It is not new that dinosaurs swallowed stones; this has been known for some time. What is new is that researchers from Okayama University of Science in Japan suggest that the shape of these stones themselves can reveal how the dinosaur's stomach worked and how it digested its food.
The stones that animals swallow are called gastroliths, or stomach stones. These stones remain inside their digestive systems. Living examples of these stones are found in birds, which do not have teeth to chew food. Therefore, the stones can reach a muscular part of the stomach called the gizzard, where the muscles contract strongly, pressing the stones onto the food and helping to break it down. This is a mechanical function in the digestive process.
For this reason, when scientists found clumps of stones inside the abdominal cavities of some dinosaur fossils, they hypothesized that they might have served a similar function.
To study this issue, the researchers in the study published in the journal Paleobiology began by analyzing modern animals, including birds and crocodiles, that naturally swallow stones, and focused on the relationship between the shape of the stones, the degree of their exposure to friction, and the strength of muscular activity within the digestive system.
The comparison showed that stones that are subjected to frequent and strong friction inside the stomach become more rounded and smooth over time, while stones that are not subjected to intense mechanical grinding retain their relatively angular and rough shapes.
Thus, the shape of the stone can serve as a kind of "fossil record" of stomach activity, as researchers can examine its roundness and erosion to determine whether it was subjected to vigorous grinding while inside the digestive system.
What do the stones say about dinosaurs?
When this method was applied to dinosaur fossils, striking differences emerged between the different groups. Herbivorous dinosaurs from the Ornithiscea group, which includes relatives of dinosaurs such as Triceratops, were associated with stones that retained a greater degree of angular shapes, which does not indicate the presence of a severe mechanical grinding process inside the stomach.
As for the giant long-necked dinosaurs, known as sauropods, their fossils also provide evidence that does not entirely agree with the traditional idea that they used their stomachs as "stone mills" to grind huge quantities of plants.
Instead of relying primarily on mechanical grinding, these animals may have benefited from other digestive strategies, including keeping food inside the digestive system for long periods and allowing fermentation processes that help extract nutrients from plants.
special breed
But one of the most interesting findings came from studying theropod dinosaurs, the group to which Tyrannosaurus rex and Velociraptor belong, and which are the ancestors of modern birds.
The results suggest that the presence of a stomach capable of performing a powerful mechanical grinding operation may have appeared at an early stage in the history of some of these dinosaurs.
This finding is of particular importance when considering the changes in the behavior of theropods, as some of their lineages gradually shifted from meat-based diets to vegetarian or mixed diets, in conjunction with changes in teeth and jaws.
This raises the possibility that an increased ability of the digestive system to mechanically process food may have helped some dinosaurs reduce their reliance on teeth and jaws for food preparation, which may have been one of the factors that facilitated the major dietary shifts experienced by some lineages, all the way to birds.
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