A groundbreaking new research project has analyzed the evolution of the placental mammal skull using 3D scans of 322 specimens housed in more than 20 international museum collections, and based on the emerging patterns, created a new model of how mammals diversified.
The team of researchers led by Prof. Anjali Goswami at the Natural History Museum gathered data on the skulls of all major groups of extinct and extant placental mammals to trace the adaptive radiation (rapid evolution that fills a variety of ecological niches) of mammals and decipher what drove their incredible rise in the aftermath of the dinosaur extinction.
"This research will transform how we understand the incredible radiation of placental mammals, a group that includes our own species, and how that critical period after the last mass extinction 66 million years ago has shaped evolution ever since," Prof. Goswami says.
The arrival of the Mammalian Age
Although the earliest mammals coexisted with dinosaurs, their diversity was limited, with the largest mammals of the Mesozoic Era growing to the size of a small dog. However, within a few 100,000 years of the dinosaurs' extinction, there is an incredible explosion of diversity among placental mammals, with the earliest ancestors of today's living groups appearing in the fossil record.
This new study, however, shows that after the initial burst of mammal diversification, the rate of evolution quickly slows. Later bursts of faster evolution do occur, but their effects diminish over time and never match the speed of that first peak. While the exact timing of these later bursts makes attribution to specific events difficult, they are most likely caused by periods of rapid or sustained climate change and global cooling throughout the Cenozoic era.
The study also reveals striking similarities, or convergence, among placental mammals, with most mammal skull shapes evolving in a similar fashion throughout the fossil record. Whales and rodents are the most notable exceptions.
What causes mammals to evolve so quickly?
One key goal of this research is to better predict how different species will react to rapid changes in their environment, such as those that are likely to occur during the current planetary emergency. The team investigated the characteristics of fast-evolving mammals and discovered that the key influencers are habitat, social behaviors, diet, parental care, and activity time.
The rate at which mammals evolve is greatly influenced by social structures. Mammals that live in groups evolve much faster than those that live alone. This is evident in ungulates that have evolved horns and antlers for fighting and social display. Water-dwelling mammals such as whales, but also manatees, seals, and walruses, are fast evolvers. Herbivores evolve at a faster rate than carnivores, most likely because they monitor changes in plants and the environment more closely than meat eaters.
"I conducted the majority of the analyses for this paper while isolating at home for several months at the beginning of the COVID-19 pandemic, so seeing the results of evolution in social versus solitary mammals really hit home," Prof. Goswami adds.
Parental care appears to be a significant factor slowing the rate of evolution. Precocial animals, such as horses and antelopes, evolve much faster than altricial mammals, such as primates, who are dependent on caregivers in infancy. When animals are active also makes a difference, with species that follow a strict schedule, whether nocturnal or diurnal, evolving slower than those that do not.
Surprisingly, the groups of mammals with the most species, rodents and bats, do not appear to evolve quickly, implying that in mammals, diversity in shape and diversity in number are not closely linked.
What were the first placental mammals like, and why haven't scientists discovered their fossils?
The team also used this new dataset to reconstruct what the earliest placental mammals might have looked like. Despite the fact that there are many fossil mammals from the right time period, scientists have struggled to identify fossils that represent the ancestors or early members of the placental mammal group, which would have lived in the late Cretaceous, just before the dinosaur extinction.
One issue is that it is difficult to predict what characteristics to expect in the earliest representatives of any of the major groups of mammals and whether scientists would recognize them in known fossils. The new reconstructions in this study show that the earliest members of all major groups of placental mammals looked very similar to each other, regardless of whether they were the ancestor of rodents and their close relatives or elephants and their close relatives. This means that identifying the earliest fossils of placental mammals may remain difficult, but these new reconstructions provide a better understanding of the subtle differences that scientists should look for in those early fossil mammals.
Prof. Goswami elaborates, "Museum collections are an invaluable resource because they allow us to forecast the future by peering into the past. Approximately one-third of the samples used in this study came from the Museum's collections, including a stunning 3D scan of Hope, the blue whale on display in Hintze Hall. These data are invaluable in understanding how past events shaped mammal evolution during the Cenozoic era, as well as which characteristics will help mammals survive the environmental challenges that lie ahead."
Journal information: Science