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The number of cell types required for the construction of a metazoan body plan can serve as an index of morphological (or anatomical) complexity; living metazoans range from four (placozoans) to over 200 (hominids) somatic cell types. A plot of the times of origin of body plans against their cell type numbers suggests that the upper bound of complexity has increased more or less steadily from the earliest metazoans until today, at an average rate of about one cell type per 3 m.y. (when nerve cell types are lumped). Computer models in which increase or decrease in cell type number was random were used to investigate the behavior of the upper bound of cell type number in evolving clades. The models are Markovian; variance in cell type number increases linearly through time. Scaled to the fossil record of the upper bound of cell type numbers, the models suggest that early rates of increase in maximum complexity were relatively high. The models and the data are mutually consistent and suggest that the Metazoa originated near 600 Ma, that the metazoan "explosion" near the Precambrian/Cambrian transition was not associated with any important increase in complexity of body plans, and that important decreases in the upper bound of complexity are unlikely to have occurred.
Paleobiology, founded to provide a forum for the greater integration of paleontology and biology, began publication in 1975. Paleobiology publishes original articles that emphasize biological or paleobiological processes and patterns including: speciation, extinction, development of individuals and colonies, natural selection, evolution, and patterns of variation, abundance and distribution of organisms in space and time. Papers concerning recent organisms and systems are also included if they aid in understanding the fossil record and the history of life. In addition to research articles of moderate length, Paleobiology provides an outlet for shorter and longer pieces. The Matters of the Record section features succinct, up-to-date discussions of new discoveries, reviews of recent conceptual advances, and brief syntheses of important topics. Paleobiology also publishes short book essays in its Book Review section. In 1999 Paleobiology began publication of an occasional series, Paleobiology Memoirs, which provides an outlet for longer works on the same topics covered in regular research articles.
Cambridge University Press (www.cambridge.org) is the publishing division of the University of Cambridge, one of the world’s leading research institutions and winner of 81 Nobel Prizes. Cambridge University Press is committed by its charter to disseminate knowledge as widely as possible across the globe. It publishes over 2,500 books a year for distribution in more than 200 countries. Cambridge Journals publishes over 250 peer-reviewed academic journals across a wide range of subject areas, in print and online. Many of these journals are the leading academic publications in their fields and together they form one of the most valuable and comprehensive bodies of research available today. For more information, visit http://journals.cambridge.org.
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Paleobiology
© 1994 Cambridge University Press