Hothouse Climate Slowed Dinosaurs’ Rise

It gets hot at the Hayden Quarry. Hot enough to keep away the biting gnats – eternal foe of the field paleontologist – and to require a brief siesta every afternoon in the cool of a nearby streambed. The August monsoons do little to help. The New Mexico desert greedily slurps every drop of moisture and within a few hours you forget that water ever falls from the sky. But that’s as it should be. It helps get you in the mindset of creatures that lived and died in the spot over 211 million years before, back when ancient aridity kept early dinosaurs down.

We often think of the Mesozoic as an endless summer when reptilian monsters stalked jungles and swamps choked with vegetation. Some dinosaurs really did live that large. But the Hayden Quarry tells a very different story. It’s from a time when the “Age of Dinosaurs” hadn’t truly begun. When tropical heat created wildly-fluctuating habitats repeatedly scorched by wildfire.

The signs of the Triassic blazes are easy to spot. Chunks of charred wood are scattered amongst the black Hayden Quarry bones, sometimes leading to moments of nervous excavation when it’s not clear whether you’re uncovering part of a little phytosaur jaw or the burnt remnants of a prehistoric conifer. (When in doubt, treat a fossil like it’s bone until you’ve proven otherwise.) And while they seem rather mundane next to the skulls, limb bones, and other vertebrate fossils that pack the quarry, the crispy remnants of prehistoric plants are what truly set the stage for this slice of time.

University of Southampton geologist Jessica Whiteside and colleagues tell the tale in a new paper in PNAS. By turning to the burnt plants, fossil pollen, and carbon isotopes of the Hayden Quarry, the researchers were able to piece together what this spot in northern New Mexico was like 211 million years ago. The site, which was then within the tropics, was a hot, arid place continuously tossed between wet and dry seasons (roughly similar to the seasonal shifts that bake the quarry today). It was so persistently dry that the local foliage often turned to tinder. Dessicated and dead plants provided the fuel for frequent wildfires that raged between 320 and 680 degrees Celsius.

Those fires altered the plant communities from season to season. Even though the forests hosted an increasing number of conifers alongside the more archaic seed ferns, Whiteside and coauthors found, the plant species present kept changing as wildfires reshuffled the ecological deck. Later the rains returned to batter that blackened ground, washing loose soil, singed wood, and bones together into the stream channels that now preserve this snapshot of Triassic life.

Yet, despite all this ecological chaos, the animals of the age lived in stable communities. They were resilient creatures that were able to carve out a living in the shifting landscape. The most diverse and disparate creatures of the tropical Triassic were pseudosuchians – crocodile cousins that included bipedal “dinosaur mimics”, huge carnivores, heavily-armored omnivores, and more. Dinosaurs, meanwhile, were only represented by a few small, sleek carnivores. There were no giant herbivores, like the long-necked sauropodomorphs found at higher latitudes, or, in fact, plant-eating dinosaurs of any kind. The plant communities in the low latitudes were too changeable to support dinosaurs that required a great deal of forage to keep their metabolisms running hot. Only little hunters could eke out a living here.

What Whiteside and colleagues found at the Hayden Quarry holds across the planet. Dinosaurs didn’t dominate the Triassic tropics. The increased carbon dioxide in the atmosphere created a hothouse climate where the sharp swings between the seasons prevented dinosaurs from getting any more than toehold at low latitudes. It was only in the wetter, lush regions away from the equator that dinosaurs started to get big and diversified. These were the centers of dinosaur evolution that produced the diversity which later took over the planet when a mass extinction decimated the protocrocs at the end of the Triassic. The true “Dawn of the Dinosaurs” didn’t start until the world had turned in their favor.

[Note – I’ve previously volunteered on Hayden Quarry excavations with several of the study authors, and will be returning there this coming August.]

Reference:

Whiteside, J., Lindström, S., Irmis, R., Glasspool, I., Schaller, M., Dunlavey, M., Nesbitt, S., Smith, N., Turner, A. 2015. Extreme ecosystem instability suppressed tropical dinosaur dominance for 30 million years. PNAS. doi: 10.1073/pnas.1505252112