The tail spikes were arranged in two pairs at the end of the tail, with the larger pair more anterior. Preserved Sharpey’s fibers indicate the orientation of ligaments that held the plates up in a vertical orientation. The most common reconstruction puts the plates in two, staggered rows extending along the back of the animal, an idea supported by known articulated specimens and the fact that no two plates on any individual were exactly the same size and shape. Once thought to have had 17 plates, a new specimen has been discovered with 18. Stegosaurus had parasagittal dermal armor along its back consisting of vertically oriented plates that varied in size and shape from the neck to the tail and two pairs of long spikes at the end of the tail. It was an herbivorous quadruped with a small head, long tail, stout forelimbs, and long, columnar hind limbs. The genus Stegosaurus can be found in the Upper Jurassic Morrison Formation of the western United States, although it has been recovered from Portugal as well. Comparisons to ornamentation involved in sexual selection of extant species, such as the horns of bovids, may be appropriate in predicting the function of some dinosaur ornamentation. Dinosaur ornamentation possibly served similar functions to the ornamentation of modern species. Based on the seemingly display-oriented morphology of plates, female mate choice was likely the driving evolutionary mechanism rather than male-male competition. Plates from all positions on the body can be classified as one of the two morphs, and previously discovered, isolated specimens possess only one morph of plates. Furthermore, the two morphs of plates do not simply come from different positions on the back of a single individual. Therefore, the dimorphism is not a result of ontogenetic change. Histology of the new specimens in combination with studies on previous specimens indicates that both morphs occur in fully-grown individuals. Without evidence for niche partitioning, it is unlikely that the two morphs represent different species. This new site demonstrates co-existence, and possibly suggests sociality, between two morphs that only show dimorphism in their plates. Taphonomy of a new quarry in Montana (JRDI 5ES Quarry) shows that at least five individuals were buried in a single horizon and were not brought together by water or scavenger transportation. In contrast, many non-sex-related individual variations are expected to show intermediate morphologies. Intermediate morphologies are lacking as principal component analysis supports marked size- and shape-based dimorphism. One morph possessed wide, oval plates 45% larger in surface area than the tall, narrow plates of the other morph. Here it is shown that dimorphism in the shape of the dermal plates of Stegosaurus mjosi (Upper Jurassic, western USA) does not result from non-sex-related individual, interspecific, or ontogenetic variation and is most likely a sexually dimorphic feature. Table corresponds to biplots in Fig 2 and S5 Fig.Ĭonclusive evidence for sexual dimorphism in non-avian dinosaurs has been elusive.
PC1, PC2, and PC3 explain about 99% of the variation (n = 25). PC1 and PC2 explain about 95% of the variation. mjosi plates except the anterior dorsal plate from SMA 0092. PCA 3: Loadings of each variable for the first three principal components for a PCA of only the most complete S. PC1, PC2, and PC3 explain about 99% of the variation (n = 39).
PCA 2: Loadings of each variable on the first three principal components for a PCA of all fairly complete S. PC1, PC2, and PC3 explain about 98% of the variation (n = 40). PC1 and PC2 explain about 83% of the variation. PCA 1: Loadings of each variable on the first three principal components for a PCA of all fairly complete S.
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