First dinosaur ichnofauna from the Bauru Group indicates Cenomanian–Turonian events led to an ‘Ornithischian Hiatus’ in the Upper Cretaceous of Southeast Brazil

Abstract

The Cenomanian–Turonian interval represents a critical period of faunal change during the Late Cretaceous. Although the main causes of this turnover are contentious, it is undeniable that these global events significantly influenced the last stages of dinosaur evolution. The Bauru Group in southeastern Brazil is known for its rich body-fossil record of crocodyliform and dinosaur remains, representing a key geological unit for understanding the evolution of continental biotas in the Upper Cretaceous of Gondwana. However, the ichnological record in these units are scarce, primarily consisting of invertebrate burrows and rare tetrapod traces. This study focuses on the newly discovered tracksite within the Cenomanian–Turonian Santo Anastácio Formation, marking the first dinosaur ichnofauna identified in the Bauru Group. The new ichnocoenosis includes four theropod morphotypes, a quadrupedal and a small bipedal ornithischian, bipedal and graviportal ornithopods, and a small, narrow-gauged sauropod trackmaker. These tracks provide new insights into the dinosaur faunas during the Cenomanian–Turonian interval and highlight the existence of clades not previously identified in the Bauru Group, particularly ornithischians. The absence of body fossils of these taxa in the younger strata of the Bauru Group indicates that the successive environmental fluctuations during the Cenomanian–Turonian may have been responsible for regional extinctions or migration of these animals to higher latitudes. This ecological shift might have allowed specialized crocodyliforms like sphagesaurians to fill the niches left by small herbivorous dinosaurs in the Bauru Group, while titanosaurians dominate the medium-to-large herbivore roles in these ancient ecosystems.