A Description of the Baby T. rex Specimen BHI 6439 (2023)

Written on 7/3/23

Link:
https://www.academia.edu/105502160/A_Description_of_the_Baby_T_rex_Specimen_BHI_6439

                                    A Description of the Baby T. rex Specimen BHI 6439

                                                                    Abstract:

The T. rex specimen BHI 6439 has been stated, and referenced, as being a young individual that differs from the cf. Dryptosaurus (or “Nanotyrannus”) specimens. Thus, showing that young T. rex specimens differed from the latter genus. However, to this author’s knowledge, there isn’t a description of the specimen yet. Only DePalma et al., (2013) referenced it. Therefore, this author decided to (as best as they could) give a description of the specimen. BHI 6439 is a semi-complete dentary that is 54.5 cm long, has 13 alveoli with teeth that are ziphodont yet robust, a tooth row that is 33 cm long, and a lingual bar that covers the first two dentary alveoli/teeth on the medial side of the bone. Using the tooth row length, the specimen would’ve been about 23 feet (7.0 meters) in length. The specimen shares all of the characteristics seen in older T. rex specimens. The specimen differs from the cf. Dryptosaurus specimen BMRP 2002.4.1 (“Jane”) in having 17 teeth that have an extreme ziphodont morphology than the teeth of BHI 6439, and a lingual bar that covers the first alveolus on the medial side instead of the first two. BMRP 2002.4.1 is about the same size as BHI 6439 at 22 feet (6.7 meters), yet differs in dentary characteristics. The latter specimen, along with the smaller T. rex specimen UCMP 119853 (2.1 meters), demonstrate that young T. rex specimens did not differ in dentary and tooth morphologies from the adults. Thus, the “Nanotyrannus” specimens belong to another genus.

Figures:

Figure 1: BHI 6439 in lateral view. Figure 1A displays the specimen as is. Figure 1B displays the terminology. The numbers above the teeth display their positions in the dentary. The white arrow locates the foramina. The blue arrow demonstrates the chin/inflection point. The numbers above the teeth display their positions in the dentary. Photo belongs to Hiroshi Kato. 

Figure 2: BHI 6439 in medial view. Figure 2A displays the specimen as is. Figure 2B displays the terminology. The numbers above the teeth display their positions in the dentary. The white arrows indicate the first and last visible dentary plates. The green arrows indicate the first alveolus, and the second tooth, are covered by the lingual bar. The red arrow indicates that the third tooth is not covered by the lingual bar. Abbreviations: “lb” is the lingual bar, “mg” is the Meckelian groove, and “idp” is the interdentary plate. Photo belongs to Peter Larson. 

Figure 3: BHI 6439 in dorsal view. Figure 3A displays the specimen as is. Figure 3B displays the terminology. The numbers above the teeth display their positions in the dentary. The white arrow points towards the 5th tooth. Blue arrows indicate the carina on the teeth. Photo belongs to Hiroshi Kato.

Figure 4: T. rex specimens FMNH PR 2081 (4A) and BHI 6439 (4B) in lateral view. White arrows point towards the foramina. The blue arrows point to the inflection points. Photo of FMNH PR 2081 comes from Brochu (2003) (p. 41 Figure 40).

Figure 5: T. rex dentaries in medial views. 5A: BHI 6439. 5B: FMNH PR 2081 from Brochu (2003) (p. 42 Figure 41). 5C: BMNH R7994 from Dalman and Lucas (2017) (p. 24 Figure 10). 5D: RSM P2523.8 from Persons IV et al., (2019) (p. 668 Figure 18B). Numbers indicate the position of the first three teeth in the dentary. The teeth in the dentary of RSM P2523.8 were numbered by the authors of Persons IV et al., (2019). Green arrows indicate the lingual bar covering the first two teeth/alveoli. The red arrows show that the lingual bar drops at the third alveolus/tooth. The position of the lingual bar doesn’t change during ontogeny. Abbreviations: “lb” is the lingual bar, “mg” is the Meckelian groove, “idp” is the interdentary plate, and “dp” are the dental plates.

