Tyrannosaurus sp. AMNH 3982. Two dorsal centra, the holotype of Manospondylus gigas. One centrum was reported lost by Osborn (1916). From an unspecified horizon, presumably the Hell Creek Formation, South Dakota. Described by Cope (1892) and discussed and figured by Osborn (1916). CM 1400. Cranial and jaw fragments, ischial fragments, ribs and chevrons. From the Lance Formation of Snyder Creek, Niobrara Co., Wyoming. CM 9401, a fragmentary lachrymal, may pertain to this specimen. MMS 51-2004. Anterior portion of braincase. From the Lance Formation (?) of northwestern South Dakota. NMMNH P-1013-1. Dentary, prearticular, articular, teeth, chevron. From the McRae Formation near Kettle Top Butte, New Mexico. Described by Gillette, Wolberg & Hunt (1986). USNM 6183. Femur, tibia and fibula. From the Lance Formation (?) of Alkali Creek, Niobrara Co., Wyoming. Lawson (1976) described a maxilla (TMM 41436-1), from the Maastrichtian Tornillo Gr. of Brewster Co., Texas, as pertaining to a juvenile T. rex. His grounds for this were the general proportions of the maxilla, a narrow bar separating the maxillary from the antorbital fenestra, and evidence that the size of the maxillary fenestra decreased with growth in T. rex relative to maxillary length. Because the maxillary fenestra of TMM 41436-1 is large, this implies that it would decrease in relative size with growth in T. rex, but increase in, e. g., Albertosaurus libratus (Lawson 1976), and hence in this specimen would come to resemble that of an adult T. rex. Although it may be doubted that the bar separating the maxillary from the antorbital fenestra is particularly narrow in T. rex, the proportions of the maxilla are as expected from growth trends in modern crocodilians. The following specimens may also pertain to Tyrannosaurus: AMNH 1011, an incomplete tooth from the Hell Creek Fm. of Montana; AMNH 5020, right fourth metatarsal from the Hell Creek Fm. near Lismas, Montana; AMNH 5021, pedal phalanx from the Hell Creek Fm. near Lismas, Montana; AMNH 5044, caudals from the Hell Creek Fm. at Sand Creek, Montana; CM 244, a phalanx from the Lance Fm. of Lance Creek, Wyoming; IVPP unnumbered specimen, a metatarsal from the Wangshi Fm. of Shandong, China (Hu 1973); NMC 9554, an incomplete cervical from the Scollard Fm. near Huxley, Alberta (R ussell 1970); NMC 9950 (now part of TMP 81.12.1), pedal phalanx from the Scollard Fm. near Huxley, Alberta (R ussell 1970); UCM 38804, partial tooth from the Laramie Fm. of Weld Co., Colorado; UNM FKK-076, tooth from the Naashoibito Fm. San Juan Basin, New Mexico; and USNM 2110, right fourth metatarsal from the Lance Fm. of Converse Co., Wyoming (Gilmore 1920); USNM 8064, right ilium from the Lance Fm. (?) of Alkali Creek, Niobrara Co., Wyoming. There are many other isolated teeth referred to T. sp. not listed here. Description Introduction The skull and jaws of Tyrannosaurus rex were figured and described by Osborn (1906, 1912). In the first of these papers he discussed the incomplete skull and jaws of the holotype (CM 9380, previously AMNH 973) but figured the elements only in lateral view, as part of a reconstruction of the skull. His later paper (1912) described additional material; AMNH 5027, AMNH 5029 (now CM 9379) and AMNH 5117. The first of these is a nearly complete articulated skull with jaws (in addition to the axial skeleton), the other two consist only of braincases. In that paper all of the elements present are figured in articulation except for the maxilla (which is also figured separately), hence the internal surfaces of the elements are neither figured nor described. Since 1912, three reasonably complete skulls of T. rex have been recovered which add greatly to our knowledge of the cranial elements. In 1967, an incomplete but articulated skull and jaw was collected by Dr. William MacMannis of Montana State University and brought to the Museum of the Rockies (Bozeman, Montana). This skull (MOR 008) was apparently eroded approximately to the midline prior to discovery and thus exposed the medial surfaces of many of the cranial elements. This skull has recently been disarticulated and completely prepared by Dr. John Horner. An incomplete skull and skeleton was discovered in 1968 by Mr. Harley Garbani for the Los Angeles County Museum of Natural History, and recovered by a field crew under the direction of Dr. J. R. MacDonald. The skull of this specimen (LACM 23844) consists of almost complete, but disarticulated, elements. In 1981 Dr. Phillip Bjork collected a third, complete and articulated skull and associated partial skeleton, which is now at the South Dakota School of Mines in Rapid City. There are two further specimens at the Tyrrell Museum, Drumheller, Alberta: one (TMP P81.12.1), on display, consists of much of the postcranial skeleton, but of the skull only a