Stomach stone hypothesis sheds light on the evolution of dinosaur digestive systems
An international research team led by Assistant Professor Ryuji Takasaki of the Faculty of Biosphere-Geosphere Science at 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 cavities. By examining the degree of wear on these gastroliths, the team sheds light on dinosaur digestion strategies. The findings are published in the journal Paleobiology.
Comparisons with gastroliths from living animals suggest that dinosaurs used multiple digestion strategies: some ground food within their stomachs as modern birds do, some chewed food within their mouths as modern mammals do, and some fermented food within their intestines. The researchers expect that combining insights from gastroliths with other dietary proxies, including tooth and jaw morphology, will provide a more comprehensive understanding of how dinosaurs processed food.
Wear patterns offer clues
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 systems, 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 those that consume invertebrates, retained gastroliths with sharp edges.
In other words, herbivorous taxa likely possessed rounded gastroliths that were severely abraded within their muscular stomachs, while carnivorous taxa retained angular gastroliths that were less abraded because their stomachs were less muscular and exerted less force on 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 that of modern birds, such as chickens, which use their muscular stomachs, or gizzards, to grind plant matter.
Different lineages, different digestion
The study also suggests that not all dinosaurs digested 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 (66 feet) 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.
Publication details
Ryuji Takasaki et al, Gastrolith shape as an indicator of digestive function and its implications for dinosaurian digestive strategies, Paleobiology (2026). DOI: 10.1017/pab.2026.10112
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