A Baby Tyrannosaurus rex Premaxillary, or First Maxillary, Tooth (2023)

Written on 1/17/23.

Link:
https://www.academia.edu/96575022/A_Baby_Tyrannosaurus_rex_Premaxillary_or_First_Maxillary_Tooth

                            A Baby Tyrannosaurus rex Premaxillary, or First Maxillary, Tooth

Abstract
In Dr. Kenneth Carpenter’s 1982 paper describing baby dinosaur dentaries and teeth, one tooth cataloged as UCMP 119853 seemed to have a morphology reminiscent of adult Tyrannosaurus rex specimens. After an extensive critique of the tooth, along with comments from other professional paleontologists (Professor Holtz, Jr., Sebastian Dalman, and Dr. Joshua B. Smith), this author believes that UCMP 119853 is a baby T. rex tooth that was situated in either the premaxillary (first or second), or the first maxillary, position. This author tends to lean more towards the premaxillary, but is still open to the possibility that the specimen is a first maxillary tooth. UCMP 119853 has a single carina that overlaps the tip of the tooth and is located in the labial and lingual positions of the crown, and the carina is denticulate. The morphology of UCMP 119853 differs from that of other young tyrannosauroid specimens that are categorized as either “Nanotyrannus,” or “juvenile T. rex specimens” (which this author categorizes as cf. Dryptosaurus aquilinguis). A comparison between UCMP 119853 and the premaxillary, and first maxillary, teeth from other tyrannosauroid genera showed that T. rex’s tooth morphology stayed consistent throughout the animal’s lifetime, and was close to that of the genus’ sister taxon Tyrannosaurus/Tarbosaurus bataar.

Figures:
Figure 1: UCMP 119853. 1A.) The fossil as shown in Carpenter (1982). 5a is the lateral, and 5b is the posterior/lingual/distal view. 1B.) Arrows indicating the location, and endpoints, of the carina. Green arrows indicate the carina is located on the lateral (labial and lingual) sides of the tooth. The blue arrows indicate to the author where the carina ended. The red arrows indicate where the carine ended according to Dr. Smith:

Figure 2: A comparison of the posterior/lingual/distal views of 2A.) UCMP 119853, 2B) BHI 3033’s first premaxillary tooth, and 2C.) BHI 3033’s second premaxillary tooth. The carina are on the lateral (labial and lingual) sides of all the teeth. Interestingly, the curvature of UCMP 119853 is close to BHI 3033’s second premaxillary tooth in particular. Photos of BHI 3033’s teeth were provided by Dr. Smith:

Figure 3: Comparisons in lateral view of 3A.) UCMP 119853, 3B.) first premaxillary tooth of BHI 3033, and 3C.) first maxillary tooth of BHI 3033. Green arrows indicate the location of the carina. Lines indicate the exterior mesial outline of the teeth. You can see that the first maxillary tooth is more elongated and thin (red lines), compared to the curvier premaxillary teeth (green lines). Photos of BHI 3033’s teeth were provided by Dr. Smith:

Figure 4: Posterior/lingual/distal views of 4A.) UCMP 119853, and 4B.) BHI 3033’s first maxillary tooth (flipped). Green arrows indicate the location of the carina. The overall curvature of the BHI 3033’s first maxillary tooth seems to be more linear compared to UCMP 119853’s, especially on the right side of each tooth. Photo of BHI 3033’s tooth was provided by Dr. Smith:

Figure 5: Lateral views of 5A.) UCMP 119853, and 5B.) first premaxillary tooth of MOR 008. Carina are indicated by green arrows. The overall shape of both teeth seem to match. Photo of MOR 008’s tooth was provided by Dr. Smith:

Figure 6: Lateral views of 6A.) UCMP 119853 and 6B.) TD-13-251 (rotated and flipped) from Stein (2021). Green arrows indicate carina in lateral (labial and lingual) view. Blue arrows show that the carina could have stopped midpoint on the crown of UCMP 119853 and TD-13-251. Photos of BHI 3033’s teeth were provided by Dr. Smith:

Figure 7: Figure 1 from Hendrickx et al., (2019) showing the views of the teeth and carina locations. Vocabulary words used here were used in this paper:

Figure 8: UCMP 124406. 8A.) The specimen as shown in Carpenter (1982). 8B.) The specimen with arrows. Red arrows show the carinae located on the distal end of the tooth. Purple arrows indicate the posterior/distal ventral ridge:

Figure 9: Comparisons between 9A.) UCMP 124406, 9B.) LACM 28471 from Molnar (1978), 9C.) FMNH PR 2902 from Gates et al., (2015), and 9D.) YPM 296 from Marsh (1892). Red arrows indicate the carinae on the posterior/distal end. Purple arrows indicate the posterior/distal vertical ridge. The morphology of the teeth stays consistent during the animal’s growth:

Figure 10: Comparisons between 9A.) UCMP 124406, 9B.) LACM 28471 from Molnar (1978), 9C.) FMNH PR 2902 from Gates et al., (2015), and 9D.) YPM 296 from Marsh (1892). Red arrows indicate the carinae on the posterior/distal end. Purple arrows indicate the posterior/distal vertical ridge. The morphology of the teeth stays consistent during the animal’s growth:

Figure 11: A comparison between 11A.) UCMP 119853, and (11B.) UCMP 124406. UCMP 119853’s carina is on the lateral (labial and lingual) sides (green arrows) that are serrated, and lacks a vertical ridge on the posterior/distal end of the crown. UCMP 124406 has distal carinae (red arrows) that lack serrations, and a distal vertical ridge (purple arrows) on the distal end of the crown:

Figure 12: 12A.) Figure 17 from Stein (2021). 12B.) Close-up of TD-13-251 and TD-13-247, comparing their sizes to each other. The two specimens are close in size, yet have different morphologies:

Figure 13: Comparison between 13A.) UCMP 119853, 13B.) TD-13-251, and 13C.) BHI 3033’s first premaxillary tooth. The green arrows represent the carina on the lateral (labial and lingual) sides. Red and blue arrows indicate possible endings of the carina. The morphology of the teeth stays consistent during the animal’s growth:

Figure 14: Comparison between 14A.) UCMP 119853, and 14B.) 2-3-year old T. bataar specimen MPC-D 107/7’s premaxillary teeth in labiodistal view (Tsuihiji et al., 2011, Figure 6C-D). The green arrows indicate the serrated carina in lateral (labial and lingual) view. In 14B.), the blue semi-circle and bar show the distal side of MPC-D 107/7’s premaxillary teeth:

Figure 15: The two teeth morphotypes from the late Maastrichtian of North America. A, C, and E is the cf. Dryptosaurus aquilinguis premaxillary tooth. B, D, and E is the T. rex premaxillary tooth. Vocabulary comes from Hendrickx et al., (2019). Illustration belongs to this author:

Tables:
Table 1: T. rex and cf. Dryptosaurus aquilinguis premaxillary tooth lengths and morphologies. The results indicated that the morphologies of the teeth in the two genera stayed consistent, aside from an increase in size during maturity. “(C)” means crown height measurement, and “(T)” means total tooth height measurement. Note: This author obtained a length of 3 cm for YPM 296, but Ford and Chure (2001) gave 2.9 cm (Table 1). This was discovered after the author already measured the specimen:

Table 2: Tyrannosauroid premaxillary tooth morphologies. The results showed that the morphology of the cf. Dryptosaurus aquilinguis premaxillary teeth were closer to the other basal tyrannosauroids than to T. rex’s. T. rex’s premaxillary tooth morphology was closer to T. bataar’s than the other taxa listed:

"Nanotyrannus" is Dryptosaurus: An Abstract (2022)

Written from 8/3/22-2/9/23.

