Preservation of Dinosaur Skin Impressions and Soft Part Anatomy

Some of the most memorable dinosaur finds in the history of dinosaur studies are of those that either have skin impressions, such as hadrosaur specimens (Edmontosaurus) discovered by the Sternbergs in Late Cretaceous strata of Alberta (Chapter 3), or otherwise show evidence of soft parts. However, not all dinosaur skin was soft, as evidenced by a few sauropods and most thyreophorans (Chapters 10 and 12). Skin impressions were typically preserved as fossils through mummification, a loss of water from a body or body part that caused some shriveling but otherwise preserved much of the soft tissue for a long enough period to have made an impression. In this case, the impression was literally that: a buried dinosaur body underwent dehydration and was surrounded by sediment that pressed around the outside of the body and made an external mold, which included an impression of its skin (Fig. 7.10). If the body decayed but the external

FIGURE 7.10 How an external mold and cast could have been made of a dinosaur body, preserving skin impressions (such as seen in Fig. 5.10).

mold remained, and sediment later filled the cavity left by the body and solidified, then the resulting fossil is a cast, which is preservationally analogous to a cranial endocast (Chapter 5). Skin impressions are currently known for ornithopods, sauropods, ceratopsians, ankylosaurs, and theropods. More will surely be found as researchers become more vigilant for their possible presence in association with dinosaur bones.

As mentioned before, anaerobic conditions are also conducive to preservation of soft parts, and recent finds of theropods with feather impressions and internal organs are evidence of a low rate of organic degradation that enabled sufficient preservation of at least the outlines of these anatomical features. Carbonization is the preservation of soft parts by a loss of volatiles, which are elements such as hydrogen, oxygen, and nitrogen that are normally gaseous at near-surface temperatures, their loss resulting in a carbon film being left behind. This mode of preservation has helped paleontologists discern at least outlines for soft tissues in fossil plants, invertebrates, and vertebrates alike. If diagenesis happened rapidly enough, so that phos-phatic minerals replaced the original organic material, a pseudomorph of soft tissues (such as muscles) can be preserved (Chapter 5). This type of preservation gives more of a three-dimensional character to the parts than mere carbonization.

Finally, recognition of biomolecules and flexible soft tissue in dinosaur bones (Chapter 8) is an example of how dinosaur paleontologists should still look for remnants of organic remains in dinosaurs. Assuming preservation in anaerobic conditions and "sealing" of biomolecules from the ravages of diagenetic processes, dinosaurs may yet yield more amino acids, proteins, and possibly nucleic acids that provide further information related to their evolutionary relationships (Chapter 6). Most recently, flexible soft tissues recovered from the center of a Tyrannosaurus femur show cell nuclei and other remarkable cellular features, suggesting that such preservation may be more common than previously thought.

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