My Prehistoric Saga: May 2022

During my laborious study of T. rex and Dryptosaurus ("Nanotyrannus") lancensis, it became evident to me that the hypothesis "tyrannosauroids lost teeth during ontogeny" became the ultimate go-to excuse as to why supposed young T. rex specimens (the D. lancensis specimens) had more teeth than the adults. This idea was created by Dr. Carr in his 1999 paper on tyrannosaur ontogeny. To my surprise, there are a number of sources that actually disprove Carr's hypothesis. When I looked at Carr's 1999 paper as well, there's one major aspect of his hypothesis that, to my knowledge, has been constantly overlooked. If tyrannosaurs lost teeth during ontogeny, wouldn't we have expected this in both the maxilla and the dentary? As I will soon demonstrate to you, this is not the case. Tyrannosauroids did not lose teeth during ontogeny. The tooth count either stayed consistent throughout the individual's lifespan, or tooth count actually seemed to have increase from one specimen to another.

Carr's Hypothesis:
In 1999, Dr. Carr published his paper on tyrannosaur ontogeny. He stated that the D. lancensis holotype, CMNH 7541, was a juvenile T. rex. in attempting to explain how a young T. rex went from having a large tooth count to a small one, Carr said that T. rex lost teeth during ontogeny. To demonstrate this, he used the maxillas of Gorgosaurus (or Albertosaurus) libratus to show that tooth count decreased as that genus grew during ontogeny (pp. 514 and 516 Table 2). Ever since then, Carr's tooth loss hypothesis has grown a lot of support and seems to be seen as conclusive ever since Carr wrote his extensive 2020 paper on T. rex ontogeny.

Carr's table of Gorgosaurus specimens supposedly losing teeth during ontogeny (Carr, 1999, p. 516 Table 2):

Interestingly, AMNH 5458, a stage 3 individual, has the same maxillary tooth count (14) as the stage 1 individuals ROM 1247 and CMN 12063. Stage 1 specimen USNM 12814 has the same maxillary tooth count (13) as the stage 3 individuals UA 10 and CMN 2120. This doesn't seem like major tooth loss due to ontogeny.

However, as I've stated before, there are a number of papers that have been published that show that tyrannosauroids did not lose teeth during ontogeny. I will talk about them in this post, as well as other key pieces of information that I think will help to demonstrate that tyrannosauroids did not lose teeth as they matured.

Currie (2003) and (2011):
The first paper that I've found that demonstrated that tooth loss did not occur in tyrannosauroids is Currie (2003). On p. 196 Figure 5, it's demonstrated from multiple genera that as the maxillary tooth row increased, tooth count either stayed the same or increased.

Tyrannosaurid Tooth Count Chart (Currie, 2003, p. 196 Figure 5):

Currie (2011) reiterated Currie's previous findings from 2003. Tooth count was variable per individual, but there was no sign of tooth loss based on the side of the individual (Abstract).

Currie (2011) on no sign of tooth loss in tyrannosaurs during ontogeny (Abstract):

Larson and Mortimer:
Peter Larson, and Mickey Mortimer from The Theropod Database Blog, said that Allosaurus,
Coelophysis, Ceratosaurus, Majungasaurus, and therizinosauroids, did not lose teeth throughout ontogeny (Larson, 2013, p. 35) (Mortimer, The Theropod Database Blog, 2013, "Validity of Nanotyrannus," "#X," para. 2). Mortimer said their tooth count was "stable or increasing." She also questions whether or not CMNH and "Jane" are special cases in terms of T. rex's tooth count, saying that this "strains the possibility," ("#X," para. 2).

Larson on Allosaurus and Coelophysis not losing teeth during ontogeny (Larson, 2013, p. 35):

Mortimer on other theropods not losing teeth during ontogeny (Mortimer, The Theropod Database Blog, 2013, "Validity of Nanotyrannus," "#X," para. 2):

Examining Tyrannosauroid Genera:
1. Gorgosaurus:
Since Carr used Gorgosaurus (or Albertosaurus) libratus first to demonstrate his hyoothesis, I will be tackling this species first. Voris et al., (2022) stated that juvenile Gorgosaurus specimens had the same tooth count in their maxillas and dentaries that the adult specimens had (13-15 in the maxilla, 14-15 in the dentary). This, along with Currie (2003), go against Carr's claim that Gorgosaurus lost teeth during ontogeny.

