the University of New Hampshire has a gingko tree growing right outside the front door! The dawn redwood (Metasequoia glyptostroboides), a primitive gymnosperm, was known only from fossils until living specimens were discovered in a remote province of China in the 1940s. 17.4.4. Evolution of Vertebrates The earliest vertebrates were jawless fishes that evolved from chordates during the Cambrian Period. Jawed fishes evolved soon afterwards and rapidly diversified. The first fish to venture on land were crossopterygians ( lobed-finned fishes ) that appeared in the Devonian Period. Lobed-fins soon evolved into legs, giving rise to the first amphibians by the end of the Devonian Period. Amphibians remained tied to water for reproduction due to their soft-bodied eggs and gill-breathing young. This was not a problem in the mild and wet coal swamps that prevailed during the Mississippian and Pennsylvanian Periods, and amphibians remained the only land vertebrates for 75 million years.
The distribution of terrestrial plant and animal fossils allows reconstruction of the southern supercontinent Gondwanaland. Linear magnetic anomalies on the seafloor can also be used to reconstruct the past positions of the continents. Magnetic polarity reversals recorded on the seafloor are symmetric about the ridge axes. The past positions of the continents during the last 150 million years can be directly reconstructed by superimposing linear magnetic anomalies of the same age, allowing us to trace the opening of the Atlantic Ocean. The magnetic polarity reversals can be used to reconstruct the age of the seafloor and the opening and growth of the ocean basins, where the seafloor has not yet been recycled by subduction. Inclination of magnetic mineral grains gives paleo-latitude. Since inclination is a function of latitude, primary magnetism that has been locked in since a rocks formation gives the paleo-latitude at the time of formation.
The inclination of magnetic mineral grains varies as a function of latitude.
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- Fall '08
- Extinction event, Cretaceous–Tertiary extinction event