2008 : WHAT HAVE YOU CHANGED YOUR MIND ABOUT? WHY?

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Vice President of Research & Collections, Denver Museum of Nature & Science; Dinosaur paleontologist and science communicator; Author, Dinosaur Odyssey: Fossil Threads in the Web of Life
The Death of the Dinosaurs

An asteroid did it . . . .

Ok, so this may not seem like news to you. The father-son team of Luis and Walter Alvarez first put forth the asteroid hypothesis in 1980 to account for the extinction of dinosaurs and many other lifeforms at the end of the Mesozoic (about 65.5 million years ago). According to this now familiar scenario, an asteroid about 10 km in diameter slammed into the planet at about 100,000 km/hour. Upon impact, the bolide disintegrated, vaporizing a chunk of the earth's crust and propelling a gargantuan cloud of gas and dust high into the atmosphere. This airborne matter circulated around the globe, blocking out the sun and halting photosynthesis for a period of weeks or months. If turning the lights out wasn't bad enough, massive wild fires and copious amounts of acid rain apparently ensued. 

Put simply, it was hell on Earth. Species succumbed in great numbers and food webs collapsed the world over, ultimately wiping out about half of the planet's biodiversity. Key geologic evidence includes remnants of the murder weapon itself; iridium, an element that occurs in small amounts in the Earth's crust but is abundant in asteroids, was found by the Alvarez team to be anomalously abundant in a thin layer within Cretaceous-Tertiary (K-T) boundary sediments at various sites around the world. In 1990, announcement came of discovery of the actual impact crater in the Gulf of Mexico. It seemed as if arguably the most enduring mystery in prehistory had finally been solved. Unsurprisingly, this hypothesis was also a media darling, providing a tidy yet incredibly violent explanation to one of paleontology's most perplexing problems, with the added bonus of a possible repeat performance, this time with humans on the roster of victims.

To some paleontologists, however, the whole idea seemed just a bit too tidy.

Ever since the Alvarezes proposed the asteroid, or "impact winter," hypothesis, many (at times the bulk of) dinosaur paleontologists have argued for an alternative scenario to account for the K-T extinction. I have long counted myself amongst the ranks of doubters. It is not so much that I and my colleagues have questioned the occurrence of an asteroid impact; supporting evidence for this catastrophic event has been firmly established for some time. At issue has been the timing of the event. Whereas the impact hypothesis invokes a rapid extinction—on the order of weeks to years—others argue for a more gradual dying that spanned from one million to several million years. Evidence cited in support of the latter view includes an end-Cretaceous drop in global sea levels and a multi-million year bout of volcanism that makes Mount St. Helens look like brushfire. 

Thus, at present the debate has effectively been reduced to two alternatives. First is the Alvarez scenario, which proposes that the K-T extinction was a sudden event triggered by a single extraterrestrial bullet. Second is the gradualist view, which proposes that the asteroid impact was accompanied by two other global-scale perturbations (volcanism and decreasing sea-level), and that it was only this combination of factors acting in concert that decimated the end-Mesozoic biosphere.

Paleontologists of the gradualist ilk have argued that dinosaurs (and certain other groups) were already on their way out well before the K-T "big bang" occurred. Unfortunately, the fossil record of dinosaurs is relatively poor for the last stage of the Mesozoic and only one place on Earth — a small swath of badlands in the Western Interior of North America — has been investigated in detail. Several authors have argued that the latest Cretaceous Hell Creek fauna, as it's called (best known from eastern Montana), was depauperate relative to earlier dinosaur faunas. In particular, comparisons are often been made with the ca. 75 million year old Late Cretaceous Dinosaur Park Formation of southern Alberta, which has yielded a bewildering array of herbivorous and carnivorous dinosaurs.

For a long time, I regarded myself a card-carrying member of the gradualist camp. However, at least two lines of evidence have persuaded me to change my mind and join the ranks of the sudden-extinction-precipitated-by-an-asteroid group. 

First is a growing database indicating that the terminal Cretaceous world was not stressed to the breaking point, awaiting arrival of the coup de grâce from outer space. With regard to dinosaurs in particular, recent work has demonstrated that the Hell Creek fauna was much more diverse than previously realized. Second, new and improved stratigraphic age controls for dinosaurs and other Late Cretaceous vertebrates in the Western Interior indicate that ecosystems like those preserved the Dinosaur Park Formation were not nearly as diverse as previously supposed. 

Instead, many dinosaur species appear to have existed for relatively short durations (< 1 million years), with some geologic units preserving a succession of relatively short-lived faunas. So, even within the well sampled Western Interior of North America (let alone the rest of the world, for which we currently have little hard data), I see no grounds for arguing that dinosaurs were undergoing a slow, attritional demise. Other groups, like plants, also seem to have been doing fine in the interval leading up to that fateful day 65.5 million years ago. Finally, extraordinary events demand extraordinary explanations, and it does not seem parsimonious to make an argument for a lethal cascade of agents when compelling evidence exists for a single agent capable of doing the job on its own.

So yes, as far as I'm concerned (at least for now), the asteroid did it.