Gorgosaurus did not lose teeth during ontogeny (Voris et al., 2022, Systematic Paleontology: Description of Juvenile Gorgosaurus Skulls: Dentition, para. 1):

One juvenile specimen of Gorgosaurus, TMP 94.143.1 (Voris, 2018) (Voris et al., 2019), has 18 teeth in its dentary, which is rather large for the species (Currie, 2003, p. 218 Figure 3). However, it had 13 teeth in its maxilla (Currie, 2003, p. 204 Figure 18 A; p. 219). This is one less maxillary tooth than the two juvenile specimens from Voris et al., (2022), but three-four more dentary teeth than the other two juvenile specimens. Upon further inspection, both TMP 2009.12.14 and TMP 2016.14.1 have 15 maxillary teeth (pers. obs.) (Voris, 2018, pp. 78-79 Figure 3.1 A-B; p. 81 Figure 3.3) (Voris et al., 2022, Figure 2 B). This is one-two more than TMP 94.143.1. TMP 2009.12.14 is also younger than the other two specimens (Voris, 2018, p. 80 Figure 3.2). Yet, a maxillary tooth count of 15 falls within the range of adult specimens, as stated in Voris et al., (2022).

TMP 2009.12.14's skull drawing (Voris, 2018, p. 81 Figure 3.3). Scale bar is 5 cm:

15 teeth are present in the maxilla here.

As for the dentary tooth count in TMP 94.143.1, one could say that the large tooth count would have shrunk when the individual matured. If that were the case, would the maxillary tooth count decrease as well? TMP 94.143.1 already has a maxillary tooth count identical to the adults', so it doesn't seem likely that the maxillary tooth count would decrease. TMP 2009.12.14 and TMP 2016.14.1 seem to have more teeth in their maxillas than TMP 94.143.1, and TMP 2009.12.14 is younger than TMP 94.143.1. Would TMP 2009.12.14 lose maxillary teeth when it's count is already the same as the adults', and the larger TMP 2016.14.1 (Voris, 2018, p. 80 Figure 3.2)? Would TMP 2016.14.1 lose teeth in its dentary, but keep its maxillary tooth count that is already larger than TMP 94.143.1's, if it were truly losing teeth with age? That wouldn't make any sense. The best conclusion is that TMP 94.143.1's maxillary and dentary tooth count is a result of individual variation. It had a larger dentary tooth count than both TMP 2009.12.14 and TMP 2016.14.1, but it had a smaller maxillary tooth count than the other two specimens.

This is also supported, surprisingly, by Carr's Table 2 from his 1999 paper. Some baby and/or juvenile specimens of Gorgosaurus had a tooth count identical to the adults. When we incorporate the adult maxillary tooth counts from Voris et al., (2022), we see that the smaller specimens from Carr (1999) that had 15 maxillary teeth fall within range of the adults' maxillary tooth count. With this in mind, it seems unlikely that TMP 94.143.1 would lose teeth in its dentary, or in its maxilla too, for that matter. This would give Gorgosaurus a higher tooth count in its dentary, resulting in 18 rather than 15.

Currie et al., (2003) give a dentary count of 15-17 for Gorgosaurus (p. 229 Table 1), so it seems even more unlikely that TMP 94.143.1 would have lost teeth due to ontogeny.

Tyrannosaurid Tooth Counts, along with Allosaurus' (Currie et al., 2003, p. 229 Table 1):

Some more information can be found in Currie (2003). A juvenile Gorgosaurus specimen, TMP 91.36.500, at 10 years of age (Erickson et al., 2006, Supplementary Information, p. 14), has 17 teeth in its dentary (Currie, 2003, p. 219), which is one less than TMP 94.143.1. Another specimen named TMP 94.12.602, an 18-year old individual (Erickson et al., 2006, Supplementary Information, p. 14), has 15 teeth in its maxilla (Currie, 2003, p. 219). This is two more than TMP 94.143.1.

