DINOSAURS STILL IN LONG-TERM NET SPECIATION DECLINE BEFORE THE K-PG BOUNDARY

A recent study in Nature Communications used ecological niche modelling (ENM) to project suitable habitat for dinosaurs during the latest stages of the Cretaceous Period (83-66 million years [Myr] ago) from detailed climate data, apparently refuting the hypothesis that dinosaurs were in a long-term decline before the Cretaceous-Paleogene (K-Pg) mass extinction event. However, we note here that: 1) suitable habitat does not necessarily equate to taxonomic diversity (i.e., number of species); and 2) lack of a decline in diversity across time intervals is not evidence against a gradual decline in net speciation through time – diversity and net speciation measure two separate things. Diversity measures the number of species within discrete time intervals, while phylogenetically inferred net speciation measures the dynamic relationship between speciation and extinction through time. Additionally, net speciation is estimated in a model framework that accounts for statistically undesired effects of shared ancestry, whereas diversity is not. Thus, diversity and net speciation are not directly comparable, and a lack of a decline in diversity does not necessarily refute a decline in net speciation through time.