Ancient mammal teeth reveal how mammals adapted after the dinosaurs disappeared

Mammals entered a transformed world after the dinosaurs vanished around 66 million years ago.

As woodland regenerated and the climate changed, mammals expanded into environments that giant reptiles had dominated for a long time.

Fossils of early mammals are known from what is now North America, but researchers have a much poorer understanding of how these animals adapted elsewhere in the world.

What ancient mammal teeth reveal

A recent study of fossilised teeth from ancient mammals in present-day South China provides new evidence.

It indicates that following the end-Cretaceous mass extinction, mammals increased in size before their teeth developed specialised functions suited to distinct diets.

This order of change may have enabled them to exploit shifting conditions while ecosystems gradually rebuilt.

Looking beyond North America

Most scientific knowledge of the opening 10 million years of the Age of Mammals is based on fossils unearthed in North America.

Asia has yielded considerably fewer fossils, creating significant gaps in the account of mammalian evolution after one of Earth’s five major mass extinctions.

Jack Tseng, the study’s lead author, is an associate professor in the Department of Integrative Biology and Museum of Paleontology at the University of California, Berkeley.

“There is a substantial imbalance in both the amount and quality of fossil records documenting the first 10 million years of the Age of Mammals,” said Tseng.

“Most known fossil sites worldwide are found in North America, while only 3 percent are in Asia, and most of those contain species that are not found elsewhere.”

Fossil teeth hold valuable clues

The researchers compiled the largest available dataset of Paleocene Asian placental mammal fossils, spanning approximately the first 10 million years after the extinction.

They examined 48 fossil specimens recovered from the Nanxiong, Qianshan and Chijiang Basins in modern-day southern China.

These specimens covered several mammal groups, including sizeable herbivores and omnivores, as well as species closely related to today’s rodents.

Using these remains, the team created a set of 200 fossil teeth from 37 species found exclusively in East Asia.

Qian Li is a professor at the Key Laboratory of Vertebrate Evolution and Human Origins at the Chinese Academy of Sciences in Beijing.

“The fact that teeth interact directly with the environment through biting and chewing behaviours makes them crucial in helping us identify potential links between their structure and ecological context,” explained Li.

Bigger first, specialized later

Researchers frequently use fossil teeth because they survive well and can reveal much about an animal’s way of life.

The form, height, sharpness and degree of wear on teeth can indicate an animal’s probable diet, including whether it crushed resilient plants, cut through meat or processed a varied diet.

Through high-resolution 3D models and computer simulations, the team assessed fine differences in tooth form and performance.

In the early Paleocene, East Asian mammals generally had comparatively large bodies and bigger teeth. Yet their teeth varied little in height or sharpness.

Over about the following five million years, this trend shifted. Tooth forms became far more diverse and progressively adapted to different methods of chewing and processing food.

The results indicate that mammal diets diversified as ecological niches emerged and were filled following the dinosaurs’ extinction.

Animals in a changing world

The team also concluded that these early mammals were apparently ecologically adaptable.

Rather than depending on a limited food supply, they probably used a broad selection of resources as their environments continued to shift.

Suyin Ting previously served as collections manager at the Museum of Natural Science at Louisiana State University.

“As ecosystems recovered and transformed following the end-Cretaceous period, so did tooth shape and function, which indicates changing evolutionary pressures and ecological opportunities,” said Ting.

Bigger first, specialized later

The researchers determined that greater tooth complexity followed increases in tooth size. They call this order a “brawn before bite” pattern.

Earlier research on mammals in North America and Europe has identified comparable patterns.

The findings indicate that this recovery route may have taken place in several regions of the world.

What the findings mean

The researchers acknowledged limitations in the study. Because most fossils belonged to herbivorous mammals, it is difficult to reach wide-ranging conclusions about every mammal group.

There are also inherent limitations to reconstructing ancient diets from teeth, while future fossil finds will help create a more complete view of Paleocene ecosystems.

“Our work provides initial insights into the changes that occurred in placental mammals outside of North America after the end-Cretaceous Period, and unearths opportunities for further research to explore some of these associations in more detail,” Tseng concluded.

“We hope the insights will be useful for developing predictive models and conservation strategies aimed at understanding how modern animals could respond to biodiversity crises in future.”

The complete study appeared in the journal eLife.