Mass Extinction at Cretaceous-Tertiary Boundary
A Review of the Dinosaur Extinction Event 65 Million Years Ago
Due to the profound amount of interest in the awe-inspiring dinosaurs, there is a spectacular amount of research available. “I hate to say it, but I think the public knows disappointingly little about the fossil record in general,” says John Alroy, a paleontologist with the University of California at Santa Barbara, “Dinosaurs and primates eat up almost all the attention, for obvious reasons.”
Taxonomic Changes at the K/T Boundary
As with most mass extinctions, the demise of the dinosaurs opened niche space to be filled by other organisms. The dinosaurs themselves were a hearty group that showed up around 228 million years ago, says Kristi Curry Rogers, a vertebrate paleontologist at Macalester College in St. Paul, Minn., and lasted until the abrupt extinction 65 million years ago, reigning for over 150 million years.
Sixty to sixty-five percent of all life on Earth was wiped out at the time of the event, with the large-bodied vertebrates on land and in water being the hardest hit, says Rogers. These organisms, such as marine reptiles and flying pterosaurs, are at the top of the food chain and thus, regardless of their survivalist instincts, are apt to die off if their food sources are depleted.
Most marine invertebrates, including the remainder of the ammonoids, were done in at this time, as well as most marine plankton groups. However, after the extinction of the large reptilian vertebrates, the Tertiary period shows evidence of evolution in mammals.
Extraterrestrial Cause for the K/T Extinction
The highlight of the dinosaur extinction theories is the impact of an asteroid on the YucatanPeninsula in Mexico. The discovery of a worldwide iridium layer, an element not typically found in Earth’s crust, is one of the most compelling pieces of evidence cited by Luis and Walter Alvarez in their groundbreaking 1980 Science paper, “Extraterrestrial Cause for the Cretaceous-Tertiary Boundary Extinction.”
“Finding a global iridium layer was significant because it indicated that something big enough to spread around the globe (and with a significant amount of iridium) happened in coincidence with the dinosaur extinction,” Rogers says. “The most parsimonious explanation was asteroid impact.”
Other evidence for an asteroid impact includes microtektites and spherules, which are essentially melted rock falling from the sky, says Rogers, and the presence of shocked quartz at the Chixulub Crater, the theorized impact site on the YucatanPeninsula which has been dated to 65 million years ago.
Other Hypothetical Causes of the K/T Extinction
Eruptions of the Deccan Traps, a large volcanic province in India, have been considered as both a cause and a contributing factor in the extinction of the dinosaurs. At the 2003 Geological Society of America conference, Sankar Chatterjee of TexasTech University presented evidence that the dinosaurs disappear just below the lava beds of the Deccan Traps. Volcanism from the Deccan Traps, he reports, may have been triggered by an asteroid impact.
Massive volcanic eruptions would have had many of the same effects as an asteroid impact, such as creating ash and debris clouds that would have covered the sun causing global cooling and making it impossible for plants to perform photosynthesis. This disruption in the food chain would have affected organisms at every level.
“There are tons of other ideas out there as well,” says Rogers. Some paleontologists believe that it was disease that took out the dinosaurs, others feel it may be a combination of many factors, but, Rogers says, an extraterrestrial impact does the best job of explaining all of the evidence at this juncture.
For more information on the K/T extinction event, the University of California at Berkeley provides a comprehensive website.
Dinosaur Fossils Receive the Most Public Attention
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Geologic Timescale of Phanerozoic Extinctions
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