Tyrannosaurus rex, Osborn, 1905

Osborn, H. F., 1917, Skeletal Adaptations of Ornitholestes, Struthiomimus, Tyrannosaurus, Bulletin of the American Museum of Natural History 35, pp. 733-771 : 762-771

publication ID	https://doi.org/ 10.5281/zenodo.1040385
DOI	https://doi.org/10.5281/zenodo.3483030
persistent identifier	https://treatment.plazi.org/id/90598799-FF0E-FF9A-FE54-A552514BFCA2
treatment provided by	Jeremy
scientific name	Tyrannosaurus rex
status

Treatment

Skeleton of Tyrannosaurus rex Osborn.

Text-figs. 17-21 View Fig. 17 View Fig. 17a View Fig. 18 View Fig. 19 View Fig. 20 View Fig. 21 , Plates XXV View Plate XXV , XXVII View Plate XXVII .

Tyrannosaurus is the most superb carnivorous mechanism among the terrestrial Vertebrata, in which raptorial destructive power and speed are combined; it represents the climax in the evolution of a series which began with the relatively small and slender Triassic carnivore Anchisaurus .

Materials. The fortunate discovery of two skeletons of exactly the same size ( Amer. Mus. Nos. 973 View Materials , 5027 View Materials ), secured by the American Museum of Natural History expeditions to Hell Creek, Montana, in the years 1902 and 1908 respectively, affords complete knowledge of all parts of the skeleton except the sternal ribs, the fore arm, and the manus. The mounted animal here represented ( Fig. 15 View Fig. 15 , Plate XXV View Plate XXV ) is chiefly composed of the specimen found in 1908, Amer. Mus. 5027 View Materials with the missing complementary parts, especially the humerus and the femur, cast from the genotype specimen Amer. Mus. 973 View Materials , discovered in 1902.

Mounting.1 The anterior, lateral, and posterior aspects of this remarkable mount are shown in Plate XXV View Plate XXV , reproduced from the superb photographs taken by Mr. A. E. Anderson in 1915 which were subsequently perfected by his skilful retouching of the negatives whereby all trace of the powerful steel frame work which supports the mounted skeleton has been removed. The absolutely perfect skull of this specimen ( Amer. Mus. 5027 View Materials , which has been described in great detail by Osborn ( 1912) 2 is not mounted with this specimen because of its great weight but is replaced by a fac simile cast. The pose of this animal has been the subject of prolonged study by the author3 with the cooperation especially of Mr. Barnum Brown and Mr Erwin Christman. It is proposed ultimately to mount the pair of skeletons in the offensive and defensive attitudes respectively.

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Resemblances to the Ornithomimidae . The resemblances which the skeleton of Tyrannosaurus presents to that of Struthiomimus and Ornithomimus are due to inheritance from a remote common ancestor of Lower Jurassic or even Triassic age. The many striking differences are due to the extremely wide divergence in the habits and adaptations of these animals, and are here represented in a form preliminary to more complete description in a memoir. The chief resemblances are the following:

1. A similar vertebral formula, namely: C 9-10, T 14-13, S 5.

2. Ornithic adaptation of the pelvic arch and sacrum and a close analogy in adaptation to bipedal locomotion, cursorial and saltatory.

3. Functional tridactylism of the pes with strong reduction of DI, and lateral compression of Mts. III between Mts. II, IV.

4. Relatively feeble development of the fore limb.

5. Retention of a complete series of sternal ribs.

The obvious ancestral resemblances cease or are masked by the widely divergent adaptations of Tyrannosaurus to exclusive carnivorous habits and aggression, and of Struthiomimus and Ornithomimus probably to herbivorous habits and defencelessness, compensated for, doubtless, by alert powers of vision and rapid locomotion. The relative scale of development of the skull and the various parts of the skeleton in these two types of animals is well shown in the accompanying figures ( Fig. 17 View Fig. 17 ), bothoneseventieth natural size.

