Recent Research: What happened to my humerus?

My paleontological loss of innocence came when I was interviewing for graduate school. I was touring the fossil mammal collections at the Field Museum in Chicago and recall being disappointed by the many drawers that were filled with teeth and partial jaws. Where were the perfect skulls and complete skeletons I had grown up admiring at the University of Nebraska State Museum? Was this really the material upon which most paleontological studies were based?

A typical mammal fossil: a lower jaw with no sign of the rest of the skull or skeleton. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

Years later, I now know that teeth and jaws - and even isolated bones - are a lot more useful than they might appear. I also know that teeth are among the most commonly preserved vertebrate remains because enamel - their hard, shiny covering - is more durable than any other material in the body. So when I go to the field, I mostly expect to find teeth and jaws. Only less commonly do I find something else: a limb bone, a skull, or perhaps even an entire skeleton.

Why is it that sometimes only a jaw is preserved and at other times, something else? I’m often asked this question when I give public presentations, and it is fundamental to interpreting the history of life. The answers come from researchers interested in taphonomy, the study of the fossilization process: everything that happens to an animal after it dies until it is collected by a paleontologist. The basic taphonomic model I was taught is that there are two main ways to become a fossil: get buried quickly and end up as a complete specimen or die on the surface to decompose, be ravaged by scavengers and the elements, and end up as one or a few isolated (disarticulated) elements. (For examples of the latter, check out the great photos in Andy Farke’s recent post on taphonomy.)

But that isn’t the whole story. A recent paper caught my attention because it used a catchy phrase to describe a third way fossils can be preserved: “stick ‘n’ peel.” This type of preservation can result in a specimen that is remarkably complete in one aspect but equally incomplete in another. An example might be one that consists of a skull, a complete and articulated (in life position) arm and hand, and a few bones of the leg.

A dead flamingo awaiting the fossilization process. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

How does something like this occur? The “stick” part of the equation happens when the fluids of decomposition and resulting fiesta of microbes cause the carcass to adhere to the ground. The “peel” aspect comes later when some agent - a predator, a water current, a flood - removes the parts that aren’t stuck to the ground. Depending on many factors, including the type of animal in question and the position in which it died, “stick ‘n’ peel” preservation can result in certain parts of the body being preserved while others are “peeled” away.

How can one determine whether stick ‘n’ peel preservation might account for the bones present in a particular specimen? That can be tricky, because other taphonomic process can result in similar patterns of preservation. The presence of small, delicate bones - which are more easily scattered or washed away - and the absence of large, solid bones - which are more commonly preserved due to their large size - are usually good clues. I’ve found specimens like this in the field before but have never stopped to contemplate how such a strange combination of elements might arise. I am certain this explanation – and the catchy title - will ‘stick’ with me the next time someone asks me why I find certain bones and not others!

- Dr. Darin Croft is an Associate Professor at Case Western Reserve University in Cleveland, Ohio

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Posted: 9/5/2016 1:44:29 PM by croftdarinadmin | with 0 comments
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