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.2020 Jan 1;6(1):eaax6250.
doi: 10.1126/sciadv.aax6250. eCollection 2020 Jan.

Growing upTyrannosaurus rex: Osteohistology refutes the pygmy "Nanotyrannus" and supports ontogenetic niche partitioning in juvenileTyrannosaurus

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Growing upTyrannosaurus rex: Osteohistology refutes the pygmy "Nanotyrannus" and supports ontogenetic niche partitioning in juvenileTyrannosaurus

Holly N Woodward et al. Sci Adv..

Abstract

Despite its iconic status as the king of dinosaurs,Tyrannosaurus rex biology is incompletely understood. Here, we examine femur and tibia bone microstructure from two half-grownT. rex specimens, permitting the assessments of age, growth rate, and maturity necessary for investigating the early life history of this giant theropod. Osteohistology reveals these were immature individuals 13 to 15 years of age, exhibiting growth rates similar to extant birds and mammals, and that annual growth was dependent on resource abundance. Together, our results support the synonomization of "Nanotyrannus" intoTyrannosaurus and fail to support the hypothesized presence of a sympatric tyrannosaurid species of markedly smaller adult body size. Our independent data contribute to mounting evidence for a rapid shift in body size associated with ontogenetic niche partitioning late inT. rex ontogeny and suggest that this species singularly exploited mid- to large-sized theropod niches at the end of the Cretaceous.

Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

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Figures

Fig. 1
Fig. 1. Femur histology of tyrannosaurid specimens BMRP 2002.4.1 and BMRP 2006.4.4.
(A) Mid-cortex of the transverse thin section of BMRP 2002.4.1. Plane-polarized light (PPL) emphasizes osteocyte lacuna density and variability in shape within the laminae, as well as longitudinal primary osteons. In CPL, there is a weak preferred fiber arrangement parallel to the transverse plane of section reflected by regional birefringence. Many primary osteons (POs) have uniformly isotropic fibers with rounded osteocyte lacunae. (B) Mid-cortex of the transverse thin section of BMRP 2006.4.4. Osteocyte lacuna density and variability in shape within the laminae are evident in PPL. CPL reveals varying birefringence associated with bone fiber orientation, but there is a weak preferred fiber arrangement parallel to the transverse plane of section reflected by regional birefringence. Many POs are composed of uniformly isotropic fibers with rounded osteocyte lacunae. (C) Longitudinal section of the mid-cortex of BMRP 2006.4.4. Vascular canals appear as near-vertical, thin, dark columns. As in the transverse section, the primary laminae between POs contain variably arranged osteocyte lacunae. In CPL, the laminae are weakly isotropic (I), corresponding to the poorly organized parallel orientation of fibers in the transverse plane. The laterally compressed osteocyte lacunae in POs are embedded within a uniformly birefringent [anisotropic (AN)] matrix in CPL, indicating that the PO lamellae are longitudinally oriented parallel-fibered bone (LP). (D) On the posteromedial side of the transverse section of BMRP 2006.4.4, there is a parallel-fibered annulus located at the periosteal surface (thickness indicated with blue line). Photographed in CPL. (E) In the transverse section on the posterolateral side, the annulus shown in (D) (blue lines) is overlain by highly isotropic woven-fibered laminae.
Fig. 2
Fig. 2. Tibia histology of tyrannosaurid specimens BMRP 2002.4.1 and BMRP 2006.4.4.
(A) Transverse mid-cortex thin section of BMRP 2002.4.1. Longitudinal POs are evident, and PPL emphasizes osteocyte lacuna density and variability in shape within laminae. CPL reveals varying birefringence associated with bone fiber orientation, but with a weak arrangement of fibers parallel to the transverse plane of section. Many POs are composed of highly isotropic fibers with rounded osteocyte lacunae. (B) Longitudinal thin section of the mid-cortex of BMRP 2002.4.1. Vascular canals appear as near-vertical, dark columns. Adjacent to the vascular canals, the POs contain laterally compressed osteocyte lacunae. CPL demonstrates that the laterally compressed osteocyte lacunae of POs are embedded within a uniformly birefringent matrix (anisotropic), indicating that the lamellae of POs are LP. Osteocyte lacunae orientation varies in the thin laminae between POs. In CPL, the laminae are weakly isotropic, corresponding to the weak arrangement of parallel fibers in transverse section. (C) In transverse thin section, the periosteal surface of BMRP 2006.4.4 on the anterior side consists of reticular POs within laminae of highly isotropic, woven tissue. (D) Within the anterior and anteromedial innermost cortex of BMRP 2006.4.4, in transverse thin section, six closely spaced LAGs are visible interstitially. Blue lines highlight the LAG trajectories.
Fig. 3
Fig. 3. The presence of an EFS at the periosteal surface of a long bone indicates skeletal maturity, while the absence of an EFS indicates that the bone is still growing at the time of death.
(A) An EFS composed of tightly stacked birefringent LAGs (between blue arrowheads) at the periosteal surface of anAlligator mississippiensis. (B) The EFS (between blue arrowheads) in an ostrich (Struthio camelus) is made of nearly avascular, birefringent parallel-fibered to lamellar primary tissue. (C) No EFS is present at the periosteal surface of the femur of BMRP 2002.4.1, (D) the tibia of BMRP 2002.4.1, (E) the femur of BMRP 2006.4.4, or (F) the tibia of BMRP 2006.4.4. All panels are shown in transverse thin section, with CPL.
Fig. 4
Fig. 4. Examples of variable CGM (blue lines) spacing in tyrannosaurids examined for this study.
(A) The variability of CGM spacing in the femur of BMRP 2002.4.1 and (B) the tibia of BMRP 2006.4.4 may imply that these individuals were approaching asymptotic body length. However, CGMs within the innermost cortices of much largerT. rex specimens (C) USNM PAL 555000 and (D) MOR 1128 demonstrate that the CGM spacing is not a reliable indicator of relative maturity status. All panels are shown in transverse thin section.
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