326-3 Decapod Body Size Evolution is Best Modeled as an Unbounded Random Walk from a Moderately Sized Origin

Abstract:

As powerful predators, scavengers, and nutrient recyclers, decapod crustaceans have played critical roles in marine ecosystems throughout their history. However, their body size evolution is poorly understood. Here, we use an updated database of decapod body sizes for more than 1000 decapod fossil and extant genera to document their size evolution through time. Despite tremendous diversification during the “Mesozoic marine revolution,ˮ the average body size of decapods has maintained moderate sizes since their Devonian origin. During this history, maximum and minimum body sizes have expanded symmetrically across six orders of size magnitude. Maximum-likelihood statistics confirm the ordinal trend is best modeled as an unbounded, unbiased random walk (URW), which is also consistent with the persistent mean and continuous bidirectional expansion of smaller and larger body sizes from a moderately sized origin. Size trends in decapod subgroups also tend to be best supported by the URW model. A directional size increase cannot be ruled out for Dendrobranchiata and Brachyura, especially during the Mesozoic. Notably, decapod body size is remarkably unchanged during both Triassic and Cretaceous mass extinctions, despite substantial diversity losses at the K/Pg. These patterns are an exception to the generalization that Cope's rule applies across most animal groups, and sheds important light on body-size decapod evolutionary history.

The first two authors contributed equally. This research has been supported by NSF grant #2322080.

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