After the publication of its discovery from the famous Hell Creek Formation (HCF) in 1905, the carnivorous dinosaur Tyrannosaurus rex (1) was met with intense scientific interest and public popularity, which persists to the present day (2). Numerous hypotheses concerning T. rex biology and behavior result from decades of research primarily focused on skeletal morphology and biomechanics [e.g., (3) and references therein]. Only within the past 15 years has bone histology been applied to investigate the aspects of T. rex life history inaccessible from gross examinations, addressing questions concerning ontogenetic age, growth rate, skeletal maturity, and sexual maturity. In 2004, two teams independently assessed the growth dynamics of T. rex using osteohistology. Their results suggest that T. rex had an accelerated growth rate compared with other tyrannosaurids and achieved adult size in approximately two decades (4, 5). The teams focused on growth curves, rather than on detailed analyses or interpretations of bone tissue microstructures. However, osteohistology is critical for establishing a baseline against which skeletal maturity and growth changes in cortical morphology related to life events in this taxon can be tested. Identifying the timing of growth acceleration and empirically quantifying juvenile T. rex growth rates are of special importance because the juvenile growth record is lost in older individuals because of bone remodeling and resorption (4, 5). Here, we examine the femur and tibia bone microstructure of two tyrannosaur skeletons of controversial taxonomic status recovered from the HCF: BMRP (Burpee Museum of Natural History) 2002.4.1, a largely complete specimen composed of nearly the entire skull and substantial postcranial material, and BMRP 2006.4.4, a more fragmentary specimen. Respectively, we estimate these specimens to be 54 and 59% the body length of FMNH (Field Museum of Natural History) PR 2081 (“Sue”) (6, 7), one of the largest known T. rex. The ontogenetic age of BMRP 2002.4.1 was previously reported by Erickson (8) as 11 years based on fibula osteohistology. However, because the fibula grows more slowly than the weight-bearing femur and tibia, it does not reflect annual increases in body size or relative skeletal maturity as accurately [e.g., (9)]. We use femur and tibia data to (i) provide detailed comparative intra- and interskeletal histological descriptions, (ii) quantify the ontogenetic age and relative skeletal maturity of these specimens, and (iii) allow empirical observation of annual growth rate, with emphasis on variability during the life history of tyrannosaurs (10). Moreover, by histologically quantifying the ontogenetic age of BMRP 2002.4.1 and BMRP 2006.4.4 and inferring skeletal maturity, we present new data that can be used to evaluate competing taxonomic hypotheses regarding these and other mid-sized tyrannosaur specimens discovered in the HCF, specifically whether BMRP 2002.4.1 (and by proxy other specimens) represents an adult “pygmy” genus of tyrannosaurid, “ Nanotyrannus.” RESULTS For detailed, orientation-specific histology descriptions, refer to the Supplementary Materials. In general, the femur and tibia cortical bones of BMRP 2002.4.1 and BMRP 2006.4.4 can be classified as a wovenparallel complex. Vascularity and osteocyte lacuna density are uniformly high throughout (Figs. 1 and 2). In the femora, the primary and secondary osteons surrounding vascular canals are frequently isotropic in the transverse section (Fig. 1, A and B) and anisotropic in the longitudinal section (Fig. 1C). Also in the transverse section, femur primary tissue exhibits moderate anisotropy regionally and weak anisotropy locally, corresponding to a loose arrangement of mineralized fibers in parallel (e.g., Fig. 1, A and B, and fig. S3B). 