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Sauropods were the largest terrestrial tetrapods (>10 5 kg) in Earth's history and grew at rates that rival those of extant mammals. Magyarosaurus dacus, a titanosaurian sauropod from the Upper Cretaceous (Maastrichtian) of Romania, is known exclusively from small individuals (<10 3 kg) and conflicts with the idea that all sauropods were massive. The diminutive M. dacus was a classical example of island dwarfism (phyletic nanism) in dinosaurs, but a recent study suggested that the small Romanian titanosaurs actually represent juveniles of a larger-bodied taxon. Here we present strong histological evidence that M. dacus was indeed a dwarf (phyletic nanoid). Bone histological analysis of an ontogenetic series of Magyarosaurus limb bones indicates that even the smallest Magyarosaurus specimens exhibit a bone microstructure identical to fully mature or old individuals of other sauropod taxa. Comparison of histologies with large-bodied sauropods suggests that Magyarosaurus had an extremely reduced growth rate, but had retained high basal metabolic rates typical for sauropods. The uniquely decreased growth rate and diminutive body size in Magyarosaurus were adaptations to life on a Cretaceous island and show that sauropod dinosaurs were not exempt from general ecological principles limiting body size.bone histology | Sauropoda | secondary osteon | nanism | island fauna S auropod dinosaurs were the largest animals that ever roamed the surface of the Earth (1, 2). Gigantic size was acquired early in the evolutionary history of the group, in the Late Triassic (3). Recent studies of bone histology have shown that sauropods attained their gargantuan sizes by an evolutionary increase in their growth rate to levels comparable to those of extant endothermic mammals (4, 5). However, not all sauropods were multi-ton animals. Some titanosaurs are known to have had relatively small body sizes by sauropod standards; e.g., the South American Neuquensaurus australis reached a body length of about 7-9 m (6, 7), and its body mass is estimated at 3,500 kg. The recently described basal macronarian Europasaurus holgeri from the Late Jurassic of Germany (8) was even smaller, with a total estimated adult body length of approximately 6.2 m and a body mass of 800 kg.Another small-bodied titanosaurian sauropod, Magyarosaurus dacus, is known from the Upper Cretaceous (Maastrichtian) continental formations of the Hat xeg Basin of Romania (9, 10). These strata contain an array of relatively small-bodied dinosaur taxa, including the basal hadrosaurid Telmatosaurus (11), and two species of the noniguanodontian euornithopod Zalmoxes (12). In a famous early evolutionary hypothesis involving dinosaurs, the small body size of these taxa prompted the brilliant Hungarian paleontologist Franz Baron Nopcsa to hypothesize that, like Mediterranean dwarf proboscideans (13), the Hat xeg dinosaurs evolved their diminutive body size on a paleo-island (14, 15). Later, however, rare larger titanosaur bones were recovered from the Hat xeg Basin as well and describ...

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Palaeopathologies are relatively common in the fossil record; however, bone histological analyses of these are rare as they are more commonly discovered and described using radiological scans. Here, we describe the bone microstructure of infectious abscesses (osteomyelitis) in the long bones of two Stegosaurus individuals from the Como Bluff area of the Late Jurassic Morrison Formation. Multiple lytic, geographical lesions filled with reactive lamellar bone occur in a femur of a juvenile specimen. A single lesion was found in an adult tibia. This lesion is characterized by the presence of spiculated periosteal reactive growth perpendicular to the bone surface and the formation of new trabeculae within the lesion.

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