Body Size Evolution & Metabolism in the Oceans

I am collaborating with Jon Payne and Matt Knope in the Payne Paleobiology Lab to document and explicate large scale trends in the body sizes of marine animals over the Phanerozoic. An organism's size is one of the most useful phenotypic traits for studying macroevolution, as it is often tightly correlated with metabolic rate, population size, extinction risk, and other traits. Furthermore, body size is easily gleaned from fossils and comparable across all taxa. Our database is the first comprehensive dataset on the evolution of body size of Phanerozoic marine bilaterian animals, including the Arthropoda, Brachiopoda, Chordata, Echinodermata and Mollusca. The dataset contains more than 17,000 genera, each with a stage-resolved stratigraphic range and a measured maximum linear dimension. Our primary source of data is the Treatise on Invertebrate Paleontology, where undergraduate researchers and high school students in the History of Life Internship Program measure the sizes of illustrated specimens. This research is attempting to answer two broad questions:
  1. How has the mean body size of marine animals changed over the Phanerozoic?
  2. What controls trends in body size: climate, atmospheric oxygen, diversification within or among clades?
The mean body size of marine bilaterian animals has increased by 2.2 orders of magnitude (a factor of 150) since the Cambrian. (One interesting exception is the Ostracoda, which show and order of magnitude decrease in size since their origination in the Ordovician and in the modern are smaller than foraminifera, on average). To test for the evolutionary mode responsible for the increase in mean biovolume, I developed a series of evolutionary branching models with unbiased and unbiased ancestor-descendant relationships. There is strong evidence for rejecting the unbiased model, or neutral, in favor of a size-biased model with active evolution towards larger body size. An analysis of trends within-lineage trends shows that differential diversification rather than within-lineage size increases seem to be driving the trend. That is, classes of marine animals that are large on average tend to diversify more than those classes with a smaller average size. Research is ongoing to determine the particular ecological, physiological, and environmental factors are responsible for this increase in size.

The differing dynamics of bivalves and brachiopods illustrate the importance of body size in regulating the metabolic activity of animals. Using living species to calculate mass-specific metabolic rates, were have shown that the bivalved molluscs were metabolically dominant over their brachiopod counterparts early in the Paleozoic. This is despite the fact that bivalves were taxonomically and numerically dominant for the whole of the Paleozoic.
  • Payne, J.L., A.M. Bush, E. Chang, N.A. Heim, M.L. Knope, and S.B. Pruss. 2016. Extinction intensity, selectivity, and their combined macroevolutionary influence in the fossil record. Biology Letters 12(10):20160202. [URL].
  • Payne, J.L, A.M. Bush, N.A. Heim, M.L. Knope and D.J. McCauley. 2016. Ecological selectivity of the emerging mass extinction in the oceans. Science 353(6305):1284-1286. [URL].
  • Smith, F.A., J.L. Payne, N.A. Heim, N.A., M.A. Balk, S. Finnegan, M. Kowalewski, S.K. Lyons, C.R. McClain, D.W. McShea, P.M. Novack-Gottshall, P.S. Anich and S.C. Wang. 2016. Body size evolution across the Geozoic. Annual Reviews of Earth and Planetary Sciences 44(1):523-553. [URL].
  • Heim, N.A., M.L. Knope, E.K. Schaal, S.C. Wang and J.L. Payne. 2015. Cope's Rule in the Evolution of Marine Animals. Science 347(6224):867-870. [URL]. Access the recommendation on F1000Prime
  • Payne, J.L., N.A. Heim, M.L. Knope and C.R. McClain. 2014. Metabolic dominance of bivalves predates brachiopod diversity decline by more than 150 million years. Proceedings of the Royal Society B, 281(1783). Early Online Publication March 2014. [PDF]

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