1 Department of Anatomy and Cell Biology, Oklahoma State University Center for Health Sciences, 1111 W.17th St., Tulsa, OK 74104, USA. 2 Department of Earth Science, Montana State University, P.O. Box 173480, Bozeman, MT 59717, USA. 3 Museum of the Rockies, Montana State University, 600 W.Kagy Blvd., Bozeman, MT 59717, USA. 4 Paleontology, North Carolina Museum of Natural Sciences, 11 W. Jones St., Raleigh, NC 27601, USA. 5 Department of Biological Sciences, North Carolina State University, 3510 Thomas Hall, Campus Box 7614, Raleigh, NC 2769, USA. 6 Chapman University, 1 University Dr., Orange, CA 92866, USA. 7 Intellectual Ventures, 3150 139th Avenue Southeast, Bellevue, WA 98005, USA. *Corresponding author. Email: holly.ballard@okstate.edu, holly.n.woodward@ gmail.com In the tibia transverse section of BMRP 2002.4.1 (Fig. 2A and fig. S4), longitudinal primary osteons are isotropic in circularly polarized light (CPL), but fibers of primary osteons encircling laminar, circular, and plexiform vascular canals are anisotropic. In contrast, primary osteons in the tibia of BMRP 2006.4.4 are frequently isotropic regardless of vascular canal orientation. Because of its proximal sampling location, the cortical shape of the tibia from BMRP 2006.4.4 in transverse section differs from that of BMRP 2002.4.1 and incorporates the fibular crest on the lateral side (figs. S2D and S8, A and F). Highly vascularized reticular woven tissue is present on the anterior and anterolateral periosteal surfaces (Fig. 2C). In both individuals, the thickest tibial cortex is located anteriorly. Of special note, within the medullary cavity of the femur and tibia of BMRP 2006.4.4, isotropic, vascularized, primary tissue is separated from the cortex by a lamellar endosteal layer. These features are morphologically consistent with medullary bone (11); however, additional studies on the systemic nature of this tissue throughout BMRP 2006.4.4 and biochemical tests on this tissue are necessary to test this hypothesis. Cyclical growth marks (CGMs), resembling tree rings in transverse thin section, were observed in the femora and tibiae of both BMRP specimens. Studies on extant vertebrates demonstrate that CGMs result from brief interruptions in osteogenesis, occurring with annual periodicity and typically coinciding with the nadir (12). The annual pauses in bone apposition are recorded as CGMs in cortical microstructure as either pronounced lines of arrested growth (LAGs) or diffuse annulus rings. On the basis of counting CGMs, BMRP 2002.4.1 was at least 13 years old at death (13 CGMs in the femur and 10 CGMs in the tibia), and BMRP 2006.4.4 was at least 15 years old at death (15 CGMs in the femur and 13 to 18 CGMs in the tibia). Typically, vertebrate long bone cortices will exhibit widely spaced CGMs within the cortex when young, corresponding to high annual osteogenesis. In subadults, CGMs become more closely spaced as osteogenesis decreases approaching adult size [e.g., (10)]. In contrast to these frequently observed patterns, the spacing of CGMs was unexpectedly variable throughout the femur and tibia cortices of both BMRP specimens. In the femur of BMRP 2006.4.4, there is an annulus at the periosteal surface on the medial side (Fig. 1D), but when followed posteriorly, the annulus is within the outer cortex, while fibrolamellar tissue makes up the cortex of the periosteal surface (Fig. 1E). Within the innermost cortex on the anterolateral side, six LAGs are closely spaced (Fig. 2D). Because of resorption from the medullary drift, these LAGs are absent within the innermost cortex of the posterior and lateral sides. Prondvai et al. (13) demonstrated that inaccurate bone microstructure interpretations are possible if the mineralized tissue is observed in only a single plane; specifically, the more slowly formed parallel-fibered mineral arrangement could be mistaken for the rapidly deposited woven-fibered mineral arrangement, which has direct bearing on growth rate interpretations. Therefore, the femur of BMRP 2006.4.4 was longitudinally sectioned in an anterolateral-posteromedial plane, and the tibia of BMRP 2002.4.1 was sectioned in a medial-lateral plane to accurately assess tissue organization and associated relative growth rates (Figs. 1C and 2B, and figs. S2, B and C, S5, and S7). In the femur of BMRP 2006.4.4, vascular canals are arranged parallel to the plane of section and to the shaft of the long bone. Adjacent to the vascular canals, bone fibers are highly anisotropic in CPL and contain osteocyte lacunae with long axes arranged parallel to the vascular canals and plane of section. Tissue of the laminae between primary osteons varies locally in degree of isotropy, with corresponding variable shape in osteocyte lacunae. On the medial side of the longitudinal section through the tibia of BMRP 2002.4.1, vascular