Box The Chronos initiative

There are a number of different geological time scales, developed by different groups of authors for different intervals of geological time, and many different ways to analyze time series data of this type. The Chronos (Greek for time) project is a web-based initiative that seeks to centralize all the various time scales and analytic tools through one web portal. This is a chronometric rather than chronostratigraphic system and thus deals with radiometric age rather than the relative order of events. Thus software is available to create your own geological time scale and to compare data from existing published sources. These facilities, together with the opportunities to build your own range charts and effect high-resolution correlation of strata, open many exciting opportunities. Real advances are now possible in dating the precise timing and rates of biological processes such as extinction and recovery rates together with the accurate timing of the origins of higher taxa and the velocity of morphological change along evolving lineages.

The site can be accessed through http://www.blackwellpublishing.com/paleobiology/.

Glossopteris

Glossopteris

Figure 2.14 Carboniferous and Permian distributions of the Glossopteris flora and the Mesosaurus fauna and the fit of Gondwana. The tight fit of Gondwana and the correspondence of fossil faunas and floras across the southern continents suggested to Wegener and others that South America, Africa, India, Antarctica and Australia had drifted apart since the Permo-Triassic. (Based on Smith, P. 1990. Geoscience Canada 15.)

Glossopteris

Figure 2.14 Carboniferous and Permian distributions of the Glossopteris flora and the Mesosaurus fauna and the fit of Gondwana. The tight fit of Gondwana and the correspondence of fossil faunas and floras across the southern continents suggested to Wegener and others that South America, Africa, India, Antarctica and Australia had drifted apart since the Permo-Triassic. (Based on Smith, P. 1990. Geoscience Canada 15.)

early 1900s, the German scientist Alfred Wegener (1880-1930) suggested that the continents moved across the Earth's surface on a liquid core, suggesting that continents could in fact drift (although not through the oceans as he thought), some 50 years before the documentation of seafloor spreading and the plate tectonic revolution confirmed his theory (Wegener 1915); such data continue to be accumulated today as an integral part of paleogeographic analysis (Fortey & Cocks 2003). Our understanding of plate movements has been greatly advanced by a number of computerized paleogeographic systems; some, such as the Paleomap Project, even taking the Earth far into the future as well as deep into the past (linked at http://www. blackwellpublishing.com/paleobiology).

Faunal and floral barriers_

Barriers of various types have partitioned bio-geographic provinces through time. The first large-bodied organisms of the Late Neopro-

terozoic Ediacara faunas may have already developed their own provinces. George Gaylord Simpson (1902-1984) distinguished three types of passages: corridors were open at all times, filters allowed restricted access, whereas sweepstake routes opened only occasionally. In continental settings the barriers may be mountain ranges, inland seas or even rain forests. Marine faunas may be separated by wide expanses of deep ocean, swift ocean currents or land. In general terms the endemicity of most marine faunas decreases with depth; the more cosmopolitan faunas are located in deep-shelf and slope environments. But in the deeper basins, populated by specialized taxa, faunas are again endemic.

Faunal or floral provinces may be fragmented relatively rapidly if a barrier arises and the biotic responses may be quite sudden. For example, rifting and basin formation can split and isolate into fragments many existing terrestrial and fringing shelf provinces, whereas the same effects in the sea may be caused by the formation of an isthmus.

In some situations, the development of a barrier for some organisms may provide a corridor for others. The emergence of the Isthmus of Panama 3 Ma connected North and South America, but at the same time it separated the Atlantic and Pacific oceans. Before this event, South America had been isolated from North America for most of the past 70 myr, and was dominated by diverse, specialized, mammalian faunas consisting of unique marsupials, edentates, ungulates and rodents. However, the Isthmus of Panama provided a land bridge or corridor between the two continents and many terrestrial and freshwater taxa were free to move north and south across the isthmus (Fig. 2.15). The great American biotic interchange (GABI) allowed the North American fauna to invade the south and destabilize many of the continent's distinctive mammalian populations (Webb 1991). South American mammals were equally successful in the north and some such as the armadillo, opossum and porcupine still survive in North America.

The emergence of the isthmus also caused changes in the marine faunas of the Caribbean. Surprisingly, not many species became extinct, and there was a diversification of mollusks (Jackson et al. 1993). The emergence of the terrestrial land bridge and marine barrier may have initiated the upwelling of nutrients in the Caribbean area, and this in turn led to an increase in species diversity. Valentine (1973) had already drawn attention to a range of plate tectonic settings, including the spreading ridges, island arcs, subduction and fault zones, and the ways they can affect biological distributions. Thus tectonic features such as spreading ridges, transform faults and subduction zones create barriers for marine faunas whereas mid-plate island volcanoes can generate a series of stepping stones assisting the migration of animals and plants across great expanses of ocean. But there may be a more important relationship between tectonics and provinciality. There is a striking correlation between provinciality and continental fragmentation through time. Intervals when continents were many and dispersed apparently were times of increased provinciality, such as the Ordovician and the Cretaceous.

Island biogeography: alone and isolated?_

Modern oceans are littered with islands. Most are transitory volcanic chains, developed above moving hotspots or at mid-oceanic ridges that will probably be subducted; some, however, are pieces of continental crust broken off adjacent continents. These lighter bits of crust are usually later imprisoned in mountain chains and can hold important paleontologi-cal data. The biogeography of modern islands is complex and it is hard to apply models based on modern islands to ancient examples (Box 2.6).

But islands and archipelagos play a number of biological roles. Most islands are isolated from the mainland, and they are important powerhouses of speciation (see p. 119). Some island chains play an important part in migrations, acting as stepping stones, where species and their larvae may move, sometimes over many hundreds or thousands of years, from one mainland to another. The vertebrate paleontologist Malcolm McKenna introduced some interesting analogies with ancient shipping. Moving island complexes that can allow the cross-latitude transfer of evolving animals and plants may have acted as "Noah's arks", just as Noah's biblical ship eventually beached on the summit of Mount Ararat with breeding pairs of all manner of contemporary life. The transit of India from Gondwana to Asia, together with its even-toed artiodactyls and odd-toed perissodactyls, is a possible example. In the longer term these complexes may function as "Viking funeral ships" (originally bound, of course, for Valhalla with decorated dead warriors) transporting exotic fossil assemblages to new locations. The occurrence of a Gondwanan Cambrian trilobite fauna in the Meguma Terrane of the Appalachians and an Ordovician trilobite fauna in Florida from the same high-latitude province, both now welded onto the North American continent, are remarkable examples.

Island biotas (faunas and floras) are often diverse, with many endemic species and commonly with evidence that these species came originally from one or more source continents. It is fascinating to study such modern islands and some, such as the Galápagos, or Aldabra, have become important sites for biologists to watch "evolution in action". It is much harder for paleontologists to

Great American Interchange

Figure 2.15 The emergence of the Isthmus of Panama promoted the great American biotic interchange (GABI) between North and South American terrestrial vertebrates together with the radiation of the shallow-water marine benthos of the Caribbean Sea. l, litopterns; n, notoungulates. (Based on Benton 2005.)

Figure 2.15 The emergence of the Isthmus of Panama promoted the great American biotic interchange (GABI) between North and South American terrestrial vertebrates together with the radiation of the shallow-water marine benthos of the Caribbean Sea. l, litopterns; n, notoungulates. (Based on Benton 2005.)

understand the role of such islands through geological time; by their very nature, being short lived and located in tectonically active areas, they are quickly lost and often destroyed.

Geological and paleontological implications: using the data_

Much of the early evidence for continental drift was paleontological, although it was

0 0

Post a comment