Link:
https://www.academia.edu/96574784/Nanotyrannus_is_Dryptosaurus_An_Abstract

                                                “Nanotyrannus” is Dryptosaurus: An Abstract

This is an updated version of an abstract written in 2022.

Dryptosaurus aquilunguis is a tyrannosauroid from the late Maastrichtian of Eastern North America, also known as Appalachia. So far, only one good specimen, the holotype ANSP 9995, has been found for the genus. A few teeth have been assigned, but no relatively complete specimens have been described yet. However, after an exhaustive examination of the controversial “Nanotyrannus”/juvenile Tyrannosaurus rex specimens, this author is going to introduce a new hypothesis: Dryptosaurus lived in Appalachia and Laramidia towards the end of the Maastrichtian. The tyrannosauroid specimens previously labeled as “Nanotyrannus” are either cf. Dryptosaurus aquilinguis, or a sister taxon to Dryptosaurus and the two genera form a clade within tyrannosauroidea. This author tends to lean more towards “Nanotyrannus” being cf. Dryptosaurus aquilinguis. Both Dryptosaurus and “Nanotyrannus” lived during the same time. Numerous publications have suggested that Laramidia and Appalachia reconnected when the Western Interior Sea subsided in the Maastrichtian. Both Laramidia and Appalachia seemed to have had similar fauna: lambeosaurs, ceratopsians, and mosasaurs. Ceratopsids, in particular, were thought to have not existed in Appalachia. However, a ceratopsian tooth has been found in the Maastrichtian-aged Owl Creek Formation, which is in Appalachia. If animals in Laramidia can be found in Appalachia, and vice versa, then Dryptosaurus could’ve migrated into Laramidia. Dryptosaurus and “Nanotyrannus” share many physical characteristics, and the “Nanotyrannus” specimens have many traits not seen in T. rex. A few traits include:

1. “Nanotyrannus” had a first maxillary tooth that was incisiform. This morphology is also seen in premaxillary teeth. The “Nanotyrannus” specimens lack serrations, and have two carinae on the distal side of the premaxillary, and first maxillary, teeth. This trait is not present in T. rex. Specimen UCMP 119853, a 8-mm long tooth that likely belongs in the premaxillary position, has one serrated carina that is located on the labial and lingual positions of the crown. This morphology is seen in larger T. rex specimens, showing that T. rex’s tooth morphology didn’t change during ontogeny. More than likely, the first maxillary tooth in young T. rex specimens wasn’t similar to the “Nanotyrannus” or Dryptosaurus specimens.

2. The maxillary, and perhaps dentary, teeth in both Dryptosaurus and “Nanotyrannus” were pinched on the labial and lingual sides in cross-section, and the denticle morphology was “hook-shaped.” T. rex’s cross-sections were oval-shaped, and the denticles were square/rectangular in shape. 

3. The dentary in the “Nanotyrannus” specimens had the lingual bar covering the first alveoli on the medial side, as seen in more basal tyrannosauroids like Appalachiosaurus, Gorgosaurus, Albertosaurus, and Bistahieversor. Since Dryptosaurus is a basal tyrannosauroid as well, it’s more than likely that it had this trait too. The derived tyrannosaurinae taxa T. rex, T. bataar, and Daspletosaurus had the lingual bar covering the first two alveoli. It seems that the lingual bar’s position can help differentiate between basal tyrannosauroids and tyrannosaurids from the derived tyrannosaurinae. The 3-4-year old T. rex specimen “Baby Bob” had the lingual bar covering the first two alveoli, as seen in the adult specimens. This alone places the “Nanotyrannus” specimens in basal tyrannosauroids or tyrannosaurids, not tyrannosaurinae.

4. The tooth count in the dentaries of baby-juvenile specimens of T. rex were the same as the adults, and the maximum count seems to be fifteen. This is also seen in T. bataar. The 3-4-year old specimen “Baby Bob” had 12 teeth in its dentary, as in the slightly larger baby/juvenile specimen BHI 6439. The 18-year old specimen BHI 3033 had 13. The 23-year old specimen CM 79057 ("Samson"), and PARC-TD-11-094/FDM-xx? (perhaps a subadult/adult individual), had 15. “Nanotyrannus” had 16-17. There is no sign of tooth loss during ontogeny.

5. The “Nanotyrannus” specimens have a non-incisiform, conical-shaped first dentary tooth with carina/carinae on the mesial and distal positions. This is also seen in Albertosaurus, but not in Gorgosaurus. T. rex’s first dentary tooth has the same morphology seen in the premaxillary teeth (incisiform, with a serrated carina on the labial and lingual faces). This is seen in the 3-4-year old “Baby Bob,” and the the 18-year old BHI 3033. The first dentary tooth never changed morphology as the T. rex individual aged, just like the premaxillary teeth.

6. Both genera have similar caudal vertebrae morphologies. In terms of the vertebrae count, it seems to be about 25 or so, as suggested by Cope. T. rex and Tarbosaurus/Tyrannosaurus bataar have 40 or more caudal vertebrae, and this is also seen in the young T. bataar specimen PIN 552-2. Whether or not this number is permanent doesn’t concern this author, but the morphology of the vertebrae does. From the middle to the distal portion of the tail, both Dryptosaurus and “Nanotyrannus” have elongated caudals that also lack transverse processes. T. rex’s caudals became shorter up to the very last caudal, and most of the caudals up until the last distal members had transverse processes. The middle-distal caudals were longer in Dryptosaurus than T. rex’s, and “Nanotyrannus’” looks to be the same.

7. Morphology of the arms of both genera are identical. The deltopectoral crest is positioned lower from the proximal end of the humerus, and faces different directions when viewed in multiple positions than in T. rex’s and other tyrannosauroids. For example, in proximal view, the deltopetoral crest in Dryptosaurus and “Nanotyrannus” faced anteriorly compared to the laterally-facing crest in T. rex. The manual phalanx 1-1 of Dryptosaurus and “Nanotyrannus” are extremely elongated, and this is an autapomorphy of Dryptosaurus. However, Megaraptor has this as well. Other tyrannosauroids like T. rex (9.85 cm), Gorgosaurus (9.8 cm), Albertosaurus (8.5 cm), and T. bataar (5.4 cm) have smaller manual phalanx 1-1 bones compared to Dryptosaurus’ (16 cm) or “Nanotyrannus’.” The manual unguals of the two genera are large and comparable in morphology and size, contra to T. rex’s and T. bataar’s short manual unguals.