There's no sign of definite tooth loss in Gorgosaurus. Two other hypotheses could be made here instead. First, Gorgosaurus could have lost teeth in its dentary just to grow more in its maxilla. Second, this is just individual variation shown in the genus. I'm going with the latter.

2. Alioramus and Qiazhousaurus:
Foster et al., (2022) stated in their paper that Qiazhousaurus shows signs of tooth loss due to ontogeny (Discussion: Tooth Count Reduction). They used the maxilla, which has 15 in it (Description: Maxilla, para. 8) (Lu et al., 2014, Comparative description, para. 3), and stated that alioramin lost teeth as they matured. They went from 18 in Alioramus altai (although Brusatte et al., 2009 gave it 17 in Diagnosis, para. 2) to 16 in Alioramus remotus (Kurzanov, 1976, p. 2) and Qiazhousaurus (Description: Maxilla, para. 8) (Lu et al., 2014, Comparative description, para. 3). For the dentary, Foster et al., (2022) said that alioramin went from 20 in Alioramus remotus (also stated in Brusatte et al., 2009, Diagnosis, para. 2) to 18 in Alioramus remotus (Kurzanov, 1976, pp. 2 and 8) and Qiazhousaurus (Discussion: Tooth Count Reduction) (Lu et al., 2014, Comparative description, para. 7).

There are problems with this, aside from the fact that Alioramus altai seems to have grown an extra tooth in its maxilla. First, Alioramus remotus and altai are the same age, at 9 years old (). Second, Alioramus remotus and Qiazhousaurus retained the same tooth count, aside from the fact that A. remotus is a baby and Qiazhousaurus is a supposed mature individual. Wouldn't Qiazhousaurus' tooth count drop even more during ontogeny? Why would it retain the dentary tooth count of a 9-year old Alioramus remotus? You could say that the maxillary tooth count did shrink, but the dentary tooth count did not. One would expect Qiazhousaurus' tooth count to be extremely low compared to the 9-year old Alioramus specimens. Alternatively, we see a rather weak decrease in tooth count, or even none whatsoever.

The best explanation is individual variation. Alioramus and Qiazhousaurus seem to be very similar to Gorgosaurus when it comes to tooth count variation. Qiazhousaurus' and Alioramus remotus' tooth counts are extremely similar, with A. remotus having only one more tooth in its maxilla. A. altai seems to be similar to the juvenile Gorgosaurus specimen TMP 94.143.1. These two specimens have the highest tooth counts for their species, but they are not extreme outliers. A. altai has two more teeth than both A. remotus and Qiazhousaurus. the maxilla does show more of a difference though, having one (or two?) more than A. remotus, and two (or three?) more than Qiazhousaurus. As I've explained above, juvenile Gorgosaurus specimen TMP 94.143.1 has three more teeth in its dentary than most other specimens. Other Gorgosaurus specimens, even more mature ones than TMP 94.143.1, had more teeth in their maxillas than TMP 94.143.1 did.

The ages of the specimens, along with a very weak trend in tooth loss, leans Alioramus and Qiazhousaurus better in the direction of tooth count variation rather than ontogenetic tooth loss. If this were not the case, Qiazhousaurus should have exhibit extreme tooth loss in both the maxilla and dentary, perhaps 11 or 13 teeth in the maxilla and dentary, for example. This is not observed, contra Foster et al., (2022).

Foster et al., (2022) claim that Qiazhousaurus and Alioramus lost teeth during ontogeny (Discussion: Tooth Count Reduction):

Alioramus remotus and altai are both 9 years old:
Brusatte et al., (2009) (Supplementary Information, p. 2):

Brusatte et al., (2009) (Diagnosis, para. 2):

Why did A. remotus lose teeth, but not A. altai, yet they're both 9 years old? Given the ages of the specimens, we should see a greater decrease in tooth count for the supposed adult Qiazhousaurus, but this is not the case. It had the same dentary tooth count in its dentary as the 9-year old Alioramus remotus.

