A remarkably complete fossil discovered in Mongolia's Gobi Desert is changing how scientists view mammal evolution during the age of dinosaurs. The newly identified species, described in the journal Nature by researchers from the American Museum of Natural History, Stony Brook University, University of Arizona, and Arcadia University, provides new evidence about an extinct group of mammals called zhelestids. For decades, many scientists thought zhelestids were closely related to modern placental mammals.
The new fossil suggests otherwise. Instead, zhelestids appear to have belonged to a very different branch of early mammals, overturning an interpretation that was based largely on fossil teeth. "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." Fossil Teeth Pointed Scientists in the Wrong Direction Zhelestids have been known to paleontologists for nearly 40 years, but most discoveries consisted of isolated teeth or incomplete remains. Their teeth were unusually specialized for eating plants compared with the sharper teeth used for eating insects that are common among many Cretaceous mammals. Those features led some paleontologists to propose that zhelestids represented an unknown group of hoofed mammals.
Scientists also debated whether they might have been placental mammals living during the Cretaceous, part of the broad group that today includes humans and most living mammal species, or whether they were part of a separate lineage that independently evolved similar characteristics. Molecular clock studies have supported the possibility that placental mammals originated well before the end of the Cretaceous. The newly described fossil, however, provides physical evidence pointing in another direction.
"Molecular models based on living animals can predict the past, but fossils provide the evidence needed to test those predictions," said study coauthor Paul Velazco, a comparative biologist at Arcadia University. A Nearly Complete Skeleton Reveals the Animal's True Identity The new species, Tamirkhan balcarceli, was discovered in the eastern Gobi Desert and is the most complete zhelestid specimen found so far. Because so much of the animal is preserved, scientists can now see what members of this mysterious group actually looked like.
Tamirkhan has the low, rounded teeth associated with zhelestids. But it also possesses long, continuously growing incisors and several distinctive skull features seen in zalambdalestoids, a group of small, shrew-like insectivore mammals. Its limbs provide another important clue.
Extract — continue reading at the source.
