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1
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Fused and vaulted nasals of tyrannosaurid dinosaurs: Implications for cranial strength and feeding mechanics

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Snively E., Henderson D. M., Phillips D. S.
Acta Palaeontologica Polonica
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2006
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tom 51
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nr 3
Tyrannosaurid theropods display several unusual adaptations of the skulls and teeth. Their nasals are fused and vaulted, suggesting that these elements braced the cranium against high feeding forces. Exceptionally high strengths of maxillary teeth in Tyrannosaurus rex indicate that it could exert relatively greater feeding forces than other tyrannosaurids. Areas and second moments of area of the nasals, calculated from CT cross−sections, show higher nasal strengths for large tyrannosaurids than for Allosaurus fragilis. Cross−sectional geometry of theropod crania reveals high second moments of area in tyrannosaurids, with resulting high strengths in bending and torsion, when compared with the crania of similarly sized theropods. In tyrannosaurids trends of strength increase are positively allomeric and have similar allometric exponents, indicating correlated progression towards unusually high strengths of the feeding apparatus. Fused, arched nasals and broad crania of tyrannosaurids are consistent with deep bites that impacted bone and powerful lateral movements of the head for dismembering prey.
2
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Variation in premaxillary tooth count and a developmental abnormality in a tyrannosaurid dinosaur

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Miyashita T., Tanke D. H., Currie P. J.
Acta Palaeontologica Polonica
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2010
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tom 55
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nr 4
Premaxillary tooth count tends to be stable amongst toothed dinosaurs, and most theropods have four teeth in each premaxilla. Only one case of bilaterally asymmetric variation is known in theropod premaxillary dentition, and there is no record of ontogenetic or individual variation in premaxillary tooth count. Based on these observations, a tyrannosaurid left premaxilla with three teeth (TMP 2007.20.124) is an interesting deviation and represents an unusual individual of Daspletosaurus sp. with a developmental abnormality. The lower number of teeth is coupled with relatively larger alveoli, each of which is capable of hosting a larger than normal tooth. This indicates that tooth size and dental count vary inversely, and instances of reduction in tooth count may arise from selection for increased tooth size. On the other hand, the conservative number of premaxillary teeth in most theropods implies strong developmental constraints and a functional trade−off between the dimensions of the premaxillary alveolar margin and the size of the teeth. In light of recent advances in the study of tooth morphogenesis, tooth count is a function of two parameters: dimensions of an odontogenic field for a tooth series, and dimensions of tooth positions. A probable developmental cause for the low tooth count of TMP 2007.20.124 is that the dimensions of the alveoli expanded by approximately a third during tooth morphogenesis. Numerical traits such as tooth count are difficult to treat in a phylogenetic analysis. When formulating a phylogenetic character, a potential alternative to simply counting is to rely on the morphological signature for developmental parameters that control the number of the element in question.
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The ligamental scar in the costovertebral articulation of the tyrannosaurid dinosaurs

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Hirasawa T.
Acta Palaeontologica Polonica
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2009
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tom 54
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nr 1
49-59
The costovertebral articulation is integral to constrain the thoracic kinematics and to infer the breathing mechanism in the respect with costal aspiration. However, the structure of the costovertebral articulation in non−avian theropods has not been studied in great detail before. This study highlights the Tyrannosauridae, which is represented by numerous complete specimens. Costovertebral articulations of ten tyrannosaurid specimens, including two nearly in−situ articulated fossils, were investigated and compared with those in extant Archosauria. For extant archosaurs, dissections were conducted to rationalize the soft tissue anatomy in tyrannosaurids. This study shows that the rib articulates ventrally or posteroventrally with the distal end of the corresponding vertebral transverse process in the tyrannosaurid ribcage. A ligament (ligamentum costotransversarium) can be reconstructed to connect the rib tuberculum to the transverse process in each articulation. The scar for lig. costotransversarium is recognizable in many theropod skeletons, and this rugosity can be used to identify the rotational axis for the rib. This result provides a cornerstone for exploring the evolution of the ribcage and breathing mechanisms across the theropod lineage leading to birds.
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Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada

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Currie P. J.
Acta Palaeontologica Polonica
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2003
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tom 48
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nr 2
Beautifully preserved, nearly complete theropod skeletons from Alberta (Canada) allow re−evaluation of the taxonomic status of North American tyrannosaurids. It is concluded that the most parsimonious interpretation of relationships leads to the separation of the two species of Albertosaurus(sensu Russell 1970) into Gorgosaurus libratusfrom the Campanian Dinosaur Park Formation and Albertosaurus sarcophagus from the upper Campanian/lower Maastrichtian Horseshoe Canyon Formation. Albertosaurus and Gorgosaurus are closely related, but can be distinguished from each other by more characters than are known to justify generic distinction within another tyrannosaurid clade that includes Daspletosaurus, Tarbosaurus and Tyrannosaurus. Daspletosaurus is known from multiple species that cover extensive geographic, ecological and temporal ranges, and it is sensible to maintain its generic distinction from Tyrannosaurus. All tyrannosaurid species have consistent ontogenetic trends. However, one needs to be cautious in assessing ontogenetic stage because many characters are size−dependent rather than age−dependent. There are relatively few osteological differences that can distinguish tyrannosaurid species at any age. For example, Nanotyrannus lancensis is probably a distinct species from Tyrannosaurus rex because there is no evidence of ontogenetic reduction of tooth counts in any other tyrannosaurid species. Some characters that are good for separating mature tyrannosaurids, such as differences in the sizes and shapes of maxillary fenestrae, are not useful for identifying the species of juveniles.
5
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Skull structure and evolution in tyrannosaurid dinosaurs

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Currie P. J., Hurum J. H., Sabath K.
Acta Palaeontologica Polonica
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2003
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tom 48
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nr 2
Tyrannosauridae can be subdivided into two distinct subfamilies—the Albertosaurinae and the Tyrannosaurinae. Previously recognized subdivisions Aublysodontinae and Shanshanosaurinae are rejected because they are based on insufficient material and juvenile specimens. Our results are based upon a phylogenetic analysis using PAUP program (Swofford 1999) of 77 skull characters and seven genera (Albertosaurus, Alioramus, Daspletosaurus, Gorgosaurus, Nanotyrannus, Tarbosaurus, and Tyrannosaurus); with Allosaurus as outgroup. Of the 77 characters used, more than half were parsimony informative. Asingle most parsimonious tree was obtained with the Tree Length being 88. The analysis of cranial characters and comparison of postcranial features reveal that Tarbosaurus bataar is not the sister taxon of Tyrannosaurus rex (contra Holtz 2001). Their similarities are partially due to the fact that both are extremely large animals. Thus, Tarbosaurus should be considered a genus distinct from Tyrannosaurus.
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