The K-T boundary separates the age of the reptiles and the age of the mammals, which was first recognized over one hundred years ago by geologists who realized that there was a dramatic change in the types of fossils deposited on either side of this boundary.  This boundary also separates two of the three eras of the Phanerozoic (see time scale at left),  which is the time in earth history that began with the origin of complex life and extends to the present.  These two eras are called the Mesozoic and Cenozoic. Dinosaurs were prevalent during the Mesozoic Era and extinct during the Cenozoic Era.  The last segment of the Mesozoic Era, from 135 to 65 millions of years ago, is called the Cretaceous Period.  The first segment of the Cenozoic Era, from 65 million years ago until the present, is called the Tertiary Period.  The abbreviation for the boundary between the Cretaceous and Tertiary periods is the K-T boundary, where K is the abbreviation for the German form of the word Cretaceous.

    This boundary corresponds to one of the greatest mass extinctions in Earth's history.  At  least 75 percent of the species on our planet, both in the seas and on the continents, were extinguished forever. The most famous of the vanquished are the dinosaurs.  However, these giants were only a small fraction of the plants and animals that disappeared. In the oceans, more than 90 percent of the plankton was extinguished, which inevitably led to the collapse of the oceanic food chain.

        Rocks deposited during the Cretaceous Period and Tertiary Period are separated by a thin clay layer that is visible at several sites around the world.  A team of scientists led by Luis Alvarez (a Nobel Prize-winning physicist) and his son Walter (a geologist) discovered that the clay layer contains a strikingly high concentration of iridium, an element that is much more common in meteorites than in Earth's crustal rocks. Like meteorites, asteroids and comets also have relatively large abundances of iridium.  Consequently, they proposed that an impacting asteroid or comet hit the Earth, generating the iridium anomaly, and causing the mass extinction event. The discovery of high iridium concentrations in the clay layer at several places around the world suggested the impact was a large one.

Squid,octopus, ammonoids and nautiloids are all cephalopods. The photo on the left shows a Jurassic ammonite. The photo on the right is the modern Nautilus. Both the nautiloids and the ammonoids lived up through the Cretaceous, but only the nautiloids survived the K-T mass extinction.