Placed here

Group One: High A/V and cancellous/compact ratios (low density); moved easily by flows with low momentum. Costae (ribs) Vertebrae Sternum Scapula Phalanges Ulna

Group Two: Low A/V ratio and intermediate cancellous/compact ratio; moved by flows with intermediate momentum. Femur Tibia Humerus Ischium Ilium Pubis Radius

Group Three: Low A/V and cancellous/compact ratios; moved by flows with high momentum. Mandible

Skull bones (parietals, frontals, etc.) Teeth

Modified from data by Voorhies (1969).

mentioned is evidence of abrasion on dinosaur bones that were transported through traction or saltation. Another clue may be gained by looking for a concentration of bones at the base of a filled channel structure, which indicates that less dense sediments were removed by flowing water. However, probably the best evidence for answering this question is gained through looking at the orientation of the bones, especially bones with appreciable lengths.

Water or air currents with sufficient momentum to move objects will tend to orientate long objects parallel to the predominant direction of the original flow, whether the flow was from water or air. Maps of bone quarries, such as the Cleveland-Lloyd or Howe quarries (Chapter 3), are extremely important in this respect because they show the distribution of skeletal parts in at least two dimensions. In such maps, long skeletal elements can be counted and plotted with vectors on a rose diagram. A rose diagram is so-called because a variety of directions indicated by the data can cause it to resemble a flower with petals. These diagrams can immediately show whether the bones have a preferred orientation, which may be a result of a unidirectional current flow (the simplest hypothesis). A rose diagram showing much scatter and no clearly definable, preferred direction to the bones may be a result of many different current flows, no currents, or redistribution of the bones by either biological or physical processes after they were initially deposited (Fig. 7.7). Indeed, some dinosaur remains show well-defined orientations that, in association with diagnostic physical sedimentary structures, suggest current activity. In contrast, other

FIGURE 7.7 Hypothetical rose diagrams of bone orientations from one flow direction (left) and variable flow directions (right) with femur used to point in direction of flow.

FIGURE 7.7 Hypothetical rose diagrams of bone orientations from one flow direction (left) and variable flow directions (right) with femur used to point in direction of flow.

accumulations have random distributions that show no evidence of a current. The latter type of deposit has been hypothesized as a product of gradual accumulation of bones in watering holes or quicksand, which has been suggested for the unusually large assemblage of Allosaurus remains in the Cleveland-Lloyd Quarry of Utah (Chapters 3 and 9). Adding another dimension (vertical) to the bone distributions can test this hypothesis. If some long bones, such as tibias and femurs, have vertical orientations, this may constitute evidence of trampling by other dinosaurs in the area that caused the bones to go from a horizontal to a more vertical position

Maps of bone distributions can give other important clues about the sedimentary situation responsible for a bone accumulation. For example, when the Howe Quarry was mapped, with its abundant bones contributed by many dinosaurs of several different species, a semi-circular pattern to the deposit became evident upon excavation (Fig. 3.11). Based on this map view, one explanation for this distribution is that the deposit represents a crevasse splay, which is a sediment body formed outside the levee of a stream from a narrow breach of the levee (Fig. 7.8). In this scenario, a river would have been carrying a variety of dinosaur remains, then the flow broke through the levee of river and the remains were deposited in the temporary channel and small "delta" that resulted from the breach.

In summary, hypotheses applied to interpreting the transport of dinosaur remains are testable through examination of at least the following information:

0 0

Post a comment