Modern forms of mammals evolved long before the demise of the dinosaurs. This startling conclusion goes against textbook wisdom that the dinosaurs had to die before mammal evolution could really get going. In the 5 September 1996 Nature, Dr Emmanuel Gheerbrant of the Université of Paris and colleagues describe fossil teeth belonging to the earliest-known representative of the order of mammals that today includes the elephants. The fossil, which they call Phosphatherium, comes from the 60-m.y.-old phosphate deposits of the Ouled Abdoun basin in Morocco. It is 7 m.y. older than the previous record-holder for the earliest elephant.
Phosphatherium probably did not look very much like a modern elephant. It was about the size of a medium-sized dog, and nobody knows whether it had a trunk, tusks or big floppy ears. But although very primitive, the elephantine affinities of Phosphatherium are apparent from its teeth, yet it lived just 5 m.y. after the dinosaurs became extinct. Given that elephants are among the most highly evolved mammals, and given the usual picture of a spurt of mammalian evolution only after the dinosaurs had perished, this implies a huge burst of evolution between 65 and 60 Ma.
Some evolutionary biologists are unhappy with this conclusion, and some are now openly suggesting that many varieties of mammal began to emerge much earlier, back in the Mesozoic, when the dinosaurs were still very much alive. An analysis of the biochemistry of modern mammals by Dr Blair Hedges of Pennsylvania State University and colleagues, published in Nature earlier this year, points to this conclusion. But some palaeontologists are also bucking conventional wisdom.
In Nature in 1993, palaeontologist Robert Martin of the Anthropological Institute in Zürich, Switzerland pointed out that the fossil record is far from complete. Lineages of animals may have originated millions of years earlier than implied by known records of the earliest-known representatives of any particular group. To put it another way, if a fossil is distinctive enough to show signs of belonging to a particular lineage, then that lineage must already have been in existence for some time. So, although Phosphatherium is very ancient for a member of the elephant lineage, elephantine ancestry probably goes deeper still. Although its earliest representatives did not look much like what we would nowadays think of as elephants, the elephant lineage may have been distinct even before the last dinosaurs became extinct.
Mammals evolved more than 200 m.y. ago, at around the same time as dinosaurs. But dinosaurs quickly came to dominate life on land, evolving a large number of varied and often gigantic forms. But mammals stayed very much in the shadows, and contemporary forms (all belonging to strange and antique lineages, all now extinct) were small and rodent-like, and never exceeded the size of a rabbit. Only when dinosaurs were gone, the story went, could mammals emerge and diversify into the varied forms we see today, ranging from bats to whales, and (of course) ourselves. But the latest work suggests that although mammals may have remained small and secretive, they were busy evolving just the same, establishing distinct lineages against the day when they could emerge and blossom into the wide range of forms we see today.
© Macmillan Magazines Ltd. NATURE NEWS SERVICE 1996
Note: This item from the Nature News Service is mounted on this Web page by special permission of Nature.
[Return to Chapter 18]
[Return to UC Davis Geology Department Home Page]