The gift that is possibly the most remarkable fossil discovery ever made keeps on giving, revealing the time of year at which the asteroid that made the Chicxulub Crater struck the Earth. The discovery could help us understand the forces that allowed a quarter of living species to survive, while the rest died.
A little over two years ago PhD student Robert DePalma and colleagues stunned the world with the announcement of a set of fossils that appeared to have died on the very day of the impact. As improbable as the discovery seemed, numerous lines of evidence support the claim.
Further research has continued at the site, located near Tanis, North Dakota. DePalma is now first author of a paper in Scientific Reports revealing the fish, turtles, and dinosaurs buried there were in the prime growth season when disaster struck. In other words, the winter that lasted many years once the asteroid hit was immediately preceded by a spring. Slightly different timing could have had important effects on the outcome.
At the end of the Cretaceous, the Tanis site was located near the shore of the Western Interior Seaway that once split North America. Fossils buried there appear to have drowned in seiches (standing waves in enclosed water bodies) triggered by earthquakes set off by the impact. Minutes later the area was blanketed in collision ejecta.
Many of the paddlefish and sturgeon fossils found at the site are young enough they must have been recently hatched. Assuming Cretaceous temperate zone fish followed the same seasonal breeding cycles as today, DePalma and co-authors conclude their deaths occurred in spring or early summer.
To test their findings, the paper’s authors examined the bones of older fish. Like tree rings, fish bones keep a record of their years of growth. “A dark layer of bone, corresponding with Spring and Summer months, arises from increased food consumption and higher metabolic rate/growth,” the paper notes. “A light band less populated with osteons is apposed during the Fall and Winter months.” Oxygen and carbon isotopes also differ by season.
Older fish at Tanis had bones whose outermost layer was dark and with growing season isotopes. Similar analysis on woody plants points to the same conclusion. If this was not evidence enough, insect behavior at the site is more consistent with the warm season at that latitude than the cold.
For the unconvinced, the authors provide yet more evidence. Sturgeons today often spend winter in salty water, returning to freshwater environments in spring. There is evidence their Cretaceous ancestors did the same. Tanis combines sturgeons with fish from families that today are exclusively freshwater, and probably were then as well.
Such a disaster would be likely to kill even more species in spring. By late fall many plants have dropped their seeds, ready to sprout the following spring. If the first warmth was a decade later a few of these seeds might still be viable, ensuring the survival of the species. A squirrel lucky enough to find not only its own store of nuts, but those of others as well, might also make it through a decade-long winter, which might be how our ancestors survived.
Consequently, the paper predicts there may have been fewer extinctions in the southern hemisphere than in the north, a pattern that would require more Gondwana research to test.
The findings could “Help us better prepare for future ecological and environmental hazards,” DePalma said in a statement.