N

_ Australia

Africa Antarctica / t

Europe

India

Tethys

_ Australia

Africa Antarctica / t

Tethys

Gondwanaland

Gondwanaland

Figure 2.18 Changing familial diversity of skeletal benthos through time in relation to plate configurations: high diversities are apparently coincident with times of greatest continental fragmentation, for example during the Ordovician, Devonian and Cretaceous-Cenozoic. A, pre-Appalachian-Variscan Ocean; H, Hispanic Corridor; I, Iapetus Ocean; U, pre-Uralian Ocean. (Based on Smith, P. 1990. Geoscience Canada 15.)

example is the "Delabole butterfly", so called because quarrymen in the village of Delabole, in Devon (England) thought they were looking at ancient butterflies. In fact, the wide-hinged fossils are spiriferide brachiopods (see p. 306), and they were bent and stretched in all kinds of ways, depending on how they were oriented in the rocks. The fossils are in Devonian sediments that were bent and stretched by the Variscan Orogeny, a great phase of mountain building that affected southern and central Europe during the Carboniferous. By measuring the fossils, these large-scale forces could be reconstructed.

Until fairly recently these and similarly deformed assemblages were of limited value to taxonomic paleontologists; now a range of microcomputer-based graphic techniques are available to "unstrain" specimens. Hughes and Jell (1992), for example, used such techniques to unstrain Cambrian trilobites from Kashmir that had been distorted by earth movements during the uplift of the Himalayan mountain belt (Fig. 2.19). Previous studies had recognized seven species among these trilobites; statistical and graphic removal of the effects of tectonism revealed only one species. The study also allowed Hughes and Jell to identify the trilobites more accurately than before and to understand how they relate to species from India and North China.

0 0

Post a comment