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, 105 (2), 582-7

Sexual Maturity in Growing Dinosaurs Does Not Fit Reptilian Growth Models

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Sexual Maturity in Growing Dinosaurs Does Not Fit Reptilian Growth Models

Andrew H Lee et al. Proc Natl Acad Sci U S A.

Abstract

Recent histological studies suggest relatively rapid growth in dinosaurs. However, the timing of reproductive maturity (RM) in dinosaurs is poorly known because unambiguous indicators of RM are rare. One exception is medullary bone (MB), which is an ephemeral bony tissue that forms before ovulation in the marrow cavities of birds as a calcium source for eggshelling. Recently, MB also was described in a single specimen of the saurischian dinosaur Tyrannosaurus rex. Here, we report two other occurrences of MB: in another saurischian dinosaur, Allosaurus, and in the ornithischian dinosaur Tenontosaurus. We show by counting lines of arrested growth and performing growth curve reconstructions that Tenontosaurus, Allosaurus, and Tyrannosaurus were reproductively mature by 8, 10, and 18 years, respectively. RM in these dinosaurs coincided with a transition from growth acceleration to deceleration. It also far precedes predictions based on the growth rates of living reptiles scaled to similar size. Despite relatively rapid growth, dinosaurs were similar to reptiles in that RM developed before reaching asymptotic size. However, this reproductive strategy also occurs in medium- to large-sized mammals and correlates with a strategy of prolonged multiyear growth. RM in actively growing individuals suggests that these dinosaurs were born relatively precocial and experienced high adult mortality. The origin of the modern avian reproductive strategy in ornithuran birds likely coincided with their extreme elevations in growth rate and truncations to growth duration.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Endosteally derived bone tissues in archosaurs. Schematics on the left represent the bone histology of actively shelling females. (A) Alligator resorbs cortical bone as a source of calcium for eggs and does not deposit MB internal to the endosteal lamellae. (B and C) Tenontosaurus (B) and Allosaurus (C) deposit MB internal to endosteal lamellae and later deposit a second layer of lamellae on the internal-most edge of the MB tissues. (D–F) Tyrannosaurus (D), Struthio (ostrich) (E), and Columba (pigeon) (F) do not deposit a second set of lamellae internal to the MB. Highlighted elements on the right (A: gray; B–F: red) indicate those sampled in this or previous studies (16, 18, 20, 44). Some histology schematics have been modified from published images: Alligator (44), Tyrannosaurus (16), Struthio (16), and Columba (45).
Fig. 2.
Fig. 2.
Age of RM and mass growth curves for Tenontosaurus (A), Allosaurus (B), and Tyrannosaurus (C). Using skeletochronology and nonlinear regression, logistic growth curves (labeled 1) were determined empirically with 95% confidence bands (dashed lines) for each species (12, 22, 23). Arrows point to specimens with MB and represent an upper bound for age and size at RM. Empirical growth curves were compared with alternative models of growth, which use the growth rates of living reptiles scaled to dinosaurian size and the von Bertalanffy (curves labeled 2) and logistic (curves labeled 3) models of mass accumulation. The effect of neonate mass on RM is not large for the von Bertalanffy models of growth in the three dinosaurs, but assuming a large neonate mass does decrease the estimated age of RM for the logistic models. Regardless, RM estimated from scaled reptilian growth rates, von Bertalanffy, and logistic models always produces much older estimates than do skeletochronological methods, which strongly suggests that dinosaurs did not grow like scaled-up living reptiles.
Fig. 3.
Fig. 3.
Growth curves and RM for Alligator (A), Loxodonta (elephant) (B), Sorex (shrew) (C), and Struthio (D). Relatively early maturity correlates with a growth strategy involving prolonged growth. Arrows indicate age at female RM. Growth and reproductive data are from published studies (, –52).

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