canals are arranged obliquely with numerous communications (fig.S5B). From the mid- to the outer cortex, vascular canals are more uniformly parallel to the bone shaft, with fewer transverse Volkmann’s canals (fig. S5C). Adjacent to vascular canals, fibers of the primary osteons are anisotropic in CPL with longitudinally flattened osteocyte lacunae. Fibers within the primary laminae vary locally in isotropy and osteocyte lacuna orientation (Fig. 2B). The lateral cortex is thinner than the medial cortex, and vascular canals are more closely spaced with fewer communicating canals (fig. S5D). DISCUSSION Limb bones exhibit moderate growth rates and tension loading Comparison of BMRP 2002.4.1 and BMRP 2006.4.4 bone fiber organization in the transverse and longitudinal sections using CPL confirms that primary tissue is generally poorly organized parallel fibered to weakly woven. Dense osteocyte lacunae and poor bone fiber organization, in combination with a rich vascular network of reticular, laminar, and plexiform primary osteons, are characteristics that empirically correspond to elevated osteogenesis ranging from 5 to 90 μ m/day (10). Nonetheless, the frequency of longitudinal vascularity, as well as regionally prevalent poorly organized parallel fiber bundles within the transverse sections, suggests that annual growth rates were nearer the lower bound (10). The BMRP individuals did, however, experience occasional periods of faster growth indicated by bands of regionally isotropic woven laminae with reticular vascularity (e.g., Figs. 1E and 2C, and figs. S6D and S8, C and D) (10). In both BMRP specimens, the majority of primary osteons as well as some secondary osteons were isotropic in the transverse section. Corresponding anisotropy in longitudinal examination confirms that the fiber bundles within osteons are longitudinally arranged (Figs. 1C and 2B, and fig. S5, B to D). Studies on long bone response to loading show that longitudinal collagen fiber orientation within secondary osteons is commonly found in habitually tension-loaded regions (14), which may also apply to primary osteon collagen fiber orientation. As such, future studies on tyrannosaurid locomotion biomechanics may benefit from incorporation of osteohistology. Relative skeletal maturity Rather than exhibiting an external fundamental system (EFS) (Fig. 3), a woven-parallel complex extends to the periosteal surface in both tyrannosaurid specimens. Thus, histology supports morphological observations that BMRP 2002.4.1 and BMRP 2006.4.4 were skeletally immature individuals at death (10). In lieu of epiphyseal fusion, which most reptile taxa lack, an EFS is the only way to conclusively confirm attainment of asymptotic adult body length from the long bones of a vertebrate. When present, the EFS occupies the periosteal surface as either closely spaced LAGs (separated by micrometers) (Fig. 3A) or as a thick, primarily avascular annulus (Fig. 3B) (10). CGMs close to the periosteal surface can sometimes be mistaken for an EFS. In the case of BMRP 2006.4.4, an annulus is present at the periosteal surface of both the femur (Fig. 1D) and tibia (fig. S8E), but when the annulus is followed around the cortex, in both cases it becomes embedded within the outer cortex and superseded by woven primary tissue (Figs. 1E and 2C). The proximity of the annulus to the periosteal surface instead suggests that BMRP 2006.4.4 died soon after growth resumed following the annual hiatus and that cortical osteogenesis was directional. Ontogenetic age On the basis of femur CGM count, BMRP 2002.4.1 was>13 years old at death, which is 2 years older than the original estimate by Erickson (8) based on fibula CGM count. The slightly larger BMRP 2006.4.4 was>15 years old. The number of CGMs missing due to medullary expansion is unknown, precluding an exact age at death for BMRP 2002.4.1 and BMRP 2006.4.4. Although the number of missing CGMs could be predicted on the basis of innermost zonal thicknesses and a process of retrocalculation [e.g., (5, 10)], the variable spacing between CGMs observed in BMRP 2002.4.1 and BMRP 2006.4.4 and other tyrannosaurs (15) renders the technique unreliable in this case, and it was not attempted. Within the innermost cortex of BMRP 2006.4.4, there is a tight stacking of six CGMs (Fig. 2D). Because the CGMs remain