Postmortem Processes: Pre-burial
After a dinosaur died, its body may have undergone a complicated series of post-death (also known as postmortem) processes that involved both biological and physical factors, perhaps well before any part of it became buried. Biostratinomy is the study of what happens to an organism between its death and its final burial. In most cases, dinosaurs would have died before their remains became buried, although a hypothesis was proposed for live burial of some dinosaurs in Late
Cretaceous deposits of Mongolia (Chapters 9 and 13). Another possibility to consider for some dinosaur body parts found in the fossil record is that a dinosaur may have lost a part before it died, and that part ended up in the fossil record but the rest of the dinosaur did not. As mentioned earlier, some dinosaur tracks show a missing toe on one foot, indicating that the dinosaur was still walking after it had its digit-depriving mishap (Chapter 14). The best example proposed for body-part loss among dinosaurs while they were still alive is their probable shedding of teeth, as seen with certain theropods (Chapter 9). However, finding convincing evidence supporting the hypothesis that a particular dinosaur body part other than a tooth was detached from a dinosaur long before it died would be extremely difficult. Nevertheless, this possibility serves as an alternative hypothesis to explain the anomalous occurrence of an expendable dinosaur body part in a deposit lacking any other sign of its owner.
Although the following sections are divided into biological and physical processes, any of the processes from each category could have happened simultaneously. For example, a dinosaur caught in a flooded stream would have been transported while it drowned, and biological processes (such as scavenging) probably would have affected the dead body while it was still moving. In other words, a spectrum of interacting events may have led to the preservation of any given dinosaur body fossil, which should be considered when interpreting all of the available evidence associated with the fossil.
Biological Processes: Decay and Scavenging
The decomposition of a dead body is largely a biological process, called necrolysis. This discussion will feature a fictional case study that illustrates the common scenario of biological processes associated with dinosaur taphonomy.
One day during the Mesozoic, a 3-metric-ton ornithopod died from a horrible, nasty, and painful viral disease. After much aimless staggering, the ornithopod collapsed on to the floodplain of an inland river. Soon after it died, its corpse underwent rigor mortis (the stiffening of musculature in the body) within the first 10 hours or so of death, followed by a relaxation of the muscles. It initially landed on either its right or left lateral surface, with its limbs stretched away from its body and its head turned toward its back as the stronger muscles on the dorsal surface of the neck contracted. The same process affected the tail, and its dorsal muscles pulled the tail toward the back of the body, with the neck and tail forming an arc.
At the same time, the anaerobic bacteria (which live in the absence of oxygen) in the gut of the animal and the aerobic bacteria (which need oxygen for respiration) from the environment outside of the body broke down the proteins and other organic compounds in the corpse to obtain their food. These simple, one-celled organisms were responsible for as much as 90% of the dinosaur's initial decomposition. Their consumption of the corpse gave off gaseous metabolic by-products that bloated the body and gave it a noticeable malodorous scent, a process called putrefaction. The gases released as waste during the aerobic decomposition of the organic matter (oxidation) were CO2 and H2O; this process recycled much organic carbon back into the ecosystem (an important phase in nutrient cycling). Anaerobic decomposition proceeded through fermentation, exemplified by the following equation:
C6H12O6 (glucose) ^ 2CO2 + 2C2H5OH (alcohol) + Energy (7.1)
where glucose (a sugar) was a simple biomolecule broken down by the bacteria. Thus, CO2 was the main gas produced, although methane (CH4) is another potential by-product of some anaerobic bacteria.
The first effluvia of these gases attracted the attention of scavenging animals, which regarded the body as a potential food source. Insects were probably the first to arrive: flies laid eggs, which soon hatched into feeding larvae, and carrion beetles began stripping away some of the outer flesh. The flies normally laid their eggs in open parts of the body, such as the nostrils, eyes, anus, or any obvious wounds. Maggots may also have already been present in any open wounds on the dinosaur's body before its death. The carrion beetles were numerous and relentless; they worked on the corpse for weeks and eventually left their distinctive, pitted gnawing marks in any exposed bone. Ants were also industrious, outnumbering the beetles as they carried away millions of pieces of the body to their nests on a nearby, vegetated riverbank.
The emission of greater volumes of the fetid gases from the corpse also alerted vertebrate scavengers to a potential meal, such as theropods, pterosaurs, small mammals, and, if this was during the Late Jurassic or Cretaceous, birds. The river flooded just enough to cover part of this area while the body was still on the floodplain, so crayfish brought in by the floodwaters joined in the feast. If the conditions were relatively hot and humid on the floodplain, then the metabolic activity of the bacteria accelerated and the corpse's bloating was more rapid than normal. This continued for nearly a week before the body exploded (aided by the numerous punctures left by scavengers) and then deflated. The result was a flattened profile to what had originally been a voluminous piece of putrid flesh.
Within six to eight weeks, all soft parts were stripped from the bones, and the bones themselves were attacked by yet more beetles and any other animals that were interested in obtaining some calcium and phosphorus in their diets (Chapter 8). More delicate parts of bones, such as the epiphyses (wide ends) of femurs and other limb bones, were especially susceptible to being broken by animals with strong jaws; some cranial bones might have been wholly consumed. The bones, once they were exposed to sunlight, also had their organic content depleted more rapidly than before, bleaching them white and making them more brittle. Other dinosaurs strolling through the area stepped on the now-weakened bones, pulverizing or otherwise fracturing them. This increased their surface area and made them more susceptible to dissolution by natural acids and further bacterial or fungal decay.
All of the preceding processes depended on temperature and humidity; for example, faster rates of decay and insect scavenging are associated with increased temperature. Assuming summertime conditions and high amounts of rainfall, within eight to ten weeks there might have been little trace of the several-ton animal, except for maybe a little more vegetation growing in its resting spot as a result of the natural fertilizer provided by its body. Unless the corpse, in whatever state, was buried by the sediments of a river flood during the preceding time frame, or parts of it were carried away by floodwaters or scavengers and subsequently buried, this ornithopod would not have made it into the fossil record in any shape or form.
Evidence supporting the preceding scenario is based on the following data:
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