We still don’t fully understand what happened to our favorite dinosaurs. Tyrannosaurus, Triceratops, Pachycephalosaurus, and all their neighbors were wiped out 66 million years ago, leaving only birds to carry on the dinosaurian legacy. It’s clear that a cosmic smack in the form of a massive asteroid was the key extinction trigger, and the aftermath of the impact continues to be investigated, but the historical questions of what occurred when are different from comprehending what made non-avian dinosaurs so vulnerable to extinction. If anything, the mystery keeps getting deeper.
One of the persistent questions about the infamous K/Pg extinction is whether dinosaurs were thriving when the asteroid struck or they were already in decline. You’d think that this would be relatively simple to answer. In strata about 75 million years old, there seems to be a riot of dinosaur species. Multiple horned dinosaurs, armored dinosaurs, hadrosaurs, tyrannosaurs, and more. By the close of the Cretaceous 66 million years ago, dinosaur communities don’t look as diverse - there’s a tyrannosaur, a horned dinosaur, a hadrosaur, and so on. But the picture isn’t nearly so simple.
The seemingly high diversity of dinosaur life 75 million years ago may be a matter of high turnover. Species that lived a million or so years apart may look like neighbors because we don’t get the geologic resolution to specify who is living with whom. Not to mention that this picture principally comes from western North America, and that the exposures of the 66 million-year-old rocks aren’t as extensive as those of the roughly 75 million-year-old window. We’re looking at a global extinction through a pinhole. Not to mention that the idea dinosaurs were shuffling through a slow decline carries along some older ideas in which dinosaurs were effectively blamed for their own extinction - that they were out-competed by superior mammals, or that they were due for extinction by becoming too large and extravagant. The upshot of all this is that paleontologists are still resolving what life was like at the end of the Cretaceous, whether dinosaurs were thriving or had lost their evolutionary gusto.
Paleontologist Alfio Alessandro Chiarenza and colleagues added the additional piece of the puzzle earlier this month. Rather than just looking at counts of dinosaur species through time, the researchers estimated the extent of habitats non-avian dinosaurs occupied between 83 and 66 million years ago in North America. This is called “ecological niche modeling,” an effort to detect how the ecology of the Late Cretaceous world might have been shifting and how those changes affected dinosaurs. In short, it’s a way to see whether dinosaurs still had places to live through time.
From an ecological perspective, dinosaurs were doing just fine at the end of the Cretaceous. If anything, the withdrawal of the Western Interior Seaway - a body of water that once split North America in two - gave dinosaurs greater areas to roam. Western North America was still a great place to be a dinosaur. In fact, dinosaurs might have even been doing better than expected.
There were probably more species of dinosaurs roaming around western North America 68 million years ago than we know. The apparent drop in diversity between 75 and 68 million years ago isn’t because dinosaurs weren’t evolving new species as rapidly. It may be because they weren’t being preserved, Chiarenza and colleagues propose. The rise and fall of a vanished sea may be the reason why.
Around 75 million years ago, the Western Interior Seaway was at a high point. Combined with mountain uplift in the western half of the continent, this created a great deal of floodplain space for dinosaurs to be preserved and available sediment to bury dinosaurs over the long north-south range of the seaway. The rich fossil deposits of Alberta’s Dinosaur Provincial Park, for example, may be thanks to these factors. But by the time of T. rex, 68 million years ago, the seaway had mostly drained off the continent and gave dinosaurs more space to roam. It also meant that the habitats best-suited to preserving dinosaurs became sparser, favoring places like the Hell Creek region of Montana but mostly failing to capture the dinosaurs to the north and south. The best places to find the latest Cretaceous dinosaurs seem to follow the vestige of the seaway as it dripped off the continent.
From an extinction perspective, the findings suggest that dinosaurs were thriving until the end. Why they were ultimately toppled from dominance may have to do with the difficulty any species would face in the wake of incredibly rapid, catastrophic disturbance - say, an asteroid strike in the wrong place, at the wrong time, and its aftermath. But what’s truly exciting about the proposal by Chiarenza and colleagues is that there are probably many unknown dinosaurs out there. In Utah’s North Horn Formation, for example, dinosaurs attributed to T. rex and Torosaurus had been found, but none complete enough to remove the shadow of doubt that these may be different species than those found in Montana. The same goes for New Mexico and Texas. The fossil sites here aren’t as rich as those in the Hell Creek, and the bones aren’t as well-preserved, yet they may offer us a very different view of dinosaur life in North America in the last days of the dinosaur reign. Time to dig in.