Fossilized droppings and vomit shed light on the reasons behind the dominance of dinosaurs.

Despite the long-standing popularity of dinosaurs in both science and culture, much remains unknown about their origins and evolution. The same can be said for other reptiles. For instance, just recently, paleontologists discovered what enabled pterosaurs to achieve gigantism, as reported by Naked Science discussed. The situation with dinosaurs is more complex, as scientists attribute their dominance on land to two competing hypotheses.

One theory suggests that dinosaurs surpassed competitors such as phytosaurs, pseudosuchians, and others due to physiological innovations like warm-bloodedness or rapid growth. Alternatively, some believe that dinosaurs were fortunate enough to occupy vacant niches left behind after a decline in diversity and the extinction of other animal groups. This occurred during the first 30 million years of these reptiles' evolution, in the upper part of the Triassic period (237-201.3 million years ago), but there is little fossil evidence from this stage.

Researchers from Northern and Eastern Europe found the answer to this question. They studied hundreds of fossilized remains of digestive systems (bromalites) from ancient animals collected from five Polish formations—regions of the former supercontinent Pangaea. Fossilized feces (coprolites), vomit (regurgitalites), and other contents from prehistoric stomachs and intestines helped paleontologists gradually reconstruct the development and spread of dinosaurs. The results of this scientific work were published in the journal Nature.

Experts searched for undigested food remnants—fragments of bones, plants, traces of parasites, insects, and more—by scanning samples (over 500 in total) using synchrotron microtomography. Additional paleoecological and climatic data helped establish the context in which ancient herbivores and predators lived. The researchers also relied on previous studies of skeletal fossils from selected locations.

Communities of vertebrates that lived 237-227 million years ago in the Carnian stage were diverse, ranging from close relatives of sharks and gibbodonts to various archosauromorphs. From the same period, scientists identified Silazaurus, an omnivorous relative of dinosaurs. However, no traces of sauropodomorphs were found. These herbivorous dinosaurs appeared in the Rhaetian stage (208.5-201.3 million years ago), coexisting with fish-eating crocodylomorphs and early theropods.

Bromalites from the transition between the Triassic and Jurassic periods preserved fragments of bones and fish scales (which were consumed by predatory theropods), as well as various plant remains. Notably, as the authors of the article pointed out, traces of ginkgophytes and ferns sharply contrasted with late Triassic conifers, which were the primary food source for herbivorous dinosaurs at that time. Paleontologists were also surprised by the presence of charcoal among the fossilized food. Apparently, ancient herbivores often consumed charcoal that remained after forest fires to neutralize the toxic effects of ferns.

The main outcome of the new research was a five-stage model for the emergence and evolution of dinosaurs. From the mid to the late Carnian stage, small omnivorous predecessors of these reptiles lived. Then, from the mid to the late Norian stage, the fauna of their ancestors became more diverse, and the first predatory dinosaurs emerged. Subsequently, closer to the beginning of the Rhaetian stage, they split into large and small forms, alongside the emergence of large ornithischians (which were omnivorous and herbivorous). By the end of the Triassic period, various sauropodomorphs appeared. By the beginning of the Hettangian stage (Lower Jurassic), dinosaurs had widely dispersed and occupied numerous ecological niches.

The reasons for the dominance of these reptiles are complex. The late Triassic environment transitioned from arid to humid during the early Jurassic phase, resulting in diverse vegetation. This transition coincided with the disappearance of many groups of non-dinosaurian quadrupeds. Herbivorous specialists like etosaurs and dicynodonts found it challenging to compete with dinosaurs, whose diet had become broader. Ultimately, by the end of the Triassic period, they were displaced by sauropodomorphs and ornithischians.

“The materials for the research were collected over 25 years, and it took us many years to piece them together into a coherent picture. This study is innovative; we aimed to understand the biology of early dinosaurs from the perspective of their dietary preferences. To avoid extinction—eat plenty of plants, somewhat in the spirit of the slogan ‘eat vegetables and live longer.’ This is exactly what early herbivorous dinosaurs did,” noted Polish paleontologist and co-author of the study Grzegorz Niedzwiedzki.