parallel about the cortex and do not merge, they either represent a single hiatus in which growth repeatedly ceased and resumed (totaling 13 years of growth) or up to 6 years where relatively little growth occurred annually (totaling up to 18 years of growth) (9, 16). This tight stacking of six CGMs is not observed in the femur of BMRP 2006.4.4, which preserves 15 CGMs. The CGM count from the partial tibia of BMRP 2006.4.4 is questionable because the proximal sampling location away from midshaft incorporates the fibular crest, introducing associated regions of remodeling and directional growth affecting apposition interpretations. Because of this and their absence in the femur, the observed grouping of six CGMs is conservatively interpreted as a single hiatus event. Similar instances of a single hiatus represented by narrowly spaced LAGs are reported in other tyrannosauroids (15). If this grouping of CGMs instead represents 6 years of protracted growth, then BMRP 2006.4.4 demonstrates the extent to which these individuals could adjust growth rate based on resource availability, in this case prolonging the ontogenetic duration of BMRP 2006.4.4 as a mid-sized carnivore. Bone tissue organization was similar across femora and tibiae, suggesting that both bones record annual increases in body size equally well. If the stacked CGMs of BMRP 2006.4.4 reflect a single hiatus, then each femur preserved more CGMs than the associated tibia. Previous studies demonstrated that intraskeletal inconsistencies in CGM counts are due to variable rates of medullary cavity expansion or cortical drift across elements (9, 17, 18) when sampled at midshaft. Therefore, our preliminary assessment of T. rex intraskeletal histology suggests that the femur is more informative than the tibia, despite regions of cortical remodeling from tendinous entheses about the cortex. Additional intraskeletal histoanalyses of tyrannosaurid specimens are necessary to test whether the femur is the preferred weight-bearing bone for simultaneous assessments of annual growth rates and skeletochronology. In addition to ontogenetic zonal thickness variability within the cortex, zonal thickness also changed with respect to cortical orientation. That is, zones were often much thinner relative to one another on one side of the transverse section and much thicker on another side (e.g., fig. S4, G and H). This pattern is particularly noticeable in the tibia of BMRP 2002.4.1 (medial cortical zones are thickest) and the femur of BMRP 2006.4.4 (posteromedial cortical zones are thickest). This observation implies that directional cortical growth occurred over ontogeny and stresses the necessity of complete transverse sections for histological analysis: Obtaining a fragment or core for study from one orientation may result in erroneous interpretations of growth rate and skeletal maturity. Variability in annual growth as a response to resource abundance Interpretations of relative maturity in nonavian dinosaurs often rely on reported trends in the thickness of cortical zones between CGMs from the inner to the outer cortex (10). Zone thickness is typically greatest within the innermost cortex, corresponding to rapid annual growth early in life. Zones become progressively thinner in the midto the outer cortex of older individuals, as annual growth rate decreases approaching asymptotic body length. These general trends provide the interpretive foundation for the two previous histologybased ontogenetic studies on Tyrannosaurus growth (4, 5). The spacing of CGMs within the outer cortices of BMRP 2002.4.1 and BMRP 2006.4.4 (Fig. 4) is narrower than between some CGMs deeper within the cortices, which suggests that, although not adults, the specimens were approaching a body length asymptote at about one-half the body length of FMNH PR 2081. However, annual zonal thicknesses between CGMs deeper within the cortices of BMRP 2002.4.1 (Fig. 4A) and BMRP 2006.4.4 (Fig. 4B) are variable, and zones do not consistently progress from widely spaced within the inner cortex to more closely spaced in the outer cortex. Because of unpredictable spacing within the cortex, reduced zonal thickness near the periosteal surface is likely an unreliable indicator of skeletal maturity in BMRP 2002.4.1 and BMRP 2006.4.4. Variable zonal thicknesses are, thus, likely