Glass Rain: The drops of solidified rock that the asteroid threw into space on impact and then fell back to Earth were among the lesser annoyances in the final half-hour of the Cretaceous.
A spooky layer of sediment reveals the season in which the dinosaurs emerged – and could also settle a dispute between researchers.
Whe looks like a frozen traffic accident,” Melanie During recalls. “In the lower sections, the fish are all oriented in one direction, just above in another. Some of the fish bodies are torn in two, others are wrapped around tree branches. I’ve never seen such violence.” This is how the Dutch vertebrate paleontologist describes what she saw when she visited an outcrop of the Hell Creek Formation in southwest North Dakota called “Tanis” in August 2017 – precisely because of those fish. These consist only of their petrified scales, heads and bones. During scraped a few of them out of the sediment to study them for her master’s thesis at the University of Amsterdam. Now she has the results together with her supervisors in Nature publishedt. There she was able to make a statement about the season in which the fish died.
Because these are not just any fish. What makes them special, however, is not their fish knowledge – they are sturgeons and paddlefish, as they still exist today – but rather the time horizon of the discovery site. The Hell Creek Formation, named for a creek in neighboring Montana, is a late Cretaceous deposit. It contains fossils of some of the greatest stars in Earth’s history, such as genera Tyrannosaurus or Triceratops. Their era ended when, 66 million years ago, an asteroid ten to fifteen kilometers across hit what is now Mexico, near Chicxulub on the Yucatán. At the same time, between 57 and 83 percent of all animal species on land and in the sea suddenly disappeared from the scene – including all dinosaur groups except the ancestors of birds. This “KT event”, which not only separates the Cretaceous (K) from the Tertiary (T) but also ended the Mesozoic, was the fifth and so far youngest major extinction since the first animals appeared. And insofar as the KT catastrophe is a prerequisite for the rise of mammals, it is also significant from our human perspective.