Bone histology confirms determinate growth and small body size in the noasaurid theropod Masiakasaurus knopfleri

ABSTRACT

Noasauridae is a clade of ceratosaurian theropods that evolved small body size independently of other non-avian 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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ACKNOWLEDGMENTS

We would like to thank members of the Mahajanga Basin Project for their notable field efforts over the past 18 years, and, in particular, for the high-quality excavations at localities MAD 93-18 and MAD 05-42. J. Groenke and V. Heisey provided valuable preparation and molding/casting efforts necessary prior to the completion of this work. R. Hikida and J. Kaufman graciously allowed us to use their respective microscopy equipment. D. W. Krause, M. Carrano, K. Curry Rogers, and N. Myhrvold provided discussions during the early stages of this work. G. Erickson, M. Sander, and H. Woodward provided constructive reviews that improved the presentation and clarity of this paper. Finally, this work was supported by the National Science Foundation (DEB-9224396, EAR-9418816, EAR-9706302, EAR-106477, EAR-116517, EAR-0446488, and EAR-0617561), the National Geographic Society, the Jurassic Foundation, the Ohio University Office of Research and Sponsored Programs, and the Ohio University Heritage College of Osteopathic Medicine.

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