How did dinosaurs digest their meals?
Since soft tissues such as stomachs and intestines almost never fossilize, scientists have had very little direct evidence about how these prehistoric animals processed food.
Now, a new study suggests that clues may have been hiding in their stomach stones all along.
Researchers led by Assistant Professor Ryuji Takasaki from Okayama University of Science have examined gastroliths—stones found inside the abdominal cavities of some dinosaur fossils—to better understand how different dinosaur groups digested their food.
Their findings, published in the journal Paleobiology, suggest that dinosaurs evolved several different digestive strategies rather than relying on a single method.
Gastroliths are stones that animals intentionally swallow. Many living birds use them inside a muscular stomach called a gizzard to help grind tough plant material because they do not have teeth for chewing.
Crocodiles also swallow stones, although they appear to use them differently.
To investigate how dinosaurs may have used gastroliths, the research team compared the shape and wear of stomach stones from living birds and crocodilians. They also considered each animal’s diet and the strength of its stomach muscles.
The scientists found a clear pattern. In plant-eating animals with powerful, muscular stomachs, gastroliths became smooth and rounded over time because they rubbed together while grinding food.
In contrast, meat-eating animals tended to have stomach stones that stayed much sharper and more angular, suggesting their stomachs did not grind food as intensely.
The researchers then applied this pattern to dinosaur fossils. They found that many toothless theropod dinosaurs had smooth, rounded gastroliths. This suggests these dinosaurs may have relied on muscular stomachs to grind plant material, much like modern chickens and other birds do today.
However, the study also found that not all dinosaurs digested food in the same way. Different groups appear to have developed different solutions to the challenge of processing plants and other foods.
Some dinosaurs likely depended mainly on their teeth and jaws to break food into smaller pieces before swallowing it. Others probably relied on muscular stomachs filled with gastroliths to grind tough vegetation. Still others may have processed food through long periods of fermentation inside an extended digestive tract, similar to the way some large plant-eating mammals digest food today.
The researchers point to examples such as Psittacosaurus, a distant relative of Triceratops, and enormous sauropods that grew more than 20 meters (66 feet) long. Although many gastroliths have been found with these dinosaurs, the stones often remain angular instead of being worn smooth.
This suggests their stomachs may not have been especially muscular. Instead, these giants may have depended on strong teeth and jaws, combined with slow fermentation in long intestines, to extract nutrients from plants.
The team says combining evidence from gastroliths with studies of dinosaur teeth and jaw structure could provide a much clearer picture of how these remarkable animals evolved such diverse ways of eating and digesting food.
