This in an interesting analysis and a thought provoking addition to the study of dinosaur macroevolutionary patterns. However, I find myself a bit skeptical of the assertion that a reduction in speciation rates means that dinosaurs were necessarily "in decline" before the end-Cretaceous.
This is a personal gripe I've had with several other studies in the past (not just with dinosaurs). The issue is that species diversity and cladogenesis are only roughly analogous to smaller scale processes. Speciation rates can indeed decline as a result of population declines, but they can also decline because geographic extend *increases*. Reducing isolation (by having mobile individuals or connected land areas) can suppress speciation. It isn't entirely clear that a clade comprised of a small number of far-ranging species is more extinction prone or "in decline" relative to a clade comprised of many isolated species. In fact, in living species, geographic range is the single best predictor of extinction risk.
The problem, of course, is that we can't get good data on population size or specific geographic range from the fossil record very often. Biodiversity becomes the measure of clade "health" through sheer practicality. It may not mean what it's often taken to mean, though.
Anyway, just some food for thought. I'd be curious to hear what others think.
Sent from my Cybernetic Symbiote
Apparently this post has been blocked on the DML. I'll try again. Apologies if the original shows up later...
---------- Forwarded message ----------
From: Ben Creisler <firstname.lastname@example.org>
Date: Mon, Apr 18, 2016 at 12:38 PM
Subject: Dinosaurs in decline tens of millions of years before their final extinction
A new paper:
Manabu Sakamoto, Michael J. Benton, and Chris Venditti (2016)
Dinosaurs in decline tens of millions of years before their final extinction.
Proceedings of the National Academy of Sciences (advance online publication)
Whether dinosaurs were in decline before their final extinction 66 Mya has been debated for decades with no clear resolution. This dispute has not been resolved because of inappropriate data and methods. Here, for the first time to our knowledge, we apply a statistical approach that models changes in speciation and extinction through time. We find overwhelming support for a long-term decline across all dinosaurs and within all three major dinosaur groups. Our results highlight that dinosaurs showed a marked reduction in their ability to replace extinct species with new ones, making them vulnerable to extinction and unable to respond quickly to and recover from the final catastrophic event 66 Mya.
Whether dinosaurs were in a long-term decline or whether they were reigning strong right up to their final disappearance at the Cretaceous–Paleogene (K-Pg) mass extinction event 66 Mya has been debated for decades with no clear resolution. The dispute has continued unresolved because of a lack of statistical rigor and appropriate evolutionary framework. Here, for the first time to our knowledge, we apply a Bayesian phylogenetic approach to model the evolutionary dynamics of speciation and extinction through time in Mesozoic dinosaurs, properly taking account of previously ignored statistical violations. We find overwhelming support for a long-term decline across all dinosaurs and within all three dinosaurian subclades (Ornithischia, Sauropodomorpha, and Theropoda), where speciation rate slowed down through time and was ultimately exceeded by extinction rate tens of millions of years before the K-Pg boundary. The only exceptions to this general pattern are the morphologically specialized herbivores, the Hadrosauriformes and Ceratopsidae, which show rapid species proliferations throughout the Late Cretaceous instead. Our results highlight that, despite some heterogeneity in speciation dynamics, dinosaurs showed a marked reduction in their ability to replace extinct species with new ones, making them vulnerable to extinction and unable to respond quickly to and recover from the final catastrophic event.