UNIVERSO DE ATENCIÓN 1 Población universo

sive. Large excavations can potentially take an entire field season (usually summer) and require the services of heavy equipment for extraction and transport of the specimen. Furthermore, after transport and deposition of the fossil to a preparator, the preparation time for extracting a dinosaur skeleton may take more than a year. Dinosaur trace fossils, especially tracks (Chapter 7), typically do not require recov- ery, but some are taken from the field site for further study or display; entire beds containing the tracks may have to be moved thousands of kilometers.

To describe a typical recovery procedure, assume a dinosaur fossil consists of skeletal material. Upon identification of partially exposed and recovery-worthy skeletal remains, the area immediately surrounding the fossil is carefully cleaned. This action is followed by a full assessment of the horizontal and vertical extent of the skeleton, which normally involves mapping the distribution of the bones on a grid (Chap- ter 3). The orientation of any bones at the surface is noted; most may be flat-lying (parallel to bedding) and predictable in their extent, but others might project outside the exposed area below the surface. Erring on the side of caution is always good, even if it means carrying out too much rock for a small amount of fossil material. Of course, the person doing the recovery uses a scientific methodology: a prediction and its accompanying evidence determine the probable extent of the skeletal material. Then that person consults with any colleagues at the site to learn their estimations to seek consensus.

Any glues needed to keep bone fragments together are then applied. But choos- ing which glues are used should be left to a professional, as not just any glue should be applied to a 65+ Ma fossil! Excavation then begins on the area around the fos- sil. In some instances, the skeletal material (especially teeth or small vertebrae) may already be loose on the ground. Such material is placed into labeled sample bags after its distribution has been noted. Whether excavation is easy or difficult depends on the surrounding rock (Fig. 4.9). If the bones are in well-cemented sand- stones, jackhammers or backhoes are not unreasonable tools for breaking up the rock. Some rocks, such as a mudstone or poorly-cemented sandstone (Chapter 7), can be picked away with rock hammers, trowels, shovels, or other hand tools. The excavation should then proceed around the prescribed area and to the perceived maximum depth for the fossil (maybe a little more, to be safe). Once this depth is reached, the excavation starts to cut underneath the fossil, although not far enough so that it collapses. This procedure causes it and the surrounding rock to form a pedestal. Water-soaked paper towels or toilet paper are then placed on the pedestal to form a barrier between the fossil and the final surrounding layer. At this stage dry plaster of Paris is mixed with water for dipping strips of burlap, which are placed around the towel-enveloped pedestal as a jacket. New materials that are less dense and more cost-effective than plaster of Paris, yet not sacrificing strength, have been proposed in recent years, but many dinosaur workers still

FIGURE 4.9 (opposite) Steps in excavation of a vertebrate fossil, in this case a partially exposed skull and other bones of a metoposaur, a large amphibian that lived at the same time (and in this case, the same region) as early dinosaurs, Chinle Formation (Late Triassic), Arizona. (A) After cleaning the area, workers estimated the extent of the fossil and dug around the defined area. (B) Digging of the rock underneath the fossil established a pedestal. (C) One worker placed wet paper towels on the top to cushion and separate the fossil from the plaster. (D) Another worker placed the plaster-soaked burlap strips for the jacket all around the pedestal. The workers then waited until the next day for the plaster to have hardened before breaking the pedestal, turning over the rock, and jacketing the underside.

prefer the plaster of Paris method, which has been in use for more than 100 years (Chapter 3).

The specimen is left while the plaster hardens completely. Only then is the sup- port under the pedestal broken so that the fossil can be turned over carefully to apply the remainder of the jacket. For later cataloguing, important information about the fossil, such as the date collected, preliminary identification, specimen number, orientation (indicated by a north arrow), and location, are written on the jacket. This also keeps the fossil from being mixed up with other, similar-looking, jacketed specimens. The snug and safe fossil is now ready for transport out of the field area, carried by people on foot (if the specimen is small enough), in land-based vehicles, or in extreme cases by helicopter.

In a preparatory laboratory, a jacketed specimen is cut open and the excava- tion begins anew, with the goal of liberating the fossil from its surrounding rock (Fig. 4.10). A preparator will use human energy and a variety of tools to separate the fossil from its entombing sediments. Just as in the field, the amount of time taken to extract bones from rock depends on the cementation of the rock and fragility of the fossil. Skeletal material is also commonly fragmented, requiring the prepara- tor to handle each small piece with care so that paleontologists can re-assemble the pieces accurately later. Preparators are among the most patient and skilled people in paleontology, some operating with the precision of surgeons.

Once the dinosaur bones are prepared, they can be placed in dynamic public dis- plays, baring their teeth or bearing their young. However, most skeletal remains of dinosaurs return to dark quarters, tucked away in storage drawers or shelves for future research. Because of its great weight, real bone is rarely mounted in a museum; supporting these hard-earned but heavy specimens and keeping them from being damaged or vandalized is an expensive technical problem. Instead, casts are made from the original bones using artificial materials, such as fiberglass. These strong,

FIGURE 4.10 Pelvis of Apatosaurus from the Morrison Formation (Late Jurassic), western Colorado, still partially encased in its protective jacket and in a preparatory lab associated with the former Museum of Western Colorado, Grand Junction, Colorado. Notice the plastic model sauropod in the background, ready to help with estimating the weight of the original animal (Chapter 1).

lightweight replications of bones are much more amenable to mounting dino- saurs in poses that may not be based on strong scientific evidence but provide for much discussion (Fig. 4.11).

