Extinction
Objectives
To explore a progression of changing life forms through time To pose possible causes of mass extinction To relate events of mass extinction and rapid adaptive radiation To learn how paleontologists demarcate geological time periods
Background
The number of species existing today is only a fraction of that produced through evolution since life began some 35 billion years ago. From the fossil record we can note times of mass extinction when perhaps 20 - 50% of the world species were wiped out, and whole families and genera disappear abruptly from the fossil record.
Perhaps the greatest mass extinction of all time occurred near the end of the Permian Period. Both marine and terrestrial biotas were devastated. By the end of the Permian, the supercontinent' Pangaea had already formed and stretched from pole to pole. Both poles had ice and a cooling trend was becoming well advanced. Since many separate land masses had pined together via continental drift, there was a great loss of coastal habitat and of shallow marine habitat overlying continental shelves. These and other factors conspired to bring many thousands of species to an end within a relatively short period of time.
Events like these resulted in a dramatic demarcation in sedimentary rock fossils. Geologists and paleontologists have used these major changes to mark the ends of geologic time spans. The great extinctions that ended approximately 245 million years ago serve to mark the end of both the Permian and Paleozoic Eras.
The changing conditions that initiate widespread extinctions will themselves change over time. Survivors find themselves in an altered world, ripe with opportunity. Many niches, old and new, may be unfilled. Mass extinctions have most likely always been followed by times of renewed speciation and proliferation of successful families
The rise of mammals may serve as a good example here. The mass extinction that marks the closing of the Mesozoic Era includes many forms of large reptiles. Hundreds of dinosaurs and other reptile species had dominated many land and marine environments throughout the Jurassic and Cretaceous Periods. Mammals, present throughout the Mesozoic Era, now responded to the increased opportunity present in a world nearly devoid of the long successful reptiles. Mammals had fewer competitors and fewer effective predators. Pangaea had broken up, a eating greater diversity in land and marine habitats. Bony fishes, birds and mammals prospered in the new age (Cenozoic), producing thousands of new life forms by adaptive radiation and speciation.
Activity
- Plot the listed data on the chart provided. (These data were gathered from fossil record evidence at sites all over- the globe. )
- Connect the datum points with a line; and respond to the list of questions. The data are from fossil record evidence accumalated from many sites over the globe.
- Identify marked decrease in number of families. There should be five places.
