Both great dinosaurian orders (Saurischia and Ornithischia) were major components of terrestrial vertebrate faunas for ~150 Myr. The prevailing view is that dinosaurs attained an evolutionary acme late in the Cretaceous, after which they gradually declined in taxonomic diversity over ~10 Myr to their extinction 63 Myr ago1-6. The postulated decline is usually supported by comparing diversity levels in 76 Myr-old and 64 Myr-old dinosaurian assemblages from North America. The resulting differences in diversity have never been compared, however, with those observed between older dinosaurian assemblages, when their extinction was not imminent. I show here that, taken as a whole, the known fossil record of North American dinosaurs shows no evidence of a decline in taxonomic diversity lasting several million years or more before their extinction.