New evidence has emerged showing that birds are living dinosaurs: gastroliths, the stomach stones that aid digestion. Toothless, beaked dinosaurs appear to have had smoothly rounded gastroliths in their stomachs, like those of today's chickens or ostriches. The finding suggests changes in the digestive system drove the evolution of carnivorous dinosaurs into birds.
A team led by Ryuji Takasaki, a professor in the Department of Paleontology at Okayama University of Science in Japan, recently reported in the international journal Paleobiology that wear patterns on gastroliths reveal dinosaurs used diverse digestive strategies like modern animals. Some dinosaurs ground food in the stomach like chickens or ducks, some tore and chewed with their mouths like crocodiles or lions, and some likely fermented food in the stomach with microbes like cows or sheep.
◇ Dinosaurs that digested with gizzard stones like chickens
The team compared gastroliths from 42 species of modern birds and four species of crocodilians. As expected, ducks and geese, which mainly eat tough plants, had well-developed stomach muscles and gastroliths worn smooth and round like beach pebbles. By contrast, seabirds and crocodiles that prey on animals had less need to grind food in the stomach, so their stomach muscles were weaker. Their gastroliths collided less and thus were angular.
Gastroliths also differed among 16 dinosaur species according to evolutionary changes in the digestive tract. Takasaki said, "We found a correlation in which, as the task of chewing and grinding food shifted from the jaws to the stomach, teeth disappeared and gastroliths became rounded."
Dinosaurs are broadly divided into Saurischia, which had lizard-like pelvises, and Ornithischia, which had bird-like pelvises. Saurischia are further split into the theropod group of carnivores that walked on two legs, such as Tyrannosaurus and Velociraptor, and the sauropod group of herbivores with long necks, such as Brontosaurus and Diplodocus.
Despite the name, modern birds evolved not from Ornithischia but from theropods, the carnivorous saurischian dinosaurs. Some lineages later shifted to herbivory or omnivory and evolved forms resembling ornithischian dinosaurs with bird-like pelvic structures. Their teeth disappeared and beaks emerged.
A representative example is Deinocheirus mirificus. When a 2.4-meter-long arm bone with sharp claws was unearthed in 1965, it was presumed to be a carnivorous dinosaur more than twice the size of Tyrannosaurus, but in 2014 a Korean research team overturned the prevailing view.
Lee Yoong-nam, director of the Geological Museum at the Korea Institute of Geoscience and Mineral Resources (KIGAM), and researcher Lee Hang-jae reported in the journal Nature, based on a fossil excavated in Mongolia with the arms, neck, torso, and leg bones preserved intact, that Deinocheirus was a herbivorous member of the ostrich dinosaur group. As many as 1,400 gastroliths were found between the rib and vertebral bones of the fossil.
In toothless theropods, the gastroliths were rounded. They likely ground plant material mechanically in the stomach like modern chickens. By contrast, Tarbosaurus, a giant theropod carnivore, had powerful teeth and angular gastroliths. That implies it tore flesh with its teeth and the stomach only handled the final processing.
◇ Different roles for gastroliths even among herbivorous dinosaurs
Even among herbivorous dinosaurs, some did not have rounded gastroliths. Instead of a muscular stomach, they relied on well-developed teeth and jaws, or on prolonged fermentation in an extended digestive tract. For example, ornithischian herbivores such as Triceratops, which had three horns on the head, possessed angular gastroliths. That indicates they ground food with powerful jaws and the stomach only performed finishing work.
Long-necked sauropod herbivores stripped leaves with nail- or pencil-like teeth and swallowed them whole without chewing, yet their gastroliths were also angular. The team inferred that in sauropods, microbial fermentation in a long digestive tract broke down the food, as in modern cows, sheep, and deer.
The team explained that the loss of teeth in dinosaurs and the rounding of gastroliths likely contributed to the evolution of birds. Carnivorous dinosaurs with powerful teeth and jaws inevitably carried more weight toward the front of the body. In contrast, toothless theropods outsourced most digestion to the stomach, shifting the center of mass toward the middle of the body. That, they said, made it easier to rise from the ground like modern birds. In other words, the secret to how dinosaurs evolved into birds was also in the stomach.
References
Paleobiology (2026), DOI: https://doi.org/10.1017/pab.2026.10112
Nature (2014), DOI: https://doi.org/10.1038/nature13874