Bone Histology in <em>Dysalotosaurus lettowvorbecki</em> (Ornithischia: Iguanodontia) – Variation, Growth, and Implications

<div><h3>Background</h3><p><em>Dysalotosaurus lettowvorbecki</em> is a small ornithopod dinosaur known from thousands of bones and several ontogenetic stages. It was found in a single locality within the Tendaguru Formation of southeastern Tanzania, possibly representing a single herd. <em>Dysalotosaurus</em> provides an excellent case study for examining variation in bone microstructure and life history and helps to unravel the still mysterious growth pattern of small ornithopods.</p> <h3>Methodology/Principal Findings</h3><p>Five different skeletal elements were sampled, revealing microstructural variation between individuals, skeletal elements, cross sectional units, and ontogenetic stages. The bone wall consists of fibrolamellar bone with strong variability in vascularization and development of growth cycles. Larger bones with a high degree of utilization have high relative growth rates and seldom annuli/LAGs, whereas small and less intensively used bones have lower growth rates and a higher number of these resting lines. Due to the scarcity of annuli/LAGs, the reconstruction of the life history of <em>Dysalotosaurus</em> was carried out using regularly developed and alternating slow and fast growing zones. <em>Dysalotosaurus</em> was a precocial dinosaur, which experienced sexual maturity at ten years, had an indeterminate growth pattern, and maximum growth rates comparable to a large kangaroo.</p> <h3>Conclusions/Significance</h3><p>The variation in the bone histology of <em>Dysalotosaurus</em> demonstrates the influence of size, utilization, and shape of bones on relative growth rates. Annuli/LAGs are not the only type of annual growth cycles that can be used to reconstruct the life history of fossil vertebrates, but the degree of development of these lines may be of importance for the reconstruction of paleobehavior. The regular development of annuli/LAGs in subadults and adults of large ornithopods therefore reflects higher seasonal stress due to higher food demands, migration, and altricial breeding behavior. Small ornithopods often lack regularly developed annuli/LAGs due to lower food demands, no need for migration, and precocial behavior.</p>