densities for teeth, compact bone, and cancellous (porous) bone (Chapter 8) in modern vertebrates average about 2.0, 1.7, and 1.1 g/cm3, respectively. Consequently, movement of teeth and compact bone would have been more likely as bedload; such dragging would have caused visible pits and fractures in most exposed bone. Such telltale marks from the physical transport of dinosaur bones have indeed been interpreted among the bones in high-energy facies. This information provides evidence for whether dinosaur remains were reworked into deposits much younger than the time when a dinosaur was alive. However, if the bones had any flesh remaining, these parts might have been cushioned from the abrasive effects of stream transport, thus the absence of fractures is not necessarily diagnostic of an autochthonous fossil.

Most modern examples of bone are cancellous, which with included organic matter are less dense than solid dahllite; loss of the organic material results in more open spaces and correspondingly less density. Dinosaur bones were similar in this respect and some, such as those of theropods, were lightly built and noticeably less dense than those of other dinosaurs (Chapters 8 and 9). Different hard parts on the same individual could also have had different densities, such as the bone composing the parietals of pachycephalosaurs (Chapter 13), or the teeth of any toothed dinosaur versus their limb bones.

Size and shape of a body or bone are also important factors in transport. Well-rounded tarsals of dinosaurs, for example, were more likely to roll along a stream bottom than their femurs or tibias. Of course, smaller bones were more susceptible to transport, with all other factors in the bones and stream being equal. Nevertheless, shape is probably more important to consider than size, because equal density of a large or small body translates into equal buoyancy regardless of size. Shape can be measured by looking at the ratio of an object's surface area to its volume, which is expressed through the simple relation of

where S is shape, A is surface area, and V is volume. Using the example of a sphere, surface area is calculated by the following equation:

and volume for that same sphere is

Using a typical orange (before peeling or squeezing) with a diameter of 10 cm (radius of 5 cm) as an example, its surface area to volume ratio can be calculated through the following procedure:

This ratio is actually the smallest that can be derived for any sedimentary particle; any particle shape deviating from a perfect sphere will result in a larger number.

The important application of this measurement to stream transport is that spherical particles are less likely to be lifted by a current than long or flat particles. In the same way, Frisbees™ (which have a high A/V) can stay airborne longer than baseballs (low A/V). Thus, long, flat particles are lifted more easily than spherical particles because of an important principle first formulated by Swiss mathematician Daniel Bernoulli (1700-82). Bernoulli was a primary contributor to hydrodynamics, the physics of water flow, which is an important science to taphonomists interested in estimating transport of bodies in water. Bernoulli discovered that a moving fluid (either water or air) caused less pressure on an object than stagnant fluid, this lower pressure providing a lift force to the object affected by the flow. This principle is exemplified by wings on aircraft, which are designed so that the pressure caused by air moving rapidly over them is less on top, causing an aircraft to lift off the ground.

Of all of the bones mentioned in Chapter 5, none are spherical, which means that all dinosaur bones had higher A/V ratios than a sphere. Bones with the largest ratios were those that were long, flat, or both, such as some cranial bones (pari-etals, frontals), the femur, humerus, tibia, and scapula. Notice that a typical ilium is shaped more like an aeroplane wing than, say, a cervical vertebra. So an ilium was more likely to be lifted in a stream and transported far away from the original death site of a dinosaur than its semispherical parts.

Consequently, the densities, sizes, and shapes of bones varied enough that all of these factors have to be taken into account when looking at a final assemblage of dinosaur bones in a deposit. In fact, some taphonomists were industrious enough to experiment with various bones of modern vertebrates, calculating A/V ratios and proportion of compact to cancellous bone (which affects density) to categorize bones on the basis of how easily they could be transported by water. These data provide a hypothetical model to test when encountering dinosaur bones in the field and assessing their possible transport (Table 7.3).

Most of the preceding discussion on transport of dinosaur bodies was based on water as a medium, but wind was also a possible (albeit less probable) agent of transport. The physics of air and its movement is aerodynamics, an essential science for people who design and fly aircraft, but one that can also be applied to any effects of air movement on any objects. For example, modern hurricanes and tornadoes have carried large, multi-ton objects for considerable distances. Living animals also have been transported hundreds or thousands of meters away from their original environment. An example of the lift forces generated by some tornadoes is illustrated by the instance of a home freezer, which probably weighed about 200 kg, that was moved 2 km by a tornado in Mississippi in 1975, and a 70 metric-ton railroad car, which weighed more than most adult sauropods (Chapter 10), that was also moved a measurable distance by a tornado.

Storms have been interpreted in the geologic record on the basis of the distinctive deposits that they leave in marine and coastal sediments. Such storm deposits, called tempestites, are common in strata formed in shallow-marine environments from the Mesozoic, so dinosaurs certainly experienced violent storms. However, no one has ever provided evidence for transport of dinosaur bodies by wind, hence this is only an idea, not a hypothesis. Of course, observations of the impact of modern hurricanes, as well as interpreted Mesozoic tempestites, could lend themselves to the hypothesis that similar inland flooding occurred from the massive amounts of precipitation and coastal storm surges that accompanied Mesozoic hurricanes or other storms. These phenomena would have increased the amount of stream discharges and correspondingly increased the likelihood of dinosaurs either drowning or having their otherwise-dead bodies washed into water bodies and later buried.

How would a paleontologist look for clues of postmortem transport (or lack of it) once dinosaur bones are found in a Mesozoic deposit? One clue already

Was this article helpful?

0 0

Post a comment