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What Is a Dinosaur? | Anatomy & Evolution | General Behavior | Where Did They Live? | Why Did They Go Extinct?

Why Did They Go Extinct?
Introduction | Alvarez Hypothesis: Origin and Evidence | Effects of the Asteroid Impact | Other Extinction Hypotheses | Deep-sea Evidence for the Impact Hypothesis | Post Extinction Recovery | References  

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Deep-sea Evidence for the Impact Hypothesis
Cretaceous foram specimensThe general acceptance of the K/T asteroid impact theory has led many scientists to focus on the specific mechanisms that may have contributed to this dramatic extinction event. Although the impact was an important factor in the extinction of so many organisms, the event has also proven to be complex. In particular, the selectivity of the extinction has puzzled many paleontologists: why did dinosaurs go extinct but not crocodiles or turtles? Why did marine reptiles, belemnites, and ammonites disappear, but not fish or sharks? Why some mammals and not others?

Tertiary foram specimensOther scientists have focused on the extinction record preserved in deep-sea sediments in order to better understand the chain of events that followed the asteroid impact. Dr. Brian T. Huber, micropaleontologist in the National Museum of Natural History Dept. of Paleobiology, has studied evidence from a deep-sea drilling core taken 500-580 km of the northeastern coast of Florida during an Ocean Drilling Program cruise. Huber studied microscopic marine organisms called foraminifera taken from the core. The specimens were extracted from both Cretaceous and Tertiary age sediments. In one 40 cm core interval, he noticed a dramatic difference between the types of planktonic (floating) foraminifera that were alive prior to the boundary event and those that lived after. Prior to the extinction, large, ornate planktonic foraminifera were abundant, but afterward most specimens belonged to smaller, less ornate species. Overall more than 90% of the Cretaceous planktonic foraminifera had gone extinct. This is comparable to the extinction rate of calcareous nannofossils, another group of microscopic fossils that are abundant in the deep-sea sediment. In addition to the foraminifera, Huber also found specimens of shocked quartz and tektites, direct evidence of the impact itself.
deep-sea core showing dar-colored impact debris

The core also offered visual clues to the changes that occurred at the time of the extinction. The sediment undergoes a dramatic color change from white Cretaceous chalk in the lower portion of the core, to a dark gray, coarse-grained tektite layer in the middle, to a whitish gray Tertiary muddy chalk in the upper part. At the top of the tektite layer is a very thin, rust-colored, iron-rich layer known as the fireball layer. This rust layer, which has been found at a number of complete K/T impact horizons around the world, contains actual particles of the asteroid along with fine soot and ash that rained down on Earth's surface after the collision. This provides further evidence supporting the asteroid impact hypothesis.

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