An international research team led by Assistant Professor Ryuji Takasaki of the Faculty of Biosphere-Geosphere Science, Okayama University of Science, has published a paper proposing a new hypothesis on the evolutionary history of dinosaur digestive systems by focusing on gastroliths—stones found within dinosaur abdominal cavity. Through examining the degree of wear on these gastroliths, the team sheds light on dinosaur digestion strategies. Comparisons with gastroliths from living animals suggest that dinosaurs utilized multiple different digestion strategies, some grind food within their stomach as modern birds do, some chew food within their mouse as modern mammals do, and some ferment food within their intestines. The researchers expect that combining insights from gastrolith with other dietary proxies including tooth and jaw morphology will provide a more comprehensive understanding of how dinosaurs processed food.
According to the research team, soft tissues such as digestive organs are extremely rarely preserved in dinosaurs, making it difficult to infer how they digested and absorbed food. To infer dinosaur digestive system, the researchers first compiled and analyzed gastrolith shape, diet, and stomach musculature in living birds and crocodilians. Their analyses suggest that gastroliths were abraded and became smoother inside stomachs. The trend was particularly clear in herbivorous taxa, whereas carnivorous taxa, including taxa consuming invertebrates, retained gastroliths with sharp edges.
In other words, herbivorous taxa likely possessed rounded gastroliths that were severely abraded within their muscular stomach, while carnivorous taxa retained angular gastroliths which was less abraded, since their stomach is less muscular and put less force to the gastroliths. By applying this framework to the dinosaur fossil record, the researchers found that many toothless theropods possessed rounded gastroliths, suggesting that they may have mechanically ground plant material inside their stomachs. This mechanism is comparable to the way modern birds, such as chickens, that use their muscular stomachs, or gizzards, to grind plant matter.
The study also suggests that not all dinosaurs digest food in the same way. Some groups relied primarily on their teeth, some processed food in muscular stomachs, and some may have relied on extended digestion within the intestinal tract. These findings indicate that different dinosaur groups likely evolved distinct digestive strategies and followed different evolutionary trajectories in the development of their digestive systems.
The researchers further propose that, whereas theropods repeatedly evolved muscular stomachs early in their evolutionary history, other herbivorous dinosaur groups such as ornithischians and sauropods may have adopted different digestive strategies. Numerous gastroliths have been discovered in Psittacosaurus , a distant relative of Triceratops , and in giant sauropods exceeding 20 meters in length. However, many of these gastroliths are angular, suggesting that these dinosaurs may not have possessed highly muscular stomachs. Instead, they may have relied on well-developed teeth and jaws, together with prolonged fermentation within an elongated digestive tract, representing a digestive system that did not depend on muscular stomachs and gastrolith-assisted grinding.
Paper
Gastrolith shape as an indicator of digestive function and its implications for dinosaurian digestive strategies
Authors
Ryuji Takasaki, Okayama University of Science
Yoshitsugu Kobayashi, Hokkaido University
Anthony R. Fiorillo, New Mexico Museum of Natural History & Science
Tsogtbaatar Chinzorig, North Carolina Museum of Natural Sciences
Gregory M. Funston, Stony Brook University
Paleobiology