By now, even schoolchildren know the story: Sixty-five million years ago, a giant asteroid slammed into Earth, ending the reign of the dinosaurs and allowing our mammalian ancestors to flourish. But a new study suggests that some of the earliest mammals were already doing well millions of years before dinos kicked the bucket. These mouse-sized creatures, known as multituberculates, appear to have started proliferating around the same time that flowering plants went through their own burst of evolution.
The multituberculates get their name from the many bumps, or tubercules, on the teeth at the rear of their jaws. They arose about 165 million years ago, and although they survived the mass extinction that killed the dinosaurs, the last species in the group died out about 34 million years ago.
Paleontologists attribute the long evolutionary success of multituberculates to their distinctive teeth. The bumpy choppers may have allowed the rodentlike creatures to munch everything from grains to insects to meatier prey. But no one knew for sure what the multituberculates actually ate.
So Gregory Wilson, a vertebrate paleontologist at the University of Washington, Seattle, and colleagues turned to a technique called orientation patch count, which involves counting the total number of distinct surfaces on the cheek teeth of the lower jaw. Living mammals with a low number of facets typically are meat eaters with bladelike teeth, and the species with complex, bumpy teeth generally are herbivores. Although carnivores usually just bite off hunks of meat and swallow them, with most digestion taking place in the gut, herbivores generally chew the tough plant matter before swallowing, grinding it up with large, bumpy teeth. Omnivores have an intermediate number of facets.
The orientation patch count technique "is a nice way to quantify the complexity of teeth," says Zhe-Xi Luo, a vertebrate paleontologist at the University of Chicago in Illinois, who was not involved in the study. "It has proven to be very effective in teasing apart differences in feeding adaptations that are otherwise rather inconspicuous."
Wilson and his team analyzed the cheek teeth of 48 fossils representing 41 different genera of multituberculates from many different eras. They used laser scans of the creature's jaws to identify facets as small as 30 micrometers across. Multituberculates that lived between 156 million and 100 million years ago had relatively few facets on their cheek teeth, suggesting that they were meat eaters, whereas members of the group that lived between 100 million and 84 million years ago had slightly more facets than did their ancestors, suggesting a generally mixed diet. Just before the dino die-off, between 84 million and 66 million years ago, some multituberculates' teeth had become complex, the researchers report online today in Nature. Of the 17 genera, or groups of closely related species, that lived during this era, five had facet counts that strongly suggest the animals either were full-blown herbivores or had a mixed diet dominated by plants, rather than feasting on insects or other small prey.
Wilson notes that the diversification of the multituberculate diet occurred during an era when flowering plants, or angiosperms, were evolving into a variety of forms, including trees, herbaceous plants, and epiphytes, a group of nonparasitic species that live on other plants and that includes today's orchids and bromeliads. That spate of evolution among angiosperms, which occurred millions of years before the mass extinction that claimed the dinosaurs, opened new ecological opportunities that multituberculates then diversified to exploit, the researchers suggest. Not only does the new study show that multituberculates began to diversify long before the dinosaurs died out, Wilson says, it reveals that species with a plant-based diet grew larger, expanding from the size of a mouse to a body mass of about 5 kilograms (slightly heftier than a large groundhog).
The new study "is a very valuable contribution," says William Clemens, a vertebrate paleontologist at the University of California, Berkeley. "This is a nice first step toward better understanding the dietary preferences of a very important group of mammals."
But Anne Weil, a vertebrate paleontologist at Oklahoma State University's Center for Health Sciences in Tulsa, is more cautious, noting the purported link between diversification in angiosperms and multituberculates is "circumstantial but interesting."
Nevertheless, Weil says, the new findings can serve as a springboard for further research—regarding, for example, why multituberculates survived the dino-killing extinctions but ultimately died out themselves. Possibly, she notes, species in the group were outcompeted by rodents, primates, and other mammals that arose a few million years before the multituberculates went extinct.