postorbital, and the other (TMP P81.6.1) has a reasonably complete skull currently under preparation. Most recently mandibular elements and a chevron reportedly from T. rex have been described from the McRae Formation of New Mexico (Gillette, Wolberg & Hunt 1986). An almost complete skeleton was recently collected by the Museum of the Rockies. The skull of LACM 23844 was discovered disarticulated, and that of MOR 008 has been disarticulated in preparation. These have provided the opportunity to study the morphology of the individual cranial bones. When originally found that of MOR 008 was eroded to the midline, so the internal relationships of the elements could be observed. The two articulated skulls studied, AMNH 5027 and SDSM 12047, have both undergone shearing and some crushing. The skull of AMNH 5027 has been sheared such that the right side has been displaced dorsally relative to the left, and vertically stretched posteriorly, so that the squamosal has become separated from the quadratojugal and the postorbital slightly separated from the jugal. That of SDSM 12047 has been sheared in the opposite sense, and crushed at least on the left side. A complete description of the cranial elements is neither feasible nor desirable as most have been well described by Osborn. Therefore the elements will be described principally with regard to five aspects: description of elements and of surfaces of elements not previously observable; features or structures that have not been previously treated (e. g. cranial sinuses); individual variation; features relating to the reconstruction of the cranial musculature, and; features relating to joint form. This study in its original form was part of the requirements for the degree of Ph. D. at the University of California, Los Angeles. It is intended that the present publication form the basis for a functional study of the skull of T. rex in preparaton, hence the inclusion of features related to joint form and muscle reconstruction. The teeth will be discussed in that study. Variation is treated in some detail in order to facilitate recognition of variable features in other taxa, and to assist in distinguishing between the kinds of features that vary and those that indicate taxonomic difference (cf. Molnar 1990). Comparison with Allosaurus fragilis is emphasized, because this is a well-known form, monographed by Madsen (1976) and Gilmore (1920). Comparison with other tyrannosaurids is desirable, but little detailed cranial osteology is yet available (except for Maleev 1974). Because comparison with other tyrannosaurids relates to the distinguishing features of Tyrannosaurus it will be largely treated in the discussion. This study depends primarily on four specimens: AMNH 5027, LACM 23844, MOR 008 and SDSM 12047. Because of variant usage in the literature it is advisable to consider the nomenclature relating to the cranial fenestrae. The term fenestra will here be used to refer only to the actual aperture. The excavation that often surrounds the aperture (as e. g. for the antorbital fenestra) will be termed the recess or fossa. Thus the antorbital fenestra will be described as laying in the antorbital fossa. Little reference is made here to circulatory or central nervous structures. These have been treated by Osborn (1912), and as little new material of the braincase has become available since that time (and that material has yet to be completely prepared and reassembled), they are not included. Detailed measurements of the elements are not presented as it was felt that too many elements were crushed or too incomplete for significant measurements to be made. Premaxilla (Fig. 1, Pl. 1): The premaxilla of T. rex has the form of a low, trapezoidal prism set on end (the tooth-bearing surface). It was figured in lateral, dorsal and ventral aspects by Osborn (1912, Pls.l and 2 and Fig. 6). The body is surmounted by a prominent recurved nasal process anteriorly and a less prominent, flattened maxillary process posteriorly. It contacts only two other elements, the maxilla posteriorly and the nasal dorsoposteriorly (but see ‘vomer’). The nasal process reaches back to the nasal, forming the dorsal margin of the naris, and the maxillary process forms the ventral margin. The premaxilla of Tyrannosaurus rex is much like that of Allosaurus fragilis with the anterior margin rising perpendicularly from the ventral border, and in this feature it differs from those of Megalosaurus hesperis (Waldman 1974) and Eustreptospondylus oxoniensis. The premaxilla bears four compressed teeth. Premaxillae are well-preserved in AMNH 5027 and LACM 23844, missing from SDSM 12047 and not yet prepared on MOR 008. The following description is based on LACM 23844. A dorsoventrally elongate foramen, opening into the oral cavity just below the palate, lays on the maxillapremaxilla