8. Both Dryptosaurus and “Nanotyrannus” were agile tyrannosauroids throughout their biogenic existence, based on their hindlimb proportions. The tibia in the “Nanotyrannus” specimens are longer than the femur. The Dryptosaurus holotype seems to have a femur and tibia that were either equal in length, or the tibia was slightly longer than the femur. Dryptosaurus’ tibia is slightly eroded, so it might’ve been longer in life. Regardless, the femur and tibia of the “Nanotyrannus” specimens may have grown to become equal in length when they reached maturity, as possibly seen in Dryptosaurus. Or, both Dryptosaurus and “Nanotyrannus” had a tibia longer than the femur. Baby-juvenile specimens of T. rex and T. bataar had femora and tibiae that were about equal in length to each other, while the adults had longer femora. The 3-4-year old ”Baby Bob” demonstrates this. Body mass could account for this. Dryptosaurus and “Nanotyrannus” were small-medium-sized tyrannosauroids, so having a longer tibia, or a femur and tibia of equal length, could’ve meant a speedier lifestyle for the two genera. Another example of this is the adult Qiazhousaurus/Alioramus sinensis specimen, which also had a longer tibia compared to its femur. Larger adult tyrannosauroids had more massive bodies, which resulted in larger femora to support their weight. Indeed, baby-juvenile specimens of large tyrannosauroids had the femora and tibiae at about an equal length, but this changed during ontology. This doesn’t seem to be the case for Dryptosaurus, nor “Nanotyrannus.”

The fact that there are baby-juvenile specimens of T. rex that have traits not seen in the “Nanotyrannus” specimens, but are seen in the older specimens, show that “Nanotyrannus” doesn’t belong in the genus Tyrannosaurus. The stratigraphic and geographic correlations, as well as the similarities in characteristics with Dryptosaurus, show that “Nanotyrannus” is a more basal tyrannosauroid than a derived tyrannosaurinae. Further analyses will be conducted in the future to explore these characteristics in more explicit detail. 

My Researchgate and Academia Accounts Update:

I've decided to delete my Researchgate account. I had a misunderstanding with the "Institution" section. I am just an undergraduate student at UMD, and I'm not associated with any program at the university. I did, however, create an Academia account to upload my work to. All of my future papers will be posted there. I already uploaded my abstract and first paper to the account.

My papers:
1. "Nanotyrannus" is Dryptosaurus: An Abstract (2022):
https://www.academia.edu/96574784/Nanotyrannus_is_Dryptosaurus_An_Abstract
2. A Baby Tyrannosaurus rex Premaxillary, or First Maxillary, Tooth (2023):
https://www.academia.edu/96575022/A_Baby_Tyrannosaurus_rex_Premaxillary_or_First_Maxillary_Tooth

Was there more than one species of Tyrannosaurus?

Recently, Paul et al., (2022) published a paper on whether or not there was more than one species of Tyrannosaurus. They used three characteristics in their analysis: One or two incisiform (small-sized) dentary teeth, the robustness of the femur, and the stratigraphic layer the specimen was discovered (Abstract; p. 3 Figures 2 and 3; p. 5, 8 and 10; p. 16 Figure 5; pp. 16-17). The conclusion was that incisiform tooth count correlates with skeletal robusticity and changes over time, and the species seem to be separated by time (Abstract; pp. 16-17). The authors hypothesized three new species within the genus Tyrannosaurus: T. rex, T. imperator, and T. regina (Abstract). T. rex has one incisiform dentary tooth, is robust, and has an adult femur circumference ratio of 2.4 or less. T. imperator is generally robust, has two incisiform dentary tooth, and has an adult femur circumference ratio of 2.4 or less. T. regina is gracile (skinny), has an adult femur circumference ratio of over 2.4, and has one incisiform dentary tooth (p. 19). 

Tyrannosaurus specimens assigned to the three species (Paul et al., 2022, p. 2 Figure 1):

W. Scott Persons IV, one of the three authors of the paper, has an entertaining YouTube video where he discusses some more about the paper, and why he thinks there are actually three species of Tyrannosaurus. In it, he explains that Dr. Bakker originally hypothesized that the number of incisiform teeth in T. rex's dentary correlated to more than one species within Tyrannosaurus, but he didn't have enough specimens to back this up so he never wrote a paper on his findings. 

Almost everybody, especially other scientists, said that the paper was bad. Online, I've seen some random people express general optimism in the paper, but overall the reception is generally negative. Professor Holtz and Dr. Hone were more kind to the paper, but of course they need more evidence to be swayed. Carr et al., (2022) quickly wrote a rebuttal and said T. rex was the only tyrannosaurid at the end of the Cretaceous in Laramidia (Conclusions, 2). That's not true because the "Nanotyrannus" specimens are actually Dryptosaurus and not baby and juvenile T. rex specimens, but let's focus on T. rex. I originally just went with the flow, knowing full well now that nothing in paleontology stays the same and almost everyone has a different opinion on what's right and wrong. In my opinion, the paper is very lacking in describing detailed traits that are convincing enough to separate Tyrannosaurus rex at the species level. Not only that, but there was no concise pattern for the traits as well. I'm more upset that the authors didn't go over the skeletons in full detail to see if any differences arose.

So, I've decided to throw my hat into the debate, and see if there is another species of Tyrannosaurus. 

Link:
Paul et al., (2022):
https://link.springer.com/article/10.1007/s11692-022-09561-5

Examining the T. rex specimens.
There have been other papers published in the past discussing the variability in the morphology of multiple T. rex specimens. There are a couple of interesting traits that do stand out. In Carpenter (1991), he examined the maxillary fenestra, the dentary, the cervical vertebrae, and the ischia, of T. rex (pp. 143-144 Figures 10.2-10.5). He concluded that the possibility of sexual dimorphism, in which the robust specimens are female, requires more specimens and work to see if any traits seen are caused by ontogeny (p. 144). Personally, the maxillary fenestra and the dentary do not show enough variation. The ischia may show differences, but I'm currently unsure about that at the moment.

As for the cervical vertebrae, there does seem to be some differences. Using some other sources, there seems to be different morphology regarding the axis (second cervical) and the third cervical, and the cervical count, amongst the vertebrae. BMNH R7994's axis curves almost completely backwards at its neural arch, and its third cervical's neural arch is thick. AMNH 5027's axis bends backwards slightly, but its neural arch is straight. The third cervical's neural arch is skinnier than BMNH's as well (Osborn, 1906, p. 287 Figure 3) (Osborn, 1917, p. 779) (Carpenter, 1991, p. 143; p. 144 Figure 10.4). However, 
Gorgosaurus shows similar variation. Therefore, using this to separate Tyrannosaurus into different might be a mistake (p. 143). Interestingly, FMNH PR 2081 ("Sue") has an atlas shaped like BMNH R7994's (Brochu, 2003, p. 53 Figure 51). 

The cervical, and dorsal, vertebrae count is interesting. BMNH 7994 has 11 cervical vertebrae (Osborn, 1906, pp. 287-288). CM 9380 has 9 cervical, and 14 dorsal, vertebrae (Osborn, 1917, pp. 765). BHI 3033 ("Stan") also has the same vertebrae count (Larson, 2008a; in Larson and Carpenter, 2008, p. 21). MOR 555/USNM 555000 seems to have 9 cervical vertebrae too (Brochu, 2003, p. 48). CM 79057 ("Samson") has been noted as having 9 cervical vertebrae as well (Larson, 2008a; in Larson and Carpenter, 2008, p. 23). AMNH 5027 seems to have 10 or 11 cervical vertebrae at maximum (Osborn, 1917, p. 765) (Carpenter, 1991, p. 144 Figure 10.4) (Brochu, 2003, p. 48), and 12 or 13 dorsal vertebrae at best (11 + 12 = 23, or 10 + 13 = 23) (Osborn, 1917, p. 763 and 765; p. 779). T. rex has 23 vertebrae before the sacral vertebrae (Osborn, 1917, p. 765). "Sue" has 11 cervical (10 cervical ribs are preserved, and the atlas doesn't have any ribs), and 12 dorsal vertebrae (14-23 are preserved, but 12-13 are not) (Brochu, 2003, pp. 60-61 Figures 57-58). Brochu says that AMNH 5027 has 11 cervical vertebrae as well (p. 48), so I will give the specimen the same vertebrae count as "Sue" (11 cervicals, 12 dorsals). 

