As is often noted, body size is both simple to measure, but also incredibly important for understanding the ecology and physiology of any organism. Unfortunately, aside from an intuitive understanding that body size has increased from the origin of life to the present, we still have a very poor understanding of how and when this increase occurred. As an example, the following picture demonstrates that the average animals of the sea floor have increased in size dramatically from the Cambrian to the Silurian. (Photo of Waldron specimen courtesy of Ken Karns.) Getting a more rigorous understanding of how and why this increase occurred requires much new data.
In recent years, significant advances have been made in compiling many fossil collections into large-scale databases that already span the history of life in marine and terrestrial habitats and for both plant and animal biotas, and that are ideal for answering such questions. One example is the Paleobiology Database, to which I and my students contribute. Unfortunately, many of the samples comprising these databases were collected without regard to particular ecological or evolutionary tests and therefore lack fundamental data on population density, body-size of individual specimens, and the total area or geological duration represented by the sample. My research here is focused on methods of estimating such “lost” data from data that is still available.
For example, several recent manuscripts (including two I contributed to: Kosnik et al. 2006 and Novack-Gottshall 2008) demonstrate that body size can be estimated for diverse fossil invertebrates spanning many sizes, shapes, and taxonomic and ecological affinities using photographs in monographs. Because of its wide applicability, this method has allowed rapid and ecologically meaningful censuses of the body sizes of taxa in my research database without having to resample the thousands of individuals represented therein.
As an example of the promise of such methods, I and a student collaborator (Novack-Gottshall 2008; Novack-Gottshall and Lanier 2008) demonstrated that the body volume (a proxy for body mass) of major phyla (brachiopods, echinoderms, and arthropods—but apparently not mollusks) increased significantly during the first 200 million years of apparent animal life. The cause of this increase was probably environmentally driven (and possibly linked to atmospheric carbon dioxide that enhanced primary productivity) because the increases were simultaneous and nearly parallel among members of these phyla. For the brachiopods (see figure below, Fig. 1 in Novack-Gottshall and Lanier 2008), this increase was especially pronounced, and also occurred independently among nearly all brachiopod subgroups!
Representative publications:
Payne, J.L., A.G. Boyer, J.H. Brown, S. Finnegan, M. Kowalewski, R.A. Krause, Jr., S.K. Lyons, C.R. McClain, D.W. McShea, P.M. Novack-Gottshall, F.A. Smith, J.A. Stempien, and S.C. Wang. 2009. Two-phase increase in the maximum size of life over 3.5 billion years reflects biological innovation and environmental opportunity. Proceedings of the National Academy of Sciences (U.S.A.) 106: 24-27 (.pdf)
Novack-Gottshall, P.M. and M.A. Lanier. 2008. Scale-dependence of Cope’s rule in body size evolution of Paleozoic brachiopods. Proceedings of the National Academy of Sciences (U.S.A.) 105: 5430-5434 (.pdf)
Novack-Gottshall, P.M. 2008. Ecosystem-wide body size trends in Cambrian-Devonian marine invertebrate lineages. Paleobiology 34: 210-228 (.pdf)
Novack-Gottshall, P.M. 2007. Using simple body size metrics to estimate fossil body volume: Empirical validation using diverse Paleozoic invertebrates. Palaios 23: 163-173 (.pdf)
Kosnik, M.A., D. Jablonski, R. Lockwood, and P.M. Novack-Gottshall. 2006. Quantifying molluscan body size in evolutionary and ecological analyses: Maximizing the return on data collection efforts. Palaios 21: 588-597 (.pdf)