What is a fossil? A fossil is the preserved remains of any once-living organism. Some fossils, such as the freeze-dried mammoths and other Ice Age mammals found in the Siberian permafrost, are almost completely preserved, and their original tissues are largely intact (fig. 2.1A). In some cases, the meat was so well preserved that Russian sled dogs and even humans could eat it when these mammoths began to thaw. An Ice Age woolly rhinoceros was found completely pickled in tar in Starunia, Poland, and most of its original tissues were preserved (fig. 2.1B). Scientists have tried to extract the DNA from these exceptionally well-preserved specimens, but their success has been limited because DNA degrades rapidly after death. Even when the animal is well preserved, contamination from bacterial DNA is present in the carcass. Despite this degradation, we have learned a lot from these specimens. They reveal the color, fur patterns, and even the stomach contents for these long-extinct beasts. Apparently, woolly mammoths had a fondness for buttercups.
Figure 2.1 ▲
There are many different kinds of fossilization. (A) In Siberia and Alaska, Ice Age mammals were freeze-dried and mummified, like this baby woolly mammoth. (B) In certain tar seeps, complete animals may be pickled in tar like this complete woolly rhinoceros from Starunia, Poland. (C) In the famous La Brea tar pits in Los Angeles, the bones are pickled in tar. (D; color figure 1) These Cretaceous ammonites still have their rainbow-colored iridescence from “mother of pearl” aragonite preserved in their shells. (E) Insects and other animals are occasionally trapped in tree sap, which can harden into a rock called amber, allowing extraordinary preservation. (F) The original woody texture of this piece of wood from the Petrified Forest in Arizona has been replaced by minerals such as silica (silicon dioxide) and some red iron oxides (“rust”). ([A, C–D, F]) Photographs by the author; [B, E] courtesy of Wikimedia Commons)
Insects and other creatures trapped in amber, including lizards and frogs, also provide famous cases of extraordinary preservation (fig. 2.1E). They are beautiful in their detail, with even the most delicate wings and bristles and hairs intact. Nevertheless, their organic material has completely degraded into just a carbon film. Contrary to the premise of the Jurassic Park/World books and movies, no original DNA is preserved in amber fossils. Not only is it impossible for dinosaur blood to survive in the gut of Mesozoic mosquitoes encased in amber, but even the original insect remains have no original DNA because amber is quite porous to chemicals from the outside.
Another common form of fossilization is permineralization. Many biological tissues (bones, wood, and so on) are full of pores and canals where a chemical can percolate through and crystallize within the fossil, preserving the original structure. This kind of preservation is common in petrified wood (fig. 2.1F), which can be completely permeated by carbonate or silica, in which the original growth rings and even cell structure can be seen.
Another common pattern in fossilization is recrystallization. Many shells are made of unstable minerals, such as aragonite (“mother of pearl”), and those minerals revert to a more stable form over time and with burial. For example, aragonite recrystallizes into calcite. Fine-grained granular calcite can recrystallize into large, coarsely crystalline calcite. The original shape and appearance of the fossil remains the same, but microscopic examination shows that the crystal texture and size has changed from its original form.
The most common form of fossilization is dissolution and replacement. Most fossil shells do not contain the original shell material. Instead, that material dissolved away, leaving a void that was replaced by new minerals (fig. 2.2). In addition, the internal cavity within the fossil can fill with crystals of calcite, silica, or even sand grains, which can form an internal mold (also known by the German word steinkern [stone cast]) that preserves the details of the shape and texture of the internal part of the shell, or even the brain cavity within a skull. The mineral in the original fossil is usually replaced by the same mineral (especially calcite). But a shell originally made of calcite may also have been replaced by silica (SiO2, or silicon dioxide), the mineral dolomite [(Ca, Mg)(CO3)2], or even sparkly crystals of “fool’s gold” (fig. 2.3A), which is the mineral pyrite (FeS2). We know that no living creature uses pyrite in its shell, so this is clearly a later replacement (fig. 2.3B).
Figure 2.2 ▲
The stages of dissolution and replacement of a fossil in a sandstone or limestone matrix. The original shell can be dissolved and replaced with a different chemical. In addition, the internal cavity of the fossil can be filled with sand or a cement made of crystals of silica or calcite. That void-filling material can weather out as an internal mold of the fossil’s internal cavity. (Illustration by Mary Persis Williams)
Figure 2.3 ▲
The original calcite mineral in shells may be replaced by some other nonbiological mineral, such as pyrite, or iron sulfide (fool’s gold): (A) brachiopods replaced by metallic pyrite; (B; color figure 2) an ammonite shell completely replaced by pyrite. (Courtesy of Wikimedia Commons)
Finally, certain kinds of fossils are preserved as flattened pancakes, with the original film of their soft tissues still visible in the process of carbonization. This type of preservation is particularly common in deepwater black shales or in swampy settings, where fossils were buried in stagnant, low-oxygen conditions and then covered in mud, which prevented scavengers from eating them. Most plant fossils preserved in coal beds are reduced to carbonized films (fig. 2.4A). And the body outlines of creatures such as ichthyosaurs (fig. 2.4B) and fish, as well as the soft tissues of creatures like trilobites and graptolites, are still preserved in many famous deepwater shales. Stagnant lake shales, such as the Eocene Green River Shale of Wyoming, Utah, and Colorado (fig. 2.4C), or the Florissant Fossil Beds in Colorado, also preserve complete insects and the leaves of plants this way.
Figure 2.4 ▲
Many fossils are preserved as they were crushed in shales, leaving only the carbonized film remains of their original tissues. (A) Fossil ferns and other plants preserved as a carbon film in sandstone. (B) Complete articulated skeleton of the marine reptile known as the ichthyosaur, with the body outline preserved as a carbonized film around it. (C) Fossil bird from the Eocene Green River Shale showing feathers and other soft tissues. (Courtesy of Wikimedia Commons)
