UMNH VP 28348 was collected at a single locality; as there is no duplication, morphological inconsistency, or size incompatibility among the identifiable elements, all bones collected from the site are regarded as pertaining to a single tyrannosaurid individual. All elements were collected as float in 2012. A test excavation that same year and subsequent visits in 2013 and 2018 did not reveal additional bones at the site. Measurements of select cranial and appendicular elements of UMNH VP 28348 are available in the supplementary information (S1 Codings and Measurements of UMNH VP 28348). Only those elements of UMNH VP 28348 that are identifiable based upon comparison with other tyrannosaurids are described.

Previous discoveries of tyrannosaurids in the Menefee Formation include tooth fragments and a metatarsal reported by Hunt & Lucas (1993), and another tooth fragment reported by Lewis (2006). Lack of overlapping material precludes referral of any of these elements to Dynamoterror dynastes. UMNH VP 28348 is the first associated tyrannosaurid skeleton reported from the Menefee Formation.

Frontals

The right and left frontals both are incomplete; however, between them, they exhibit nearly the entire frontal morphology, with the exception of the rostral end of the nasal process (Figs. 1–7). Many fine details are preserved, allowing thorough comparisons with other tyrannosaurid frontals. Unfortunately, no frontals are known for the Late Cretaceous Appalachian outgroups of Tyrannosauridae, Appalachiosaurus montgomeriensis and Dryptosaurus aquilunguis (Carr, Williamson & Schwimmer, 2005; Brusatte, Benson & Norell, 2011). However, comparisons were possible with B. sealeyi (Lehman & Carpenter, 1990; Carr & Williamson, 2000, 2010), which is the closest outgroup of Tyrannosauridae (Carr & Williamson, 2010; Brusatte & Carr, 2016), and with numerous members of Tyrannosauridae, including the albertosaurines Albertosaurus sarcophagus and G. libratus (Currie, 2003) and the tyrannosaurines Alioramus altai (Bever et al., 2013), Daspletosaurus sp. (Currie, 2003), L. argestes (UMNH VP 20200) (Loewen et al., 2013), Teratophoneus curriei (BYU 8120/9396, UMNH VP 16690) (Carr et al., 2011; Loewen et al., 2013), N. hoglundi (Fiorillo & Tykoski, 2014), Tarbosaurus bataar (Hurum & Sabath, 2003), Tyrannosaurus rex (LACM 23845, LACM 150167) (Osborn, 1912; Brochu, 2003; Carr & Williamson, 2004; Larson, 2008), and an unnamed tyrannosaurine from the Aguja Formation of Texas (Lehman & Wick, 2012).

Figure 1: Frontals of UMNH VP 28348 in rostral view. Photographs and 3-D models of right (A, B) and left (C, D) frontals of UMNH VP 28348. Abbreviations: bnp, basal of nasal process; cnc, caudal extent of nasal cavity; plp, prefrontolacrimal process; pnp, prefrontonasal process; rpos, rostral part of postorbital suture; vls, ventrolateral part of lacrimal suture. Scale bars equal five cm.

Figure 2: Frontals of UMNH VP 28348 in dorsal view. Photographs and 3-D models of right (A, B) and left (C, D) frontals of UMNH VP 28348. Abbreviations: bnp, basal of nasal process; mls, medial-most point of lacrimal suture; plp, prefrontolacrimal process; pnp, prefrontonasal process; rpos, rostral part of postorbital suture; scr, sagittal crest; stf, supratemporal fossa. Scale bars equal five cm.

Figure 3: Frontals of UMNH VP 28348 in ventral view. Photographs and 3-D models of right (A, B) and left (C, D) frontals of UMNH VP 28348. Abbreviations: bnp, basal of nasal process; ccr, crista cranii; cnc, caudal extent of nasal cavity; cpos, caudal part of postorbital suture; es, ethmoid scar; gpos, groove between rostral and caudal parts of postorbital suture; lss, laterosphenoid suture; mls, medial-most point of lacrimal suture; obf, olfactory bulb fossa; oss, orbitosphenoid suture; plp, prefrontolacrimal process; pnp, prefrontonasal process; sog, supraorbital groove; vls, ventrolateral part of lacrimal suture. Scale bars equal five cm.

Figure 4: Frontals of UMNH VP 28348 in medial view. Photographs and 3-D models of right (A, B) and left (C, D) frontals of UMNH VP 28348. Abbreviations: bnp, basal of nasal process; cnc, caudal extent of nasal cavity; ifs, interfrontal suture; obf, olfactory bulb fossa; oss, orbitosphenoid suture; scr, sagittal crest. Scale bars equal five cm.

Figure 5: Frontals of UMNH VP 28348 in lateral view. Photographs and 3-D models of right (A, B) and left (C, D) frontals of UMNH VP 28348. Abbreviations: bnp, basal of nasal process; cpos, caudal part of postorbital suture; gpos, groove between rostral and caudal parts of postorbital suture; ow, orbital wall; ps, parietal suture; rpos, rostral part of postorbital suture; scr, sagittal crest; sog, supraorbital groove; stf, supratemporal fossa; vls, ventrolateral part of lacrimal suture. Scale bars equal five cm.