Alioramus altai had 17 teeth in the maxilla, not 18 (Brusatte et al., 2009, Diagnosis, para. 2):

3. Daspletosaurus:
Carr et al., (2017) stated that Daspletosaurus horneri exhibited ontogenetic tooth loss. They explained that the tooth count in the maxilla started at 15, then increased to 17, and then decreased back to 15, as the individuals increased in age. However, they also stated that the dentary, for any specimen that was preserved with one, had 17 teeth in them (Discussion: Ontogenetic tooth count reduction, para. 1). The dentary tooth count did not increase then decrease during ontogeny for the species. This is a clear case of individual variation in Daspletosaurus horneri. The tooth count in the maxillary varies between individuals, but the dentary never changes in tooth count. Once again, if tooth count decreased during ontogeny for tyrannosaurs, then this should be present in both the maxilla AND the dentary from the youngest individual to the oldest individual. This is still not the case.

Carr et al., (2017) on ontogenetic tooth loss in Daspletosaurus (Discussion: Ontogenetic tooth count reduction, para. 1):

The authors used the Gorgosaurus specimen TMP 94.143.1 (TMP 94.143.0001 in the paper) to say that Daspletosaurus torosus lost teeth in the maxilla during ontogeny as well. This specimen is not Daspletosaurus torosus anymore (see above). Second, they note that the most mature D. torosus specimen, CMN 8506, had 14 teeth in the maxilla, while slightly younger specimens, AMNH FARB 5346 had 15, and MOR 395 had 16 (Discussion: Ontogenetic tooth count reduction, para. 2). As we've seen in Alioramus and Qiazhousaurus, this is individual variation, especially if the dentary had an identical tooth count regardless of the age of the specimens. Carr et al., (2017) only say that the maxillas fluctuated in tooth count for the mentioned D. torosus specimens, so the dentaries for those individuals may have the same tooth count.

According to Russell (1970), CMN 8506, which is the holotype specimen for D. torosus, had 14-15 teeth in its maxilla, and 15-16 teeth in its dentary (p. 17). This specimen is 24 years old (Erickson et al., 2006, Supplementary Materials, p. 16). There are other Daspletosaurus torosus specimens that we can examine as well. According to Dalman and Lucas (2016), TMP 1999.55.170 has 17 teeth in its dentary (pp. 21-22 Figure 5), and is 14 years old (Erickson et al., 2006, Supplementary Materials, p. 15). However, this specimen had the same tooth count in its dentary as the adult D. horneri specimen. In Dalman et al., (2018), specimen TMP 97.12.223 has about 14 teeth in its maxilla (pers. obs. on p. 132 Figure 11), while the specimen TMP 2001.36.01 has 16 teeth in its maxilla (pers. obs. in p. 133 Figure 12). TMP 12.223 is 17 years old (Erickson et al., 2006, Supplementary Materials, p. 15), while TMP 2001.36.01 is 21 years old (Erickson et al., 2006, Supplementary Materials, p. 16). The adult D. torosus specimens have a tooth count either very close to (CMN 8506 compared to TMP 1999.55.170 and AMNH FARB 5346), identical to (TMP 2001.36.01 compared to MOR 395, since both have 16 maxillary teeth), or even greater than (TMP 2001.36.01 compared to AMNH FARB 5346), the younger specimens, and vice versa. This is individual variation.

TMP 12.223's maxilla (Dalman et al., 2018, p. 132 Figure 11):

14 teeth are present.

TMP 2001.36.01's maxilla (Dalman et al., 2018, p. 133 Figure 12):

16 teeth are present.

If both the maxillas and the dentaries do not experience major tooth count reduction during ontogeny, then it's more than likely a result of individual variation. Both D. torosus and horneri supports this.

4. Tarbosaurus:
Carr et al., (2017) said that tooth loss during ontogeny occurred for tyrannosaurinae (Discussion: Ontogenetic tooth count reduction, para. 3). However, so far, this is not present in Daspletosaurus torosus or horneri. This is also not the case for Tarbosaurus. Adult specimens have 13 teeth in the maxilla, and 15 in the dentary (Hurum and Sabath, 2003, pp. 186-187). The 2-3-year old Tarbosaurus specimen MPC-D 107/7 had the same tooth count as the adults (Tsuihiji et al., 2011, pp. 7 and 17). The authors of that paper also stated that this means that tooth reduction does not occur ontogenetically in tyrannosaurids (Abstract; p. 17).

