Ever since we were little, we have been taught that the extinction of the dinosaurs was the result of an asteroid impacting the Earth. This created an impact winter, where ash and dust fill the atmosphere and block out the sun, which the dinosaurs could not survive. This theorized event is known as the Alverez hypothesis, and involves an asteroid hitting what is now the Yucatan Peninsula in Mexico, creating the Chicxulub crater 66 million years ago (Mya)[1,2]. However, it was hypothesized that the dinosaurs may have been in decline prior to the impact, and were destined to go extinct anyways, with the asteroid impact accelerating their decline. This theory, however, is heavily debated due to worldwide fossil disparities and weak statistical analysis. To try to help settle the debate, Sakamoto, Benton and Venditti set out to develop a statistical method to determine if dinosaurs were in fact already in decline before their sudden eradication .
Using a statistical model based on phylogenetic trees for the three major dinosaur clades, groups of dinosaurs with a single common ancestor, the authors found that their model predicts that dinosaurs as a whole were in decline beginning around 24 million years before the asteroid impact (~90 Mya). This time frame is based on the number of speciation events, calculated from when the emergence of new species in the phylogenetic tree is overtaken by the number of extinction events. As such, if there is a net loss of species, then this is indicative of an overall extinction. However, individual dinosaur clades were actually in decline prior to 24 million years prior to the Chicxulub impact. Sauropodomorphs, which include Brachiosaurus and Diplodocus, were declining as early as 114 Mya. Theropods, such as Tyrannosaurus rex and Velociraptor, were beginning to decline even earlier, with extinction rates exceeding speciation events by 120 Mya. The final clade, Ornithischians, as a whole began declining around the same time as the Sauropodomorphs; however, two subclades within this clade actually bucked the trend and were becoming more diverse at the time of the asteroid impact. The Hadrosorids, duck billed dinosaurs, and Ceratopsids, frilled and horned dinosaurs such as Triceratops, were likely adapting to changes in the plants available to eat, though the specifics of what they were eating is still debated .
The modeling done by Sakamoto, Benton, and Venditti shows that the dinosaurs were in decline prior to the mass extinction event and they were likely destined to die out anyways. This may be why they didn’t survive the impact winter created by the Chicxulub impact; they were not diversified enough at the time of the asteroid to properly adapt to the changing world. This allowed other animal types, i.e. our mammalian ancestors, could swoop in and take over. Lucky for us!