Was there more than one species of Tyrannosaurus?

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

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. 

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. However, I would say that this is a case for a subspecies within T. rex, not speciation. For example:

T. rex subspecies 1:
Proposed name: Tyrannosaurus rex eversor: "The tyrant lizard king destroyer" in Latin.
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.

T. rex subspecies 2:
Proposed name: Tyrannosaurus rex victoris: "The tyrant lizard king conqueror" in Latin.
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 rex.

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 T. rex specimens, it seems that these could be a new species of Tyrannosaurus. I would offer the name Tyrannosaurus princeps, the "tyrant lizard prince" in Latin. Every king starts out as a prince before they become king, so I think this has a nice ring to it. Of course, I am also fine with having these four specimens being labelled as T. rex, making the species appear earlier in time than previously thought.

Proposed name: Tyrannosaurus princeps: "The tyrant lizard prince" in Latin.
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. I would propose the name Tyrannosaurus princeps for those. However, the specimens from the late Maastrichtian, I believe, are T. rex. However, based on the vertebrae count, possible sexual dimorphism, and lacrimal morphology, I believe that there was possibly a subspecies of T. rex. I propose the names Tyrannosaurus rex eversor and Tyrannosaurus rex victoris. 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):

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

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. Lythronax and Tarbosaurus can be placed within Tyrannosaurus as Tyrannosaurus argestes and Tyrannosaurus bataar. 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.