Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Section A2: 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 CHAPTER 25 PHYLOGENY AND SYSTEMATICS

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 the fossil on or near the surface before it is destroyed by erosion too. 3. The fossil record is a substantial, but incomplete, chronicle of evolutionary history Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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 as mass extinctions and adaptive radiations. 4. Phylogeny has a biogeographical basis in continental drift Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The continents drift about Earth’s surface on plates of crust floating on the hot mantle. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 25.3a

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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

A second major shock to life on Earth was initiated about 180 million years ago, as Pangaea began to break up into separate continents. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 25.4

Each 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, which were continguous during the Mesozoic era. The great diversity of marsupial mammals in Australia that fill so many ecological roles that eutherian (placental) mammals do on other continents is a product of 50 million years of isolation of Australia from other continents. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The fossil record reveals long quiescent periods punctuated by brief intervals when the turnover of species was much more extensive. These brief periods of mass extinction were followed by extensive diversification of some of the groups that escaped extinction. 5. The history of life is punctuated by mass extinction Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 25.5

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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Factors that may have caused the Permian mass extinction include: disturbance to marine and terrestrial habitats due to the formation of Pangaea, 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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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: 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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 25.6

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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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 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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings