Tyrannosaurus rex Osborn 1905
Creators: name: Persons, Scott W
name: Currie, Philip J.
name: Erickson, Gregory M.
Publisher: name: Zenodo
Year: 2019
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Dinosauria
Tyrannosauridae
Tyrannosaurus
Tyrannosaurus rex

description: Henry Fairfield Osborn first introduced <i>Tyrannosaurus rex</i> to the scientific community and the public in 1905 (Osborn, 1905). It has reigned as the quintessential giant carnivorous dinosaur ever since (Hone, 2016). However, as studies of <i>T. rex</i> and the rest of the Tyrannosauridae have progressed, it has become increasingly apparent that <i>T. rex</i> is far from typical and representative of the theropod lineage as a whole. Tyrannosaurids constitute a highly derived and anatomically unusual group, possessing wide and deep skulls, stout teeth, elongate hindlimbs with arctometatarsalian pedes, and reduced forelimbs with only two functional digits (Brusatte et al., 2010). Phylogenetically, tyrannosaurids are members of the Coelurosauria, which otherwise consists of predominantly small-bodied theropods. <i>Tyrannosaurus rex</i> stands out in representing the terminal extreme for gigantism within Tyrannosauridae (Erickson et al., 2004; Holtz, 2004). The iconic size of <i>T. rex</i> has led to myriad questions pertaining to its development (Erickson et al., 2004; Erickson and Currie, 2006), how its great size evolved (Erickson et al., 2004), and the ecological and physical upper bounds of terrestrial predator existence (Alexander, 1985; Henderson and Snively, 2004; Holtz, 2008). Nevertheless, since <i>T. rex</i> ’s discovery other enormous carnivorous dinosaurs belonging to the non-coelurosaurian tetanuran lineages Allosauroidea and Spinosauridae have been found that are purported to be nearly as large as <i>T. rex</i>, if not larger (Currie, 1996; Dal Sasso et al., 2005), suggesting <i>T. rex’ s</i> giantism does not stand out among nonavian theropods. ABBREVIATIONS: AMNH = American Museum of Natural History, New York, New York; BHI = Black Hills Institute of Geological Research, Hill City, South Dakota; CM = Carnegie Museum of Natural History, Pittsburgh, Pennsylvania; FMNH = Field Museum of Natural History, Chicago, Illinois; FSAC = Faculté des Sciences Aïn Chock, Casablanca, Morocco; MOR = Museum of the Rockies, Bozeman, Montana; MSNM = Museo di Storia Naturale di Milano, Milano, Italy; MUCPv-CH = Museo de la Universidad Nacional del Comahue, El Chocón collection, Neuquén, Argentina; RSM = Royal Saskatchewan Museum, Regina, Saskatchewan, Canada; TMP = Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada Grant sponsor: National Science Foundation; Grant numbers: EAR 0207744, DBI 0446224. *Correspondence to: W. Scott Persons, Department of Biological Sciences, CW 405, Biological Sciences Bldg., University of Alberta, Edmonton, Alberta T6G 2E9, Canada. Tel: 780-492-1252 E-mail: persons@ualberta.ca Received 19 May 2018; Revised 11 November 2018; Accepted 10 December 2018. DOI: 10.1002/ar.24118 Published online 00 Month 2019 in Wiley Online Library (wileyonlinelibrary.com). RSM P2523.8 (RSM: Royal Saskatchewan Museum, Regina, SK, Canada) was discovered in an iron-stained, heavily cemented sandstone lens of the Frenchman Formation (Maastrichtian, Upper Cretaceous) near Eastend, Saskatchewan, Canada in 1991. However, excavation did not begin in earnest until 1994. The extremely hard matrix surrounding the fossils, in combination with the size of the specimen, led to a prolonged period of preparation and delays in disseminating its discovery and importance regarding theropod giantism. Although some measurements of the limb bones have been disseminated in previous studies (Larson, 2008a; Benson et al., 2014; Campione et al., 2014), including inaccurate values (owing to being made prior to full preparation of the specimen]), RSM P2523.8 has never been formally described and its skeletal proportions scientifi- cally quantified. DESCRIPTION Overview RSM P2523.8 is