Field photos of the Khugenetsavkhlant badlands in the Eastern Gobi Desert of Mongolia, which is where Tamirkhan balcarceli was discovered.Michael Novacek/© AMNH
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.
“This discovery illustrates why fieldwork remains indispensable to understanding life’s history,” said the study’s lead author Andres Giallombardo, who found the specimen as a graduate student on a Museum-sponsored expedition in 2004. “It was the thrill of a career to find a new species so completely preserved that also solves a longstanding scientific problem and a reminder that the Gobi Desert, which is well known for fossils, continues to change science.”
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.
It’s a reminder that appearances, and especially teeth, can be deceiving.
“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 study coauthor Maureen O’Leary, a paleontologist at Stony Brook University and research associate at the Museum. “While teeth remain among the most informative fossils available, this work demonstrates that teeth cannot always tell us how the whole animal looked.”
Named Tamirkhan balcarceli, the new species was likely about 6-7 inches long from nose to tail, with long hind limbs that gave it a rabbit-like silhouette. Because of this resemblance, paleontologists have sometimes informally referred to zhelestids as “Cretaceous rabbits,” despite their distant relationship to modern rabbits.
The fossil also underscores why the Gobi Desert remains one of the world’s greatest windows into prehistoric life. Its rocky outcrops have yielded some of the best-preserved dinosaur and early mammal fossils ever discovered, as highlighted in the new Museum exhibition Fossils of the Flaming Cliffs.
For elusive animals like zhelestids, which are known from only a handful of locations across Central Asia, each new specimen helps complete a much larger evolutionary puzzle.
“The Gobi is one of the only places where we routinely recover such remarkably complete Cretaceous mammals,” said coauthor Michael Novacek, a curator in the Museum’s Division of Paleontology who has co-led annual expeditions to the Gobi since 1990 in partnership with the Mongolian Academy of Sciences. “These extraordinary fossils continue to transform our understanding of mammalian evolution.”
To figure out exactly where the new species belongs, the researchers turned to MorphoBank, a collaborative database that allows scientists to compare hundreds of anatomical characteristics across living and extinct mammals.
Developed over more than a decade, the platform has become one of the most comprehensive resources for studying early mammal evolution.