Tyrannosaurids did not lose teeth during ontogeny (Tsuihiji et al., 2011):
Abstract:

P. 17:

5. Tyrannosaurus:

The hypothesis that tooth loss occurred for tyrannosaurinae/tyrannosaurids started because young tyrannosaur specimens from the same time period, and place, as T. rex have been found. Young T. rex specimens are, admittedly, hard to find. In fact, finding young tyrannosaurids is rare because the young reach a mature size quickly, have a "high survivorship," and an "increase in midlife mortality" (Buckley et al., 2010, p. 1228 para. 1). However, using the baby and juvenile specimens of T. rex that we do have, the baby specimen RSM P2347.1 has been reconstructed with 13 teeth in its maxilla (pers. obs. from a photo from Jack Milligan's Twitter post). This is the same as the adult specimen CM 79057 ("Samson")
(Erickson et al., 2006, Supplementary Materials, p. 13) (Carr et al., 2011, p. 5, Discussion) (Deak and McKenzie, 2016, slide 9; from Horner, 2011). A larger baby specimen, BHI 6439, has 13 teeth in its dentary (pers. obs from a photo sent to me by Sebastian Dalman). An older specimen (I'm placing it as a juvenile), TMM 41436-1, has room for 11 teeth in its maxilla (pers. obs. in Wick, 2014, Figure 1). Another juvenile specimen, "Tinker," at 14 years of age (Erickson et al., 2006, Supplementary Materials, p. 13), has 12 teeth in its dentary (pers. obs. from a photo sent to me by Dalman). The subadult specimen "Stan," at 18 years of age (Erickson et al., 2006, Supplementary Materials, p. 13), has 11 teeth in its maxilla and 13 teeth in its dentary (Larson, 2013, p. 37) (pers. obs. in Dalman and Lucas, 2016, p. 25). As stated before, "Samson," a 23-year old adult (Erickson et al., 2006, Supplementary Materials, p. 13), has 13 teeth in its maxilla and 15 in its dentary (Carr et al., 2011, p. 5, Discussion) (Deak and McKenzie, 2016, slide 9; from Horner, 2011).

RSM P2347.1 has the same maxillary tooth count as "Samson," which indicates that T. rex did not lose teeth during ontogeny in the maxilla. BHI 6439 and "Stan" have less teeth in their dentaries than "Samson," yet "Samson" is older than both specimens. Instead of losing teeth, it seems that this is another case of individual variation. Also, like Gorgosaurus, perhaps T. rex's tooth count increased during ontogeny? More than likely, this is individual variation, just like Gorgosaurus. Just like the other tyrannosauroids examined in this post, there is no evidence to suggest that T. rex lost teeth during ontogeny.

RSM P2347.1's maxilla (Jack Milligan's Twitter post):

Tooth Count: Teeth were not found, but reconstruction shows that about 13 can fit (pers. obs.).

Top view of T. rex specimen BHI 6439's and Dryptosaurus lancensis' dentaries (pic. provided by Dalman):

Tooth count: 13 (pers. obs).

TMM 41436-1's maxilla (Wick, 2014, Figure 1) (Scale bar is 10 cm):

Tooth count: 11 teeth are present (pers. obs.).

"Tinker's" Dentary (pic. provided by Dalman) (Measuring tape is presumably in inches):

Tooth count: 12 (pers. obs.).

BHI 3033's ("Stan's") dentary (Dalman and Lucas, 2016, p. 25):

Tooth count: 13.

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

Tooth count: 15.

I used specimens that have been catalogued in museums, or are kept in museums as casts. Here's a link to another post I made on T. rex ontogeny using as many T. rex specimens as I could find. There is still no evidence of tooth loss:

https://psdinosaurs.blogspot.com/2021/11/my-ontogenetic-growth-series-for-t-rex.html