BMNH 7994's cervical vertebrae (Osborn, 1906, p. 287 Figure 3) (Note: No. 5866 is BMNH R7994):

AMNH 5027's cervical vertebrae (Osborn, 1917, p. 779):

BMNH R7994 (A) and AMNH 5027 (B) cervical vertebrae (Carpenter, 1991, p. 144 Figure 10.4) (Based on Osborn, 1906 and 1917, Carpenter seems to have messed up the naming of the vertebrae):

The sacral vertebrae, and ilia, in AMNH 5027 are skinnier than CM 9380's (Osborn, 1917, p. 768)(Notes: Picture is reversed; No. 973 is CM 9380):

Now, I generally believe that we should see if there is any hint of sexual dimorphism in T. rex before we try to separate the specimens at the species level. Carr (2020) said that there was no evidence of sexual dimorphism in the specimens (p. 93, Conclusions,  number 24). Mallon (2017) said that no sexual dimorphism could be seen in nine dinosaur species (Abstract). This was also stated in Hone et al., (2020) (Abstract). In an article, Saitta (2022) states that the old ways of finding sexual dimorphism in dinosaurs, by using statistical data and p-values, may not provide answers to this question (para. 4, 15, 19-20). Interestingly, Tereshchenko (2021) says that male dinosaurs had taller neural spines, and robust limb bones. Female dinosaurs had a wider pelvis, and a greater abdominal volume (big belly). He came to these conclusions by studying neoceratopsia. The similarities seen in those specimens can be correlated in T. rex. If this is true, then the T. rex specimens with wider pelvis bones are females, while the ones with skinnier pelvic bones are males, as shown in Osborn (1917) (Abstract; Sexual variability in dinosaurs: Sexual dimorphism in ceratopsian dinosaurs (Ornithischia: Neoceratopsia); Sexual variability in dinosaurs: Assessment of some published data on sexual dimorphism in ceratopsian dinosaurs, para. 3). 

Possible sexual dimorphism in T. rex based on pubic bone robustness (Tereshchenko, 2021, Sexual variability in dinosaurs: Assessment of some published data on sexual dimorphism in ceratopsian dinosaurs, para. 3):

Using the cervical vertebrae for AMNH 5027 from Osborn (1917) shows that male T. rex specimens have a skinnier axis and third cervical vertebrae. BMNH, CM 9380, and "Sue" could be females. 

Another curious trait that may help is the morphology of the lacrimals. CM 9380, BHI 3033, CM 79057, and possible MOR 555/USNM 555000, have a lacrimal that curves downward posteriorly (Carpenter, 1991, p. 142) (Larson, 2008, p. 108) (Urban and Lamanna, 2006, p. 233 Figure 2). However, AMNH 5027 and FMNH PR 2081 have lacrimals that are straight horizontally (Larson, 2008, p. 108 Figure 8.6) (Carpenter, 1991, p. 142 Figure 10.1) (Brochu, 2003, pp. 8-9 Figure 2; p. 15 Figure 7). 

T. rex lacrimal morphology differences between AMNH 5027 (first) and BHI 3033 (second) (Larson, 2008, p. 108 Figure 8.6) (Notice how BHI 3033's lacrimal bends downward posteriorly): 

Tyrannosauroid lacrimals from Urban and Lamanna (2006) (Figure 2) (B is CM 9380, and D is MOR 555/USNM 555000):

If we take the different vertebral counts, sexual dimorphism in the skeletons, and the lacrimal morphologies, into consideration, then there may be a chance that there is some speciation in the T. rex specimens. I originally hypothesized that this could be a case for either speciation or subspecies within Tyrannosaurus, but I will go for speciation now. For example:

Tyrannosaurus species 1:
Cervical: 9.
Dorsal: 14.
Lacrimal morphology: Bends posteriorly downward.
Specimens:
1.) CM 9380 (Holotype): Female.
2.) BHI 3033 ("Stan"): Male.
3.) MOR 555/USNM 555000 ("Nation's rex"): Male.
4.) CM 79057 ("Samson"): Probably female, given the robustness of the specimen.

Tyrannosaurus species 2:
Cervical: 11 (at best).
Dorsals: 12 (at best).
Lacrimal morphology: Straight horizontally.
Specimens:
1.) BMNH R7994: Female.
2.) AMNH 5027: Male.
3.) FMNH PR 2081: Female.

There seems to be a consistent pattern in terms of vertebra count, gender, and lacrimal morphology, within the specimens of Tyrannosaurus.

As for speciation, there are probably four specimens in North America that may be a different species of Tyrannosaurus. The first is CM 9401, a lacrimal. The second is YPM VPPU 023469, a premaxillary tooth. The third is TSJC 2008.1, which consists of a tooth and a femur fragment. The fourth specimen is NMMNH P-1013/P-3698, a giant dentary. All of these specimens come from the Campanian to early Maastrichtian, which is earlier than the other T. rex specimens which come from the very late Maastrichtian. For more information, check this link out here:
https://psdinosaurs.blogspot.com/2019/12/did-tyrannosaurus-appear-during.html

Since these specimens appear earlier than the other Tyrannosaurus specimens, it seems that these could be a new species for the genus as well. 

Specimens:
1. CM 9401 (Urban and Lamanna, 2006, p. 232):

2. YPM VPPU 023469 (Dalman et al., 2018, p. 128 Figure 3 E-H):

3. TSJC 2008.1:

Figure 1: Tooth (A), denticles (B), and femur fragment (C) (Berry, 2008, p. 12):

Figure 2 (p. 13):

4. NMMNH P-1013/P-3698 (Larson and Carpenter, 2008, p. 42):

In summation, I believe that the Campanian-aged Tyrannosaurus specimens could be another species of the genus, one that is far older than the other specimens. However, the specimens from the late Maastrichtian, I believe, are T. rex. Of course, this is only if one wants to take this super seriously. I think it's an interesting idea, but I'm not willing to go all the way with this yet.

