This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: About 252 million years ago, at the end of the Permian Period, a large proportion of species on Earth rapidly became extinct. After this mass extinction, many groups of organisms began to recover, while reptiles known as archosauromorphs—the group that includes dinosaurs and crocodiles—proliferated.
The proliferation of these reptiles gradually transformed life on land, laying the foundations for the ecosystems that characterized the Mesozoic Era (i.e., the interval between 252 and 66 million years ago). The diversification of archosauromorphs is an example of an evolutionary radiation, a relatively rapid increase in the number and variety of species descending from a common ancestor, often following a major extinction. Many paleontologists and biologists have tried to understand this evolutionary radiation, typically using evolutionary family trees (phylogenies) that map the inferred relationships among different archosauromorph species.
They have often found—as in other major radiations—an "early burst" pattern, in which many new species are generated early on and the rate of origination then slows. However, exactly what processes drive such early-burst patterns remains unclear. An international team of researchers based at the University of Liège (Belgium), the Argentine Museum of Natural Sciences "Bernardino Rivadavia" and University of Buenos Aires (Argentina), the University of Fribourg and ETH Zurich (Switzerland), the University of Gothenburg (Sweden), and the University of Birmingham (U.K.) recently analyzed data extracted from fossil records to reconstruct changes in archosauromorph speciation and extinction rates.
Their findings, presented in a paper published in the Proceedings of the Royal Society B, suggest that archosauromorph evolution underwent an early burst of speciation, followed by declining speciation and increasing extinction rates. "Much of the biodiversity on this planet came about through what are called major radiations of particular groups of species, often following the extinction of the dominant group that came before," Roland B. Sookias, first author of the paper, told Phys.org.
"A classic example is mammals after the dinosaurs went extinct, but dinosaurs and their relatives themselves radiated after stem mammals (i.e., mammal-like reptiles) were wiped out by the huge end-Permian extinctions (some 255 million years ago), probably caused by volcanic activity. "Understanding these radiation events thus helps us understand the origins of the amazing biodiversity around us and predict the effects of future mass extinction events, like the one humans are causing now." Major evolutionary radiations are often expected to follow a classical pattern marked by an early burst, a period during which many new species and evolutionary lineages emerge. After this early burst, the formation of new species tends to slow, yet the reasons for this remain unclear.
"One hypothesis is that this pattern occurs because 'niche space,' the different ecological opportunities available, gets filled up," said Sookias. "Another—based on the unified neutral theory of biodiversity and biogeography developed by Hubbell in 2001—is that, by chance, there are more speciation events than extinction events at the beginning because there are more individual animals or plants per species (the population is higher), but once there are more species, each species has a smaller population, so it is more likely to go extinct—this means diversification slows down." Another factor that could influence the rate of speciation (i.e., how quickly new species form) is animals' body size. This is because larger animals generally tend to reproduce more slowly than smaller ones.
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