Allosaurus, Marsh, 1877

Osborn, Henry Fairfield, 1912, Crania of Tyrannosaurus and Allosaurus, Memoirs of the American museum of Natural History 1, pp. 1-30 : 44-46

publication ID	https://doi.org/ 10.5281/zenodo.2784073
persistent identifier	https://treatment.plazi.org/id/9C2F87CB-D517-FFC5-1DA6-F89AFD02F2FB
treatment provided by	Jeremy
scientific name	Allosaurus
status

Treatment

Comparison with Skull of Allosaurus .

Text Figs. 9-11 View Fig. 9 View Fig. 10 View Fig. 11 , 26 View Fig. 26 , 27 View Fig. 27 .

In addition to the descriptions of the skull of Allosaurus published by the writer1 as well as to the description by Hay of the skull of Creosaurus , 2 the following observations and new characters brought out in the comparison with the skull of Tyrannosaurus are of interest.

Comparison of Figs. 25 View Fig. 25 , 26 View Fig. 26 , 27 View Fig. 27 all reproduced to J scale, exhibits both the difference in size and in proportion of the Upper Jurassic (or Lower Cretaceous) and Upper Cretaceous Thero- poda. The substantial resemblance in the morphology of the anterior aspect of the cranium is shown in the comparison of Figs. 7 View Fig , 8 View Figure ( Tyrannosaurus ), and Figs. 9-11 View Fig. 9 View Fig. 10 View Fig. 11 ( Allosaurus ).

1. Skull of Allosaurus .

Allosaurus surely has a more primitive skull, with some characters which may indicate that it is not in the direct line of ancestry of Tyrannosaurus .

Primitive characters of Allosaurus : (1) Allosaurus is slender-skulled, less robust; (2) cranial region longer and lower; (3) larger number of maxillary teeth (circa 15); (4) rugosity on pos terior border of lachrymal separated from postorbital by a deep supraorbital notch; (5) post- frentals loosely articulated, forming upper border of supraorbital notch; (6) supraorbital rugosity not developed; (7) nasal rugosity not pronounced; (8) parietal crest not so prominently devel oped; (9) supraoccipitals narrow, rectangular, bearing large keel for ligamentum, nuchce; (10) above the occipitals a pair of pits lodging rounded prominences possibly represent a portion of the periotic mass; (11) quadrates movably articulated with squamosals; (12) postorbitals without an infraorbital process leaving a very large, widely open orbital fenestra; (13) exoccipi-

tals closing in over foramen magnum, suturally separate from supraoccipitals; (14) posttemporal fenestra closed; (15) there is a deep cavity between the basioccipitals and basisphenoids in Allosaurus which has been closed up in Tyrannosaurus , in which the basioccipital and basisphen- oidal plates are closely compressed. The wide supraorbital notch in Allosaurus ( Fig. 26 View Fig. 26 ) is reduced to a narrow cleft in Tyranno- saurus ( Fig. 25 View Fig. 25 ), seen just in front of the supraorbital rugosity. With this powerful fore-and-aft compression and development of the great rugosities for muscular attachment, the loose articulation of the frontals and the freely moving joint between the quadrates and the quadrato-jugals, especially noted in Allosaurus , are replaced by the ﬁrm sutural ﬁxation of the corresponding parts in Tyrannosaurus .

At the same time the supporting arches of the side of the cranium formed by the adlachry- mals, the postorbitals, and the squamosals are greatly expanded and strengthened, thus reduc- ing the open areas both of the orbital and posttemporal fenestrae. At the same time the fenestrae in front of the orbit (f1, f2, f“) are also reduced by the ingrowth of their osseous

borders. In brief, the Tyrannosaurus skull is an abbreviated and highly powerful offspring of the Allosaurus skull, with reduced fenestration and correspondingly expanded bony structure. Among the progressive characters of the Tyrannosaurus skull in addition to those enumer- ated above are: (1) development of horizontal infraorbital bar from the postorbitals; (2) anchy- losis or coalescence, of adlachrymals and supraorbitals; (3) development of prominent supra- orbital rugosity; (4) development of nasal rugosities; (5) coalescence of supraoccipitals and exoccipitals in median line; (6) vertical reduction of supraoccipitals and corresponding expan- sion of parietals. 2. Abbreviation of Tyrannosaur Skull. The chief mechanical progression in adaptation to the excessively powerful and destructive functions of the Tyrannosaurus skull is antero-posterior abbreviation, analogous to that in the more powerful mammalian Carnivora , such as the Felidae and short-faced Canidae . Thus the dental series of the premaxillo-maxillary is numerically reduced from 20 in Allosaurus to 16 on each side in Tyrannosaurus ; the face and jaws are correspondingly shortened and deepened. The cranium, especially in the supraorbital region, is also compressed; thus the adlachrymal rugosity of Allosaurus ( Fig. 26 View Fig. 26 ) is approximated to the great supraorbital rugosity in Tyranno- saurus; the prefrontals, which are loosely articulated and exposed above the orbits in Allosaurus ( Figs. 26 View Fig. 26 , 27 View Fig. 27 ) are reduced and thrust inward in Tyrannosaurus while the adlachrymals and postorbitals are brought together. The Albertosaurus skull, which will shortly be redescribed by Mr. Barnum Brown, will show many transitional characters between those of Allosaurus and Tyrannosaurus . The Theropod skull ﬁnds its nearest analogue in the Sauropod skull ( Fig. 12 View Fig. 12 ). Important differences, it is true, are observed in the presence of a pineal fenestra in Diplodocus and a com- pletely closed pituitary sac and infundibulum. These resemblances and differences will be the subject of a special discussion of the theropod skull in the writer’s Monograph on the Sauro- poda.