Shortly after the dinosaurs went extinct, most mammals weighed less than ten kilograms. But within a few million years, giant species weighing several tons emerged. This development was particularly impressive in the case of the Brontotheres, also known as thunder-hoofed animals. These rhino-like herbivores were among the first to produce giant species. Using fossils, paleontologists have now uncovered the evolutionary mechanisms underlying the evolution of Brontothera.
In the Eocene, around 56 to 33.9 million years ago, the so-called brontothera, also known as thunder-hoofed animals, lived in various parts of the world. They resembled rhinos, but are more closely related to today’s horses. Fossils from North America, Asia and Eastern Europe show that the Brontothera have multiplied their body size within a few million years. While the first known specimens weighed around 18 kilograms and were only about the size of a deer, later species weighed several tons. Similar developments were evident in numerous other lineages of mammals during the Eocene – albeit mostly more slowly and less extreme than in thunder-hoofed animals.
Three hypotheses on growth in size
“However, the evolutionary processes underlying the rapid increase in size of animal lineages are poorly understood,” writes a team led by Oscar Sanisidro from the University of Alcalá in Madrid. One hypothesis, coined by the American paleontologist Edward Cope and known as Cope’s Law, states that living things in general have a tendency to increase in size over time because larger animals outperform in intra- and interspecies competition. Another hypothesis states that the increase in size was mainly due to the fact that the animals repeatedly adapted to new adaptive zones that built on one another and gradually preferred larger species.
A third hypothesis assumes that the species differentiate in an undirected manner, whereby an increase in size can come about because, on average, more larger than smaller species survive – for example, because they can occupy ecological niches in which there is less competitive pressure. To determine which of these hypotheses best describes brontothera evolution, Sanisidro and his team analyzed a data set of 276 brontothera fossils spanning the period from the emergence of brontothera to their extinction around 33.9 million years ago. Using this data, they modeled trait evolution and analyzed how the lineage diversified.
Undirected diversification
The result: “The third model, in which changes occur mainly in speciation events without a preferred direction, is at least four times more likely than the other scenarios,” say the researchers. “The undirected character of the size evolution in the group is particularly illustrated by the fact that up to the most recent Eocene, smaller species of brontothera emerged again and again, some of them even under 100 kilograms.” This contradicts the thesis of successive adaptive zones that always larger animals prefer. Furthermore, the analyzes showed that body mass within individual species hardly increased over time – in contrast to the predictions of Cope’s law.
But why did more smaller than larger species become extinct? To answer this question, Sanisidro and his colleagues examined two important ecological environmental factors: niche saturation and species-specific predation pressure. To do this, they included information on 680 herbivores and carnivores that inhabited the same habitats as the Brontotherans at the same time. “We found that niche saturation was the best explanatory variable for diversification bias in brontotheres,” the researchers said. Species in more densely occupied ecological niches diversified more often, but also died out more quickly. Newly emerging large species had the advantage of being able to use ecological niches that were still sparsely occupied and in which there was less competitive pressure. Therefore, their risk of extinction was lower.
Extinction due to environmental change
Towards the end of the Eocene, brontotheran body mass evolution slowed. In addition, more species became extinct than new ones, so that the diversity of species in this animal group decreased. The researchers attribute this to changes in climate and vegetation: “The second half of the Eocene saw a gradual transition to drier and less forested conditions in the northern hemisphere, resulting in unprecedented selective pressures on the last grazing, forest-dwelling individuals Brontotheren led,” say the authors. Further investigations, which also include climate models and the putative diet of the brontothera, could clarify to what extent environmental changes have led to the extinction of the brontothera.
Source: Oscar Sanisidro (Universidad de Alcalá, Madrid, Spain) et al., Science, doi: 10.1126/science.ade1833