Figure 6: Comparison between cf. Dryptosaurus specimen BMRP 2002.4.1 and T. rex specimen BHI 6439 in dorsal view. 6A is the original photo. 6B is a close up, showing the 17th alveoli for BMRP 2002.4.1 and the 13th alveoli for BHI 6439. Despite having a close tooth row length, BHI 6439 has fewer teeth than BMRP 2002.4.1. This demonstrates that young T. rex individuals would’ve had a tooth count similar to the adults. The teeth in the T. rex specimen are also more robust, while the teeth in the cf. Dryptosaurus specimen shows an extreme ziphodont morphology. Photo belongs to Hiroshi Kato.

Figure 7: Dentary of cf. Dryptosaurus specimen BMRP 2002.4.1 in dorsal view. 6A is the original photo. 6B shows the number of teeth. 6C is the tooth count of BHI 6439 (also in dorsal view). Tooth row length for BMRP 2002.4.1 is 31.5 cm. The tooth row length for BHI 6439 is 33 cm. Photo belongs to Hiroshi Kato.

Figure 8: cf. Dryptosaurus specimen BMRP 2002.4.1 dentary from Dalman and Lucas (2017) (p. 24 Figure 9B and B’). 8A is the whole dentary. 8B is a close-up of the anterior tip. Numbers indicate the position of the teeth. Green arrows show that the lingual bar covers the first alveolus only. Red arrows show that the lingual bar drops at the second alveolus, and continues past the third alveolus. Abbreviations: “lb” is the lingual bar, “mg” is the Meckelian groove, and “idp” is the interdentary plate.

Figure 9: Close-ups of cf. Dryptosaurus specimen BMRP 2002.4.1 and T. rex specimen BHI 6439 (reversed). In 9A, the green arrow indicates that the lingual bar covers the first alveolus only. The red arrows show that the lingual bar drops at the second alveolus, and past the third one. In 9B, the green arrows show that the lingual bar covers the first alveoli and second tooth. The red alveoli shows that the lingual bar drops at the third tooth. Abbreviations: “lb” is the lingual bar, “mg” is the Meckelian groove, and “idp” is the interdentary plate.

Figure 10: Close-ups of the anterior tips of basal tyrannosauroidea and tyrannosauridae dentaries in medial views. Figures belong to Dalman and Lucas (2017). 10A is cf. Dryptosaurus specimen BMRP 2002.4.1 (p. 24 Figure 9B’). 10B is the Alioramus altai holotype IGM 100/1844 (p. 23 Figure 6B) (reversed). 10C is Gorgosaurus specimen FPDM-V8062 (p. 19 Figure 2B). 10D is Albertosaurus specimen TMP 1999.50.40 (p. 21 Figure 3B) (reversed). 10E is Bistahieversor specimen NMMNH P-27469 (p. 29 Figure 9A’). The numbers indicate the placement of the teeth/alveoli. The green arrows indicate that the first alveolus is covered by the lingual bar only. The red arrows indicate that the second and third alveoli are not covered by the lingual bar.

Figure 11: Skull and mandible bones of Appalachiosaurus holotype RMM 6670 compared to cf. Dryptosaurus specimen BMRP 2002.4.1. Photo of Appalachiosaurus belongs to Joe Songer. 11A shows the photo as is. 11B shows a close-up of the anterior tip of the dentary of Appalachiosaurus in medial view (rotated). 11C shows the anterior portion of the dentary of BMRP 2002.4.1 in medial view. Numbers indicate the position of the teeth/alveoli. Green arrows indicate that the lingual bar covers the first alveolus only. The red arrows indicate that the lingual bar drops past the second, third, and fourth alveoli. All basal tyrannosauroids and tyrannosaurids share this characteristic.

