It's a small, weathered tooth -- nothing flashy at first glance. But the scientists who found it buried in Brazil's Tremembé Formation know better. This single canine belonged to a predator that once hunted in South America's ancient wetlands, long before the arrival of modern carnivores.
This fossil, described in a study published in the Revista Brasileira de Paleontologia by Caio César Rangel and colleagues, is a lower canine belonging to an extinct group of metatherians known as sparassodonts. These mammals are close relatives of modern marsupials. Nowadays, most marsupials are herbivores, but back in the day, marsupial predators ruled South America. The new specimen, found in southeastern Brazil, adds a rare and chilling entry to their family tree: an apex predator with features never before seen in the region.
"This record increases the diversity of large mammals in the Tremembé Formation," the authors wrote, "and provides valuable insights into the final stages of the Paleogene period in intertropical South America".
The fossil comes from the late Oligocene epoch, dated between 29 and 21 million years ago. At that time, Earth's climate was cooling, reshaping habitats and pushing some species to extinction while giving others a foothold to evolve. In South America, this meant the rise and fall of several lineages of metatherians -- the branch of mammals that includes opossums, kangaroos, and their extinct cousins.
Among these were the sparassodonts, a now-vanished clade of carnivorous mammals. These sparassodonts filled the ecological roles typically played by dogs, cats, and bears elsewhere. The newly described specimen -- catalogued as MHNT.VT.2075 -- is identified as a member of Proborhyaenidae, a family of large, meat-eating sparassodonts that bore some of the most fearsome adaptations in the Cenozoic world.
"The procumbence observed in the canine associated with evident wear in the crown," the researchers explain, "suggests a frequent use probably related to the capture or active processing of prey in an adult individual."
The canine is 5.73 centimeters long, open-rooted, and lined with deep grooves -- especially a pronounced sulcus along its inner face. These features are consistent with those found in other proborhyaenids, a group that includes the formidable Proborhyaena gigantea, a predator likely rivaling today's big cats in size. But unlike earlier finds in Bolivia and Argentina, this fossil hails from Brazil's Taubaté Basin -- a humid lowland depression.
To modern eyes, a saber-tooth marsupial may seem alien. They weren't placental mammals like lions or tigers. The metatherians were basically an "evolutionary experiment" in mammalian carnivory. They bore saber-like canines and powerful jaws but belonged to the same lineage as kangaroos.
"Sparassodonta is an extinct clade of metatherians endemic to South America that presented the main terrestrial mammalian predators on this continent during Cenozoic times," said Dr. Rangel and his colleagues.
The new fossil confirms that Proborhyaenidae extended farther east than previously known. Until now, sparassodonts in Brazil had been limited to rare and fragmentary remains. The Tremembé find now puts this predator on the map.
Its features hint that it may be related to the lineage that eventually produced Thylacosmilus atrox, the "marsupial saber-tooth." The new tooth shares some features with thylacosmilids, including its size and curved shape. But it lacks other critical traits -- such as the closed roots of adult lower canines seen in Thylacosmilus.
Rangel and colleagues conducted a comprehensive phylogenetic analysis, placing MHNT.VT.2075 within the Proborhyaenidae, outside the saber-toothed lineage. "This new record supports the presence of a large sparassodont predator in the swamp/lacustrine depositional environment of the Tremembé Formation," they concluded.
The Tremembé Formation itself is the only fossil-rich unit in Brazil from the Late Oligocene that preserves mammal remains. Over the past decades, it has yielded a rich mosaic of life: bats, armadillos, rodents, extinct ungulates, ancient crocodiles -- and now, a top-tier predator.
This lone tooth deepens our picture of the ecological web in ancient Brazil. The presence of a large carnivore implies an ecosystem capable of sustaining big herbivores. The formation has already revealed such prey: tapir-like astrapotheres, horse-like litopterns, and armored notoungulates. Into this world of strange herbivores, MHNT.VT.2075 descended with precision bites.
As climate cooled and South America remained isolated from other continents, its native fauna took eccentric paths. Sparassodonts like this proborhyaenid were mammals with familiar functions but unfamiliar faces. No living carnivore descends from them. They left no heirs.
But their teeth remain.
And in doing so, this one battered canine breathes new life into an ancient swamp monster.