Published online by Cambridge University Press: 25 September 2013
The problem of the origin of dinosaurs has historically had three dimensions. The first is the question of whether Dinosauria is monophyletic, and of its relationships to other archosaurs. This question was plagued from the beginning by a lack of relevant fossils, an historical burden of confusing taxonomic terms and a rudimentary approach to devising phylogenies. The second dimension concerns the functional and ecological adaptations that differentiated dinosaurs from other archosaurs, a question also marred by lack of phylogenetic clarity and testable biomechanical hypotheses. The third dimension comprises the stratigraphic timing of the origin of dinosaurian groups with respect to each other and to related groups, the question of its synchronicity among various geographic regions, and some of the associated paleoenvironmental circumstances. None of these dimensions alone answers the question of dinosaur origins, and they sometimes provide conflicting implications. Since Dinosauria was named, one or another set of questions has historically dominated academic discussion and research. Paradigms have shifted substantially in recent decades, and current evidence suggests that we are due for more such shifts. I suggest two changes in thinking about the beginning of the “Age of Dinosaurs”: first, the event that we call the (phylogenetic) origin of dinosaurs was trivial compared to the origin of Ornithodira; and second, the “Age of Dinosaurs” proper did not begin until the Jurassic. Re-framing our thinking on these issues will improve our understanding of clade dynamics, timing of macroevolutionary events, and the effects of Triassic climate change on terrestrial vertebrates.