Were dinosaurs on the way out before their doom? Or were they going strong all the way to their end?

[lpetrich]

Dinosaurs have long caught the public imagination, much more than most other of the Earth's past biota. Some of them were just plain big, and many of them look rather weird by the standards of present-day biota, much more than most pre-Holocene Cenozoic megafauna. The mammoth, for instance, was a furry elephant, but sauropods were not only elephant-sized or bigger, they looked weird: an elephant-like body with a long neck, a small head, and a long tail.

Their demise has also caught the public imagination, with "dinosaur" suggesting something ponderous but obsolete. But were dinosaurs on the way out in the late Cretaceous? Or were they doing very well before the disaster that killed them?

"Dinosaur" here is "non-avian dinosaur", because it is now well-established that birds are descended from dinosaurs. Not from sauropods, of course, but from some smaller two-legged carnivores and omnivores: theropods. Tyrannosaurs are the best-known theropods, but some were much smaller.

Dinosaur diversification rates were not in decline prior to the K-Pg boundary | Royal Society Open Science

Determining the tempo and mode of non-avian dinosaur extinction is one of the most contentious issues in palaeobiology. Extensive disagreements remain over whether their extinction was catastrophic and geologically instantaneous or the culmination of long-term evolutionary trends. These conflicts have arisen due to numerous hierarchical sampling biases in the fossil record and differences in analytical methodology, with some studies identifying long-term declines in dinosaur richness prior to the Cretaceous–Palaeogene (K-Pg) boundary and others proposing continued diversification. Here, we use Bayesian phylogenetic generalized linear mixed models to assess the fit of 12 dinosaur phylogenies to three speciation models (null, slowdown to asymptote, downturn). We do not find strong support for the downturn model in our analyses, which suggests that dinosaur speciation rates were not in terminal decline prior to the K-Pg boundary and that the clade was still capable of generating new taxa. Nevertheless, we advocate caution in interpreting the results of such models, as they may not accurately reflect the complexities of the underlying data. Indeed, current phylogenetic methods may not provide the best test for hypotheses of dinosaur extinction; the collection of more dinosaur occurrence data will be essential to test these ideas further.

In short, new dinosaur species were emerging from earlier ones all the way to their end in the Cretaceous-Paleogene (K-Pg) disaster.

 

[Politesse]

Not just thriving, they were in many cases lynchpin species. Their very success is I think part of what precipitated a mass extinction event. If you keep in mind what has happened historically when we lose megafauna, consider the impact of just, say, the sauropods going extinct. Something ate those 50 ton beasts, what would they eat in their absence? Other things were kept in check from being eaten by them. Other things lived off of their waste. Others no doubt lived in landscapes transformed by their presence. It is not hard to imagine the entire ecosystem collapsing if even just a few genera of giant dinosaurs suddenly died off.

I snapped this photo in 2007: the K-T boundary exposure near Gubbio, where the tell-tale iridium layer was first discovered. You can see how heavily it has been predated upon by geologists and rockhounds!

 

[Politesse]

Something else to point out about the possible K-T event (and for that matter the Permian extinction that opened the niche for the dinosaurs to inhabit in the first place):

SCALE.

The disruption of the ancient ecosystem hinted by the few traces of evidence we have simply staggers the mind. We imagine our own extinction as though it would be a significant historical circumstance. But dinosaurs, at the point when they surrendered the planet to their feathered cadet branch, had already been the dominant land animals for something on the order of 230 million years, a much longer period of time than has elapsed since their extinction. We've been around for two at the outside, and really enjoyed our prominence in the global animal hierarchy for a only few hundred thousand years at most. A modern chicken is closer to a being a tyrannosaur, in both time and genetics, than a tyrannosaur was to the first predatory dinosaurs to which she was distantly a descendant.

The number of dinosaur species populating the planet at 65 million years is simply unknown; there simply aren't a lot of good fossil beds to give us a window on the right time frame. But if it was anything like the nearest time period (about ten million years before) that we do have good samples for, the number of extinct dinosaur species was enormous - projected to at least six hundred species, maybe as many a thousand (source: le Louff 2012, a great general exploration of this topic). And that's just dinosaurs; they weren't they only clade to seemingly experience a mass die-off at this time.

 

[BH]

The asteroid and deccan trap disaster did not kill off the dinosaurs that are called terror birds. They were huge and only went extinct afterwards.

 

[Politesse]

For anyone curious about this general period, I highly recommend a docuseries, 恐竜超世界, recently produced by NHK and released in English as "Amazing Dinoworld". The two-part miniseries uses flawlessly beautiful animation to recreate the late Cretaceous bioscape at land and at sea, focusing on the last 5-10 years of surprising discoveries pertaining to dinosaurian feathers and reptilian placentas.

 

[skepticalbip]

I only watched the preview but it looks like the producers consulted with paleontologists but not with paleo-climatologists. During the late Cretaceous there were no ice caps. The arctic was temperate to tropical and home to forests, palm trees, crocodilians, etc. The opening scene (and description) of critters trudging through arctic ice and snow was a bit of a disappointment.

 

[Politesse]

Those aren't ice caps, just snow. It used to be thought that dinosaurs lacked the ability to survive in snowy environments, but recent research has shown this to be untrue, hence that segment of the documentary. Some species even specialized in temperate environments, dwelling in arctic spaces year-round with tangible physiological adaptations to the cold evident in their remains. If curious, you can catch up on some of this growing body of literature at the following links:

https://www.smithsonianmag.com/history/the-strange-lives-of-polar-dinosaurs-180347471/
https://www.sciencedaily.com/releases/2015/09/150922104639.htm
https://www.nature.com/articles/s41598-018-19362-6
https://bioone.org/journals/journal...0022-3360(2005)079[0997:TFPDFT]2.0.CO;2.short
https://www.app.pan.pl/article/item/app20110033.html

Which is not to say the documentary doesn't get a bit carried away on a few points (Their pachyrhinosaurs are... wrong), but for the most part their reconstructions are sound, and certainly compelling to watch.