Tooth Morphology:
In addition to tyrannosauroids losing teeth, Carr has proposed that T. rex's first maxillary tooth went from being unserrated to serrated (Tsuihiji et al., 2011, p. 17). This is because D. lancensis' first maxillary tooth is identical to its premaxillary teeth, and it seems to be lacking serrations (Larson, 2013, pp. 33-35 Figure 2.14) (Larson's Twitter post) (Molnar, 1978, p. 77) (Carr and Williamson, 2004, p. 517). D. lancensis' teeth are morphologically similar to Gorgosaurus' (Lambe, 1917, p. 19) (Gilmore, 1946, p. 15) (Larson, 2013, pp. 33-35). In Carpenter (1982), there is a first maxillary tooth from a baby tyrannosaur, UCMP 119853, either from the Hell Creek, or Lance, Formation (p. 128 Figure 5; p. 130). It's identical to the first maxillary tooth seen in the subadult T. rex specimen "Stan's" maxilla, with the serrations on the sides (lateral view) of the tooth, the front/anterior/mesial serrations do not reach the base of the tooth (Samman et al., 2005, pp. 762 and 768), and it has no ridges to hold any of the serrations (Smith, 2005, p. 875 Figure 8 F-G). This is not the case in Dryptosaurus lancensis', and Gorgosaurus', first maxillary tooth (Lambe, 1917, p. 19) (Larson, 2013, pp. 33-35 Figure 2.14) (Larson's Twitter post) (Voris et al., 2022, Systematic Paleontology: Description of Juvenile Gorgosaurus Skulls: Dentition, para. 2). It seems that baby T. rex specimens had teeth with a morphology identical to the adults', just like juvenile and adult Gorgosaurus specimens.

Tyrannosaur tooth UCMP 119853 (ascribed to T. rex) from either Hell Creek or Lance Formation (Carpenter, 1982):

P. 128 Figure 5 (A is side view and B is the posterior (back) view). scale bar is 2 mm:

Description of the tooth (p. 130):

Subadult T. rex "Stan's" 4th premaxillary and 1st maxillary teeth (Smith, 2005, p. 875 Figure 8 F-G; both are side views) (Arrows indicate serrations):

Dryptosaurus ("Nanotyrannus") lancensis (D), T. rex (A and C), and Gorgosaurus' (B), first maxillary teeth (Larson, 2013, p. 35 Figure 2.14):

Dryptosaurus lancensis "Jane's" first maxillary tooth (posterior/rear view):

D. lancensis specimen LACM 28471 first and second maxillary teeth (Molnar, 1978, p. 76 Figure 5):

Note: Premaxillary tooth is the first maxillary tooth (Carr and Williamson, 2004, p. 489).

To back this up, the baby Tarbosaurus specimen MPC-D 107/7 has a serrated first maxillary tooth just like the adults (Tsuihiji et al., 2011, p. 17) (Smith, 2005, p. 872). Both Tyrannosaurus and Tarbosaurus are phylogenetically close to each other, and are both tyrannosaurine. It seems that, when we remove the D. lancensis specimens from the genus Tyrannosaurus, T. rex has an identical tooth count and morphology to Tarbosaurus'.

Tsuihiji et al., (2011) on baby Tarbosaurus MPC-D 107/7's first maxillary tooth (p. 17):

Voris et al., (2022) stated that two juvenile specimens had tooth morphology identical to the adults. All specimens had a first maxillary tooth that is incisiform, just like the premaxillary teeth (Systematic Paleontology: Description of Juvenile Gorgosaurus Skulls: Dentition, para. 2):

Note: They said that juvenile T. rex specimens had an incisiform first maxillary tooth because they lumped the D. lancensis specimens into T. rex.

Dryptosaurus aquilunguis, Jinbeisaurus, and Alioramus altai, all had a first maxillary tooth that was incisiform (Brusatte et al., 2011, p. 9) (Cope, 1869, pp. 100-101) (Wu et al., 2019, p. 9). The major changes that tyrannosaur teeth go through are size-related (smaller to larger) (Buckley et al., 2010, p. 1244), and the serration/denticle count decreases as the teeth grow larger, as seen in Tarbosaurus for example (Tsuihiji et al., 2011, p. 17) (Hurum and Sabath, 2003, p. 187). In fact, Buckley et al., (2010) said that juvenile and adult Albertosaurus teeth differed in size mainly, and in another analysis, juvenile and adult Gorgosaurus teeth were grouped together by 100% (p. 1244).