represented by an associated but disarticulated skeleton (Fig. 1). Of special note is the abundance of incompletely coossified elements that were previously used as indices of somatic immaturity for the taxon (Currie et al., 2003). These include quadrates that are not fused to the quadratojugals, surangulars that are not fused to the articulars, and prominently visible open sutures among the basisphenoid, exoccipital-opisthotic, and prootic elements. Some fusions within RSM P2523.8 ’s skull are also not bilaterally symmetrical. For instance, the right jugal osteoderm [the rugose cheek horn common to many tyrannosaurs (Currie et al., 2003)] is fused, with the suture fully obliterated. Nevertheless, its left counterpart is completely unfused (Fig. 1). The fourth sacral centrum is not fused to either the third or the fifth sacral centra. Numerous vertebral neural arches are not fused to the centra (Fig. 2), and the scapula is not fused to the coracoid (Fig. 3). As is common among tyrannosaurs (Brusatte et al., 2010), RSM P2523.8 is riddled with skeletal pathologies. These include rugosities and deformations surrounding the neural canals of caudal vertebrae 15–17, a thick ingrowth of interdental bone within the eighth alveolus of the left maxilla (Fig. 4) and an incompletely healed and fractured dorsal rib head from the right side of the animal (Fig. 5). ABBREVIATIONS: AMNH = American Museum of Natural History, New York, New York; BHI = Black Hills Institute of Geological Research, Hill City, South Dakota; CM = Carnegie Museum of Natural History, Pittsburgh, Pennsylvania; FMNH = Field Museum of Natural History, Chicago, Illinois; FSAC = Faculté des Sciences Aïn Chock, Casablanca, Morocco; MOR = Museum of the Rockies, Bozeman, Montana; MSNM = Museo di Storia Naturale di Milano, Milano, Italy; MUCPv-CH = Museo de la Universidad Nacional del Comahue, El Chocón collection, Neuquén, Argentina; RSM = Royal Saskatchewan Museum, Regina, Saskatchewan, Canada; TMP = Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada Grant sponsor: National Science Foundation; Grant numbers: EAR 0207744, DBI 0446224. *Correspondence to: W. Scott Persons, Department of Biological Sciences, CW 405, Biological Sciences Bldg., University of Alberta, Edmonton, Alberta T6G 2E9, Canada. Tel: 780-492-1252 E-mail: persons@ualberta.ca Received 19 May 2018; Revised 11 November 2018; Accepted 10 December 2018. DOI: 10.1002/ar.24118 Published online 00 Month 2019 in Wiley Online Library (wileyonlinelibrary.com). RSM P2523.8 (RSM: Royal Saskatchewan Museum, Regina, SK, Canada) was discovered in an iron-stained, heavily cemented sandstone lens of the Frenchman Formation (Maastrichtian, Upper Cretaceous) near Eastend, Saskatchewan, Canada in 1991. However, excavation did not begin in earnest until 1994. The extremely hard matrix surrounding the fossils, in combination with the size of the specimen, led to a prolonged period of preparation and delays in disseminating its discovery and importance regarding theropod giantism. Although some measurements of the limb bones have been disseminated in previous studies (Larson, 2008a; Benson et al., 2014; Campione et al., 2014), including inaccurate values (owing to being made prior to full preparation of the specimen]), RSM P2523.8 has never been formally described and its skeletal proportions scientifi- cally quantified. DESCRIPTION Overview RSM P2523.8 is represented by an associated but disarticulated skeleton (Fig. 1). Of special note is the abundance of incompletely coossified elements that were previously used as indices of somatic immaturity for the taxon (Currie et al., 2003). These include quadrates that are not fused to the quadratojugals, surangulars that are not fused to the articulars, and prominently visible open sutures among the basisphenoid, exoccipital-opisthotic, and prootic elements. Some fusions within RSM P2523.8 ’s skull are also not bilaterally symmetrical. For instance, the right jugal osteoderm [the rugose cheek horn