Bone histology of the stegosaur Kentrosaurus aethiopicus (Ornithischia: Thyreophora) from the Upper Jurassic of Tanzania.

Using bone histology, a slow growth rate, uncommon for most dinosaurs, has been interpreted for the highly derived stegosaur Stegosaurus (Ornithischia: Thyreophora) and the basal thyreophoran Scutellosaurus. In this study, we examine whether this slow growth rate also occurs in the more basal stegosaur Kentrosaurus from the Tendaguru beds of Tanzania. The bone histology of six femora of Kentrosaurus representing an ontogenetic series from subadult to adult was studied, as well as one scapula. The primary bone is mainly highly vascularized fibro-lamellar bone with some reticular organization of the vascular canals. In addition to LAGs and annuli, distinctive shifts in the pattern of vascularization occur, which have been interpreted as potential growth marks. The variation in the development of growth marks may reflect annual climatic fluctuations. The overall bone depositional rate, and hence growth rate in Kentrosaurus appears to be higher than in Stegosaurus and Scutellosaurus. Considering that Stegosaurus is the larger-sized of the two stegosaurs, this would be contrary to an earlier supposition that small-bodied dinosaurs have slower growth rates than larger ones. Our finding of rapid rates of bone deposition in Kentrosaurus suggests that slow growth rates previously reported in Scutellosaurus and Stegosaurus are not a phylogenetic characteristic of the Thyreophora. Thus, slow growth rates are not plesiomorphic for the Thyreophora. We propose that the slow growth rates documented in the highly derived Stegosaurus could have been secondarily derived or alternatively that Kentrosaurus is the exception having increased growth rates.