Juvenile and adult Albertosaurus teeth differed in size mainly (Buckley et al., 2010, p. 1244):

Juvenile and adult Gorgosaurus teeth were grouped together by 100% (Buckley et al., 2010, p. 1244):

2-3-year old Tarbosaurus specimen MPC-D 107/7/s serration count, and Tarbosaurus' teeth do not go from being unserrated to serrated (Tsuihiji et al., 2011, p. 17):

Adult Tarbosaurus serration count (Hurum and Sabath, 2003, p. 187):

The same goes for the premaxillary teeth. Using T. rex and Dryptosaurus as an example, in Stein (2021), there is a T. rex tooth that is identical to subadult, and adult, T. rex premaxillary teeth (Stein, 2021, p. 39 Figure 17, B). However, the specimen, TD-13-251, is small. It seems that the specimen could be a baby or a juvenile. Next to it, Stein shows a Dryptosaurus premaxillary tooth, TD-13-247, that is almost the same size as TD-13-251. This could be a baby or juvenile Dryptosaurus specimen.

Baby or juvenile T. rex premaxillary tooth TD-13-251 (Stein, 2021, p. 39 Figure 17, B):

Baby or juvenile T. rex premaxillary tooth TD-13-251, and baby or juvenile Dryptosaurus premaxillary tooth TD-13-247 (Stein, 2021, p. 39 Figure 17, B and C):

T. rex premaxillary tooth (Dalman et al., 2018, p. 132 Figure 10):

Subadult T. rex specimen "Stan's" premaxillary teeth (numbers 1 and 3) (Smith, 2005, p. 870 Figure 3 B, both in lingual/posterior views):

We can see that T. rex's premaxillary teeth seem to stay very wide throughout the animal's life. The ridges, with serrations on them, are located on the sides/lateral views of the teeth, and they don't seem to reach the base of the tooth (Smith, 2005, pp. 868-870, Figure 3, B; pp. 879-880). As for Dryptosaurus, the premaxillary teeth, and first maxillary tooth, have ridges on the posterior end of the teeth that are opposite of each other. Sometimes they either lack, or have very few, serrations (Carpenter, 1982, p. 130) (Molnar, 1978, p. 77) (Bakker et al., 1988, p. 24) (Larson, 2013, pp. 33-35). Interestingly, Dryptosaurus' ridges seem to meet the base of the teeth (pers. obs. in Molnar, 1978, p. 76 Figure 5, A; Carpenter, 1982, p. 128 Figure 7; Larson, 2013, p. 35 Figure 2.14), which is unseen in T. rex's.

Dryptosaurus specimen LACM 28471 first and second maxillary teeth (Molnar, 1978, p. 76 Figure 5, A and C):

Note: Premaxillary tooth is the first maxillary tooth (Carr and Williamson, 2004, p. 489). Notice that the ridges reach the base of the tooth.

UCMP 124406 tooth ("Aublysodon") (Carpenter, 1982, p. 128 Figure 7):

This tooth matches LACM 28471's first maxillary tooth, so I'm placing it as Dryptosaurus. The ridges also reach the base, and the ridges are located on the posterior end of the tooth (Figure 7, A).

T. rex's premaxillary teeth also lack a vertical/lingual/posterior ridge that runs down the middle of the tooth on the rear of it (Dalman et al., 2018, p. 134). One tyrannosauroid premaxillary tooth from Zanno et al., (2015) has this same feature, along with the ridges on the posterior end of the tooth that reach the base (pp. 132 and 134, Figure 2, G). The authors assigned the tooth to T. rex, but I'll assign it to Dryptosaurus . this ridge also seems to be visible in UCMP 124406, so it seems that Dryptosaurus had the posterior ridge on its premaxillary teeth, unlike T. rex. Gorgosaurus also has this ridge on the posterior side of its premaxillary teeth, and it's on the juvenile and adult specimens (Voris et al., 2022, Systematic Paleontology, Description of Juvenile Gorgosaurus Skulls, Dentition, para. 2). Seems that the lingual/posterior ridge doesn't disappear during ontogeny.