Figure 6: Comparison among derived tyrannosaurine frontals. Dynamoterror dynastes, UMNH VP 28348, in lateral view. The postorbital suture is highlighted in purple. Scale bar equals five cm for A–D. BYU 8120/9396, right frontal of Teratophoneus curriei in lateral view, shown as a photograph (E), photograph with the postorbital suture highlighted in purple (F), and a line drawing with the postorbital suture highlighted in purple (G). Photograph of BYU 8120/9396 courtesy of Rod Scheetz (BYU) and is used here with permission. Additional anatomical information from UMNH VP 16690 (Fig. 3E in Lythronax argestes in lateral view, shown as an image with the postorbital suture highlighted in purple (H) (modified from Fig. 2E in Digital models and line drawings of the right (A, B) and left (C, D) frontals of, UMNH VP 28348, in lateral view. The postorbital suture is highlighted in purple. Scale bar equals five cm for A–D. BYU 8120/9396, right frontal ofin lateral view, shown as a photograph (E), photograph with the postorbital suture highlighted in purple (F), and a line drawing with the postorbital suture highlighted in purple (G). Photograph of BYU 8120/9396 courtesy of Rod Scheetz (BYU) and is used here with permission. Additional anatomical information from UMNH VP 16690 (Fig. 3E in Loewen et al. (2013) ). UMNH VP 20200, right frontal ofin lateral view, shown as an image with the postorbital suture highlighted in purple (H) (modified from Fig. 2E in Loewen et al. (2013) ), and a line drawing with the postorbital suture highlighted in purple (I).

Figure 7: Frontals of UMNH VP 28348 in caudal view. Photographs and 3-D models of right (A, B) and left (C, D) frontals of UMNH VP 28348. Abbreviations: lss, laterosphenoid suture; ps, parietal suture; scr, sagittal crest; stf, supratemporal fossa. Scale bars equal five cm.

The length, width, and depth of the frontals of UMNH VP 28348 were measured after the methodology of Currie (2003). The overall dimensions of the right frontal of UMNH VP 28348 are similar to those of LACM 23845 (Table 2 in Currie (2003)), a subadult specimen of Tyrannosaurus rex (Molnar, 1980; Carr & Williamson, 2004) (S1 Codings and Measurements of UMNH VP 28348). Although complete length cannot be measured for the frontals of UMNH VP 28348 due to breakage of the nasal processes, width and depth can be measured for the right frontal. The ratio of depth to width for the right frontal is 0.57, comparable to frontals of subadult and adult Daspletosaurus sp. (0.50–0.66) and Tyrannosaurus rex (0.59–0.67, including LACM 23845 (0.62)), calculated from the measurements provided by Currie (2003; Table 2). In tyrannosaurids, the depth of the frontal exhibits positive allometry relative to width during the course of ontogeny (Currie, 2003). Coupled with the measurements and depth/width ratio, this suggests that UMNH VP 28348 represents a subadult or adult individual.

The frontals are described together as a single unit, though it is noted whether a feature is most clearly preserved on the right, left, or both frontals. Given the complexity of the morphology of the frontals, the description is divided into six sections, detailing the rostral, dorsal, ventral, medial, lateral, and caudal surfaces.

Ventral surface The detailed description of the frontals of Alioramus altai (see Fig. 9 in Bever et al. (2013)) was useful for interpreting the ventral surfaces of the frontals of UMNH VP 28348. Details of the rostral region of the ventral surface are well preserved on both frontals. This region is dominated by a rostrocaudally elongate, mediolaterally wide fossa that is defined rostrally by the prefrontal suture, medially by the interfrontal suture, laterally by the crista cranii, and caudally by the ethmoid scar (Fig. 3). When the frontals are placed in articulation, the right and left fossae form the caudal extent of the olfactory region of the nasal cavity, which was lined with mucous membrane when the animal was alive (Witmer & Ridgely, 2009; Bever et al., 2013). Lateral to the fossa is the crista cranii, a thick, sharply defined ridge that extends rostrolaterally to caudomedially, delineating the ventral and lateral surfaces of the frontal (Fig. 3). The rostral fossa is separated from the olfactory bulb fossa by the ethmoid scar, a subtle mediolaterally oriented ridge that branches off of the crista cranii (Fig. 3). Only a small portion of the olfactory bulb fossa is preserved on the left frontal, while the entire fossa and the surrounding features of the caudal region of the ventral surface are intact on the right frontal. The large olfactory bulb fossa is defined rostrally by the ethmoid scar, medially by the interfrontal suture, laterally by the crista cranii and orbitosphenoid suture, and caudally by the parietal suture (Figs. 3A and 3B). The orbitosphenoid suture is a subcircular, ventromedially facing concavity lined with a complex set of delicate ridges, bumps, and depressions. The orbitosphenoid suture widens caudally where it merges with the laterosphenoid suture on the caudoventral surface of the frontal.

Medial surface The medial surface is occupied entirely by the flat, vertical interfrontal suture (Fig. 4). The suture itself is better preserved on the left frontal, and consists of a series of overlapping fine, V-shaped ridges with the V’s opening rostrally.