to be observed in ontogenetically older T. rex individuals. To test this hypothesis, we examined femur and tibia thin sections from T. rex specimens USNM PAL (National Museum of Natural History) 555000, MOR (Museum of the Rockies) 1125, MOR 1128, MOR 1198, and CCM (Carter County Museum) V33.1.15. In all individuals, variability in annual zonal thicknesses was observed. In particular, compared to zone spacing within the mid-cortex, noticeably thinner zones are present within the innermost cortex of USNM PAL 555000 (Fig. 4C) and MOR 1128 (Fig. 4D). These results contradict the mathematically predictable zonal spacing in T. rex long bones reported by Horner and Padian (5), which used some of the same specimens reassessed in the present study. Results further suggest not only that BMRP 2002.4.1 and BMRP 2006.4.4 had not yet entered the accelerated growth period proposed for this taxon (4, 5) but also that the accuracy of the generalized T. rex body mass curve from Erickson et al. (4) would be affected by undetected individual variation in annual growth. Variable LAG spacing is reported in ornithomimids, ornithopods [(19) and references therein], and other tyrannosauroids (15) and may correlate with annual resource abundance (12, 19). Our data suggest that this trait also characterizes T. rex: Because the level of bone tissue organization within zones remained the same from the innermost cortex to the periosteal surface in the BMRP specimens, growth rates were within a similar range from year to year. To produce these extremes in annual bone apposition, the duration of the growth hiatus must have varied annually. On the basis of the larger T. rex specimens examined here for comparison, the adjustment of annual growth hiatus duration in response to resource abundance is a physiological characteristic observed throughout T. rex ontogeny. Regardless of cause, unpredictable CGM spacing, Published as part of Holly N. Woodward, Katie Tremaine, Scott A. Williams, Lindsay E. Zanno, John R. Horner & Nathan Myhrvold, 2020, Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy " Nanotyrannus " and supports ontogenetic niche partitioning in juvenile Tyrannosaurus, pp. 1-8 in Science Advances (eaax 6250) (eaax 6250) 6 on pages 1-7, DOI: 10.1126/sciadv.aax6250, http://zenodo.org/record/3749024, {"references":["1. H. F. Osborn, Tyrannosaurus and other Cretaceous carnivorous dinosaurs. Bull. Am. Mus. Nat. Hist. 21, 259 - 265 (1905).","2. S. L. Brusatte, M. A. Norell, T. D. Carr, G. M. Erickson, J. R. Hutchinson, A. M. Balanoff, G. S. Bever, J. N. Choiniere, P. J. Makovicky, X. Xu, Tyrannosaur paleobiology: New research on ancient exemplar organisms. science 329, 1481 - 1485 (2010).","3. P. L. Larson, K. Carpenter, Tyrannosaurus Rex, the Tyrant King. (Indiana Univ. Press, Bloomington, 2008).","4. G. M. Erickson, P. J. Makovicky, P. J. Currie, M. A. Norell, S. A. Yerby, C. A. Brochu, Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs. Nature 430, 772 - 775 (2004).","5. J. R. Horner, K. Padian, Age and growth dynamics of Tyrannosaurus rex. Proc. R. soc. Lond. B Biol. sci. 271, 1875 - 1880 (2004).","6. J. R. Hutchinson, K. T. Bates, J. Molnar, V. Allen, P. J. Makovicky, A computational analysis of limb and body dimensions in Tyrannosaurus rex with implications for locomotion, ontogeny, and growth. PLOs ONE 6, e 26037 (2011).","7. P. J. Currie, Allometric growth in tyrannosaurids (Dinosauria: Theropoda) from the Upper Cretaceous of North America and Asia. Can. J. Earth sci. 40, 651 - 665 (2003).","8. G. M. Erickson, Assessing dinosaur growth patterns: A microscopic revolution. Trends Ecol. Evol. 20, 677 - 684 (2005).","9. H. N. Woodward, J. R. Horner, J. O. Farlow, Quantification of intraskeletal histovariability in Alligator mississippiensis and implications for vertebrate osteohistology. PeerJ 2, e 422 (2014).","10. K. Padian, E. - T. Lamm, Bone Histology of Fossil Tetrapods: Advancing Methods, Analysis, and Interpretation (University of California Press, Berkeley, 2013), p. 285.","11. M. H. Schweitzer, W. Zheng, L. Zanno, S. Werning, T. Sugiyama, Chemistry supports the identification of gender-specific reproductive tissue in Tyrannosaurus rex. sci. Rep. 6, 23099 (2016).","12. M. Kohler, N. Marin-Moratalla, X. Jordana, R. Aanes, Seasonal bone growth and physiology in endotherms shed light