g. Farke, 2004). However, selleck chemical Triceratops is virtually (along with Avaceratops) the only neoceratopsian with a solid frill, which is also the shortest among large neoceratopsians (Fig. 3). Other large neoceratopsians have substantial openings in their frills, which would have been of little use in defense. It now turns out that the adult Triceratops is in fact what has been called Torosaurus, and its frill is not only

fenestrated but also quite thin, as in other neoceratopsians (Scannella & Horner, in press). We hypothesize that, because it is so similar to young Triceratops, the adult form of Avaceratops may turn out to have been fenestrated as well. And horns vary widely; chasmosaurines had orbital horns of various sizes selleck inhibitor and orientations, but most centrosaurines had small orbital horns, and nasal horns of variable size that show no obvious function in combat (Farke et al., 2009). Farke (2004) used restored scale models of Triceratops to determine how individuals might have fought each other, interlocking horns, and Farke et al. (2009) showed that injuries occurred significantly more often on skull bones that would have been expected according to his predictions. However, even if this function is plausible, it has not been proposed and tested for other chasmosaurines, although it was absent in centrosaurines (Farke et

al., 2009). The most recent published phylogenies of neoceratopsians (Xu et al., 2002; Dodson, Forster & Sampson, 2004; Fig. 3) show no directional pattern of improvement MG-132 chemical structure of either brow horns or nose horns. Hence there is no evidence for adaptation to a particular function, and other hypotheses also need to be considered as a general explanation

for the evolution of horns and frills. For stegosaurs, as Main et al. (2005) have shown, the elaboration of plates and spikes shows no phylogenetic trends in adaptation to proposed functions of thermoregulation (Galton & Upchurch, 2004b). The possible function of defense has been rejected by several authors (Buffrenil et al., 1986; Main et al., 2005): the plates consist of a thin layer of compact bone surrounding a central core of well-vascularized, lattice-like (spongy) trabecular bone that would be crushed easily by the teeth of any large theropod. A possible function in deterring predators by making the animal appear larger has been suggested, but again it would not explain why Stegosaurus has large plates and those of the contemporaneous Kentrurosaurus and others are much smaller. Pachycephalosaur domes have been assumed to have been used in head-butting, ever since Colbert’s (1955) casual suggestion (review in Maryanska, Chapman & Weishampel, 2004). However, histological studies have shown that the columnar cell structure of these domes would not have deflected the forces incurred in battering, as reasonably proposed by Sues (1978) on the basis of biomechanical models of gross anatomy.