Noasauridae is a clade of ceratosaurian theropods that evolved small body size independently of other nonavian theropods. The best-preserved and most complete noasaurid is Masiakasaurus knopfleri from the Maastrichtian-aged Maevarano Formation in Madagascar. An abundance of skeletal material from several individuals spanning a wide range of ontogeny makes Masiakasaurus an ideal candidate for the analysis of growth. We histologically sampled a growth series of elements consisting of four femora and three tibiae. Bright-field and circularly polarized light microscopy were used to distinguish between slowly and rapidly growing forms of bone. To simultaneously estimate age at death and reconstruct growth trajectories, we measured the perimeters of growth lines in each specimen and fitted models to these data using a novel application of mixed-effects regression. Our histological results show an external fundamental system in the largest tibial specimen and confirm that Masiakasaurus grew determinately, matured at small body size, and is not the juvenile form of a larger-bodied theropod. Parallel-fibered bone is unusually prominent and suggests relatively slow growth. Moreover, our quantitative analysis shows that the average individual took about 8-10 years to get to the size of a large dog. Although Masiakasaurus grew 40% faster than crocodylians, it grew about 40% slower than comparably sized non-avian theropods. Slowed growth may have evolved as a means to minimize structural and maintenance costs while living in a semiarid and seasonally stressful environment. Dimorphism does not appear related to asymptotic size or growth rate but seems to reflect the degree of skeletal maturity.

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