Research article:
Giallombardo, A., Hoffman, E.A., O’Leary, M.A. et al. Cretaceous zhelestid mammals are zalambdalestoids. Nature 657, 973–980 (2026). https://doi.org/10.1038/s41586-026-10861-5 (NOT open access)
Snippet:
More than 20 years after it was first discovered in Mongolia’s Gobi Desert, a remarkably complete fossil is changing how scientists think about the evolution of mammals during the age of dinosaurs.
The little mammal, about the size of a chipmunk, makes its formal debut today in a study describing the ancient species published in the journal Nature based on work led by a team of scientists from the Museum, Stony Brook University, University of Arizona, and Arcadia University.
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Paleontologists have long debated about the evolutionary relationships of a group of extinct mammals called zhelestids, mostly known from isolated teeth or small jaw fragments. Unlike the sharp, insect-eating teeth common among many mammals from the Cretaceous Period, zhelestids had low-crowned teeth that appeared well suited for eating plants.
Those teeth led scientists to propose that zhelestids were part of an unknown group of hoofed mammals, and some researchers suggested that these animals were among the earliest placental mammals, the group that today includes humans and most mammals. Molecular clock studies support this idea, concluding that placental mammals originated long before the end of the Cretaceous.
Unlike earlier zhelestid discoveries, the newly described fossil preserves much more of the animal’s skull and skeleton, giving researchers their first real look at what a zhelestid actually looked like.
This specimen’s rounded teeth fit the pattern scientists expected for a zhelestid. But there also were surprises, including long, ever-growing front teeth, specific skull features, elongated hind legs, and specialized ankle bones. These characteristics closely resemble those of another Cretaceous group called zalambdalestoids—small, shrew-like insectivore mammals.
Taken together, the features suggest that zhelestids were not close relatives of placental mammals after all. Instead, they belonged to the zalambdalestoid branch of the mammal family tree. The distinctive rounded teeth of zhelestids are likely a case of convergent evolution, when unrelated animals evolve similar traits because they adapt to similar lifestyles.