Dr. Velazco Part of Team Rewriting Chapter of Mammal Evolution

A remarkably preserved fossil from Mongolia's Gobi Desert is reshaping scientists' understanding of mammal evolution during the age of dinosaurs.

The new species, described recently in the journal Nature by a team of scientists including Dr. Paul Velazco, associate professor of Biology, reveals that a group of extinct mammals known as zhelestids were not close relatives of modern placental mammals as many researchers thought for decades. Instead, they belonged to an entirely different branch of early mammals, overturning a longstanding interpretation based primarily on fossil teeth.

"This discovery illustrates why fieldwork remains indispensable to understanding life's history," said the study's lead author Andres Giallombardo of the American Museum of Natural History, 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."

Zhelestids have been known from isolated teeth and fragmentary fossils for nearly 40 years. Their distinctive teeth, which are more specialized for eating plants than the sharp, insect-eating teeth found in many Cretaceous mammals, led paleontologists to suggest that zhelestids represented an unknown group of hoofed mammals.

Researchers have debated whether these animals were examples of Cretaceous placental mammals-the group that today includes humans and most living mammals-or a separate lineage that evolved similar features independently. The idea that placental mammals existed during the Cretaceous is supported by molecular clock studies, which suggest that placentals originated long before the end of the Cretaceous.

The fossil described in the new study provides evidence to the contrary.

"Molecular models based on living animals can predict the past, but fossils provide the evidence needed to test those predictions," Velazco said.

Discovered in the eastern Gobi Desert, Tamirkhan balcarceli is the most complete zhelestid found yet and solves the mystery of what this group looked like. Although Tamirkhan possesses the low, rounded teeth characteristic of zhelestids, it also has long, ever-growing incisors and a suite of distinctive skull features found in a group of small, shrew-like insectivore mammals called zalambdalestoids.

"Tamirkhan's hind legs are long and slender with distinctive ankles, traits that are unmistakably zalambdalestoid," added Shawn Zack, a paleontologist at the University of Arizona and co-author on the study.

Based on these findings, the researchers conclude that zhelestids were not placental mammals or particularly placental-like, but a subset of the zalambdalestoid group. The discovery also shows that the mammal species in the Cretaceous were not as anatomically diverse as might have been anticipated based on the teeth alone. Their distinctive rounded teeth-common to plant-eating mammals-are likely a case of convergent evolution, where unrelated animals evolve similar traits because they adapt to similar lifestyles.

The study's evolutionary conclusions were made possible through a comprehensive mammalian phylogenetic research platform called MorphoBank. First published in 2013, the database has expanded over more than a decade to include an ever-growing number of key fossil species, providing the most comprehensive framework yet assembled for analyzing early mammal relationships.

Other authors on the study include Maureen O'Leary of Stony Brook University, Michael Novacek of the American Museum of Natural History, and Eva Hoffman of Yale University.