Until

recently, a salient exception appeared to be the stegosaurs, whose low diversity was anomalous to this general model. However, new discoveries of stegosaurs have increased our knowledge of their diversity: Carpenter check details (2001) estimates that at least five stegosaur species are now known from the Morrison Formation of the western United States (although Galton & Upchurch (2004b) recognize only three). This would appear to simplify the problem, but there is an additional caveat: the species are not all contemporaneous (K. Carpenter, pers. comm., 2004), and there may be geographic differentiation as well within the Morrison. The lack of contemporaneity could have several explanations, including insufficient stratigraphic sampling to establish that more of these species lived at the same time than it now appears. However, another approach is phylogenetic. If these five species turned GSK-3 inhibitor out to be morphotypes of a single anagenetic lineage, there would indeed be no evidence for contemporaneity. Would the hypothesis of species recognition thereby be weakened (Fig. 7, left)? In fact, such a result would weaken a hypothesis of anti-hybridization, but it would not

weaken or test the hypothesis of positive assortative mating (Paterson, 1993). However, if phylogenetic analysis revealed that these species indeed represented different lineages, and their ‘ghost ranges’ indicated that they must have diverged from others

at an earlier time, then at one time the test of contemporaneous species would have been passed (Fig. 7, right). filipin It is not impossible that such a pattern could also indicate other processes than species recognition, such as sexual or social selection, but in concert with non-directional evolutionary change the indication would be rather more strongly in favor of species recognition. Phylogenetic analysis and further biostratigraphic sampling can test this hypothesis. Finally, we return to the test of the Mate Recognition Hypothesis that Sampson (1999) proposed. We found that in every criterion, mostly related to higher rates of speciation and habitat shifts, the concept of ‘species recognition’ could be substituted for the terms related to sexual selection without any apparent difference in results. The exception was his fourth criterion (speciation will often be correlated with vicariance events rather than the formation of peripheral isolates), which we suggest is untestable in the fossil record, and in any case would not discriminate between sexual selection and species recognition as a cause.