PADIAN, K.; HORNER, J.R.; Univ. of California, Berkeley; Montana State University: “Bizarre” morphological structures in dinosaurs: adaptation, sexual display, or neither?

Many dinosaur groups are instantly recognized by “bizarre” horns, frills, plates, and so on. But why did they evolve? Two general explanations are function and display. Functional explanations (and some display explanations) depend on natural selection; most display explanations depend on sexual selection, but many roles are possible (attracting mates, repelling rivals, discouraging potential predators, etc.). Using phylogenetic, histological, and functional evidence, we show that neither natural selection nor sexual selection accounts very well for the known patterns. Variations in bizarre structures within a clade do not fit a pattern consistent with a single functional explanation; rather, functional explanations for structures in individual dinosaurs do not generally extend to related species. A weak sexual dimorphism has been demonstrated in some dinosaurs, but it seldom goes beyond the kind of size differences seen in living reptiles (such as crocodiles and monitor lizards) and mammals, in which the features in question are rarely used in agonistic combat or sexual display. We propose instead that the observed “bizarre” structures in dinosaurs can be explained more generally by species recognition, a kind of display supported both by fossil and recent evidence. The expected morphological patterns of species diversification differ sharply between the causes of selection (whether natural or sexual) and species recognition. The first ordinarily results in linear trends; the second evinces no particular pattern except differentiation. This pattern is seen in guilds of living ungulates. Its inference is most strongly supported when several closely related species are sympatrically present, or can be shown to have been recently sympatric as ghost taxa.