Bill Clemens had been excavating fossils in eastern Montana’s Hell Creek Formation for more than 10 years, focusing primarily on the small mammals that scurried around the feet of dinosaurs and other Mesozoic Era creatures, when, in 1980, Walter and Luis Alvarez at the University of California, Berkeley, proposed that the dinosaurs and most of life on Earth were wiped out at the end of the Mesozoic by an asteroid or comet impact.
As a paleontologist who had unearthed myriad fossils from before and after the putative impact 66 million years ago, Clemens knew the story wasn’t that simple. Dinosaur fossils in the Hell Creek Formation, where he and his team dug every summer, began to peter out hundreds of thousands of years before the end of the Mesozoic.
And while the large dinosaurs like T. rex and Triceratops disappeared at the end of the Cretaceous — the so-called Cretaceous-Paleogene or K-Pg boundary, which is defined as the end of the Mesozoic — other dinosaurs, which evolved into modern birds, survived and began to flourish. Similarly, many small mammals that Clemens focused on, as well as reptiles and amphibians, weathered the cataclysm and quickly evolved to repopulate the continents.
Clemens, who died peacefully of metastatic cancer at his home in Berkeley on Nov. 17 at the age of 88, became one of the most persuasive voices against the impact hypothesis. He represented many biologists and paleontologists who, seeing continual turnover of life in the fossil record, challenged the catastrophism of physicists like Luis Alvarez, geologists like his son, Walter, and, increasingly, the public, which found the impact hypothesis very compelling. While the debate between the two camps became, at times, rancorous, Clemens continued to calmly press for a more nuanced view: that other global changes — a cooling climate and extensive volcanic eruptions in what is modern day India among them — contributed to Earth’s last mass extinction.