contact. The walls of this channel are smooth, and occupy one-third of the area of the maxillary contact surface of the premaxilla (Fig. 1). The corresponding face of the maxilla is too poorly preserved for the channel to be traced. Six or seven large foramina are set into the external surface of this bone. Three are aligned parallel to, and just above, the ventral margin and the others lay dorsal to them. The palatal surface of the premaxilla bears a ridge along its medial border. When both premaxillae are articulated these ridges contribute to the formation of a single ridge along the symphysis which extends posteriorly to the anterior apex of the vomer. This ridge separates the tooth depressions of left side of the premaxillary part of the palate from those of the right. Maxilla (Pl. 2): Described by Osborn (1906) and later figured (Osborn 1912, Pl. 1 and Figs.22 and 23) the maxilla of T. rex agrees in general form with those of the other well-known species of large theropods. The maxilla is roughly triangular with the apex directed forward: it is deeply emarginate from behind. This emargination forms the antorbital fenestra, anterior to which is the maxillary (previously termed the second antorbital) fenestra. Both fenestrae are set in a distinct common fossa that is more pronounced than in e. g. Albertosaurus. The floor of this fossa is inset from the general surface level by 2 to 3 cm. The maxillae are well-preserved in AMNH 5027, LACM 2384, SDSM 12047 and UCMP 118742. The maxillae contact the premaxillae anteriorly, the nasal dorsally, and behind the nasal the lachrymal meets the upper arm of the maxilla above the antorbital fenestra, and medially the shelf-like palatal process joints the vomer anteriorly and palatine posteriorly. The maxilla of LACM 23844 bears a small foramen along the ventral margin of the antorbital fossa between the maxillary and antorbital fenestrae. This foramen is not found in the maxillae of AMNH 5027 or CM 9380, but opens on the posterior margin of the maxillary fenestra in UCMP 118742. The maxillary fenestra varies in form. In AMNH 5027 it is (roughly) trapezoidal in outline (on the left side) as it is in SDSM 12047 and UCMP 118742, but in CM 9380 it is oval (almost triangular), and in LACM 23844 it is triangular. It is intermediate in form on the right side of AMNH 5027. The sculpture of the maxilla terminates abruptly at the edge of the antorbital fossa. TMP P81.6.1 is unique in that the maxilla bears no discernible sculpture. A series of prominent foramina parallels the ventral margin. The maxillae of AMNH 5027 and SDSM 12047 exhibit, just behind the 12th tooth, a sharp lateral flexure of 30° to the long axis. However, in the maxillae of LACM 23844, this flexure is both more subdued, of only 10°, and more anterior, at the 9lh tooth. The maxillary flexure is doubtless related to that of the jugal, which together form of the postorbital expansion of the skull: this feature is treated in the discussion. The maxillae of LACM 23844 and UCMP 118742 bear along their dorsal (nasal) margins a series of chambers — one small and three large — separated by incomplete partitions. The extent and form of these chambers is shown in Fig. 2. Their development varies: in LACM 23844, the third from the front is largest, but in UCMP 118742 the second is largest. A small foramen on the medial face of the maxilla of CM 9380 (Osborn 1912, Fig. 23), just posterodorsal to the maxillary fenestra, apparently communicates with these chambers. A foramen on the lateral face at this position is figured by Osborn (1912, Fig. 22) in this specimen, which is not present in AMNH 5017, LACM 23844 or SDSM 12047. The dorsal portion of the bar separating the maxillary from the antorbitai fenestra houses a large triangular chamber apparently not connected to the other four. In AMNH 5027, CM 9380 and SDSM 12047 (on the right side) the maxilla bears 12 alveoli but in LACM 23844, and possibly on the left is SDSM 12047, it has only 11 alveoli. In anterior view the teeth incline slightly laterally as they do in the Komodo dragon, Varanus komodoensis. The medial (palatal) shelf of the maxilla had a long contact with the palatine. In LACM 23844 and UCMP 118742 the palatal process of the maxilla is relatively more dorsal in position, c. 130% further from the ventral margin than in AMNH 5027 and CM 9380. Although SDSM 12047 exhibits some distortion this process appears to be in a position comparable to that of AMNH 5027, rather than to that of LACM 23844. The medial face of the maxilla bears a number of shallow depressions like those found in the palate of alligatorids. The depressions are assumed to have accomodated the tooth crowns when the mouth was closed, as in alligatorids. The anteriormost is in the palatal part of the premaxilla, and is followed posteriorly