Links:
Paul et al., (2022):

https://link.springer.com/article/10.1007/s11692-022-09561-5
Carr et al., (2022):
https://link.springer.com/article/10.1007/s11692-022-09573-1
Carpenter (1991):

https://www.researchgate.net/publication/295458205_Variation_in_Tyrannosaurus_rex

Osborn (1906):

https://digitallibrary.amnh.org/bitstream/handle/2246/1473//v2/dspace/ingest/pdfSource/bul/B022a16.pdf?sequence=1&isAllowed=y

Osborn (1917) (Skeletal Adaptations of Ornitholestes, Struthiomimus, Tyrannosaurus):

https://digitallibrary.amnh.org/bitstream/handle/2246/1334//v2/dspace/ingest/pdfSource/bul/B035a43.pdf?sequence=1&isAllowed=y
-V2:

https://zenodo.org/records/1040385

Larson (2008a; in Larson and Carpenter, 2008):

https://books.google.com/books?id=5WH9RnfKco4C&pg=PR19&dq=One+hundred+years+of+Tyrannosaurus+rex:+The+skeletons.&hl=en&newbks=1&newbks_redir=0&source=gb_mobile_search&sa=X&ved=2ahUKEwiivJu45vD3AhU0p3IEHbV1AtoQ6AF6BAgEEAM#v=onepage&q=One%20hundred%20years%20of%20Tyrannosaurus%20rex%3A%20The%20skeletons.&f=false

Link 2:

https://zenodo.org/record/3808759#.YokK4yUpCEc

Brochu (2003):

https://www.researchgate.net/publication/249022959_Osteology_of_Tyrannosaurus_rex_Insights_from_a_Nearly_Complete_Skeleton_and_High-Resolution_Computed_Tomographic_Analysis_of_the_Skull
Carr (2020):

https://peerj.com/articles/9192/
Mallon (2017):

https://www.cambridge.org/core/journals/paleobiology/article/abs/recognizing-sexual-dimorphism-in-the-fossil-record-lessons-from-nonavian-dinosaurs/76D9931163D564D386E86ACF686E586D
Hone et al., (2020):

https://peerj.com/articles/9134/

Saitta (2020):

https://theconversation.com/did-male-and-female-dinosaurs-differ-a-new-statistical-technique-is-helping-answer-the-question-173634
His paper on Stegosaurus:

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0123503
His paper on sexual dimorphism:

https://www.researchgate.net/publication/323557099_Approaching_sexual_dimorphism_in_non--avian_dinosaurs_and_other_extinct_taxa

Tereshchenko (2021):

https://link.springer.com/article/10.1134/S0031030120120047
Abdominal cavity definition:

https://biologydictionary.net/abdominal-cavity/
Larson (2008):

https://books.google.com/books?id=5WH9RnfKco4C&printsec=frontcover&dq=larson+and+carpenter+(2008)+tyrannosaurus&hl=en&newbks=1&newbks_redir=0&source=gb_mobile_search&sa=X&ved=2ahUKEwiWtZf0tID4AhVwlnIEHfeECjMQ6AF6BAgMEAM#v=onepage&q=Bakker&f=false

Other Candidates:
1. Lythronax argestes:
Lythronax's skull (Loewen et al., 2013, Figure 2). Scale bars are 10 cm, but 50 cm for B:

Lythronax is a very interesting genus of tyrannosaurinae. Loewen et al., (2013) listed a lot of characteristics that the species has that are also seen in Tyrannosaurus:

Results: Diagnosis:
1. "Differs from Appalachiosaurus, Alioramus and all other tyrannosauroids except Bistahieversor, Tyrannosaurus and Tarbosaurus in having a concave lateral margin of dentary and a broad postorbital process of jugal" (relative to total jugal length)."

2. "Differs from all other tyrannosauroids except Tyrannosaurus and Tarbosaurus in having a laterally expanded caudal portion of the skull, such that the orbits are directed rostrodorsally" (back of the skull is wide).

Results: Descriptions and Comparisons:
Para. 1:
3. "Based on the strongly sigmoidal lateral margin of the maxilla and jugal, as well as the mediolateral width of the frontal, the posterior portion of the skull of Lythronax was substantially expanded mediolaterally, with anterolaterally directed orbits, features otherwise present only in Tyrannosaurus and Tarbosaurus among tyrannosaurids."
Para. 2:
4. "Similar to Bistahieversor (11), Tarbosaurus (12–13) and Tyrannosaurus (12–13), a reduced number of maxillary alveoli (11) are present relative to other tyrannosaurids such as Albertosaurus (14–16), Daspletosaurus (15–16), and Gorgosaurus (14–15)."
Edit: Bistahieversor had 13 maxillary teeth (Tyrannosauroidea central, Literature review 2: Loewen et al., 2013, para. 2).
5. "The maxillary dentition of Lythronax is also notably heterodont; the first five teeth are much larger than the remaining posterior six teeth."

6. "In overall morphology, the maxilla of Lythronax is notably robust and sigmoidal in lateral contour, with a well-developed palatal shelf comparable to that of Tyrannosaurus."
7. "The jugal of Lythronax is similarly robust (to Tyrannosaurus), with a sinusoidal lateral profile and wide postorbital process."
Para. 3:

8. "The posterior end of the dentary is also dorsoventrally flared, indicating a deep post-dentary region, as in Tarbosaurus and Tyrannosaurus, and unlike the condition in Albertosaurus, Daspletosaurus, and Gorgosaurus."
9. "Lythronax further resembles Tyrannosaurus in possessing a dorsally concave surangular shelf."
Para. 4:
10. "The pubis of Lythronax is notable for its dorso-ventrally expanded pubic boot, proportionally most similar to those of Tarbosaurus and Tyrannosaurus, and contrasting with the less-expanded condition in taxa such as Albertosaurus, Daspletosaurus, and Gorgosaurus."

Figure 4:
11. Lythronax is phylogenetically closer to T. rex while Teratophoneus is closer to Bistahieversor:

12. Lacrimal processes:
When I investigated the skull of Lythronax, I noticed that it also had lacrimal processes on its nasals (Figure 2, D):

T. rex specimens BHI 3033, FMNH PR 2081, and LACM 23845, have these (Hurum and Sabath, 2003, p. 169 Figure 5; p. 170) (Currie, 2003a, p. 200) (Brochu, 2003, p. 16).

13. Skull width:
The skull width, as seen in Figure 2 and noted in the Results section, for Lythronax's skull is very wide at the end. The end of T. rex's skull is very wide (Hurum and Sabath, 2003, p. 164).

14. Jugal flange:
There is also a jugal flange, or horn, on the jugal (Figure 2, F; Results: Descriptions and Comparisons, para. 2). T. rex specimen "Scotty" has this as well (Persons IV et al., 2019, pp. 659 and 661 Figure 9). 

15. Maxillary fenestra:
As for the maxillary fenestra, the premaxillary fenestra placed the maxillary fenestra farther away from the maxillary strut (Figure 2, C). This, interestingly enough, is present in FMNH PR 2081 (Brochu, 2003, p. 11 Figure 3; p. 16 Figure 8).

16. Lacrimals:
The lacrimals are not preserved in Lythronax (see Figure 2), so I don't know why they were recreated with a cornual process. Whether or not Lythronax had them is up in the air. Adult T. rex and Tarbosaurus 
specimens didn't have them (Carr and Williamson, 2004, p. 500 Figure 10). However, some juvenile T. rex specimens (LACM 23845) didn't have them (Molnar, 1980, p. 103) (Carr and Williamson, 2004, p. 500 Figure 10), but a baby/juvenile Tarbosaurus specimen (GIN 100/66) has lacrimal horns (Currie, 2003a, p. 200). If this specimen of Lythronax had cornual processes on its lacrimals, then it could have been immature when it died. Unfortunately, we only have this one specimen, and no paper seems to have given an age estimate for it either, so this is purely hypothetical.

17. Lingual bar:
Lythronax's lingual bar covers the first two teeth in the interior of the dentary, as in T. rex (Dalman and Lucas, 2017, pp. 23-24 Figure 10B).

In my opinion, there's enough traits here to place Lythronax within the genus Tyrannosaurus, calling Lythronax argestes Tyrannosaurus argestes. So far, the only substantial trait that separates it from T. rex, as well as Tarbosaurus, is "possessing a dorsally expanded ascending process of the astragalus (with the height of the ascending process greater than the width of the astragalus and calcaneum)" (Results: Diagnosis). The astragalus, or talus, is a bone in the foot (Armstrong, 2010). If the lacrimals had a cornual process, then that would be another trait that would separate Lythronax from Tyrannosaurus. If not, then I think Lythronax would fit rather well into the genus Tyrannosaurus. For now, I will call Lythronax argestes Tyrannosaurus argestes.