Figure 12: Close-ups of the anterior tip of the dentaries derived tyrannosaurinae taxa in medial views. Figures belong to Dalman and Lucas (2017), and Fiorillo and Tykoski (2014). 12A is Daspletosaurus torosus specimen TMP 2001.36.1 (Dalman and Lucas, 2017, p. 24 Figure 10A’). 12B is T. bataar specimen ZPAL MgD-I/5 (p. 24 Figure 10C’). 12C is Nanuqsaurus holotype DMNH 21461 from Fiorillo and Tykoski (2014) (Figure 3) (reversed). 12D is Lythronax (or T. argestes?) holotype UMNH VP 20200 (Dalman and Lucas, 2017, p. 24 Figure 10B’). 12E is Zhuchengtyrannus (or T. magnus?) holotype ZCDM V0031 (p. 24 Figure 10E’). 12F is T. rex specimen BMNH R7994 (p. 24 Figure 10D’). Numbers represent the tooth/alveoli placement. Nanuqsaurus’ alveoli were numbered by the authors of Fiorillo and Tykoski (2014). Green arrows indicate that the lingual bar covers the first two alveoli/teeth. Red arrows indicate that the lingual bar drops at the third tooth/alveolus. Unlike the basal tyrannosauroids and tyrannosaurids in Figure 11, all derived tyrannosaurinae here have the lingual bar covering the first two alveoli.

Figure 13: Photos of an adult and baby T. rex specimen dentaries. 13A is a figure of the dentary of the adult T. rex specimen NMMNH P-3698 from Larson (2008) (p. 41 Figure 1.24) (called UMNH 110000 in the paper). 13B is a close-up of the anterior tip of the dentary in medial view. 13C is BHI 6439. Numbers indicate the position of the alveoli/teeth. Green arrows demonstrate that the lingual bar covers the first two alveoli. The red arrows show that the lingual bar drops at the third tooth/alveolus, and continues past the fourth tooth/alveolus. This characteristic is seen in other T. rex specimens (see Figure 5), and other derived tyrannnosaurinae taxa (Figure 10).

Figure 14: Daspletosaurus dentaries in medial views. 14A is an X-ray reconstruction of the dentary of the cf. D. horneri hatchling MOR 268 in medial view from Funston et al., (2021) (Figure 7). 14B is a close-up of the anterior portion of the dentary in medial view. 14C is the anterior portion of the dentary of the D. torosus specimen TMP 2001.36.1. Numbers indicate the position of the teeth/alveoli. The green arrows indicate that the lingual bar covers the first two alveoli/teeth. The red arrows indicate that the lingual bar drops past the third alveolus. The position of the lingual bar stays the same in different species of the same genus, regardless of age.

Figure 15: Sketch of the two different dentary morphologies seen within tyrannosauroidea in medial views. 15A is the basal tyrannosauroidea and tyrannosauridae dentary morphology, with the lingual bar covering the first alveolus only. This sketch was based on BMRP 2002.4.1. 15B is the derived tyrannosaurinae dentary morphology, with the lingual bar covering the first two alveoli. This sketch was based on BHI 6439. Numbers represent the amount of alveoli the lingual bar covers. Abbreviations: “lb” is the lingual bar, “mg” is the Meckelian groove, and “idp” is the interdentary plate. Sketch belongs to this author.

Tables:

Table 1: Position of the lingual bar within multiple genera of tyrannosauroidea. Basal tyrannosauroidea and tyrannosauridae taxa have the lingual bar covering the first alveoli only. Derived tyrannosaurinae genera have the lingual bar covering the first two alveoli.

Table 2: A comparison between the dentaries of T. rex specimen BHI 6439, and cf. Dryptosaurus aquilunguis specimen BMRP 2002.4.1. BHI 6439 has 13 teeth that are robust in morphology, and a lingual bar covering the first two alveoli/teeth on the medial side of the dentary. BMRP 2002.4.1 has 17 teeth that have an extreme ziphodont morphology, and a lingual bar that covers the first alveoli on the medial side of the bone. Both specimens have a close body size (6.7 to 7.0 meters), yet the two dentaries show major differences in morphology. The morphology of the dentary for BMRP 2002.4.1 is closer to basal tyrannosauroidea and tyrannosauridae genera, while BHI 6439 matches the morphology of derived tyrannosaurinae taxa.