For example, the horns of ceratopsians might satisfy all four (five) criteria listed above for both MRH and SRH, but would not pass the test of high sexual dimorphism required for sexual selection; on the other hand, they appear to pass the two tests of the species recognition hypothesis (non-directional variation of bizarre structures and several sympatric species). Moreover, without a clear demonstration of sexual dimorphism, the MRH reduces to the social

selection hypothesis (Hieronymus et al., 2009). Our purpose is not to insist that species recognition has been the only cause of the evolution of bizarre structures in dinosaurs, nor that adaptation, social selection and sexual selection have been unimportant in dinosaurian evolution. We merely ask in each case: how would we test this? We conclude that the hypotheses of mechanical function and sexual display that have predominated for decades as general explanations DZNeP nmr of the evolution DNA Damage inhibitor of these structures in dinosaurian clades are unfounded. When we test the hypothesis that presumed functions of these structures have evolved in their clades, we find no evidence; hence the notions that these structures are ‘adaptations’ fail the criteria proposed by evolutionary biologists (Greene, 1986; Williams, 1992; Rose & Lauder, 1996; Padian, 2001). Furthermore,

sexual dimorphism has not been strongly established for any bizarre structures in dinosaurian lineages, even though mild dimorphism has been statistically demonstrated in at least one lineage and may be plausible in others.

If criteria of sexual behavior other than those based on sexual selection (which requires sexual Sitaxentan dimorphism: Darwin (1871) are to be proposed, they should be justified on grounds that are more stringent than weak analogies to very different living organisms. We stress that no evolutionary hypothesis can be regarded as a ‘default’ explanation (i.e. if a certain class of explanation fails, then another one is automatically strengthened or must be accepted by default). Hypotheses must be independently tested, or they are not scientific. In many or most cases, definitive tests will not be possible. We have proposed two tests of a Species Recognition hypothesis, and there may be others. In our view, most dinosaurian bizarre structures pass these tests, but they do not pass the tests of adaptation or of sexual display. The importance of social selection (Hieronymus et al., 2009) remains to be tested in dinosaurs beyond individual species. This does not mean that these structures were not adaptive or used in attracting mates; we simply have no evidence on these points at present. Our hypothesis is that the Species Recognition Hypothesis is simpler and more general in explaining the evolution of bizarre structures in dinosaurs than those of mechanical function, social selection, or sexual selection/mate recognition.