Digestive contents and food webs record the advent of dinosaur supremacy
The early radiation of dinosaurs remains a complex and poorly understood evolutionary event1,2,3,4. Here we use hundreds of fossils with direct evidence of feeding to compare trophic dynamics across five vertebrate assemblages that record this event in the Triassic–Jurassic succession of the Polish Basin (central Europe). Bromalites, fossil digestive products, increase in size and diversity across the interval, indicating the emergence of larger dinosaur faunas with new feeding patterns. Well-preserved food residues and bromalite-taxon associations enable broad inferences of trophic interactions. Our results, integrated with climate and plant data, indicate a stepwise increase of dinosaur diversity and ecospace occupancy in the area. This involved (1) a replacement of non-dinosaur guild members by opportunistic and omnivorous dinosaur precursors, followed by (2) the emergence of insect and fish-eating theropods and small omnivorous dinosaurs. Climate change in the latest Triassic5,6,7 resulted in substantial vegetation changes that paved the way for ((3) and (4)) an expansion of herbivore ecospace and the replacement of pseudosuchian and therapsid herbivores by large sauropodomorphs and early ornithischians that ingested food of a broader range, even including burnt plants. Finally, (5) theropods rapidly evolved and developed enormous sizes in response to the appearance of the new herbivore guild. We suggest that the processes shown by the Polish data may explain global patterns, shedding new light on the environmentally governed emergence of dinosaur dominance and gigantism that endured until the end-Cretaceous mass extinction.
Dinosaurs evolved in the mid-part of the Triassic, as indicated by the earliest unequivocal dinosaur fossils in upper Carnian deposits8 and the remains of close dinosaur ancestors in the Middle Triassic9. However, terrestrial ecosystems dominated by dinosaurs of various trophic levels and taxonomic affinities, a structuring that would persist until the end-Cretaceous mass extinction, did not appear until the Early Jurassic, some 30 million years later10. Many non-dinosaur tetrapods (for example, most temnospondyl amphibians, procolophonid parareptiles, rhynchosaurs, phytosaurs and pseudosuchians, and some therapsids) became extinct during this interval, leading to the rise of dinosaurs being considered one of the most classic examples of a macroevolutionary biotic replacement. Two main contrasting models have been proposed to explain this event. The traditional ‘competitive replacement model’ argues that dinosaurs outcompeted their rivals because of more efficient physiologies, new anatomical adaptations or different feeding habits11,12. By contrast, the ‘opportunistic replacement’ model focuses on the role of stochastic processes that would have enabled the early radiation of dinosaurs following a diversity decline, or total extinction, of other groups13,14,15. There are still various opinions on the impact of the mass extinction at the end of the Triassic on the evolutionary success of dinosaurs7,16. New findings and more accurate chronostratigraphic dating have improved our understanding of the patterns of early Mesozoic tetrapod evolution17. However, no single hypothesis seems capable of explaining the rise of dinosaurs fully and critical questions about how dinosaurs established their dynasty on land remain largely unanswered18,19,20,21,22,23,24.
