Actual remains of plants or animals in the abdominal region of a dinosaur seemingly represent unambiguous evidence supporting hypotheses about what dinosaurs ate. However, considering that fossilization of any dinosaur part was a rare event (Chapter 7), finding a specimen preserved with parts or all of a recent meal in the location of its former innards is always a surprise. The rare reports of dinosaur stomach remains provide a glimpse of a dinosaur's last meal that, despite being a sample of one, can be compared to other evidence of feeding behavior for that given species. Of course, taphonomy is an all-too-important consideration when discussing what composes these stomach remains: animals with mineralized tissues are much more likely to have been preserved than those with soft tissues or plants. Indeed, stomach remains from dinosaurs are mostly vertebrates. The consumption of insects or other invertebrates by dinosaurs is unknown, and the study of plant material associated with herbivorous dinosaur body fossils is a study in frustration.
Digestion in dinosaurs was most likely facilitated by a series of organs specialized for the task, along with a little help from anaerobic bacteria.
Although the fossil record for herbivorous dinosaurs' stomach remains is poorly documented, a review of digestion in herbivores is warranted. Because plant material in many cases has low nutritional yield for large volumes, much plant material will reside in an alimentary canal for a relatively long time. However, a long residence does not necessarily mean that digestion is inefficient; on the contrary, digestive efficiency has evolved to a high degree in modern herbivores. But considering that herbivorous dinosaurs were the largest land animals that ever lived, they should have evolved comparable or superior digestive efficiency.
Decomposition of plant material with the assistance of anaerobic bacteria in the alimentary canals of terrestrial animals probably developed early among plant-eating reptiles. Modern herbivores that exemplify this process include ruminants, mammals that have large, multi-chambered stomachs (rumens) that physically mash plant material into a compacted mass called a bolus (sometimes called a cud). The bolus is then regurgitated into the mouth for more chewing, then swallowed again. Bacteria within the digestive tracts of herbivores also chemically break down some of the organic compounds in plants that otherwise cannot be digested. For example, the decomposition of cellulose, a common organic compound in plants, first produces sugars through fermentation (Chapter 7), then the formation of acetic, propionic, butyric, and formic acids, followed by amino acids, vitamins, CO2, and CH4. In the last stage of decomposition, bacteria reduce CO2 to form CH4. After fermentation, partially digested material is mixed with microbial cells from the bacteria, where it passes into the rest of the gastrointestinal tract. This constitutes the main source of protein and vitamins for a ruminant.
A ruminant would quickly die from malnutrition if it did not have a symbiotic relationship with its gut bacteria. Modern carnivorous animals have similar requirements for digestion. As a result, paleontologists assume that decomposition aided by anaerobic bacteria was also the case for carnivorous dinosaurs. Modern carnivorous analogues to theropod digestion, such as crocodilians and birds, have a stomach divided into a proventriculus, which produces enzymes for chemical breakdown and precedes a muscular gizzard that further aids digestion.
Modern methanogenic bacteria, such as Methanobacterium thermautotrophicum, have a cumulative effect of producing methane on a globally measurable scale, if present in enough herbivores that digest large amounts of plant material. Of the current global methane budget, about 80% is related to methane produced by bacteria, of which many are hosted by the guts of domesticated cattle. Termites also are significant contributors to the global methane budget (in fact, more so than cattle) and they also host bacterial colonies that assist their digestion of wood. During the Mesozoic, large herbivorous dinosaurs, along with termites and other wood-digesting organisms, were probably the purveyors of voluminous gaseous emissions that would have saturated the atmosphere of that time.
The rarity of plant material as stomach contents may actually be an artifact of preparation methods. The matrix entombing an herbivorous dinosaur may have contained disseminated plant fragments that were formerly in the gut of the animal. Thus far, only one specimen of an herbivorous dinosaur (Edmontosaurus) was reported with stomach remains consisting of plant material. Described in 1922, this specimen's abdominal area contained seeds, twigs, and needles from a species of conifer. Unfortunately, the plant material was taken out of the specimen during its preparation, and a thorough investigation of its taphonomy was not undertaken. As a result, an alternative hypothesis is that this plant material may not represent actual stomach remains, but rather fragmented debris washed into an open cavity of the hadrosaur's dead body.
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