The evolution of the complex mammalian molar — a tooth that can both slice and grind — has been long regarded as the key innovation that prompted the exceptional success of mammals since the Cretaceous. A new fossil discovery lends evidence to the controversial theory that this tooth, the tribosphenic molar, evolved more than once, according to paleontologist Oliver Rauhut and colleagues at the Museo Paleontológico Egidio Feruglio, Argentina and Freie Universität, Germany, who published work in the March 14 Nature.
Although the Jurassic was a critical stage in the evolution of mammals, the fossil record of Jurassic mammals is extremely sparse. The new specimen, a tooth-bearing lower jaw from the Patagonia region of Argentina, is the first find of a Jurassic mammal in South America.
“Our mammal has lower molars with fully developed talonids, basin-like structures which acted together with the upper molars as mortar-and-pestle. Even though the upper teeth of the new mammal are not known, it can be inferred that they possessed a structure called a protocone, which fitted into the talonid,” says co-author Thomas Martin of Freie Universität. This arrangement is characteristic of the tribosphenic molar, he adds. The scientists identified the jaw as belonging to a southern clade of mammals, indicating that the tribosphenic molar may have evolved separately in the Southern Hemisphere.
“The oldest known tribosphenic molars from the Northern Hemisphere are from the early Cretaceous period, about 20 million years later than our find. This supports the hypothesis that the tribosphenic molar evolved much earlier on the southern continents than on the northern continents,” Martin says. Mammals alive today are believed to have originated in the Northern Hemisphere, while those in the group that evolved in the Southern Hemisphere are now extinct, except for egg-laying mammals such as the platypus.
“Previous fossil records of Jurassic mammals of South America were not just inadequate — they were deplorable,” says Zhe-Xi Luo of the Carnegie Museum of Natural History. With an article that appeared in the Jan. 4, 2001, Nature, Luo was among the first researchers to propose the dual origin of the tribosphenic molar. “Paleontological studies do not get any better or more exciting than what Rauhut and colleagues have accomplished: the discovery of an interesting fossil from a crucial geographic location and crucial time, providing a critical yet previously missing piece to the global puzzle of the mammalian evolution.”
Geotimes contributing writer