Fossilised footprints discovered in Brazil show that a giant mammal resembling a large dog with rat-like features walked alongside dinosaurs 130 million years ago.
The tracks were uncovered in the Botucatu Formation, a geological layer stretching across the Brazilian states of São Paulo, Paraná, and Santa Catarina. Scientists say the footprints indicate an animal measuring approximately 1.55 metres in length, a size far greater than previously expected for mammals living in the Mesozoic era prior to the extinction of the dinosaurs.
According to the study, the largest footprints recorded at the site reach up to 11.4 centimetres in length and 13.4 centimetres in width. Researchers also measured a spacing of 40 centimetres between individual track marks, a combination of dimensions and stride distance that enabled the team to calculate the overall scale of the creature.
Researchers identified the physical characteristics of the track impressions left in the ancient soil as the primary evidence supporting their findings. While the precise species responsible for the prints remains unknown, structural analysis confirmed that the impressions resemble typical mammalian footprints expanded to a significantly larger scale. The tracks show no similarity to recognized footprint patterns of prehistoric dinosaurs or crocodiles.
Challenging beliefs about prehistoric mammals
The discovery has the potential to reshape established scientific knowledge regarding mammals from the Mesozoic era. For decades, early mammals from this period were widely understood to have been small, discreet creatures that remained minor components of ecosystems to avoid being hunted by larger reptilian predators. The new evidence supports a hypothesis that larger mammals occupied more prominent positions higher up the ecological food chain than previously assumed.
Although footprint evidence cannot definitively confirm the animal's diet, researchers believe the creature was an omnivore. The scientific team noted that an animal of this size could have preyed upon smaller mammals, invertebrates, and potentially even small dinosaurs.
The Botucatu Formation is an extensive geological region located in southeastern and southern Brazil. Formed during the Early Cretaceous period, the site originally consisted of a vast dune field within an ancient desert environment. Over millions of years, windblown sands consolidated into sandstone formations, preserving tracks and trace fossils left by animals traversing the ancient dunes.
The Mesozoic era, often referred to as the Age of Reptiles, spanned from approximately 252 million to 66 million years ago and includes the Triassic, Jurassic, and Cretaceous periods. While dinosaurs dominated land environments throughout this era, early mammals evolved alongside them, with most known fossil specimens representing small, nocturnal, shrew-sized organisms.
Research institutions and future fossil searches
The research findings were published in early August in the scientific journal Journal of South American Earth Sciences. The investigation was conducted by a multi-institutional team of researchers from five Brazilian higher education centers: the State University of Minas Gerais (UEMG), the Federal University of Rio Grande do Norte (UFRN), the State University of Rio de Janeiro (UERJ), the SATC University Center (UniSATC), and the Federal University of São Carlos (UFSCar).
The collaborating universities represent major scientific and academic hubs distributed across Brazil, ranging from the northeastern state of Rio Grande do Norte to southeastern and southern academic institutions in Minas Gerais, Rio de Janeiro, Santa Catarina, and São Paulo. The joint research brought together specialists in geology, paleontology, and fossil trace analysis.
Scientists emphasized that significant questions remain unanswered because skeletal fossils of the creature have not yet been recovered. Researchers explained that while fossilised footprints allow for reliable estimates of body length and gait, complete bones remain necessary to reveal critical physical details, including skull shape, tooth structure, and overall skeletal framework.
