An Ancient Mammal with New Teeth Discovered

  A team of Chinese and American scientists discovered a new mammal from the 165 million-year-old lakebeds of the Jurassic Period in Northern China. This new find sheds light on the earliest mammalian evolution, especially the convergent evolution of the important tribosphenic teeth among early mammals.Mammals have very diverse teeth. Groups of mammals are distinguished by what they eat with – the teeth. Giraffes and zebras are plant eaters, cats eat meat, aardvarks feed on termites, and many primates prefer fruits. Mammals have specialized feeding adaptations of a great variety, thanks to their different teeth.But these diverse modern mammals are all descendants from some ancient mammals - living with the dinosaurs in the Mesozoic - with tribosphenic teeth (word etymology: "sphen" is Greek for shearing and cutting; "tribo" is Greek for grinding and pounding. The "tribo-sphenic" molars have both a cutter and a grinder.). The earliest marsupial and placental mammals and their kin all had tribosphenic teeth capable of not only cutting, but also grinding. The combined shearing and grinding tooth structure made more versatile feeding functions possible, and are therefore important for early mammalian diversification.In the November 1st, 2007 issue of the international scientific journal NATURE, a team of American and Chinese scientists announced the discovery of a new mammal of the Jurassic Age in China with “pseudo-tribosphenic” teeth. The new discovery suggests that mammals were far more diverse in the age of dinosaurs than previously thought.The pseudo-tribo-sphenic mammal teeth are superficially similar to the tribo-sphenic teeth in having a cutter and a grinder; but the cutter and grinder are arranged in just the opposite positions. Dr. Zhe-Xi Luo, a curator of paleontology at Carnegie Museum of Natural History and a member of the international team that made the discovery, noted that: “In the pseudo-tribo-sphenic, the grinder is in front of the cutter; in the true tribo-sphenic teeth of the ancestors of marsupials and placentals, the cutter is in front of the grinder.”“The story of the earliest mammals is a story of their teeth”, said Dr. Luo. “By tracing their evolution in the rich fossil record of the Mesozoic, we get to understand how these cutting and grinding teeth evolved over and over again.”Because tribosphenic teeth are such a unique and intricate structure, paleontologists used to believe that there must have been a single origin in the Mesozoic. However, the pseudo-tribosphenic molars from the new fossils in China show that similar structures to combine cutting and grinding had evolved several times.Under natural selection, organisms descending from different ancestors can evolve analogous structures and similar adaptations, a phenomenon known as convergent evolution. “The pseudo-tribosphenic teeth and the true tribosphenic teeth are great examples of convergent evolution and a great manifestation of how dental and feeding adaptation can be achieved by different lineages of mammals,” said Luo.With their versatile functions for chewing different kinds of food, the tribosphenic and pseudo-tribosphenic teeth are key evolutionary innovations that enabled some of the earliest mammals to diversify in the Mesozoic times. They are considered by many paleontologists to be more advanced than the primitive mammal teeth that were limited to cutting, not grinding or crushing.Fossil mammals with the tribosphenic and pseudotribosphenic teeth actually belonged to two distinct clans: the pseudotribosphenic are more closely related to monotremes; and one group of true tribosphenic mammals is closer to placentals and marsupials and their close kin.