An analysis of fossil teeth from mammals that lived in China following the most recent major mass extinction suggests size came before both shape and function as diets diversified.
This discovery, published previously as a Reviewed Preprint in eLife and appearing today as the final Version of Record, sheds light on how biodiversity recovered in different regions of the world following the end-Cretaceous extinctions around 66 million years ago. eLife's editors say it is an important contribution to our understanding of ancient mammal evolution in Asia, proposing a 'brawn before bite' hypothesis based on diverse analytical approaches.
The end-Cretaceous Period is one of five significant mass extinctions in Earth's history. It led to the demise of large dinosaurs and gave way to a period of global greenhouse conditions and a dramatic rise of placental mammals – otherwise known as the 'Age of Mammals'. Most of our knowledge about how mammals recovered from the extinction comes from fossil sites in North America, while far less is known about how they adapted to a changed environment in other parts of the world.
"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," says first author Jack Tseng, Associate Professor in the Department of Integrative Biology and Museum of Paleontology, University of California, Berkeley, and Research Associate in the Department of Vertebrate Paleontology, Natural History Museum of Los Angeles County, US. "Most known fossil sites worldwide are found in North America, while only 3% are in Asia and most of those contain species that are not found elsewhere. As far as we know, previous analyses of recovery dynamics following the end-Cretaceous Period have not included samples from Asia."
To address this gap, Tseng and colleagues developed the largest existing dataset of Asian placental mammal samples from the Paleocene epoch, the first 10 million years (approximately) of the Age of Mammals. The dataset included 48 specimens found across three of the most fossil-rich Paleocene sedimentary sites in paleotropical Asia: the Nanxiong, Qianshan, and Chijiang Basins, all located in present-day south China. The specimens were from members of the Pantodonta (large herbivorous mammals), Arctostylopidae (stocky herbivorous and omnivorous mammals), and Anagaloidea (a group closely related to rodents).
From these specimens, the team created a new dataset of 200 teeth, representing 37 species endemic to east Asia. "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," explains author Qian Li, Professor at the Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China.
Using high-resolution 3D modeling and simulations, the team quantified variations in the shape and function of these fossil teeth. Their analyses revealed that, during the early Paleocene, mammals in east Asia were relatively large and had a high average tooth size. There was also little variability in the height and sharpness of their teeth around the same time. However, this variability increased over the next five million years, peaking around mid-way through the epoch. At this point, teeth became increasingly specialised for different forms of chewing and food processing, which the authors say reflects the likely diversification of diets and ecological niches within mammal communities as time went on.
Their findings also suggest these early mammals were ecologically flexible, meaning they were able to exploit a wide range of food resources and adapt to changing environmental conditions.
"The overall increase we saw in mammals' dental complexity and curvature over time coincided with an increase in drought-tolerant flora in south China and specifically with paleoenvironmental reconstructions in the Nanxiong Basin," says author Suyin Ting, former Collections Manager at the Museum of Natural Science, Louisiana State University, Baton Rouge, US. "As ecosystems recovered and transformed following the end-Cretaceous period, so did tooth shape and function, which indicates changing evolutionary pressures and ecological opportunities."
This increasing dental complexity and curvature lagged behind tooth size during the early Paleocene, before both shape and function achieved their maximum linkage towards the end of the epoch. This so-called 'brawn before bite' pattern mirrors a similar one seen in the jaw structures of the mammals' North American and European counterparts in previous studies. Together, these findings suggest that an initial size-driven recovery followed by evolutionary and ecological changes in parallel is a global phenomenon.
