Fossilized footprints uncovered in southern Brazil may belong to the largest mammal known to have lived alongside dinosaurs, according to a new study. The discovery, made in a formation of ancient desert sand dunes, is reshaping what scientists thought they knew about the size of early mammal relatives during the Early Cretaceous Period, when dinosaurs still ruled the planet.
The study was led by Pedro Victor Buck of the Paleontology Laboratory in the Department of Exact and Earth Sciences at the State University of Minas Gerais, Brazil, and published in the Journal of South American Earth Sciences. It describes two newly identified trackways, labeled UERJ IC-170 and UERJ IC-171, found in the Botucatu Formation of the Paraná Basin, a region that once formed part of a massive desert covering southern Brazil.
Giant four-toed tracks challenge understanding of mammal size
Each trackway contains four-toed hindfoot prints that are larger than any mammal-like tracks previously recorded in the same rock formation. The prints are short and rounded, and the narrow gap between the left and right tracks suggests the animal walked on all four legs. Based on these features, researchers assigned the tracks to a fossil footprint group called Chelichnopodidae.
For years, paleontologists generally believed that mammals and their closest extinct relatives, known as mammaliaforms, remained small, usually under roughly two pounds, throughout most of the Mesozoic Era.
A new study reveals footprint evidence of the largest mammal to live during the age of dinosaurs in Brazil. pic.twitter.com/bzwJ2e7Y0N
— Tom Marvolo Riddle (@tom_riddle2025) August 21, 2026
According to Buck and his co-authors, the newly discovered footprints push back against that assumption, indicating that some of these animals grew far larger, and did so earlier in their evolutionary history than previously believed. The Botucatu Formation had already yielded two other unusually large mammal-like tracks, named Aracoaraichnium leonardii and Brasilichnium anaiti, described in earlier research.
Combined with recent fossil discoveries from other parts of Gondwana and Laurasia, the new tracks add to growing evidence that several mammaliaform lineages broke away from their typically small size well before the end of the Cretaceous Period.
Findings show largest mammals lived right alongside dinosaurs
The two specimens were collected from a quarry near Turvo, Santa Catarina State, and are now stored at the Laboratório de Paleontologia in Rio de Janeiro. Researchers noted that identifying the exact animal behind the footprints is difficult, since wind-blown desert sand often blurs fine details in fossil tracks. Even so, the size and shape of the prints point to a mammaliaform far larger than the largest mammals living during the age of dinosaurs, according to the study.
Buck and his colleagues concluded that the findings broaden the known range of body sizes among Early Cretaceous mammaliaforms and offer new insight into how these ancient creatures evolved and adapted across the Gondwana supercontinent.
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