The decline case is quantitative
A 2021 Bayesian diversification study of major dinosaur families inferred that net diversification became negative roughly 76 million years ago, with cooling climate and changes among herbivores proposed as contributing pressures.
The counter-case focuses on sampling
Other modelling shows that apparent declines can track where fossil-bearing rocks are exposed and sampled rather than actual global biodiversity. Ecological niche modelling has found no clear climatically driven diversity collapse immediately before the boundary.
Both can be partly true
Some clades or regions may have declined while others remained diverse. A globally averaged yes-or-no answer may hide a mosaic of ecological trends during the final ten million years of the Cretaceous.
What the asteroid changes either way
Even if diversity was trending downward, non-avian dinosaurs were still present immediately before the K–Pg boundary. A pre-existing decline would make the impact a coup de grâce, not remove it as the primary trigger of the abrupt final extinction.
Research record
Condamine et al. (2021) — dinosaur biodiversity decline before impactPeer-reviewed · open sourceChiarenza et al. (2019) — modelling does not support climatically driven declinePeer-reviewed · open sourceNature Ecology & Evolution (2024) — jury remains out on dinosaur declineResearch synthesis · open sourceWere Dinosaurs Already Dying Out Before the Asteroid?
Direct answer: We do not yet have a single uncontested answer. Some analyses support a multi-million-year decline; others show that fossil sampling can mimic one. Dinosaurs were nevertheless still present when the Chicxulub impact occurred.
Contested pre-impact trajectory; strongly supported abrupt terminal extinction. SCRIBE keeps those two questions separate.