Adaptive Divergences and Contrasts.

Chief Characters of the Tyrannosaurus Skeleton.

In a previous description of the Tyrannosaurus skeleton ( 1906) 1 a preliminary description was given of the parts preserved in the genotype ( Amer. Mus. 973 View Materials ). The present description is based upon the parts preserved in both the specimens ( Amer. Mus. Nos. 973 View Materials , 5027 View Materials ); Pre-sacral vertebrae ( Plate XXVII View Plate XXVII ). There is little doubt that the total number of pre-sacral vertebrae is twenty-three, but the line of division between the cervicals and the thoracics is not positively demarkated. The formula is either:

Cervicals 9, Thoracics 14 =23, or Cervicals 10, Thoracics 13 = 23. On the whole, close examination of C 10 appears to indicate that this should be regarded as the posterior cervical, since the capitulum of the rib is depressed as in C 9-5, whereas in the first thoracic, D 1, the capitular and tubercular articulations of the rib are widely separated as in the thoracics D 2-D 9.

Cervicals. The cervical series, C 1-10, is extremely abbreviated by the fore-and-aft compression of the vertebrae. C 1 (atlas) is extremely short, C 2 (axis) has a somewhat more elongate centrum, with a bifid spine, coalesced intercentrum, and a monocipital rib, C 3 exhibits a powerful spine and a bicipetal rib (as partly restored). In C 5-C 10 the neural spines gradually diminish in height; lateral cavities (pleurocoelia) appear at the sides of the centra; the tubercular processes of the ribs expand into a broad tuberculo-capitular head, in which the capitulum is abbreviated and no longer articulates with the posterior face of the centrum in front. The centra of the last cervical and the first dorsal, D1, are more or less coalescent.

Thoracics or dorsals. The ten successive vertebrae exhibit the following characters: (1) the centra progressively elongate; (2) the faces are amphiplatyan or slightly opisthocoelous; (3) lateral cavities (pleurocoelia) are present in D 2, are faint in D 3, D 5, present in D 6, D 13, rising gradually from the middle of the centrum in D 4 to the top of the centrum in D 13; (4) the capitular articulations of the ribs rise from near the base of the centrum in D 1 to the middle of the neural arch in D13; (5) the articulations for the tubercles of the ribs are elevated on the metapophyses (transverse processes) throughout, diminishing in size from D 1-D 12, in which the capitular and tubercular processes are brought nearer together. The under surfaces of the metapophyses are sculptured and pitted with two facets or coelia; (6) the neural spines increase steadily in size from D 1- D 13; (7) between D 7 and D 13 there is a tendency of the adjacent neural spines to coalesce at the summits; (8) in D 9-D12 there are traces of a neurocentral suture; (9) the metapophyses of D 12-D 13 tend to unite with the antero-internal face of the iliac crest ( Fig. 17 View Fig. 17 ); (10) the centrum of D 13 tends to unite with that of the first sacral ( Fig. 17 View Fig. 17 ).

As a whole the pre-sacral series, the cervico-thoracic, is greatly abbreviated so that the extremely heavy and powerful skull is brought very near the powerful sacral arcade.

Sacral vertebrae ( Figs. 19 View Fig. 19 , 20 View Fig. 20 , Plate XXVII View Plate XXVII ). Of the two superbly preserved sacral arcades that of the first specimen ( Amer. Mus. 973 View Materials ) shows best the normal condition of the five sacral vertebrae since the second specimen ( Amer. Mus. 5027 View Materials ) is unnaturally compressed and deformed. It is noteworthy that the five vertebrae composing the sacrum coalesce by their centra, also that S 2, S 3 are laterally compressed, as shown normally in Amer. Mus. 973 View Materials . S1 is directly opposite the anterior (pubic) peduncle of the ilium, while S 3 is opposite the posterior (ischiac) peduncle of the ilium. These are probably the primary sacrals, while S 4, S 5 are probably caudo-sacrals. The sutures between the sacral centra and sacral ribs are still apparent in the ventral view of Amer. Mus. 973 View Materials . An instructive view of the sacro-iliac arcade is shown in Plate D, in which it is observed also that the anterior (pubic) and posterior (ischiac) peduncles of the ilium coincide with S 1 and S 3 respectively.

