An extraordinary fossil location has revealed the speed of evolution after the extinction of the non-avian dinosaurs. Within 100,000 years the number of mammalian species represented at a Colorado site more than doubled, and larger-bodied creatures appeared over the next million years. No previous site has recorded the aftermath of a mass extinction in anything like such detail.
An estimated 75 percent of all the species on the planet went extinct in the aftermath of the asteroid strike that caused the Chicxulub crater. Ash and sulfur dioxide cleared from the skies within a few years, but the ocean acid-alkaline balance took 80,000 years to return to normal, and temperatures even longer. For the survivors, the world was rich in newly opened ecological niches.
Inevitably, this led to a great expansion of species, with mammals, birds and flowering plants being among the greatest beneficiaries. Until now, we have lacked a detailed record of the process, however, particularly one with a clear timeline. That is just what Dr Tyler Larson Of the Denver Museum of Nature and Science and co-authors have described in Science.
At Corral Bluffs, just outside Colorado Springs, a single outcrop reveals the last hundred thousand years of the Cretaceous (K) and first million years of the Paleogene (Pg) era without the discontinuities seen elsewhere. Thousands of fossils outstandingly presevered within egg-shaped structures called concretions have already been recovered. Moreover, the site is layered with volcanic ash that can be accurately dated, giving us precise measurements on when its 150 stratigraphic layers were deposited.
The age of the rocks had led fossil hunters to search Corral Bluffs before, but without success. It was only when Lyson was visiting South African colleagues, who look out for concretions rather than bits of weathered bone, that he realized the problem. Within a week of his return, he was finding a skull every 15 minutes.
The authors consider the find so significant they produced a PBS Nova TV special about it, whose online release was timed to coincide with the peer-reviewed paper.
“Large bodied mammals disappear at the K-Pg boundary but returned to near KPgE levels within 100,000 years,” the paper reports. Judging by the teeth, herbivores, including some nut eaters, appeared where once mammals had been omnivorous or fed largely on insects. Thereafter Lyson found two intervals around 300,000 and 700,000 years after the extinction where even bigger mammals emerged, no longer having to look out for large dinosaurs.
“Our understanding of the asteroid’s aftermath has been spotty,” Lyson said in a statement. “These fossils tell us for the first time how exactly our planet recovered from this global cataclysm.”
Similarly, leaf mass per area and diversity of large plants took just 300,000 years to recover from the disaster. Among the fossils Lyson and co-authors report is the oldest known member of the bean family, dated to 65.35 million years ago, contradicting the theory legumes appeared in South America soon after the collision but took 7 million years to reach North America.
The extinction event’s survivors had to face significant temperature changes. Lyson estimates 5.1ºC (9.2ºF) of warming over the first 60,000 years after the event. Subsequent swings of up to 3.3ºC (6ºF) were seen over the next million years, inducing major changes in plant richness and the mix of animal fossils. The most dramatic subsequent warming, around 300,000 years after the big event, coincided with the most intense phase of the Deccan Traps eruption, but even the most rapid climatic shift recorded over this period was at 50 times slower than current rates.