New research sheds light on how dinosaurs and other reptiles evolved following one of Earth’s most severe mass extinctions at the end of the Permian Period, about 252 million years ago. An international team of scientists analyzed fossil records to track changes in the speciation and extinction rates of a group of reptiles called archosauromorphs, which include the ancestors of modern-day crocodiles, birds, and dinosaurs. Their findings, published in the Proceedings of the Royal Society B, suggest that these early reptiles experienced a rapid burst of new species forming shortly after the extinction, followed by slower speciation and rising extinction rates. The study, led by Roland B. Sookias, proposes that this pattern could be explained by ecological factors, such as the filling of newly available habitats after the extinction, or by the neutral theory of biodiversity, which suggests that as more species evolve, each species may have smaller populations, making them more prone to extinction. The researchers used a statistical tool called PyRate to estimate speciation and extinction rates without needing a complete family tree of the species. They also used the length of the femur, a bone in the leg, as an indicator of body size, which can help estimate an animal’s overall mass. Interestingly, the study did not find a strong link between body size and the rates of speciation or extinction. The trends observed were the opposite of what was expected—larger animals seemed to go extinct more slowly and diversify more quickly than smaller ones. This result highlights the complexity of evolutionary processes and the need for further research to understand the factors driving species survival and diversification. The peak of early diversification among archosauromorphs occurred earlier than previously thought, according to the study. This suggests that different analytical methods can lead to varying conclusions and may reveal a "cryptic" diversification event that happened earlier than previously estimated. Such events might involve species that are not well-represented in the fossil record, making them harder to detect using traditional methods. The findings contribute to the broader understanding of how life diversifies after mass extinctions and may help scientists predict how modern ecosystems might respond to environmental changes. The researchers plan to expand their work by examining other groups of animals and different extinction events, both before and after the Permian extinction, to gain a more comprehensive view of evolutionary history.