Info

(Converting m3 to kg) Step 2. M = (71 m3 x 1000 kg/m3)(1.0 m/s) Step 3. M = 71,000 kg x m/s

These calculations pertain to how dinosaurs or parts of them were moved, because such movement would have taken momentum. For example, a dinosaur such as our unfortunate example in the previous section, which weighed about 3000 kg and died on an emergent floodplain, would have required a flow with a momentum greater than 3000 kg x m/s to move it 1.0 meter in 1.0 second. Even higher momentums would have been needed to move the same mass any appreciable distance from the death site of the dinosaur within a given time.

What evidence can we use as the basis for calculations of how fast a stream was flowing during the Jurassic, and how can we work out its cross-sectional area? Although exact values cannot be calculated, as with modern streams, the approximate speed and cross-sectional area of a Mesozoic stream can be calculated from indirect evidence preserved in the lithofacies. For calculating speed, numerous experiments conducted with modern sediments of different sizes and known densities have shown that certain velocities are needed to lift and otherwise transport sediments with these parameters. This information can then be applied to dinosaur parts. Additionally, distinctive bedforms are made in water by flows of a certain velocity as applied to sediments of certain grain sizes. Bedforms, such as ripple marks or dunes, are sediment bodies with a definable geometry. If distinctive bedforms are associated with dinosaur bones, they give paleontologists an estimate of what flow existed when the dinosaur parts were buried. This information may not tell us exactly how fast the water was flowing immediately before the parts were buried. However, in conjunction with the sizes and shapes of the dinosaur bones, bedforms can at least provide some limits for working out the competence of the flow, and

FIGURE 7.6 Sandstone bed formed from filling of a river channel and containing dinosaur bones near Morrison, Colorado.

thus the discharge and momentum. A bedform also can be a filled fluvial channel, which can give an approximation of the cross-sectional area of a stream (Fig. 7.6).

Dinosaur-dependent factors that affected movement of its body or body parts include the shapes and sizes of the parts being moved, as well as whether the dinosaur was recently dead (pre-putrefaction), dead for several weeks (during putrefaction), or mostly or entirely stripped of flesh (post-putrefaction), all of which affected its density. Density, in turn, is related to the buoyancy of a dinosaur's body or body parts once it was in a water body. Buoyancy, which is the lift force generated by a fluid (whether it is moving or not), is caused by pressure exerted by a liquid on all sides of an immersed object. This pressure makes an object weigh less while maintaining the same mass that it has in air. This principle, first articulated by Greek scientist Archimedes (of "Eureka!" fame, c. 287-212 bce), states that a floating or immersed body in a fluid will experience a lift force equal to the weight of the displaced fluid. For example, the immersion experiment conducted on dinosaur models in Chapter 1 showed a displacement of volumes for each model; these models had lift forces exerted on them equivalent to the weight of whatever water volume was displaced. Using a human body that weighs 64.8 kg as an example (see Eqn 1.2, Chapter 1), if it displaces 72,000 cm3 of water, the weight in water is

= 64,800 g - 72,000 g = -7200 g = -7.2 kg where Wi is immersed weight. A negative number in the final value means that a person should float when immersed in water, because they weigh less than the mass of the water displaced. This is why objects less dense than water float: in the preceding example, the density is 0.9 g/cm3 (64,800 g/72,000 cm3), whereas fresh water has a standard density of 1.0 g/cm3. Conversely, an object with a density greater than 1.0 g/cm3 will sink, unless the water is made denser, which changes the buoyant force. For example, ocean water, which is saline (containing a relatively high concentration of dissolved elements) in comparison to fresh water, has more buoyant properties than fresh water because it is denser. The Great Salt Lake in Utah or the Dead Sea in Israel are more saline than ocean water, so these are accordingly more buoyant than ocean water. Buoyancy allows steel ships weighing thousands of tons to float. When those ships strike icebergs, the rapid influx of water subsequently increases their density, causing them to sink. Likewise, buoyancy is what caused some multi-metric-ton dinosaur carcasses to float, depending on the respective densities of the dinosaurs (or their parts) and the bodies of water carrying them.

The density of a typical vertebrate land animal may vary from about 0.7 to 1.1 g/cm3, meaning that some recently dead animals can float. However, their flotation might be similar to icebergs in that most of their bodies will be below the surface of the water. The implication here is that a recently dead dinosaur body would have dragged along the bottom of a river that was shallower than half of the dinosaur's body width (limbs included) because of its limited ability to float. Therefore, the dinosaur body would have behaved much like a sedimentary particle undergoing traction or saltation. In contrast, a bloated (post-putrefaction) carcass that retained large volumes of internal gases may have been much less than 1.0 g/cm3 (Fig. 7.5). As a result, more of the animal would have been above a stream surface in suspension, which would have provided the potential for it to move greater distances from its original site of death. This possibility allows for largely complete dinosaur bodies to have moved considerable distances but later accumulated in stagnant areas. For example, carcass flotation has been invoked as the cause of monospecific beds of remains for the theropod Coelophysis (Chapter 9) in Late Triassic fluvial deposits of New Mexico.

Yet another factor in dinosaur buoyancy is whether the dinosaur had feathers or not (Chapters 5, 9, and 15). Modern birds are relatively more buoyant than other animals because of air trapped between their feathers, as well as their less dense skeletal structures. Consequently, some birds can float for long distances in a water body until their feathers become waterlogged, which can finally cause sinking. Similar scenarios have been proposed for fossil birds, beginning in the Late Jurassic (Chapter 15). The occurrence of relatively small, feathered theropods in Cretaceous lake deposits is also suggestive of such floating and sinking to lake bottoms (Chapter 9).

Once they were floating, numerous dinosaur bodies in a stream also may have created "dinosaur jams," similar to log jams where felled trees clog a stream. Such jams may have contributed to massive deposits of nearly entire dinosaur skeletons, providing an explanation for the numerous, well-preserved sauropods (Chapter 10) and theropods (Chapter 9) seen in the Late Jurassic Morrison Formation at Dinosaur National Monument in Utah, as well as mixed assemblages of dinosaur remains in the Late Cretaceous Judith River Formation of Montana.

Figuring out the preceding scenarios for whole dinosaur bodies is relatively easy in comparison to another possibility, the movement of detached body parts with flesh or bone material. Just to simplify these possibilities, bone (without flesh) can be used as an example. Dinosaur bone was composed originally and primarily of the mineral dahllite (Chapters 5 and 8), which has a density of 3.15-3.2 g/cm3. This guarantees that this mineral by itself, even in ocean water, would have sunk. However, skeletal material is not composed entirely of solid mineral material: average

Was this article helpful?

+1 0

Post a comment