Time Course of Biodiversity through Geological Time

Our knowledge of the patterns of change of biological diversity through geologic time is mainly based on the information contained in the fossil record. The available fossil evidence suggests that the diversity of families of multicellular marine organisms (Figure 8) rose steadily through the Cambrian and Ordovician period, attaining a plateau about 440 million years ago and then was punctuated by a great wave of extinction in the Permian (290-245 million years ago). After this, it steadily increased to the present.

Terrestrial organisms first appeared about 440 million years ago, with the invasion ofthe land by the ancestors of plants, fungi, vertebrate animals, and arthropods. In this period, each group increased rapidly in diversity from that time onward. At the species level, terrestrial vascular plants began to diversify markedly around 400 million years ago and declined during the worldwide Permian extinction event that also affected marine organisms profoundly. After this, similar to marine organisms, plants began to diversify around the Mid-Cretaceous Period, some 100 million years ago, with the flowering plants (angiosperms) becoming an extremely diverse and dominant group up to the present.

Thus the fossil records of both marine and terrestrial multicellular eukaryotes indicate maximum diversity at

Figure 8 Time course of (a) the number of families of marine animals, insects, tetrapods, and fish through the last 600 million years and (b) the number of land-plant fossil species in three major groups (angiosperms, gymnosperms, and pteridophytes) through the last 400 million years. V, Vendian; Ca, Cambrian; O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Permian; Tr, Triassic; J, Jurassic; K, Cretaceous; T, Tertiary; and Q, Quaternary. Reprinted, with permission, from the Annual Review of Environmental and Resources, Volume 28 © 2003 by Annual Review.

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Million of years

Figure 8 Time course of (a) the number of families of marine animals, insects, tetrapods, and fish through the last 600 million years and (b) the number of land-plant fossil species in three major groups (angiosperms, gymnosperms, and pteridophytes) through the last 400 million years. V, Vendian; Ca, Cambrian; O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Permian; Tr, Triassic; J, Jurassic; K, Cretaceous; T, Tertiary; and Q, Quaternary. Reprinted, with permission, from the Annual Review of Environmental and Resources, Volume 28 © 2003 by Annual Review.

the present time (Figure 8). The present level of marine diversity is estimated to be about twice the average over the past 600 million years, and that of terrestrial diversity is perhaps also about twice its historical average since organisms first invaded the land. The trend has been continually upward despite the five major extinction events that have occurred over the past 570 million years, which essentially covers the history of multicellular organisms (Figure 8). An interesting result emerging from the described trends in diversity and our current understanding of extinction levels is that current species diversity, in spite of being the highest in the history of life, represents only c. 1-10% of the total diversity that has ever existed on Earth.

Nevertheless, the pattern of temporal increase in biodiversity through the Phanerozoic era we have described must be taken with caution. On the one hand, there is the 'pull-of-the-recent effect' whereby young rocks are more likely than old rocks to be well preserved, and thus the most recent occurrences of species are more likely to be found than the older occurrences. On the other hand, a considerable fraction of the recent marine faunas are known from a few, restricted localities. Although paleontologists have amassed an impressive amount of information and analyzed it with sophisticated statistical methods, the patterns emerging from these analyses, although exciting and compelling, are still in need of further confirmatory work.

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