Although two-thirds of the total may have survived, only a small fraction of the species on land actually made it through into the Tertiary period
, he says.
The thin layer of iridium-and-quartz-rich sediment dates to the transition of the Cretaceous and Tertiary periods
, which mark the end of the Mesozoic Era and the beginning of the Cenozoic Era.
In recent years, a controversy overprehistoric impacts has been making waves in the scientific community as geologists debate whether a large impact 65 million years ago could possibly have initiated a round of mass extinctions concurrent with the boundary between the Cretaceous and the Tertiary periods
(SN: 5/16/87, p.
Because iridium has been found at the geologic boundary between the Cretaceous and Tertiary periods
in concentrations exceeding typical iridium levels in the crust, some scientists have speculated that a meteorite had slammed into the earth, spraying the planet with a fine layer of meteoric iridium and other debris.
Kitchell studied various species of marine plankton, or diatoms, and found that those that had adapted to northern climates survived the mass extinction between the Cretaceous and Tertiary periods
(65 million years ago), while plankton that had adapted to low- and mid-latitude sites did not.
These minerals are found in the layer of clay that was laid down all over the world 65 million years ago at the boundary of the Cretaceous and Tertiary periods
, when the dinosaurs and about three-fourths of all animal species became extinct.
The basin dates from about 65 million years ago--the border between the Cretaceous and Tertiary periods
, when the asteriod is believed to have hit.
Since high levels of iridium, normally rare in the earth's crust, were discovered in the "K-T" layer marking the geologic boundary between the Cretaceous and Tertiary periods
, scientists have been exploring the possibility that the iridium came from a meteorite or comet shower.