Evidence has been presented that the attribute that allowed dinosaurs to outlive previously dominant reptiles was cold adaptation. If this is true, feathers were probably an essential feature to their early survival. They may have been key.
A visitor to the Earth in the Late Triassic might have paid little attention to dinosaurs, being just one reptile family among many, and far from the largest one. Yet 202 million years ago a mass extinction occurred that took out most of the competition. Soon after it ended, dinosaurs were everywhere.
The end-Triassic extinction is thought to have been caused by major volcanic eruptions in the Central Atlantic Magmatic Province, which spewed greenhouse gasses and heated the planet. However, a new paper in Science Advances proposes this warm era was punctuated by brief bouts of extreme cold, and it was dinosaurs' capacity to survive these that set themselves apart from their rivals. Insulating feathers were probably a big part of this.
For most of the era of dinosaur dominance, the world was much warmer than today, thanks to a combination of a carbon dioxide-rich atmosphere, and an arrangement of continents that saw less sunlight reflected back to space. This has shaped our perception of dinosaurs.
However, Dr Dennis Kent of Columbia University and co-authors note most of the Triassic dinosaur fossils we have come from high latitudes, suggesting they were initially less well-suited to equatorial conditions than nondinosaurian species such as phytosaurs and Lagerpetids.
Being far from the equator doesn't always mean cold, however. During most of the Jurassic and Cretaceous temperatures were still fairly warm at these latitudes most of the year. Moreover, the paper notes, “The Late Triassic and earliest Jurassic are characterized as one of the very few times in Earth history in which there is no evidence of polar glacial ice sheets.”
That doesn't mean it never got cold. The paper reports evidence for stones in the Junggar Basin in now northwest China that appear to have been ice-rafted to their locations.
“This shows that these areas froze regularly, and the dinosaurs did just fine,” said Kent in a statement. At the time the Basin was around 71 degrees north. Today a winter ice sheet north of the Arctic Circle would be normal, but the fact this occurred during a super greenhouse era is more unexpected.
The authors attribute the freezing to aerosols thrown into the atmosphere by the same volcanoes supplying carbon dioxide. Such aerosols reflect incoming light and, at first, cool the planet more than the accompanying CO2 warms it. However, aerosols have a much shorter half-life than CO2. Consequently, the eruptions would produce brief bouts of extreme cold, punctuating longer periods of warmth.
“Transient but intense volcanic winters associated with massive eruptions and lowered light levels led to the end-Triassic mass extinction,” the paper reads. All medium-to-large reptiles died out except the dinosaurs, who filled the gap, just as mammals did 135 million years later.
The authors attribute the dinosaur adaptability to their insulation, which we know as feathers. Debate continues as to whether all dinosaurs were feathered, and if not, which ones weren't but if the paper is right, feathering was widespread early on. Only much later did the ancestors of birds discover another use for their winter coats.
“Severe wintery episodes during volcanic eruptions may have brought freezing temperatures to the tropics, which is where many of the extinctions of big, naked, unfeathered vertebrates seem to have occurred,” Kent said. “Whereas our fine feathered friends acclimated to colder temperatures in higher latitudes did OK.”