Update (3/14/26):
I realized that the lacrimals for Lythronax have not been found. They have been reconstructed to look like Daspletosaurus, or even more basal tyrannosaurids like Gorgosaurus. Therefore, I'll keep Lythronax as its own species for now. If we discover the true lacrimals for this taxon, it might change my opinion. On the other hand, all of the other characteristics for Lythronax demonstrate that it belongs in the derived tyrannosaurini. 

Link:
Loewen et al., (2013):

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0079420

Hurum and Sabath (2003):

https://www.app.pan.pl/archive/published/app48/app48-161.pdf

Currie (2003a):

https://app.pan.pl/archive/published/app48/app48-191.pdf

Brochu (2003):

https://www.researchgate.net/publication/249022959_Osteology_of_Tyrannosaurus_rex_Insights_from_a_Nearly_Complete_Skeleton_and_High-Resolution_Computed_Tomographic_Analysis_of_the_Skull

Persons IV et al., (2019):

https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.24118
Molnar (1980):

https://www.jstor.org/stable/1304167

Carr and Williamson (2004):

https://www.academia.edu/2291683/Diversity_of_late_Maastrichtian_Tyrannosauridae_Dinosauria_Theropoda_from_western_North_America

Armstrong (2010):

https://www2.palomar.edu/users/warmstrong/borrego2.htm

Dalman and Lucas (2017):

https://www.dinosaur.pref.fukui.jp/archive/memoir/memoir016-017.pdf
Tyrannosauroidea central. Literature review 2: Loewen et al., 2013, para. 2:

https://tyrannosauroideacentral.blogspot.com/2013/11/literature-review-2-loewen-et-al-2013.html?m=1

2. Tarbosaurus bataar:
Tarbosaurus has probably been the number one tyrannosauroid genus that has been either lumped, or split into, Tyrannosaurus ever since it was discovered. Maleev (1955a) and (1955c) placed the Tarbosaurus holotype, PIN 551-1, into the genus Tyrannosaurus as Tyrannosaurus bataar (Maleev, 1955a, p. 1) (Maleev, 1955c, p. 2 Figure 2). Later on, multiple scientists have either placed Tarbosaurus into Tyrannosaurus (Carpenter, 1992, pp. 254-256) (Carr, 1999, p. 499) (Holtz, 2001, p. 70), or separated them as distinct genera (Hurum and Sabath, 2003, p. 188) (Currie, 2003a, p. 225). 

Honestly, Tarbosaurus' skull looks a lot like Tyrannosaurus'. There are a couple of traits that Tarbosaurus are suppose to not have that are present in T. rex, but apparently they are:

1. Lacrimal processes:
According to Hurum and Sabath (2003), Tarbosaurus does not have lacrimal processes on its nasals (p. 169 Figure 5; pp. 170 and 188).

Tarbosaurus (A3) and Tyrannosaurus (B) nasals (Hurum and Sabath, 2003, p. 169 Figure 5):

Tarbosaurus specimen ZPAL MgD-I/4's lacrimal (Hurum and Sabath, 2003, p. 170 Figure 5):

T. rex specimen BHI 3033 ("Stan") lacrimal (Hurum and Sabath, 2003, p. 171 Figure 6):

However, the three-year old Tarbosaurus specimen MPC-D 107/7, interestingly, had the lacrimal processes on its nasals (p. 10 Figure 8, A).

3-year old Tarbosaurus specimen MPC-D 107/7's nasals with the arrow indicating the presence of the lacrimal process:

Carr (1999) said that the presence of the lacrimal process is located in the younger specimens of Gorgosaurus libratus, but it disappears when the individual matures (p. 500; p. 501 Figure 2, B-D). However, juvenile and adult specimens of T. rex have these, as noted above. Also, Voris (2018) shows that most Gorgosaurus specimens kept the lacrimal processes (p. 80 Figure 3.2; p. 81 Figure 3.3).

Gorgosaurus' ontogeny chart (Voris, 2018, p. 80):

Juvenile Gorgosaurus specimen TMP 2009.12.14's skull drawing (Voris, 2018, p. 81 Figure 3.3). Scale bar is 5 cm. Lacrimal process is present here:

Juvenile Gorgosaurus specimen TMP 94.143.01 skull drawing with lacrimal processes (p. 80 Figure 3.2):

Older specimen USNM 12814 skull drawing with lacrimal processes (p. 80 Figure 3.2):

Tsuihiji et al., (2011) say that this feature is lost in the adult specimens (p. 10 Figure 8). However, this may not be entirely true. A cast of a Tarbosaurus skull, presumably a subadult or adult, seems to have the lacrimal processes (Lee's Twitter pic):

Close-up of the lacrimal and rear end of the nasals:

As far as I'm concerned, if a particular specimen has lacrimal processes, then they'll stay there throughout the animal's lifetime (when it was alive). This is seen in Tyrannosaurus, Gorgosaurus, and now Tarbosaurus.

On the other hand, there is one T. rex that does not have the lacrimal processes, and it is fully grown. This specimen is LACM 23844 (Currie, 2003a, p. 200) (Molnar, 1991, p. 177 Plate 1, 1-2).

LACM 23844 not having the slots in its lacrimal for the lacrimal processes (Currie, 2003a, p. 200):

T. rex LACM 23844's lacrimal (Molnar, 1991, p. 177 Plate 1, 1-2):

T. rex specimen BHI 3033 ("Stan") lacrimal (Hurum and Sabath, 2003, p. 171 Figure 6):

As you can see, there are no slots in LACM's lacrimal for the lacrimal processes, unlike in "Stan's" lacrimal. Apparently, both Tyrannosaurus and Tarbosaurus can either have, or lack, the lacrimal processes.

2. Skull width:
Tarbosaurus' skull is narrower than T. rex's (Hurum and Sabath, 2003, pp. 164 and 166). To see how wide, or not, Tarbosaurus' skull was, I measured ZPAL MgD-I/4's skull in Hurum and Sabath (2003).

ZPAL MgD-I/4's skull (Hurum and Sabath, 2003, p. 165 Figure 1):

Skull width: 56 cm (at best) (Measured on 6/17/22 in Hurum and Sabath, 2003, p. 165 Figure 1, B).

Interestingly, ZPAL's skull width seems to be equal to Lythronax's (check Loewen et al., 2013, Figure 2, B). 

3. Tooth count and serrations:
Tarbosaurus and Tyrannosaurus have the same tooth count, contra Hurum and Sabath (2003) (pp. 186-188). T. rex specimen CM 79057 ("Samson") has 13 teeth in its maxilla (Carr et al., 2011, p. 5). "Samson," and PARC-TD-11-094/FDM-xx?, both have 15 teeth in their dentaries (Deak and McKenzie, 2016, slide 9; from Horner, 2011) (Carr et al., 2011, p. 5) (Stein, 2021, pp. 36 and 37, Figure 16). This is the same as Tarbosaurus (Hurum and Sabath, 2003, pp. 186-188). 