Baby Dryptosaurus premaxillary tooth FMNH PR 2902 (Zanno et al., 2015, p. 134 Figure 2, G). Scale bar is 1 mm:

Gorgosaurus' premaxillary teeth also have the lingual/posterior ridge, and this is present on juvenile and adult specimens (Voris et al., 2022, Systematic Paleontology, Description of Juvenile Gorgosaurus Skulls, Dentition, para. 2).

Some tyrannosauroid premaxillary teeth, YPM VPPU 023387 and 023475, from Dalman et al., (2018), have the groove on them as well, while one tooth, YPM VPPU 023469, doesn't and matches the morphology of T. rex's premaxillary teeth (p. 128 Figure 3; p. 134). This is interesting because, not only do we have some evidence that points to T. rex being in the Campanian-aged Two Medicine Formation (p. 126), but Dryptosaurus' premaxillary teeth matches another tyrannosauroid that are different from T. rex's.

Dryptosaurus' maxillary teeth were also ziphodont and labiolingually-compressed (Brusatte et al., 2011, p. 8). Carr (2020) said that "Nanotyrannus" also had "ziphiform" teeth (p. 48, Bite force and maturity: Tooth morphology). Seems that, if "Nanotyrannus" was Dryptosaurus, the teeth would have stayed ziphodont throughout the genus' lifetime.

In short, tooth morphology does not change drastically during ontogeny, like going from unserrated to serrated. As the individual animal grew larger, tooth serration size increased while serration/denticle count decreased.

Conclusions:
In an examination of multiple different tyrannosauroid genera, ontogenetic tooth loss was not observed and is demonstrated as well as most paleontologists may think. The best explanation for the evidence is that tooth count fluctuated between different specimens in a single genus and species of tyrannosauroid, resulting in individual variation. One specimen may have had a larger maxillary tooth count while another would have a larger dentary tooth count. Both of these differed regardless of the ages of the individuals. On the other hand, other specimens would have a tooth count identical to other specimens, especially in the dentary. This was also independent of age. Tooth morphology also doesn't change incredibly during ontogeny either, with adult specimens having an identical tooth shape as their juvenile counterparts. The same goes for T. rex when the Dryptosaurus ("Nanotyrannus") lancensis specimens are taken out of that genus. When this happens, T. rex has a very similar tooth morphology, and count, as Tarbosaurus'. Tooth size and serration/denticle counts are the only major features that seem to change during ontogeny for the teeth in tyrannosauroids.

Since there is no evidence of ontogenetic tooth loss, or a drastic change in tooth morphology, I will maintain my original hypothesis that the Dryptosaurus lancensis specimens do not belong in the species Tyrannosaurus rex. Checking the tooth count and tooth morphologies would be helpful in identifying, and separating, the two genera.

Links:
Carr (1999):

https://core.ac.uk/download/pdf/227005733.pdf

Currie (2003):

https://www.app.pan.pl/archive/published/app48/app48-191.pdf
Erickson et al., (2006) (Supplementary Materials):

https://science.sciencemag.org/content/sci/suppl/2006/07/11/313.5784.213.DC1/Erickson.SOM.pdf

Currie (2011):

https://www.researchgate.net/publication/40662060_Allometric_growth_in_tyrannosaurids_Dinosauria_Theropoda_from_the_Upper_Cretaceous_of_North_America_and_Asia

Larson (2013) (PP. 15-53):
https://www.geokniga.org/bookfiles/geokniga-tyrannosaurid-paleobiology.pdf
Mortimer, Mickey. The Theropod Database Blog. "Validity of Nanotyrannus." 2013. "#X," para. 2, "#7":
http://theropoddatabase.blogspot.com/2013/09/validity-of-nanotyrannus.html?m=1

Gorgosaurus:
Voris et al., (2022):
https://www.tandfonline.com/doi/full/10.1080/02724634.2021.2041651
Voris et al., (2019):

https://www.researchgate.net/publication/337602721_Reassessment_of_a_juvenile_Daspletosaurus_from_the_Late_Cretaceous_of_Alberta_Canada_with_implications_for_the_identification_of_immature_tyrannosaurids

Voris (2018):

https://prism.ucalgary.ca/bitstream/handle/1880/109240/ucalgary_2018_voris_jared.pdf?sequence=1&isAllowed=y