Paleobiology of North American stegosaurs: Evidence for sexual dimorphism

Abstract

The discovery in central Montana of the first ever multi-individual, monotaxic stegosaur site in North America prompted a revaluation of previous Stegosaurus material. Of particular interest was the plate morphology and function, as the plates of these specimens come in two morphs. This study examined a majority of known Stegosaurus material in collections across the USA and in Switzerland using a combination of quantitative techniques, histological analysis, and CT scanning. Several major insights into the biology of North American stegosaurs were made. First, in agreement with previous research, the taxa “Hesperosaurus mjosi” was found to be similar enough to specimens of Stegosaurus such that it should be considered a species within this genus. Second, dimorphism in the plates of S. mjosi was confirmed, with one morph being wide and ovular and the other being tall and narrow. The wide morph plates reach sizes 45% larger in surface area than do the tall morph plates. This variation was determined to be sexual dimorphism by the elimination of alternate hypotheses. These included individual, ontogenetic, and interspecific variation as well as the possibility that one individual possessed both morphs of plates. There appear to be no intermediate morphs. Both morphs are known from sexually mature, young adults and fully-grown, old adults. Taphonomy of the Montana site suggests that these two morphs comprised a social group and the only dimorphic feature is plate shape. Isolated specimens possess only one morph of plate or the other and plates of both morphs can be identified along the entire plate series from neck to tail. This is the strongest evidence for sexual dimorphism in dinosaurs yet presented and the first to rigorously test all alternate hypotheses for the observed dimorphism. Third, a simple model was developed to estimate the relative intensities of predation risk to sexual selection in a species based on the degree of dimorphism observed in their horny structures. By applying this to S. mjosi and available data on extant bovids, it was found that modern analogs for S. mjosi include the mountain goat (Oreamnos americanus), American bison (Bison bison), African or Cape buffalo (Syncerus caffer), and the genus of antelope Gazella. The comparison to extant bovids was further explored. A tail spike found at the central Montana quarry was preserved with fossilized keratin extending off of the tip. By developing a proxy for heat flow into or out of horny structures based on the surface areas of bony core and keratin sheath, it was found that Stegosaurus likely thermoregulated to the same extent as would be expected for an extant, tropical bovid of the same body size. Fourth, an examination of some of the variation within Stegosaurus suggests that the genus likely exhibited high species richness and varying degrees of sexual dimorphism between species within the genus. Overall these results provide evidence for multiple functions of plates including inter- and intraspecific display, predator deterrence, and thermoregulation. The relative importance of these different functions probably varied between species of Stegosaurus. Future paleontological and stratigraphic work on Morrison Formation stegosaurs, in combination with comparative approaches to extant species, will likely provide new insights into the details of the paleobiology of Stegosaurus.