Discussion and Conclusions

Abstract

That different groups of ornithopods had substantially different jaw mechanisms at a time of major evolutionary change among plants suggests that these mechanical changes may have been controlled in large part by the additions of niche space through evolutionary events in contemporary terrestrial plants (see also Krassilov 1981). There are substantial increases in number of families of pteridophytes (ferns) during the Mesozoic, from the four families that cross the Permo-Triassic boundary, to eight at the Triassic-Jurassic boundary, and at least 15 during the late Jurassic (Banks et al. 1967 a, b). Among what are commonly called gymnosperms, there is a similar radiation, from six families at the Permo-Triassic boundary to 19 by the close of the Jurassic (Alvin et al. 1967). By far the most profound radiation to occur among terrestrial plants during the Mesozoic pertains to the angiosperms, which begin as one family sometime during the Barremian-Aptian to 51 families three or four stages later during the Cenomanian (Chesters et al. 1967). Several extinctions also occur during the Mesozoic, including the remaining arborescent lycopod family, the Pleuromeiaceae, half of the six sphenopsid families that crossed the PermoTriassic boundary, and the gymnosperm families Taenopterydales, Glossopterydales, Corystospermales, and Peltaspermales, during the late Triassic-early Jurassic (Andrews 1961; Thomas 1981). The Caytoniales, Czekanowskiales, Nilssoniales, and Bennettitales have their origin, become significant members of the terrestrial flora, and become extinct during the Mesozoic (Hughes 1976). The production and/or reduction of prospective adaptive zones through the diversification and extinction of these plant groups undoubtedly had a major impact on feeding mechanics and strategies of ornithopods and other terrestrial herbivores, similar to that of Oligocene/Miocene grasses on several groups of contemporary ungulates and rodents (Stirton 1935, 1947; Simpson 1953; Van Valen 1960). Ornithopods, in turn, appear to have exerted selection on fructification dispersal (Weishampel 1984).KeywordsPower StrokeMuscle ScarTerrestrial HerbivoreMaxillary SegmentAdductor MandibulaeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.