common to many tyrannosaurs (Currie et al., 2003)] is fused, with the suture fully obliterated. Nevertheless, its left counterpart is completely unfused (Fig. 1). The fourth sacral centrum is not fused to either the third or the fifth sacral centra. Numerous vertebral neural arches are not fused to the centra (Fig. 2), and the scapula is not fused to the coracoid (Fig. 3). As is common among tyrannosaurs (Brusatte et al., 2010), RSM P2523.8 is riddled with skeletal pathologies. These include rugosities and deformations surrounding the neural canals of caudal vertebrae 15–17, a thick ingrowth of interdental bone within the eighth alveolus of the left maxilla (Fig. 4) and an incompletely healed and fractured dorsal rib head from the right side of the animal (Fig. 5). Diagnosis RSM P2523.8 is referred to <i>T. rex</i> based on the following suite of anatomical characteristics: (1) premaxillary teeth arranged in a laterally oriented tooth row and with D-shaped cross-sections; (2) a maxilla with a rounded rostral margin and a convex ventral margin; (3) “keyhole-shaped” and anteriorly directed orbits; (4) a corneal boss posterodorsal to the orbit; (5) a broad and elongate quadratojugal process and pneumatic excavation of the squamosal; (6) a maximal skull width estimated to be nearly two-thirds that of the premaxilla-occipital condyle length; (7) a long and strap-like scapula with accentuated proximal expansion; (8) a pubic shaft showing concave anterior curvature; and finally (9) an anteriorly projecting sheet of bone spanning roughly half the dorsoventral length of the ischium. Histology Transverse petrographic microscopy analysis reveals that the diaphysis of the right fibula to be almost entirely composed of dense Haversian bone (Fig. 6A,B). Between two and five tightly spaced lines of arrested growth, collectively constituting an External Fundamental System (Cormack, 1987), are locally present nearest the lateral periosteal surface of the element (Fig. 6A). Our histological sampling of the fibula of RSM P2523.8 reveals porous, poorly organized bone matrix composing the cortex spanning from the medial periosteal surface to approximately 20% of the centroid (Fig. 6C). There is no definitive evidence of medullary bone associated with ovulation (Schweitzer et al., 2005). Although reminiscent of previous descriptions of medullary bone in <i>T. rex</i>, the inner region of the element shows extensive bioerosion in the form of radiating Wedl tunnels (Trueman and Martill, 2002) (Fig. 6D). The diagenetic activity appears to have remodeled the former secondary osteons, whose partial borders are evident from the histology, leading to unorganized mineral globules with only superficial similarities with medullar bone structure (Schweitzer et al., 2005). Skull Elements The right premaxilla is incomplete (Fig. 7), but its total size is estimated at approximately 140 mm in anteroposterior length (based on comparison with BHI 3033). The premaxilla includes the symphysis and the outer walls of four alveoli. In lateral view, the right maxilla of RSM P2523.8 has a strongly convex alveolar margin (Fig. 8). The maxillary fenestra spans 122 mm at its greatest anteroposterior length, while the antorbital fenestra spans 280 mm. Ten of the presumed 11 alveoli are preserved. On the ventral surface of the maxillary shelf, there are pits above dentary tooth positions 2–10. As in other tyrannosaurids, other than <i>Albertosaurus sarcophagus</i> (Currie, 2003), only the first four pits are pronounced. The nasals are fused and are nearly complete, lacking only the slender tips of the narial processes (Fig. 9). The nasals are 790 mm long, with the missing tips likely adding an additional 10 mm (based on comparison with BHI 3033). The external surfaces of the nasals are heavily sculptured. The contact with the maxilla is asymmetrical and is not fused. However, independent movement of either bone would have been restricted by interfingering staircase sutures, as is the case in