by three in the maxilla, decreasing in size posteriorly. These five are large and distinct. The impressions fade out at the mid-portion of the maxilla but become noticeable again posteriorly as three smaller, shallow, confluent depressions that together form a longitudinal groove. These impressions are clear in AMNH 5027, LACM 23844 and UCMP 118742, and the anterior impressions may be seen in Osborn’s figure (1912, Fig. 23) of the maxilla of CM 9380 but impressions were not seen on SDSM 12047. The spacing of the anterior five impressions matches the spacing of the five anterior dentary crowns. Lachrymal (Pl. 1): The lachrymal has a form like that of the letter “L” inverted, with its lower ramus forming the dorsal part of the preorbital bar. It meets the nearly horizontal upper ramus at an angle of 60°. Along almost the entire length of its upper ramus, the lachrymal joins medially with the nasal and it contacts the maxilla at its anterior termination. It meets the prefrontal and the frontal posteriorly and, apparently, the ethmoid medially along the upper portion of the descending ramus. The jugal contacts the ventral termination of the descending ramus. The inflated, roughly cylindrical upper ramus shows no indication of having borne a lachrymal horn as in earlier large theropods, such as Allosaurusfragilis, Ceratosaurus nasicornis and Yangchuanosaurus shangyouensis. The lachrymal is figured by Osborn (1912, Pls. 1 and 2) in both lateral and dorsal aspects, in articulation with the other cranial elements. This description relies heavily on LACM 23844; less so on AMNH 5027, MOR 008 and SDSM 12047. The horizontal ramus of the lachrymal is extensively excavated into a series of at least three chambers (Fig. 3). The small lateral foramen opens into the central of these. The descending ramus is flat, thin and presumably solid; medially it bears a prominent, oblique, slightly curved ridge extending from posterodorsal to anteroventral. A smaller chamber located just lateral to the top of this ridge opens broadly anteriorly and communicates posteriorly with the orbit via a narrow canal (0.04 cm in diameter in MOR 008). Its location and relationships suggest that this canal accomodated the lachrymal duct, and the chamber possibly a nasal sinus. As on the maxillae the surface sculpture terminates abruptly, marking the dorsal border of the antorbital fossa. This fossa does not extend onto the descending ramus. No sculpture is apparent on the lachrymal of TMP P8 1.6.1. Nasal: The general form and appearance of the nasals is shown by Osborn (1912, Pls. 1 and 2). The nasals resemble those of Albertosaurus libratus but are more rugose in most specimens. Anteriorly the nasal touches the premaxilla both above and below the external naris. Laterally, over most of its length, it joints the maxilla and, posteriorly, the anterior ramus of the lachrymal. Tapering posteriorly, the nasals are strongly constricted between the lachrymals and posteriorly have only restricted contact with the frontals. A small contact with the prefrontals may have existed immediately lateral to the junction with the frontals, but this cannot be verified on any known specimen. The nasals are strongly arched in transverse section, and have a dorsal surface that is usually strongly rugose at the midline. These rugosities are absent in TMP P81.6.1, and less pronounced in LACM 23844 and SDSM 12047 than in AMNH 5027. They are most emphatically developed in MOR 008, where prominent cusps are present (Pl. 3). This specimen also shows several well-developed foramina (to 1 cm in diameter). The dorsal surface is distinctly depressed at the level of the anteriormost contact with the lachrymals. There is no indication of internal chambers. The premaxillary processes of the nasals of LACM 23844 are broken just anterior to the nasal symphysis, however sufficient remains to indicate that they were oval, nearly cylindrical rods like those of MOR 008, but unlike the flattened premaxillary processes of AMNH 5027. Postorbital (Pl.4): The postorbital, in articulation, is figured by Osborn (1912, Pl. 1): it is preserved in AMNH 5027, LACM 23844, MOR 008, SDSM 12047 and TMP P81.12.1. Essentially a flat, broad, vertical plate it is, in its upper portion mildly concave internally and convex externally. The postorbital of T. rex is much broader than in the earlier Allosaurus fragilis, Ceratosaurus nasicornis and Yangchuanosaurus shangyouensis. It is even slightly broader than in Alectrosaurus olseni: it is matched in broadness, however, by that of the unrelated Camotaurus sastrei. This vertical plate sweeps forward ventrally in a gentle curve, to

Published as part of Molnar, Ralph E., 1991, The Cranial Morphology of Tyrannosaurus rex, pp. 137-176 in Palaeontographica Abt. A 217 (4 - 6) on pages 139-171, DOI: 10.5281/zenodo.3251815