on dinosaur physiology. Nature 487, 358 - 361 (2012).","13. E. Prondvai, K. H. W. Stein, A. de Ricqles, J. Cubo, Development-based revision of bone tissue classification: the importance of semantics for science. Biol. J. Linn. soc. 112, 799 - 816 (2014).","14. J. G. Skedros, S. D. Mendenhall, C. J. Kiser, H. Winet, Interpreting cortical bone adaptation and load history by quantifying osteon morphotypes in circularly polarized light images. Bone 44, 392 - 403 (2009).","15. L. E. Zanno, R. T. Tucker, A. Canoville, H. M. Avrahami, T. A. Gates, P. J. Makovicky, Diminutive fleet-footed tyrannosauroid narrows the 70 - million-year gap in the North American fossil record. Commun. Biol. 2, 64 (2019).","16. M. H. Caetano, J. Castanet, Variability and microevolutionary patterns in Triturus marmoratus from Portugal: age, size, longevity and individual growth. Amphibia Reptilia 14, 117 - 129 (1993).","17. I. Griffiths, Skeletal lamellae as an index of age in Heterothermous Tetrapods. Ann. Mag. Nat. Hist. 4, 449 - 465 (1961).","18. J. M. Hutton, Age determination of living Nile crocodiles from the cortical stratification of bone. Copeia 2, 332 - 341 (1986).","19. T. M. Cullen, D. C. Evans, M. J. Ryan, P. J. Currie, Y. Kobayashi, Osteohistological variation in growth marks and osteocyte lacunar density in a theropod dinosaur (Coelurosauria: Ornithomimidae). BMC Evol. Biol. 14, 231 (2014).","20. T. D. Carr, Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). J. Vertebr. Paleontol. 19, 497 - 520 (1999).","21. C. W. Gilmore, New carnivorous dinosaur from the Lance formation of Montana. smithson. misc. collect. 106, 1 - 19 (1946).","22. R. T. Bakker, M. Williams, P. J. Currie, Nanotyrannus, a new genus of pygmy tyrannosaur, from the latest Cretaceous of Montana. Hunteria 1, 1 - 30 (1988).","23. T. D. Carr, T. E. Williamson, Diversity of Late Maastrichtian Tyrannosauridae (Dinosauria: Theropoda) from western North America. Zool. J. Linnean soc. 142, 419 - 523 (2004).","24. A. K. Rozhdestvensky, Growth changes in Asian dinosaurs and some problems of their taxonomy. Paleontol. Zh. 3, 95 - 109 (1965).","25. C. A. Brochu, Osteology of Tyrannosaurus rex: Insights from a nearly complete skeleton and high-resolution computed tomographic analysis of the skull. J. Vertebr. Paleontol. Memoir 22, 1 - 138 (2003).","28. T. R. Holtz Jr., The Dinosauria, D. Weishampel, P. Dodson, H. Osmolska, Eds. (University of California Press, Berkeley, 2004), pp. 111 - 136.","29. T. Tsuihiji, M. Watabe, K. Tsogtbaatar, T. Tsubamoto, R. Barsbold, S. Suzuki, A. H. Lee, R. C. Ridgely, Y. Kawahara, L. M. Witmer, Cranial osteology of a juvenile specimen of Tarbosaurus bataar from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia. J. Vertebr. Paleontol. 31, 497 - 517 (2011).","33. L. M. Witmer, R. C. Ridgely, The Cleveland tyrannosaur skull (Nanotyrannus or Tyrannosaurus): new findings based on CT scanning, with special reference to the braincase. Kirtlandia 57, 61 - 81 (2010).","32. N. L. Larson, in Tyrannosaurus rex, the Tyrant King, P. Larson, K. Carpenter, Eds. (Indiana Univ. Press, Bloomington, 2008), pp. 1 - 56.","34. J. R. Horner, M. B. Goodwin, N. Myhrvold, Dinosaur census reveals abundant Tyrannosaurus and rare ontogenetic stages in the Upper Cretaceous Hell Creek Formation (Maastrichtian), Montana, USA. PLOs ONE 6, e 16574 (2011).","35. T. R. Holtz Jr., Taxonomic diversity, morphological disparity, and guild structure in theropod carnivore communities: implications for paleoecology and life history strategies in tyrant dinosaurs. J. Vertebr. Paleontol. 24, 72 A (2004).","36. T. R. Holtz Jr., Tyrannosaurus rex, the Tyrant King, P. Larson, K. Carpenter, Eds. (Indiana Univ. Press, Bloomington, 2008), chap. 20, pp. 371 - 396.","37. A. Kane, K. Healy, G. D. Ruxton, A. L. Jackson, Body size as a driver of scavenging in theropod dinosaurs. Am. Nat. 187, 706 - 716 (2016).","38. D. A. Russell, Tyrannosaurs from the Late Cretaceous of western Canada. National Museum of Natural sciences, Publications, in Paleontology 1, 1 - 34 (1970).","39. P. Dodson, Functional and ecological significance of relative growth in Alligator. J. Zool. 175, 315 - 355 (1975).","40. J. E. Peterson, K. N. Daus, Feeding traces attributable to juvenile Tyrannosaurus rex offer insight into ontogenetic dietary trends. PeerJ 7, e 6573 (2019)."]}