Small body size and extreme cortical bone remodeling indicate phyletic dwarfism in Magyarosaurus dacus (Sauropoda: Titanosauria) - PubMed (original) (raw)

Small body size and extreme cortical bone remodeling indicate phyletic dwarfism in Magyarosaurus dacus (Sauropoda: Titanosauria)

Koen Stein et al. Proc Natl Acad Sci U S A. 2010.

Abstract

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.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Fig. 1.

Photographs of some of the sampled titanosaur bones from the Maastrichtian of Romania. (A–D) Magyarosaurus dacus humeri, specimens (A) MAFI Ob. 3092 (smallest recorded body size, 45% maximum size), (B) FGGUB R.1246 (65% maximum size), (C) MAFI v.13492 (76% maximum size), (D) FGGUB R.1048 (largest known specimen), and (E) “_Magyarosaurus_” hungaricus, MAFI Ob.3104. (Scale bar, 100 mm.)

Fig. 2.

Fig. 2.

Micrographs of long bone histology. (A–E) Long bone histology of Magyarosaurus dacus under crossed polarizers. (A) Micrograph of a midshaft section of the smallest available specimen of Magyarosaurus dacus (MAFI Ob.3092, 46% max size). (B) Close-up of A: largely interstitial laminar primary bone in the outermost cortex. The vascular canals are oriented circumferentially as in laminar fibrolamellar bone, but the bone matrix between the vascular canals consists largely of parallel-fibered and lamellar bone, with only a minute fraction of fibrous (or woven) bone tissue. (C) Micrograph of a midshaft section of MAFI v.13492 (76% max. size). The cortex is completely remodeled, in some areas several generations of secondary osteons can be seen crosscutting each other. (D) Closeup of C: cortex dominated by several generations of secondary remodeling. (E) Micrograph of a midshaft section of the largest available M. dacus humerus (FGGUB R.1048). (F) Close-up of E: Note the secondary osteons of the third generation, and truncated secondary osteons at the outer bone surface. (G and H) Long bone histology of ‘M.’ hungaricus under polarized light. (G) Micrograph of a midshaft section of ‘M.’ hungaricus (MAFI Ob.3104). The specimen is strongly remodeled, but the interstitial primary tissue is of the highly vascularized laminar fibrolamellar kind, with well developed primary osteons in the middle cortex, and poorly developed primary osteons with no lamellar bone infilling in the outermost cortex. Note that secondary osteons of the first generation are less well developed than in the largest M. dacus specimens. (H) Close-up of G: Secondary osteons crosscutting well developed primary osteons in the middle cortex. (I) Laminar fibrolamellar bone of Apatosaurus (BYU 72517014). (J) Alligator (SMNS 10481) long bone histology showing lamellar-zonal bone. (Scale bars: A, B, and D–H, 200 μm; C, 1,000 μm; I and J, 500 μm).

Fig. 3.

Fig. 3.

Plot of histologic ontogenetic stage (HOS) (35) vs. body size as expressed by femur length in Magyarosaurus dacus, compared with Europasaurus, Apatosaurus, Alamosaurus, and Phuwiangosaurus. The samples of Magyarosaurus dacus derive from humeri that were normalized to femur length. The single “M”' hungaricus sample is also included. Data for Alamosaurus were obtained from a previous report (38), supported by own data. Data for Phuwiangosaurus were obtained from another report (37).

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