Caudal vertebra ( Fig. 17 View Fig. 17 , Plate D). The anterior caudal, C 1 ( Fig. 17 View Fig. 17 ), partly unites by its caudal rib with the horizontally expanded internal plates of the ilium ( Fig. 17 View Fig. 17 , Amer. Mus. 5027 View Materials ). The caudal series so far as preserved in the specimen ( Amer. Mus. 5027 View Materials ) is represented in the shaded vertebrae figured in Plate D, namely C 1-C 15, C 17,?C 21-22. The intermediate vertebrae as well as the terminal vertebrae, C 23-53, are conjecturally restored. As in other carnivorous dinosaurs the caudals gradually diminish in size and increase in relative length. The centra are amphiplatyan or slightly procoelous. Neuro-central sutures are observed in Cd 2, Cd 11. The tail is an elongate balancing organ for the anterior part of the body, having considerable flexibility and exhibiting no indications of the extreme rigidity observed in the inflexible tail of the Ornithomimidae .

Sternal ribs. Remnants of the doubly paired, or more probably triply paired series of abdominal ribs are preserved with specimen Amer. Mus. 973 View Materials and have already been described ( Osborn, 1906). Brown observes:1 “As yet we cannot definitely say as to the triply paired disposition of the abdominal ribs in Tyrannosaurus although I doubt not that this was the placement. It is true in all other forms in which the abdominal plates are found in position, and when the median series of abdominal ribs of Tyrannosaurus are placed in position the ends do not reach the costal ribs, so that it appears that there were two additional abdominal rib segments on each side.”

Problematic Dermal plates. The dermal plates previously described by Osborn (1906) are still unique in the American Museum collections; they are totally unlike those of the armored herbivorous dinosaurs or the armored Ceratopsia. In a type specimen of Ceratosaurus recently mounted in the U. S. National Museum there is part of a row of dermal plates present above the tips of the neural spines. It is, therefore, quite possible that plates of the character attributed by Osborn to Dynamosaurus (= Tyrannosaurus ) extended down over the sides of the body.

Pelvic arcade ( Fig. 17 View Fig. 17 , Plate XXVII View Plate XXVII ). The pelvic arcade is distinguished by the following characters: (1) the antero-posterior extension of the ilium; (2) the firm articulation of the ilium with the sacral ribs ( Fig. 16 View Fig. 16 , S 1-S 5); (3) the articulation of the ilium with the metapophyses of the three posterior sacrals ( Fig. 16 View Fig. 16 , S3-S5); (4) the anterior expansion of the iliac plates ( Fig. 16 C View Fig. 16 ); (5) the postero-intemal iliac horizontal plates ( Fig. 16 D View Fig. 16 ).

The pubis, firmly coalesced with the ilium and ischium, terminates in an enormously expanded peduncle. The ischium is comparatively slender with contracted rod-like extremity.

Fore limb. The great disparity between the fore limb and hind limb is illustrated in the contrast ( Fig. 18 View Fig. 18 ) between the humerus (B) and the femur (A1, A2, A3) of the genotype specimen ( Amer. Mus. 973 View Materials ).

As to the number of digits in the still unknown manus of Tyrannosaurus Gilmore regards it as probable that Tyrannosaurus will prove to be functionally didactyl with a vestigial D III as in the genus Gorgosaurus , in which D III is reduced to a vestige being represented only by Mtc. III without phalanges. It has recently been shown by Gilmore and Lambe that the manus of the Theropoda is greatly reduced not only in size but in the number of its parts.