfere.   We suggested the mystery coating might be one of the most impregnable known to man—greasy fingerprints and peanut butter and jelly.  The bone is sturdy and it has been handled by at least 50,000 children!   He went back and remixed his solutions. From the email we received at the end of the week, it looks like we’re moving forward again:

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Posted in DNA

Ancient DNA Centre photos

andylab1The experience Andy and his colleagues are getting with analyzing ancient bones, and using clean room facilities, body suits, facemasks, etc. to avoid contamination are the very skills that will be needed in twenty years when astronauts return from Mars  with samples to test for traces of ancient life.  [previous post: Ancient DNA Centre]

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

andylab2 andylab3 andylab4

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DNA tests continue

We sent a bone sample for DNA testing to the McMaster Ancient DNA Centre, McMaster University, Hamilton, Ontario two weeks ago.   Andy Clack, a PhD student in the Centre, sent this encouraging reply:


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.

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Another successful dig

group_2339A foray to the site August 14-15 netted 16 more segments of bone through the heroic efforts of  20 volunteers.      Most of the elements appear to be portions of juvenile ribs.  Positive assignment to the “toddler” or “baby”  awaits cleaning and a closer look. A couple of unidentified pieces are intriguing, but time for cleaning and study was curtailed by the storms that moved through the region all day.

Holmes and I were accompanied on the trip out by Meghann Mahoney, UI Museum of Natural History and Jan Ailes, Education Facilitator, Indian Creek Nature Center, Cedar Rapids.   We were joined in preparing the site for Saturday’s dig by our long-time Bobcat operator Will Mott, Council Bluffs, Bill Wiechman from the Greater Shenadoah Historical Society,  and Mary Brenzel the co-PI’s sister, who drove up from Fayetteville, Arkansas to help.

 Jan_Mary Meghann_Jan snapping_turtle Holmes_siteprep Bob_Athen bailing digging_start  final_bones break-time

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