EMBARGO TIMES

1900 London time (GMT) on 3 January (the day before publication).

1400 US Eastern Standard Time on 3 January in Pittsburgh time.

January 3 , 2001

Tribosphenic Mammal Discovery

Pittsburgh, Pennsylvania... To biologists who study mammals, you are what you eat with. Mammal teeth can not only reveal an animal's diet but also its place on the family tree.

In the January 4th, 2001 issue of the international science journal NATURE, a team of international scientists have proposed a new theory of evolution of the ancestral mammalian tooth structure, known as tribosphenic teeth.

The story of the earliest mammals is a story of their teeth.

The earliest marsupial and placental mammals all had the tribosphenic teeth capable of not only cutting, but also grinding (Image No. 1). ("Sphen" is Greek word for shearing and cutting; "tribo" is the Greek work for grinding and pounding. The "Tribo-Sphenic" molars have both a cutter and a grinder.)

With their versatile functions for chewing different kinds of food, the tribosphenic teeth are a key evolutionary innovation that enabled marsupials and placentals 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 cut, but not for grinding or crushing.

Because the tribosphenic teeth are such a unique and intricate structure, paleontologists have thought for several decades that it must have a single origin in the Mesozoic time. However, Drs. Luo, Cifelli, and Kielan-Jaworowska suggest that the tribosphenic molars evolved twice in the Mesozoic time. "To have it evolved once was good, but to have it evolved twice is even better," says Luo.

They recognize that the fossil mammals with this type of teeth actually belonged to two distinct clans (Image No. 2): the southern clan (the "australosphenidans"), which are related to living monotremes; and the northern clade (the "boreosphenidans"), which include placental mammals and marsupials, and their close kins. They propose that it independently originated in both southern mammals (such as the fossil relatives of the platypus), and in the earliest known relatives of marsupials and placentals which, in Meszoic time, were only known from northern continents.

In the Jurassic and Early Cretaceous (160 to 110 million years ago), the continents and oceans of the world were in very different places from where they are today (Image No. 3). Australia, the Antarctica, Africa, South America, India and Madagascar were all clustered together as unified southern land-masses, known as the Gondwana.

It is turns out that the newly discovered tribosphenic mammals from Madagascar and from Australia belonged to the same Gondwana land-mass as the fossil and living monotremes. And they were far separated the northern continents where the Mesozoic marsupials and placentals were found. The relationships of southern tribosphenic mammals, as proposed by Dr. Luo and his co-workers fit nicely with the geography of the world in the Mesozoic times.

Contact Leigh Kish for available images.