Fossilized dinosaur gut shows that sauropods barely chewed

Artist impression of Diamantinasaurus matildae feeding. Credit: Travis Tischler

Plant fossils found in the abdomen of a sauropod support the long-standing hypothesis that these dinosaurs were herbivores, finds a study published in Current Biology. The dinosaur, which was alive an estimated 94 to 101 million years ago, ate a variety of plants and relied almost entirely on its gut microbes for digestion.

“No genuine sauropod gut contents had ever been found anywhere before, despite sauropods being known from fossils found on every continent and despite the group being known to span at least 130 million years of time,” says lead author Stephen Poropat of Curtin University.

“This finding confirms several hypotheses about the sauropod diet that had been made based on studies of their anatomy and comparisons with modern-day animals.”

Knowledge of the diet of dinosaurs is critical for understanding their biology and the role they played in ancient ecosystems, say the researchers. However, very few dinosaur fossils have been found with cololites, or preserved gut contents.

Sauropod cololites have remained particularly elusive, even though these dinosaurs may have been the most ecologically impactful terrestrial herbivores worldwide throughout much of the Jurassic and Cretaceous periods, given their gigantic sizes.

Due to this lack of direct evidence when it comes to diet, the specifics of sauropod herbivory—including the plant taxa they ate—have been largely inferred based on anatomical features such as tooth wear, jaw morphology, and neck length.

In the summer of 2017, the staff and volunteers at the Australian Age of Dinosaurs Museum of Natural History were excavating a relatively complete subadult skeleton of the sauropod Diamantinasaurus matildae from the mid-Cretaceous period, which was found in the Winton Formation of Queensland, Australia.

During this process, they noticed an unusual, fractured rock layer that appeared to contain the sauropod’s cololite, which consisted of many well-preserved plant fossils.

Australian Age of Dinosaurs Collection Manager Mackenzie Enchelmaier holds up sauropod gut content fossil. Credit: Stephen Poropat

Analysis of the plant specimens within the cololite showed that sauropods likely only engaged in minimal oral processing of their food, relying instead on fermentation and their gut microbiota for digestion.

The cololite consisted of a variety of plants, including foliage from conifers (cone-bearing seed plants), seed-fern fruiting bodies (plant structures that hold seeds), and leaves from angiosperms (flowering plants), indicating that Diamantinasaurus was an indiscriminate, bulk feeder.

“The plants within show evidence of having been severed, possibly bitten, but have not been chewed, supporting the hypothesis of bulk feeding in sauropods,” says Poropat.

The researchers also found chemical biomarkers of both angiosperms and gymnosperms—a group of woody, seed-producing plants that include conifers.

“This implies that at least some sauropods were not selective feeders, instead eating whatever plants they could reach and safely process,” Poropat says. “These findings largely corroborate past ideas regarding the enormous influence that sauropods must have had on ecosystems worldwide during the Mesozoic Era.”

Although it was not unexpected that the gut contents provided support for sauropod herbivory and bulk feeding, Poropat was surprised to find angiosperms in the dinosaur’s gut.

“Angiosperms became approximately as diverse as conifers in Australia around 100 to 95 million years ago, when this sauropod was alive,” he says. “This suggests that sauropods had successfully adapted to eat flowering plants within 40 million years of the first evidence of the presence of these plants in the fossil record.”

Lead author Stephen Poropat at the Judy excavation site in Australia in June 2017. Credit: Stephen Poropat

Based on these findings, the team suggests that Diamantinasaurus likely fed on both low- and high-growing plants, at least before adulthood.

As hatchlings, sauropods could only access plants found close to the ground, but as they grew, so did their viable dietary options. In addition, the prevalence of small shoots, bracts, and seed pods in the cololite implies that subadult Diamantinasaurus targeted new growth portions of conifers and seed ferns, which are easier to digest.

According to the authors, the strategy of indiscriminate bulk feeding seems to have served sauropods well for 130 million years and might have enabled their success and longevity as a clade. Despite the importance of this discovery, Poropat pointed out a few caveats.

“The primary limitation of this study is that the sauropod gut contents we describe constitute a single data point,” he explains. “These gut contents only tell us about the last meal or several meals of a single subadult sauropod individual,” says Poropat.

“We don’t know if the plants preserved in our sauropod represent its typical diet or the diet of a stressed animal. We also don’t know how indicative the plants in the gut contents are of juvenile or adult sauropods, since ours is a subadult, and we don’t know how seasonality might have affected this sauropod’s diet.”