CM 79057 ("Samson") dentary and tooth count (Deak and McKenzie, 2016, slide 9; from Horner, 2011):

PARC-TD-11-094/FDM-xx? dentary (Stein, 2021, p. 37, Figure 16):

Note: First two alveoli are covered by the lingual bar (A).

Stein's statement on the dentary having 14-15 tooth positions (Stein, 2021, p. 36):

PARC-TD-11-094/FDM-xx? with arrows showing 15 teeth (Kawabe and Hattori, 2021; photo from Randall, 2021):

As for the serrations, Hurum and Sabath (2003) said that both T. rex and Tarbosaurus had the same serration count: 3 per 1 mm on the premaxillary, and 2 per 1 mm on the maxillary and dentary teeth (pp. 187). Dong (1979) said that some of Tarbosaurus' teeth had serrations on the anterior (front) side that reach the midline of the tooth, while on the posterior side they do reach the base of the tooth (p. 2). This is identical to T. rex's maxillary teeth (Sammon et al., 2005, pp. 762, 764, and 768). 

4. Lingual bar:
Tarbosaurus' lingual bar covers the first two alveoli in the interior of its dentary, as in T. rex (Dalman and Lucas, 2017, pp. 23-24 Figure 10, B).

5. Maxillary fenestra:
Tarbosaurus' maxillary fenestra is right on the maxillary strut, just like Tyrannosaurus' (Hurum and Sabath, 2003, p. 165 Figure 1).

6. Bone count:
Tarbosaurus and T. rex have a similar vertebrae count: 9 cervicals, and 40-45 caudals (Maleev, 1974, pp. 12, 26, and 29) (Brochu, 2003, p. 90) (Osborn, 1917, pp. 765) (Larson, 2008a; in Larson and Carpenter, 2008, p. 21).

It seems to indicate that Tarbosaurus could be placed into the genus Tyrannosaurus, as some scientists have done in the past.

Links:
Maleev (1955a):
https://paleoglot.org/files/Maleev_55a.pdf
Maleev (1955c):
https://paleoglot.org/files/Maleev_55c2.pdf
Maleev (1974):
https://paleoglot.org/files/Maleev_74.pdf
Carpenter (1992): 
https://www.researchgate.net/profile/Kenneth_Carpenter3/publication/314988830_Tyrannosaurids_Dinosauria_of_Asia_and_North_America/links/58c8026ea6fdcca657f63102/Tyrannosaurids-Dinosauria-of-Asia-and-North-America.pdf?origin=publication_detail
Link 2: 
https://www.researchgate.net/publication/314988830_Tyrannosaurids_Dinosauria_of_Asia_and_North_America
Carr (1999):
https://zenodo.org/record/3372241#.X7K6PyVOmEc
Link 2:
https://core.ac.uk/download/pdf/227005733.pdf
Holtz (2001):
https://www.academia.edu/293183/7_The_Phylogeny_and_Taxonomy_of_the_Tyrannosauridae?auto=download
Hurum and Sabath (2003):
https://www.app.pan.pl/archive/published/app48/app48-161.pdf
Currie (2003a):
https://app.pan.pl/archive/published/app48/app48-191.pdf
Tsuihiji et al., (2011):
https://people.ohio.edu/witmerl/juvenile_tyrannosaur/2011_Tsuihiji_et_al._Tarbosaurus_juvenile_skull_PROOF.pdf
Voris (2018):
https://prism.ucalgary.ca/bitstream/handle/1880/109240/ucalgary_2018_voris_jared.pdf?sequence=1&isAllowed=y
Lee (paleeoguy) Twitter pic:
https://mobile.twitter.com/paleeoguy/status/685557446799167488
Molnar (1991):
https://zenodo.org/record/3251815#.YmQoviUpCEc
Carr et al., (2011):
https://www.researchgate.net/publication/233899056_A_new_genus_of_short-skulled_tyrannosaurid_from_the_Upper_Cretaceous_upper_Campanian_Kaiparowits_Formation_of_UtahDeak and McKenzie (2016):
https://www.researchgate.net/publication/309340780_HYPOTHETICAL_DIVERGENT_EVOLUTION_OF_TWO_APEX_PREDATORS_FROM_THE_HELL_CREEK_FORMATION_NANOTYRANNUS_LANCENSIS_AND_TYRANNOSAURUS_REX
Stein (2021):
https://www.aaps-journal.org/pdf/JPS.C.2021.0001.pdf
Kawabe and Hattori (2021) (Abstract):
https://www.tandfonline.com/doi/full/10.1080/08912963.2021.1965137
Randall (2021):
https://www.dailymail.co.uk/sciencetech/article-9919241/Fossils-T-rex-complex-nerve-sensors-tips-jaws-study-finds.html
Dong (1979):
https://zenodo.org/record/162178#.YoMv-yUpCEf
Midline/median line definition:
https://www.lexico.com/en/definition/median_line
Sammon et al., (2005):
https://www.app.pan.pl/archive/published/app50/app50-757.pdf
Dalman and Lucas (2017):
https://www.dinosaur.pref.fukui.jp/archive/memoir/memoir016-017.pdf

Conclusions:
In all honesty, I do believe that there are other species of Tyrannosaurus. Tarbosaurus can be placed within Tyrannosaurus as Tyrannosaurus bataar, so I will call the taxon Tarbosaurus/Tyrannosaurus bataar. I would love to put Lythronax within Tyrannosaurus as Tyrannosaurus argestes, but I would love to have the lacrimals preserved before making my final decision. Within the specimens attributed to T. rex, on one hand, there are some specimens that are earlier in time compared to most T. rex specimens. These earlier specimens can be placed as a new species of Tyrannosaurus. I personally would call them Tyrannosaurus princeps, the "tyrant lizard prince." However, they could just be early specimens of T. rex, making the species appear way earlier than previously estimated. Either way is fine with me. On the other hand, there could be a subspecies of T. rex that can be separated on the bases of vertebrae count and possible gender. 

Update (1/31/23):
Gregory S. Paul's preprint from August of 2022 says that Tyrannosaurus might've been more than one species after all. Also, Tyrannosaurus might've had some sort of sexual dimorphism regarding the skull. Regardless of whether or not this holds true in the end, I'm going to see where this journey leads to. Here's the link:
https://www.researchgate.net/publication/362522911_Observations_on_Paleospecies_Determination_With_Additional_Data_on_Tyrannosaurus_Including_Its_Highly_Divergent_Species_Specific_Supraorbital_Display_Ornaments_That_Give_T_rex_a_New_and_Unique_Life_Ap

Update (7/4/25): T. rex and T. imperator:
I'd propose that Tyrannosaurus imperator was possibly a valid species or subspecies of Tyrannosaurus.  

Tyrannosaurus rex Species Info.:
Cervicals: 9.
Dorsals: 14.
Lacrimal morphology: Bends posteriorly downward.
Specimens:
1.) CM 9380 (Holotype): Female.
2.) BHI 3033 ("Stan"): Male.
3.) MOR 555/USNM 555000 ("Nation's rex"): Male.
4.) CM 79057 ("Samson"): Probably female, given the robustness of the specimen.

Tyrannosaurus imperator Species Info.:
Cervicals: 11 (at best).
Dorsals: 12 (at best).
Lacrimal morphology: Straight horizontally.
Specimens:
1.) BMNH R7994: Female.
2.) AMNH 5027: Male.
3.) FMNH PR 2081: Female.

