Mammal Teeth Evolved Twice

Jan. 4, 2001 -- The specialized teeth that enabled ancient, shrew-like creaturesto flourish and gave rise to all modern mammals evolvedindependently in two animal groups living continents apart, a studysuggests.

Scientists said the finding could dramatically alter theoriesabout the pace of early mammals’ global advance in the waning daysof the dinosaurs more than 65 million years ago.

It may also shed light on the relationships between extinct andliving mammal lineages, including humans, which also possess theteeth.

For the study, three paleontologists analyzed fossilized teethand bones of mammals that lived between 65 million and 200 millionyears ago.

Separate Evolution

By comparing the teeth’s characteristics, they concluded thatthe advanced molars that ensured the small creatures’ successevolved in two mammal groups that arose separately on northern andsouthern continents.

Previous theories had suggested that these specializedtribosphenic molars — which can simultaneously shear food and crushit in mortar-and-pestle fashion — had evolved only once, in theNorthern Hemisphere.

However, recent finds in Madagascar and Australia cast doubt onthat idea, suggesting a southern origin for the dual-functionmolars.

The latest study was published in today’s issue of thejournal Nature. It was led by Zhe-Xi Luo of the Carnegie Museum ofNatural History.

Luo said the primordial animals’ advanced teeth gave them anevolutionary advantage by allowing them to eat a wider range offoods than other more primitive mammals.

Other scientists hailed the new study but cautioned that morefossils are needed to support or refute its conclusions.

“This is a bold hypothesis that takes the dental evidence aboutas far as it can go,” said Leonard Krishtalka, a professor ofecology and evolutionary biology at the University of Kansas.

Luo and two colleagues scrutinized fossils of 21 early mammalsranging in size from shrews to small raccoons.

After Pangea Broke Apart

The scientists said the dual north-south evolution of theadvanced tribosphenic teeth occurred after the supercontinent ofPangea split into southern and northern continents about 160million years ago.

The scientists’ revised mammal evolutionary tree suggests thatplacental and marsupial mammals originated in the northerncontinents, while three different types of mammals appeared in thesouthern continents.

Of the three southern lineages, only one remains — themonotremes, which are represented today by the egg-laying platypusand echidna, the scientists said.

One of the recent fossil finds that challenged mammal evolutiontheories was a jaw fragment found in Madagascar in the late 1990sthat contains three of the advanced molars. The167-million-year-old fossil is 25 million years older than allprevious tribosphenic fossils — and it was found in the southern,not the northern, hemisphere.

The co-discoverer of that fossil, John J. Flynn, a curator atthe Field Museum of Natural History in Chicago, said the new studyis “the first volley of what will be a very interesting debate”over the evolution of mammals.