A dinosaur's last supper: The intestines of a sauropod that lived 100 million years ago reveal how they ate.

Ever since the first dinosaur fossils were discovered, scholars have maintained that sauropods, the largest of all, were herbivores. Logic dictated this, along with knowledge of food chains, the shape of their teeth, the length of their necks, and their size and slowness, which would have prevented them from chasing other animals. But there was little direct evidence, such as coprolites (fossilized feces) or, even less, cololiths (fossilized digestive contents). Until now, a group of researchers detailed in the journal Current Biology the discovery of the collolith of a young sauropod that lived in what is now northeastern Australia. In its stomach was the last thing it had eaten, a varied diet consisting of conifers, ferns, and leaves of the first angiosperms, flowering plants. Furthermore, they observed that it barely chewed what it ate.

“There has been a scientific consensus on the plant-based diet of sauropods for over 150 years. However, definitive gut contents of a sauropod have never been found before, hence the significance of our fossil,” says Stephen Poropat, a researcher at Curtin University (Australia) and first author of this study, in an email. “This finding confirms several hypotheses about sauropod diet that had been formulated based on studies of their anatomy and comparisons with extant animals,” he comments.

Discovered in 2015 a few kilometers from Winton, in the state of Queensland, the sauropod specimen was a juvenile that already measured eleven meters. It was a Diamantinasaurus matildae , a species that lived in Australian lands during the Cretaceous. In this case, it is estimated that it died between 94 and 101 million years ago. When they began extracting the fossil, paleontologists discovered a strange rocky outgrowth in what would be the abdominal area. Measuring 2x1 meters, with a thickness of up to one meter and a volume of 100 liters, it is the first confirmed sauropod collolith. And despite the time that has passed, it tells almost everything about the diet of the largest animals to have walked the Earth's surface.

“Our sauropod preserves remains of at least four different types of plants in its gut: araucaria [a genus of conifers], austrosequoia [related to modern sequoias], seed ferns, and angiosperms [flowering plants],” says the Australian researcher. Although many modern-day herbivores have specialized in grasses, they would not have appeared yet, much less in that part of the world. The work allows for even more detail on the diet: of the conifers, what they found most were bracts, leaves transformed to protect the fruit. Of the ferns, they found fruits of an already extinct species. And of the angiosperms, leaves of several species. In any case, Poropat adds, “the diet of our sauropod is quite varied.”

So they were generalists, with both high- and low-level browsing, which gives many clues about their environment. As hatchlings, the sauropods only had access to plants close to the ground, but as they grew, so did their food options. Furthermore, the prevalence of small shoots, bracts, and pods in the collolith implies that young Diamantinasaurus fed on the shoots of conifers and seed ferns, which are easier to digest.

The fact that they ate angiosperm leaves is relevant to the researchers, since these types of plants had appeared on the planet not so long ago. In the fossil record, the first flowering plants were found in what is now the Iberian Peninsula about 130 million years ago . By the time it reached the stomach of the young D. matildae , Australia was still connected to what would become Antarctica, but the two had long since separated from Gondwana . So the angiosperms must have taken their time getting there. “Angiosperms had spread more or less throughout the world before 100 million years ago, and in the flora of the Winton Formation [where the cololith was found] they were codominant with conifers and seed ferns, which attests to their success,” Poropat points out. But it’s also an example of the adaptability of the dinosaurs, who knew how to incorporate them into their diet.

The researchers point out here that, long before large herbivorous mammals, it was the great dinosaurs that prepared this planet for flowering plants: angiosperms are known for developing physical or chemical defenses against herbivores, for regenerating and reproducing rapidly, and, as this Australian researcher points out, “for housing their seeds in fruits that, when consumed and eventually excreted by herbivores, are widely scattered in piles of prepared fertilizer (the feces).”

Analysis of the plant remains from the collolith provides one final clue about sauropods: the presence of only lightly chewed leaves and virtually whole shoots show that this young dinosaur barely chewed what it ate, leaving the processing and digestion to its gut flora. This hypothesis has long been proposed because sauropods do not have teeth adapted for chewing: all their teeth are the same and are adapted for cutting vegetation, not crushing it. "The gut contents of our sauropod support this idea because many of the plants present in it can still be identified because they have not been crushed," concludes Poropat, who also warns that it would be risky to say that all sauropods ate the last thing this young dinosaur had shortly before dying.