Part 2: Tyrannosaurus mcraensis.
(3/14/26) Well, it's been a while since I made this post. I forgot that I hypothesized that the Campanian-aged specimens of T. rex could be their own species within the genus. Now, we have one! Dalman et al., (2024) said that the specimen NMMNH P-1013/P-3698 was a new species of Tyrannosaurus called Tyrannosaurus mcraensis. Compared to other Tyrannosaurus dentaries, the dentary of T. mcraensis is very slender and the chin/symphysis (sym) is also smaller. 

NMMNH P-3698 (Dalman et al., 2024, Figure 3):

NMMNH P-3698 compared to other Tyrannosaurus dentaries (Figure 6). Scale bars are 10 cm:

Full Figure:

Now, I began to study the physical characteristics of Tyrannosaurus dentaries. I noticed that CMNH 9380 (Figure 6I), and BHI 3033 (Figure 6L), had wider chins that were more... let's say "square-shaped." FMNH PR 2081 (Figure 6K), LACM 150167 (Figure 6J), and MOR 1125 (Figure 6H) had smaller, protruding chins. Let's call them "triangle-shaped" chins, since they look like spears. You can see this better with "Sue's" right dentary (Brochu, 2003, p. 41 Figure 40C-D):

AMNH 5027 also has a skinny, "triangle-shaped" chin on its dentary (Osborn, 1912, p. 22 Figure 18):

I also think something interesting is going on with the postorbitals (Dalman et al., 2024, Figure 6A-F). Briefly, MOR 980 (Figure 6D), "Sue" (Figure 6E), and RSM P2523.8 ("Scotty") (Figure 6F) have similar postorbitals compared to the other specimens. This can be seen for "Sue" in Brochu (2003) as well (p. 27 Figure 23). I might explore that later.

For now, one could hypothesize that Tyrannosaurus rex had a larger, "square-shaped" chin/symphysis on its dentary than Tyrannosaurus imperator. T. mcraensis, and T. imperator, had smaller, protruding, "triangle-shaped" symphyses on their dentaries. Did the symphysis on the dentary of Tyrannosaurus adapt to become larger over time? Paul et a., (2022) said that T. rex specimens were located in the "upper, and possible middle" strata of their formations. Same goes for T. regina. T. imperator specimens were located in the "lower, lower middle and possibly middle" strata of their formations. The authors also placed MOR 1125 in T. imperator (Conclusion and Systematic Paleontology). Since I believe that BHI 3033, who the authors placed as T. regina, was a specimen of T. rex, I believe that T. regina was a synonym of T. rex. For now, I believe that North America had (possibly) three species of Tyrannosaurus: T. mcraensis, T. imperator, and T. rex. Perhaps T. imperator could've been a subspecies, but I'll consider it a species of Tyrannosaurus for now. CM 9401 could've been T. mcraensis, but I think it should be catalogued as Tyrannosaurus sp. This also goes for other earlier Tyrannosaurus specimens.

(3/18/26) The main body of the dentaries for T. imperator seem to be more linear on the ventral side than the dentaries for T. rex. The same goes for the dentary for T. mcraensis. In fact, NMMNH P-3698 seems to be slightly convex on the ventral side. The dentaries for T. rex seem to be more concave on the ventral side (Dalman et al., 2024, Figure 6G-L). Dalman et al., (2024) said that the posterior of NMMNH P-3698 had "a convex posteroventral margin," (Systematic paleontology, Diagnosis). I would add that the entire ventral side of the dentary is rather convex compared to the other Tyrannosaurus specimens. It seems that the ventral region, and the anterior portion, of the dentary might contain autapomorphies for determining different species of Tyrannosaurus. 

(3/21/26) LACM 150167 was probably a male specimen. Its femoral circumference (470 mm) is about the same as MOR 980 (483 mm), and close to BHI 3033 (505 mm). Both latter specimens were gracile (Paul et al., 2022, Table 1). We now have two male specimens for T. imperator.

(4/2/26) LACM 23844 seems to fit with T. imperator. It has a straight lacrimal, and a dentary with a slender symphysis (Molnar, 1991, Plates Plate 1-2, and 12). 

Left dentary of LACM 23844 (Molnar, 1991, Plate 12) (Scale bar is 10 cm):

Paul et al., (2022) didn't know whether to put this specimen in T. rex or T. regina. This specimen, and LACM 150167, lived at the end of the Maastrichtian (higher level). The T. rex specimen "Samson" appeared in the lower level strata (Table 1, Conclusion and Systematic Paleontology). I'm placing both LACM specimens in T. imperator, but it seems that T. imperator and T. rex coexisted. The two species can be identified using the characteristics we've accumulated though. There's no femoral circumference for LACM 23844 in Paul et al., (2022), but the photos I've seen of its mounted skeleton suggests that it was probably a male. 

Tyrannosaurus LACM 23844 vs. a Triceratops (Natural History Museums of Los Angeles County, Dueling Dinos):

Thus, we probably have three male, and three female, specimens of T. imperator. 

Updated Tyrannosaurus Species Info.:

Tyrannosaurus mcraensis:

Dentary:
-Ventral side is slightly convex.
-Dentary symphysis morphology: Skinny, protruding, and "triangle-shaped."
Specimens:
1.) NMMNH P-1013/P-3698.

Tyrannosaurus imperator:
Dentary:
-Ventral side is more linear.
-Dentary symphysis morphology: Skinny, protruding, and "triangle-shaped."
Vertebra:
-Cervicals: 11 (at best).

-Dorsals: 12 (at best).

Lacrimal morphology: Straight horizontally.
Specimens:
1.) BMNH R7994: Female.
2.) AMNH 5027: Male.
3.) FMNH PR 2081: Female.

4.) MOR 1125: Female.
5.) LACM 150167: Male.
6.) LACM 23844: Male.

Tyrannosaurus rex:
Dentary:
-Ventral side is more concave.
-Dentary symphysis morphology: Wide, and "square-shaped."
Vertebra:
-Cervicals: 9.
-Dorsals: 14.
Lacrimal morphology: Bends posteriorly downward.
Specimens:
1.) CM 9380 (Holotype): Female.
2.) BHI 3033 ("Stan"): Male.
3.) MOR 555/USNM 555000 ("Nation's rex"): Male.
4.) CM 79057 ("Samson"): Probably female, given the robustness of the specimen.

Tarbosaurus/Tyrannosaurus bataar could serve as the Asian species of Tyrannosaurus.

Links:
Dalman et al., (2024):
https://www.nature.com/articles/s41598-023-47011-0
Brochu (2003):
https://www.researchgate.net/publication/249022959_Osteology_of_Tyrannosaurus_rex_Insights_from_a_Nearly_Complete_Skeleton_and_High-Resolution_Computed_Tomographic_Analysis_of_the_Skull
Osborn (1912):
https://digitallibrary.amnh.org/items/10190e09-d3c8-4eeb-81e2-1ba8291425b4
-V2:
https://babel.hathitrust.org/cgi/pt?id=mdp.39015034374440&seq=11
Paul et al., (2022):
https://link.springer.com/article/10.1007/s11692-022-09561-5
Molnar (1991):
https://zenodo.org/record/3251815#.XU8SYLaZP-Y
-V2:
http://treatment.plazi.org/id/6E4987EAFFEDFF83C269FCFA784991B6
Natural History Museums of Los Angeles County. Dueling Dinos:
https://nhmlac.org/experience-nhm/exhibitions-natural-history-museum/dueling-dinos