large specimens of <i>Daspletosaurus torosus</i>, <i>Tarbosaurus bataar</i>, and other specimens of <i>T. rex</i> (Hurum and Sabath, 2003). As in other tyrannosaurids, a shallow concavity on the medial surface of the jugal has an opening that connects with the jugal sinus (Fig. 10). There is a large circular pneumatic fenestra in the posteroventral corner of the antorbital fossa. A large rugosity is present on the posteroventral corner of the left jugal. This rugosity, which indicates the presence of a blunt jugal “horn,” is absent on the right jugal. It is known from other tyrannosaur specimens that this rugosity is a separate ossification that typically becomes fused to the jugal during ontogeny. The left lacrimal is nearly complete (Fig. 11). It measures 360 mm in anteroposterior length and 375 mm in dorsoventral height. The two arms meet at an acute angle of slightly less than 90 degrees. Supraorbital ossifications are fused to rugosities on both postorbitals (Fig. 12). The postorbital boss is 6 cm long, 14 cm high, and 7 cm thick (lateromedial). The right squamosal is exceptionally well preserved (Fig. 13). There are two large pneumatophores in the posteromedial half of the medial pneumatic chamber. The larger of these pneumatophores (measuring 78 mm in dorsoventral height and 55 mm in anteroposterior length) appear in the region dorsal to the quadrate cotyle. A smaller anteroventral pneumatophore (measuring 18 mm in dorsoventral height and 35 mm in anteroposterior length) invades the medial anteroventral portion of the squamosal. The quadrates are tall (386 mm in dorsoventral height) (Fig. 14), but are not fused to the quadratojugals. This is atypical for mature tyrannosaurids. Relative to the anteroposterior length of the quadrate wing of the pterygoid, the pterygoid wing of the quadrate is unusually long (22 cm) for a tyrannosaurid. Ventromedial to the pterygoid process, a 50 <i>×</i> 30 mm pneumatic sinus enters the anterior surface. The anterolateral–posteromedial axis of the articular condyles for the lower jaw is 18 cm wide. The quadrate–quadratojugal fenestra is large with a dorsoventral diameter of roughly 10 cm. The quadratojugal of RSM P2523.8 extends approximately 15 cm into the lateral temporal fenestra (Fig. 15). However, the dorsal edge of the quadratojugal shows only a shallowly overlapping contact with the long quadratojugal process of the squamosal. The braincase is nearly complete and has suffered only slight taphonomic distortion (Fig. 16) with the cultriform process and most of the left paroccipital process being the only notable absences. The parietals and frontals are tightly bound to each other and to the rest of the braincase by complex interdigitating sutures. However, these are not fused. The orbitosphenoid, septosphenoid, sphenethmoid, and part of the orbital septum are fused. The frontals are partially fused, and are dorsoventrally deep between the supratemporal fossae and interorbital region. As a result, the contacts with the nasals and prefrontals slope anteroventrally. The lacrimal suture is 11 cm deep dorsoventrally and the postorbital suture is 9 cm deep dorsoventrally. The fused parietals measure 195 mm across the midline, 270 mm between the laterosphenoid contacts, and roughly 600 mm between the distal ends of the suture boundary between the paroccipital processes. The nuchal crest is incomplete on the left side, but was clearly asymmetrical, being thinner on the right side (37 mm anteroposteriorly compared with 43 mm). The parietals form a pronounced sagittal crest. The laterosphenoid extends dorsolaterally into the postorbital process roughly 21 cm from the midline and more than 5 cm beyond its contact with the frontal and parietal. The dorsal surface of the postorbital process of the laterosphenoid is smooth where it contacts a trough-like surface in the postorbital. The supraoccipital is fused indistinguishably onto the occipital surface of the parietal, as is the case in the other <i>T. rex</i> specimens— TMP 