However, as noted above, the extent of variation in the supposedly dimorphic features was statistical (as opposed to presence/absence features of true dimorphism), and although they may have supported more conspicuous sexually dimorphic features in soft part anatomy that is not preserved, the statistical argument on the basis of hard parts is insufficient. The kind of variation appears much more akin to the sort of differences that characterize male and female crocodiles, which differ from each other mainly at

adult size, where it is www.selleckchem.com/ALK.html mostly a matter of relative robusticity (Webb et al., 1978; Chabreck & Joanen, 1979). If dimorphism were important in small basal ceratopsians, it should be emphasized or at least detectable in larger, more derived forms, but this does not seem to be the case. Lehman (1990) suggested a pattern of sexual dimorphism in Chasmosaurus and related species that could be traced through later ontogeny, but the small sample sizes, incomplete preservation, and lack

of association of much of this material, as Lehman noted, makes it difficult to evaluate hypotheses about sexual differences, even if they are accepted. Ryan et al.’s (2001) study of a ceratopsian bone bed, where dimorphism could be presumed to emerge, turned up no significant patterns. A recent review of Ceratopsia (Dodson et al., 2004) did not accept sexual dimorphism as a general feature in this clade of dinosaurs. Soft-part features and behaviors that are not preserved in extinct taxa may well have contributed to sexual selection (e.g. Sampson, 1997). selleck screening library However, to invoke them for extinct groups of dinosaurs is outside the pale of homological and analogical comparison. As for fossil birds, which are dinosaurs, we have almost no information about dimorphism; long tail feathers

in the basal avialian Confuciusornis are suggestive (Chiappe et al., 1999), but this is not enough to establish evolutionary polarity. Because dimorphism (and not just inter-sexual difference) is generally low in other reptiles (Fig. 5), the EPB does HSP90 not support sexual dimorphism in non-avian dinosaurs on the grounds of homological comparison. Vrba (1984) used the example of degree of horn differentiation, which is usually greater in alcelaphine bovids (hartebeest, wildebeest, etc.) than in the related aepycerotines (impalas), to suggest an explanation for the greater species diversity through time of the former clade. Sampson (1999) suggested that sexual selection, not just natural selection, could be the motor of enhanced diversity in certain subclades over others. He proposed a Mate Recognition Hypothesis (MRH) by which selection for positive recognition of mates could lead to increased differentiation of populations and eventually greater rates of speciation in some lineages over others.