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Transcript
PHYLOGENY AND SYSTEMATICS Part 2 The Fossil Record and Geological Time (continued) 3. The fossil record is a substantial, but incomplete, chronicle of evolutionary history 4. Phylogeny has a biogeographical basis in continental drift 5. The history of life is punctuated by mass extinctions The fossil record is a substantial, but incomplete, chronicle of evolutionary history The discovery of a fossil depends on a sequence of improbable events. • First, the organism must die at the right place and time to be buried in sediments favoring fossilization. • The rock layer with the fossil must escape processes that destroy or distort rock (e.g., heat, erosion). • The fossil then has only a slight chance that it will be exposed by erosion of overlying rock. • Finally, there is only a slim chance that someone will find and recognize the fossil on or near the surface before it is destroyed by erosion too. A substantial fraction of species that have lived probably left no fossils, most fossils that formed have been destroyed, and only a fraction of existing fossils have been discovered. • The fossil record is slanted toward species that existed for a long time, were abundant and widespread, and had hard shells or skeletons. • Still, the study of fossil strata does record the sequence of biological and environmental changes. Phylogeny has a biogeographical basis in continental drift The history of Earth helps explain the current geographical distribution of species. • For example, the emergence of volcanic islands such as the Galapagos, opens new environments for founders that reach the outposts, and adaptive radiation fills many of the available niches with new species. • In a global scale, continental drift is the major geographical factor correlated with the spatial distribution of life and evolutionary episodes such as mass extinctions and adaptive radiations. • The continents drift about Earth’s surface on plates of crust floating on the hot mantle. Plate Boundaries Plate Boundaries The San Andreas Fault About 250 million years ago, all the land masses were joined into one supercontinent, Pangaea, with dramatic impacts on life on land and the sea. • Species that had evolved in isolation now competed. • The total amount of shoreline was reduced and shallow seas were drained. • Interior of the continent was drier and the weather more severe. • The formation of Pangaea surely had tremendous environmental impacts that reshaped biological diversity by causing extinctions and providing new opportunities for taxonomic groups that survived the crisis. • A second major shock to life on Earth was initiated about 180 million years ago, as Pangaea began to break up into separate continents. Each continent became a separate evolutionary arena and organisms in different biogeographic realms diverged. • Example: paleontologists have discovered matching fossils of Triassic reptiles in West Africa and Brazil, These land masses were connected during the Mesozoic era when these reptiles thrived. • The great diversity of marsupial mammals in Australia that fill so many ecological roles that placental mammals do on other continents is a product of 50 million years of isolation of Australia from other continents. The history of life is punctuated by mass extinction The fossil record reveals long quiet periods punctuated by brief intervals when the turnover of species was much more extensive. • These brief periods of mass extinction were followed by adaptive radiation and extensive diversification of some of the groups that escaped extinction. A species may become extinct because: • its habitat has been destroyed, • its environment has changed in an unfavorable direction • evolutionary changes by some other species in its community may impact our target species for the worse. • As an example, the evolution by some Cambrian animals of hard body parts, such as jaws and shells, may have made some organisms lacking hard parts more vulnerable to predation and thereby more prone to extinction. • Extinction is inevitable in a changing world. • During crises in the history of life, global conditions have changed so rapidly and disruptively that a majority of species have been swept away. • The fossil record records five to seven severe mass extinctions. THE PERMIAN EXTINCTIONS The Permian mass extinction (250 million years ago) claimed about 90% of all marine species. • This event defines the boundary between the Paleozoic and Mesozoic eras. • Impacting land organisms as well, 8 out of 27 orders of Permian insects did not survive into the next geological period. • This mass extinction occurred in less than five million years, an instant in geological time. What Caused the permian extinctions ? Factors that may have caused the Permian mass extinction include: • disturbance to marine and terrestrial habitats due to the formation of Pangaea, • Global warming - Massive volcanic eruptions in Siberia that may have released enough carbon dioxide to warm the global climate • changes in ocean circulation that reduced the amount of oxygen available to marine organisms. The cretaceous extinctions goodbye to the dinosaurs - hello mammals The Cretaceous mass extinction (65 million years ago) doomed half of the marine species and many families of terrestrial plants and animals, including nearly all the dinosaur lineages. • This event defines the boundary between the Mesozoic and Cenozoic eras. Hypotheses for the mechanism for this event include: • Global Cooling - The climate became cooler, and shallow seas receded from continental lowlands. • Large volcanic eruptions in India may have contributed to global cooling by releasing material into the atmosphere and blocked sunlight needed by plants and other photosynthetic species Walter and Luis Alvarez proposed that the impact of an asteroid would produce a great cloud that would have blocked sunlight and severely disturbed the climate for several months. • Part of the evidence for the collision is the widespread presence of a thin layer of clay enriched with iridium, an element rare on Earth but common in meteorites and other extraterrestrial debris. • Recent research has focused on the Chicxulub crater, a 65-million-year-old scar located beneath sediments on the Yucatan coast of Mexico. • Regardless of the cause, global cooling contributed to the large scale extinction of gymnosperm plants, which were the main source of food for many dinosaur species. The shape of the impact crater indicates a fireball and spread of debris in a geographic pattern that matches the rate of extinction Critical evaluation of the impact hypothesis as the cause of the Cretaceous extinctions is ongoing. • For example, advocates of this hypothesis have argued that the impact was large enough to darken the Earth for years, reducing photosynthesis long enough for food chains to collapse. • The shape of the impact crater implies that debris initially inundated North America, consistent with more severe and temporally compacted extinctions in North America. • Less severe global effect would have developed more slowly after the initial catastrophe, consistent with variable rates of extinction around the globe. Although the debate over the impact hypothesis has muted somewhat, researchers maintain a healthy skepticism about the link between the Chicxulub impact event and the Cretaceous extinctions. • Opponents of the impact hypothesis argue that changes in climate due to continental drift, increased volcanism, and other processes which could have caused mass extinctions 65 million years ago. • It is possible that an asteroid impact was the sudden final blow in an environmental assault on late Cretaceous life that included more gradual processes. Adaptive radiation of surviving species While the emphasis of mass extinctions is on the loss of species, there are tremendous opportunities for those that survive. • Survival may be due to adaptive qualities or sheer luck. After a mass extinction, the survivors become the stock for new radiations to fill the many biological roles vacated or created by the extinctions. • Example: after the extinction of the dinosaurs, there was a large adaptive radiation of mammals and the subsequent diversification into many new species.