In 2004, a graduate student named Andres Giallombardo was working the rocky badlands of Mongolia's eastern Gobi Desert on an expedition sponsored by the American Museum of Natural History. The small fossil he found there would take more than two decades to tell its full story.
That story was told in July in the journal Nature, and it settles a puzzle that palaeontologists have argued over for nearly 40 years.
"This discovery illustrates why fieldwork remains indispensable to understanding life's history," said Giallombardo, the study's lead author. "It was the thrill of a career to find a new species so completely preserved that also solves a longstanding scientific problem."
A mystery made of teeth
The puzzle concerns zhelestids, a group of small mammals that lived in the Late Cretaceous, alongside the dinosaurs. Their molars were first reported in the late 1980s, and since then scientists have mostly had isolated teeth and jaw fragments to work with.
Those teeth were unusual. Many Cretaceous mammals had sharp, high-cusped teeth suited to crunching insects. Zhelestid teeth were low and rounded, more like those of later plant-eating hoofed mammals.
Teeth are hard-wearing and often all that survives, so palaeontologists lean on them heavily. Some researchers proposed that zhelestids were early relatives of hoofed mammals, or even among the earliest placental mammals, the group that includes humans, whales, bats and most living mammals. Without a skeleton, nobody could test the idea properly.
What the whole animal revealed
The new species, named Tamirkhan balcarceli, is the most complete zhelestid ever found: a skull with teeth and a partial hind limb, all from a single individual.
Its teeth are classic zhelestid. The rest of the animal tells a different story. It has long, ever-growing front incisors, distinctive skull features, and fused lower hind-limb bones with elongated foot bones. Those are hallmarks of zalambdalestoids, a well-known group of small, shrew-like Cretaceous insect-eaters.
"Tamirkhan's hind legs are long and slender with distinctive ankles, traits that are unmistakably zalambdalestoid," said co-author Shawn Zack of the University of Arizona.
The team concludes that zhelestids were neither placental mammals nor especially placental-like. In the words of the paper, they were "a dental variant within Zalambdalestoidea". Their plant-grinding teeth were most likely a case of convergent evolution, in which unrelated animals arrive at similar features because they adapt to similar diets.
"More than 200 years ago, French naturalist Georges Cuvier famously argued that a single tooth could allow scientists to predict the anatomy of an entire animal," said co-author Maureen O'Leary of Stony Brook University. "While teeth remain among the most informative fossils available, this work demonstrates that teeth cannot always tell us how the whole animal looked."
Why 'Cretaceous rabbit'?
Tamirkhan measured about 15 to 18 centimetres from nose to tail, roughly the size of a chipmunk. Its long hind legs gave it a rabbit-like silhouette, which is why palaeontologists have sometimes informally called zhelestids "Cretaceous rabbits".
The nickname describes its looks, not its family. According to the museum, zhelestids were only distantly related to modern rabbits, and the study does not suggest they were rabbit ancestors.
What changed, and what didn't
The finding reclassifies one group of ancient mammals. It does not settle when placental mammals first appeared. Molecular clock studies, which estimate timings from the DNA of living species, suggest placentals may date back into the Cretaceous. Zhelestids had been seen as possible fossil evidence for that idea, but the new study indicates they do not fill that role.
"Molecular models based on living animals can predict the past, but fossils provide the evidence needed to test those predictions," said co-author Paúl Velazco of Arcadia University.
To place the animal on the family tree, the researchers used MorphoBank, a shared database of anatomical features from living and extinct mammals that has been built up since 2013.
Co-author Michael Novacek, a curator at the American Museum of Natural History who has co-led annual Gobi expeditions with the Mongolian Academy of Sciences since 1990, said: "The Gobi is one of the only places where we routinely recover such remarkably complete Cretaceous mammals."
The study, "Cretaceous zhelestid mammals are zalambdalestoids", is published in Nature, volume 657, DOI 10.1038/s41586-026-10861-5. Its authors are Andres Giallombardo, Eva A. Hoffman, Maureen A. O'Leary, Paúl M. Velazco, Shawn P. Zack and Michael J. Novacek, from the American Museum of Natural History, Stony Brook University, the University of Arizona, Arcadia University and Yale University.