Other dinosaur fossils, such as a well-outlined nest with eggs or tracks in rapidly eroding (poorly cemented) rocks, might require recovery techniques that are sim- ilar to those for skeletal material. Eggshell material presents a special problem for recovery because the fragments can be so small and numerous that their scientific value might be limited (Chapter 8). Nevertheless, the recent resurgence of studies of dinosaur eggs and juveniles has helped increase recognition of such fossils, so they are now more likely to be recovered. In contrast, most dinosaur tracks are stud- ied in place, although casts are made occasionally with latex or some other mater- ial that does not damage the fossil. Natural casts of tracks (Chapter 14) might be loose and on their own, which encourages their collection for further study or use as display or teaching specimens. However, entire trackways are sometimes extracted, such as one from east Texas that was split into three parts: one part was taken to the American Museum of Natural History in New York, one to National Museum (Smithsonian) in Washington, and one to University of Texas in Austin. The part that went to the American Museum was then used in combination with skeletal mounts of analogous trackmakers for an imaginative linking of dinosaur body and trace fossil evidence.

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FIGURE 4.11 Tyrannosaurus rex mount, which uses

artificial casts of the bones and thus allows for the unusual pose of the display; Denver Museum of Science and Nature, Colorado. Author (imitating the pose in the foreground) for scale.

SUMMARY

Many tools, physical and mental, are required for dinosaur studies, so people who are serious about learning dinosaur paleontology must know what to take into the field, both in their backpacks and brains. In their backpack, they should carry a variety of tools that allow them to find, measure, and record information about dinosaur fossils safely, effici- ently, and effectively. Because dinosaurs are contained primarily

in rocks, a basic knowledge of geological principles is needed to under- stand the local setting for their fossils and the larger-scale factors that affected their distribution. Geological principles, such as original hori- zontality, superposition, cross-cutting relationships, lateral continuity, and inclusions, can be applied while driving by a road cut that exposes strata, or while hiking in a wilderness area, lending a new dimension to interpreting the natural history of an area. Similarly, identification of guide fossils can immediately indicate the relative age (era, period, or epoch) of the rocks in a particular area. The combination of these low-cost obser- vations with laboratory measurements of naturally decaying radioactive elements and their by-products gives a more complete picture of the immensity of geologic time. These observations also make allowances for demonstrating the considerable changes in dinosaurs and other fossil species through time. Knowing the mathematical and other scientifically-based reasoning behind radiometric age dating, as well as the cross-checks made through relative age dating methods, shows that the geologic time scale is based on reality and is a well-tested, accurate representation of the ages of rocks. Geologists and paleontologists use these dating methods every day, not just for finding and documenting dinosaur fossils but also espe- cially for prospecting for the minerals and fossil fuels that make possible the lifestyle choices of industrialized nations.

With some basic knowledge of paleontological and geological prin- ciples, as well as a lot of energy, dinosaur fossils are discovered, recovered, brought back, and studied by paleontologists for the public appreciation of their inherent knowledge and beauty. The education required to find and study dinosaur fossils is well worth the long hours of studying geo- logical principles, radiometric age dating, and plate tectonics that are nec- essary to form a more eclectic picture of dinosaur lives and afterlives. Finally, the existence of dinosaurs over 165 million years can be viewed against the background of the all-encompassing theory of plate tectonics, which would have affected dinosaur populations throughout most of the Mesozoic Era. Through the interactions of the lithosphere and astheno- sphere, plate movement is responsible for phenomena as diverse as earthquakes, volcanism, and the occurrence of island chains. Because plate tectonics causes the movement of continents either away from one another or closer together through the course of geologic time, it is the main driving force behind the proximity of continents.

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DISCUSSION QUESTIONS

1. Is one scientific field necessarily bet- ter than another? List what sciences have been men- tioned so far in this book and rank them in

order of what you perceive as most scientific to least scientific. What evidence do you have to justify such a list? Compare your list with other students in the class to see if they share your consensus.

2. Practice mapmaking in your everyday life, Draw a map for someone who needs directions and include the three features that should be on every map. Has someone ever given you poor verbal directions or a badly drawn map? If so, what would have helped to prevent the considerable time you spent being lost?

3. A stratigraphic sequence has a limestone bed at the base, which is overlain by a shale, which in turn is overlain by a sandstone; the entire sequence is cross-cut by a fault. For this sequence, what is the order (from oldest to youngest) of the geologic events represented? What if you find inclusions of the sandstone in the shale? Would you change your assessment, and if so, why?

4. You go to an area with dinosaur fossils and find fragments of Coelophysis, a theropod previously known only from the Late Triassic, in the same stratum that contains the remains of Allosaurus, a theropod only known from the Late Jurassic. What are at least two hypotheses to explain your observation? How can both be falsified? 5. How could a transgression occur without a change in sea level (which is caused by more water in the world’s oceans)? How could a regression occur without sea-level changes?

6. What is the minimum number of alpha decays that occurred between the parent element of 238_{U and the final stable daughter} element of 206_{Pb? How did you arrive at this number?}

7. How is compound interest in savings accounts similar to radiometric age dating? Provide mathematical proofs through some examples. 8. What alternative explanations could account for the sameness of age

dates derived from the five different radiometric methods given in Table 4.4? What evidence would be needed to falsify the accuracy of these age dates?

9. How can plate tectonics be responsible for the following circumstances in both relative and absolute age dating:

a. Stratigraphic sequences that have the oldest fossils at the top and the youngest fossils at the bottom.

b. Strata tilted into a vertical position.

c. Volcanic ash layers that show older radiometric ages than found in underlying strata.

d. Unconformities that show considerable angles between strata below and above the unconformity.

e. Younger radiometric ages for metamorphic rocks than found in surrounding sedimentary rocks in a mountain range.

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You are watching a documentary on television about dinosaurs and you get