It’s a good question because – apart from any insights the dino-bird controversy may offer – all the dinosaurs went extinct, yet mammals and reptiles suffered only partial losses.
From “Were Dinosaurs Undergoing Long-Term Decline Before Mass Extinction? (ScienceDaily, May 1, 2012), we learn,
The findings, published online May 1 in Nature Communications, suggest that in general, large-bodied, “bulk-feeding” herbivores were declining during the last 12 million years of the Cretaceous. But carnivorous dinosaurs and mid-sized herbivores were not. In some cases, geographic location might have been a factor in the animals’ biological success.
The researchers found that hadrosaurs and ceratopsids, two groups of large-bodied, bulk-feeding herbivores-animals that did not feed selectively-may have experienced a decline in biodiversity in the 12 million years before the dinosaurs ultimately went extinct. In contrast, small herbivores (ankylosaurs and pachycephalosaurs), carnivorous dinosaurs (tyrannosaurs and coelurosaurs), and enormous herbivores without advanced chewing abilities (sauropods) remained relatively stable or even slightly increased in biodiversity.
As a complication, hadrosaurs showed different levels of disparity in different locations. While declining in North America, the disparity of this dinosaur group seems to have been increasing in Asia during the latest Cretaceous.
“These disparity calculations paint a more nuanced picture of the final 12 million years of dinosaur history,” Brusatte said. “Contrary to how things are often perceived, the Late Cretaceous wasn’t a static ‘lost world’ that was violently interrupted by an asteroid impact. Some dinosaurs were undergoing dramatic changes during this time, and the large herbivores seem to have been mired in a long-term decline, at least in North America.”
Hmm. If large plant-eaters were declining, the future can’t have been rosy for large meat-eaters like the tyrannosaurs, even if they were not in absolute decline. In other words, the whole mega-ecology the biggest species were living in was probably fragile.
These findings, while useful, don’t help with the puzzle of what happened to the many small dinosaurs who also went extinct.
See also:
Jury’s still out on dino to bird transition?
Dinosaurs doomed by egg-laying?