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Carnegie Museum of Natural History Media Relations 4400 Forbes Avenue Pittsburgh, PA, 15213 |
Contact: Ellen James (Carnegie Museum of Natural History) Press Embargo by NATURE Until: November 1, 2007 Discovery of An Ancient Mammal with New Teeth
Pittsburgh, Pennsylvania... 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 (Image No. 1). 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 (Image No. 2) 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 (Image No. 3). 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 (Image No. 2). 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.” (Image No. 1) “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 (Image No. 3). The new mammal Pseudotribos robustus is an insectivore feeding on worms and insects (Image No. 4). Its skeleton is about 12 cm long, and its estimated weight would be about 20 to 30 grams. This small animal had very robust limbs, and it lived on the ground but was also capable of power digging. Co-existing with this mammal are very rich freshwater arthropods, salamanders, other mammals (the swimming Castorocauda and gliding Volaticotherium) and several dinosaurs. The fossil was discovered in 2004 from the Ningcheng County of Inner Mongolia Region of China and is now deposited in the Institute of Geology, Chinese Academy of Geological Sciences (Beijing). For further information, please contact: Dr. Qiang Ji
Illustration: Mark A. Klingler, Carnegie Museum of Natural History 1.37
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Illustration: Mark A. Klingler, Carnegie Museum of Natural History 1.78
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Illustration: Mark A. Klingler, Carnegie Museum of Natural History 759
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Illustration: Mark A. Klingler, Carnegie Museum of Natural History 3.71
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Image
No. 1: Fossil site of the new Jurassic mammal Pseudotribos
robustus.
The fossil is from the lakebeds of the Jiulongshan Formation of Ningcheng
County, Inner Mongolia Region of China. The formation is about 165
million years old. Co-existing with Pseudotribos are such
mammals as the swimming Castorocauda and gliding Volaticotherium,
salamanders, pterosaurs and dinosaurs. 
Image
No. 2: The
new mammal
Pseudotribos is unique in its “pseudo-tribosphenic” teeth
(schematic figures in lower row). In the pseudo-tribosphenic lower
molar teeth, the grinding and crushing structure (pseudo-talonid) is
in front of the cutting structure (trigonid). By contrast, in true
tribosphenic teeth (schematic figures in upper row), the grinding and
crushing structure (talonid) is behind the cutting structure. These
structures are in opposite arrangements, but similar in the combined
cutting and grinding tooth functions. This is a good example of organisms
from different ancestors evolving similar structures under natural
selection, a phenomenon known as convergent evolution. 
Image No.
3: Position
of the new Jurassic mammal Pseudotribos
robustus on the mammalian evolutionary tree. Its
jaw and skeletal structures suggest that Pseudotribos is
more closely related to monotremes than to marsupials and placentals.
Its upper molar is similar to the tribosphenic
ancestors of marsupials and placentals, but its lower molar has an
opposite arrangement of cutting (trigonid) and grinding (pseudotalonid)
structures. 
Image No.
4: The
new Jurassic mammal Pseudotribos is about 12 cm (~4
inches) long, and weighed an estimated 20 to 30 grams. It has very
robust limb bones, possibly for digging. Its teeth have versatile
feeding functions, and it can feed on insects and some plants. In
this artistic
reconstruction,
Pseudotribos is portrayed as foraging among the fallen ginkgo
leaves and the scattered shells of conchostracan arthropods on the
shore
of a shallow freshwater lake. 
