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Wednesday, July 23, 2014

Fragments of the Fossil Record: "Thigh Bone Disconnected from the Hip Bone..."

I still remember as though it was yesterday...

Like a camp of rogue militants being tracked, the secluded site was interrupted suddenly by an unidentified chopper closing in. Everyone scrambled as the downwind thrust of the propeller kicked up more dust than could be warded off by canvases on hand, and indignation grew for the uninvited guest. The engine faded while the researchers regained their sight, only to be met by an elderly man with an ornamented cane. What could he possibly want, worth jeopardizing the operation at hand? He merely wished to broker a business deal like none before; he sought to resurrect dry bones from the dust and bring imagination to life.

Such was the world's baptism into the cult of popular paleontology. Never again would we be unmoved by the sight of a dinosaur skeleton encased in solid rock, or fail to appreciate the paleontologist who reconnected our species with ancient forms of life. On that day in 1993, when Jurassic Park hit the big screen, we all learned something about digging for fossils. But this movie did for geology what Indiana Jones had done for archaeology: exaggerated the discipline to make it exciting and accessible to all of us. Therefore, I hope it's not too late to unlearn something about digging for fossils—namely, that skeletons regularly appear intact.

The Fragmented Nature of the Fossil Record

No doubt, Dr. John "Jack" Horner of Montana State University, on whom the character of Dr. Alan Grant is loosely based, has personally seen a few nearly complete skeletons emerge from from dig sites around the world. But any paleontologist would be quick to point out that intact skeletal remains are by far the exception to the rule. We have been spoiled as spectators by the perfectly preserved Archeopoteryx and the exceptional specimens of the Green River Formation. But for every set of bones found in 'life' position, there are thousands found disconnected, broken, weathered, and scattered throughout the sediments.

Example of a nearly complete juvenile dinosaur (Bolong yixianensis, a species of Iguandontia), found in northeastern China. Image is Figure 2 from Zheng et al. (2014).
Single tooth from a large crocodile.
Note penny for scale.
Several years ago, I had the privilege of assisting with vertebrate fossil collections from the Cretaceous rocks of Bryce Canyon National Park. In less than two months, we had recovered more than 11,000 individual fossils, of which precisely zero were 'intact'. Single specimens of teeth (fish, crocodile, shark, dinosaur), vertebrae, bony scales (especially gar fish), and turtle shell comprised the vast majority. Dinosaur bones were not uncommon, but in all but one case, the bone was so badly weathered down that we could not determine from which limb the fragment actually came (in this one case, the joint was preserved, so that the bone was recognizably a partial tibia). Perhaps the closest exception to this pattern was dense cluster of turtle shell. Of the dozens of individual fragments, we were able to piece together almost half of one turtle's carapace:

Lower carapace of a Late Cretaceous turtle, recovered from southern Utah.
The rarity of well preserved skeletons is explained by a subdiscipline of paleontology called taphonomy, which focuses on the conditions surrounding death and burial of a fossilized organism. Taphonomists analyze the morphological and chemical details of fossils, asking questions like "Was the organism exposed long at the surface or was the burial instant?" or "Was the water alkaline/acidic, oxic/anoxic, still or flowing?" These clues help to reconstruct the paleoevironments in which ancient life lived and ultimately died, aiding our understanding, for example, of how certain animals even behaved.

The vast majority of fossilized bone exhibits at least some evidence of weathering at the surface. Though fossilization commonly involves "rapid" burial, skeletons can and often did spend many days or weeks exposed to the elements before nature locked them away. Exceptional preservation sells well, both at the gift shop and in education, but it remains exceptional. Certain environments, like stratified, alkaline lakes provide the ideal conditions for complete, intact skeletons. But most organism underwent a more brutal decay process after death, which explains the imbalanced levels of preservation, particularly in terrestrial fossil site.

A taphonomic perspective on Young-Earth Creationism

Once we gain a proper perspective on the nature of the fossil record, it becomes clear why paleontologists continue to reject the proposals of so-called 'Flood Geology'. If the majority of fossil specimens were buried suddenly and catastrophically only ~5,000 years ago, then intact skeletons (even partial ones) should be widespread and common. While raging waters do have the potential to dismember fragile lifeforms, it is not to the extent that we actually find in sediments today: nearly every tooth pulled from the jaw, nearly every vertebrae unhinged from the next—basically Dem Bones sung in reverse. Connective tissues like cartilage are sufficiently strong to keep most skulls, limbs, and backbones together, as is evident in the tragic results of natural catastrophes today.

On the other hand, the abundance of weathered fragments of only the most resilient bones (vertebrae, teeth, shells) fits perfectly within the conventional framework of paleontology. The Cretaceous rocks of Bryce Canyon National Park, along with most terrestrial sites from which dinosaurs are recovered, were formed largely by rivers and floodplains. Sedimentary structures help us to interpret features like migrating dune formations, levee overflow, and clay-rich lowlands, where the bony remains of rotting corpses are most likely to accumulate. Perhaps you've hiked past the remains of something like a deer or a snake, where only a pile of disconnected bones remains. If so, you can imagine that in a typical river plain, occasional floods are capable of burying the recently deceased for fossilization. These remains will scarcely resemble that famed velociraptor from the badlands of Hollywood-staged Montana, but such is the true face of vertebrate paleontology and the fragmented nature of the fossil record.

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