1981.6.1, FMNH PR2081 and MOR 555 — but unlike other tyrannosaurids, which tend to have pronounced knob-like processes that extend conspicuously posterior to the parietal. The basipterygoid process is 10 cm below the level of the basal tuber. The basipterygoid processes are 24 cm across. The basioccipital and exoccipitals are indistinguishably fused in the occipital condyle. The prootic is fused anteriorly with the basisphenoid and laterosphenoid, and the line of contact cannot be seen. However, the exoccipital-opisthotic, prootic, and basisphenoid sutures are visible. The left ectopterygoid is better preserved than the right (Fig. 17), and is inflated to a thickness of more than 7 cm. A large, 73 <i>×</i> 31 mm, medially oriented pneumatic opening leads into a sinus that hollows out the bone and extends into at least the base of the jugal process. There also seems to be a small, 18 <i>×</i> 9 mm pneumatophore in the base of the hooked jugal process. The minimum length of the right dentary of RSM P2523.8 is 810 mm, but the posterior edge is missing (Fig. 18). At the level of the sixth alveolus, the left dentary is 94.5 mm wide mediolaterally. The tooth row consists of 14 alveoli on each side and is 595 mm long. The mylohyal foramen of the splenial has a dorsoventral diameter of 5.5 cm. The surangulars are 645 mm long, with maximum depths of 250 mm, but in spite of their great sizes, neither is fused with the articular (Fig. 19). The adductor shelf protrudes 4 cm over the lateral surface. As in most other tyrannosaurids, the main vertical body of the surangular is pierced by sporadic openings. The insertion on the articular for the depressor mandibular musculature is 20 cm across and faces more posteriorly than dorsally. Posteromedial to the glenoid is a large pneumatophore that is 25 mm in diameter. SIZE ESTIMATION To quantify the size of RSM P2523.8 and compare it to that of other <i>T. rex</i> specimens (<i>n</i> = 11), a series of skeletal measurements were taken. To help ensure consistency of measurement technique, all measurements on all specimens were made by one of the authors (P.J.C.). All measurements were made by hand using standard digital calipers or measurement tape, as the size of the measured element dictated (Table 1). The standard in neontology for contrasting size across animals of disparate proportions is to use body mass (Campione et al., 2014; Erickson, 2014). Campione et al. (2014) offered a revised equation for using minimal femoral circumference (MFC) to estimate total body mass across nonavian bipeds, dinosaurs in particular (log 10 <i>M</i> = 2.754 <i>·</i> log 10 <i>C</i> − 0.683, where <i>M</i> is body mass and <i>C</i> is minimum femoral circumference). One limitation of such approaches is that the hollow femoral shafts of theropods are prone to taphonomic distortion. The mid-femur shaft of RSM P2523.8 is slightly damaged, but its minimum transverse diameter (199 mm) is not deformed and indicates a femoral circumference of approximately 590 mm (based on the ELLR method of tyrannosaur femoral circumference estimation, Bradley et al., 2015). The body mass of RSM P2523.8 is thus estimated to be ~8,870 kg (<i>±</i> 25%). Following from the 590 mm femoral circumference estimation, the relative femoral section moduli (Erickson et al., 2002; Therrien and Henderson, 2007) of RSM P2523.8 is calculated at 650,289 cm 3 and that of FMNH PR2081, the next largest <i>T. rex</i> specimen, is 617,781 cm 3. Section moduli therefore suggest that the RSM P2523.8 femur had a little more than 1.05 times the bending strength of FMNH PR2081. Snively (2018) suggested that comparing the
description: Published as part of <i>Persons, Scott W, Currie, Philip J. &amp; Erickson, Gregory M., 2019, An Older and Exceptionally Large Adult Specimen of Tyrannosaurus rex, pp. 1-17 in The Anatomical Record 302</i> on pages 1-16, DOI: 10.1002/ar.24118, http://zenodo.org/record/3251817
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