So for a couple of years now, I have stated that "Nanotyrannus" was actually the genus Dryptosaurus, or a sister taxon to Dryptosaurus and the two genera form a clade within tyrannosauroidea. Looks like my perseverance has paid off!
I have been alerted that professional paleontologist Nicholas Longrich (or Nick Longrich) wrote a message on Facebook on October 18. He is uploading a preprint to BioRXiv about "Nanotyrannus." In his message, he states that there are 77 characteristics that separate "Nanotyrannus" from T. rex, that the specimens show signs of slow maturity instead of rapid growth associated with T. rex, and that "Nanotyrannus" is a tyrannosauroid and not a tyrannosaurinae like T. rex!
Longrich's message from Facebook:
Pic 1:
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https://m.facebook.com/story.php?story_fbid=pfbid02LVfzbMwkXg7feHtuCP78YhwqzfipLjLjfDCbj3gibcVhCS6JhGTSCYXmazNtrNHol&id=556574124
I'll be waiting for his preprint "with great interest."
Here's my own work on "Nanotyrannus" for comparison:
"Nanotyrannus" is Dryptosaurus:
The preprint is here (Longrich and Saitta, 2023):
V2:
Review:
1. Highlights:
1.) First off, the authors said that tyrannosauroids reached peak diversity in the late Cretaceous, which means that there is ample opportunity for more than one tyrannosaur genus to coexist in one place. The best examples are Gorgosaurus and Daspletosaurus in North America, and Tarbosaurus (or Tyrannosaurus bataar) and Alioramus (pp. 7-8; p. 9 Table 3). They also raise the question as to how likely it would be for only one genus of large predator to reign uncontested in its environment, saying that having two genera of tyrannosaurs in the same environment is more than likely to occur (p. 10).
2.) Second, no other specimens fill in the morphological gaps between the "Nanotyrannus," and T. rex, specimens (pp. 10 and 15; p. 15 Figure 3).
3.) The authors then go on to state 77 morphological differences between the "Nanotyrannus" and T. rex specimens (pp. 10-14). Some of the traits have been mentioned elsewhere in other publications (p. 10). I will say that the authors mentioned the lingual bar (p. 13), but they didn't state that it covers the first alveoli in "Nanotyrannus" and the first two in T. rex. The authors said that the bar is "deep posteriorly" in "Nanotyrannus," but is "narrow and tapered relative to the anterior of the toothrow" in T. rex (p. 13).
The placement of the lingual bar near the anterior of the dentary is a huge trait to help identify tyrannosauroid and tyrannosaurid, or tyrannosaurinae, taxa. I've stated this on the blog many times, and in my paper that described BHI 6439, but I'll reiterate it here: the lingual bar covers the first alveoli in basal tyrannosauroids and tyrannosaurids, but it covers the first two in tyrannosaurinae. This was first mentioned in Dalman and Lucas (2017). The lingual bar covers the first alveoli in "Nanotyrannus," but the first two alveoli in T. rex. This is seen in BHI 6439 (a baby T. rex specimen) and FMNH PR 2081 (an adult T. rex specimen).
Figure 5 from my "A Description of the Baby T. rex Specimen BHI 6439" paper: T. rex dentaries in medial views. 5A: BHI 6439. 5B: FMNH PR 2081 from Brochu (2003) (p. 42 Figure 41). 5C: BMNH R7994 from Dalman and Lucas (2017) (p. 24 Figure 10). 5D: RSM P2523.8 from Persons IV et al., (2019) (p. 668 Figure 18B). Numbers indicate the position of the first three teeth in the dentary. The teeth in the dentary of RSM P2523.8 were numbered by the authors of Persons IV et al., (2019). Green arrows indicate the lingual bar covering the first two teeth/alveoli. The red arrows show that the lingual bar drops at the third alveolus/tooth. The position of the lingual bar doesn’t change during ontogeny. Abbreviations: “lb” is the lingual bar, “mg” is the Meckelian groove, “idp” is the interdentary plate, and “dp” are the dental plates.
All in all, the characteristics in the "Nanotyrannus" specimens do not appear as juvenile characteristics of T. rex. Using other tyrannosaur taxa, like Tarbosaurus/Tyrannosaurus bataar, baby and juvenile tyrannosaurs have characteristics seen in mature individuals. Although some characteristics in "Nanotyrannus" could potentially be juvenile in nature, a comparison with juvenile
Tarbosaurus/Tyrannosaurus bataar, and Gorgosaurus, specimens, demonstrate that they do not undergo massive transformations during ontogeny like T. rex supposedly would. Thus, the growth series provided by Carr (2020) is incorrect (pp. 15-16).
I have also stated that the teeth, and dentary, characteristics of T. rex did not change during ontogeny either. The baby T. rex premaxillary tooth UCMP 119853 had an identical morphology to the premaxillary teeth seen in the adults (carina on the labial and lingual sides, and being serrated). It was also distinct from the "Nanotyrannus" premaxillary teeth (two carinae on the distal side of the tooth, unserrated, and a distal ventral ridge in the middle of the tooth).
Figure 11 from my "A Baby Tyrannosaurus rex Premaxillary, or First Maxillary, Tooth" paper. 11A is UCMP 119853 (T. rex premaxillary tooth, 8 mm), and 11B is UCMP 124406
(cf. Dryptosaurus/"Nanotyrannus" premaxillary tooth, 6.6 mm). 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:
The larger baby T. rex specimen, BHI 6439, had 13 teeth in its dentary, and the lingual bar covered the first two alveoli. This is seen in adult specimens as well. "Nanotyrannus" had 16-17 teeth in its dentary, with the lingual bar covering the first alveoli only. This is seen in BMRP 2002.4.1 ("Jane") (and "Zuri," but I haven't used that specimen yet. I will in the future).
Figure 9 from my "A Description of the Baby T. rex Specimen BHI 6439" paper: Close-ups of cf. Dryptosaurus specimen BMRP 2002.4.1 and T. rex specimen BHI 6439 (reversed). In 9A, the green arrow indicates that the lingual bar covers the first alveolus only. The red arrows show that the lingual bar drops at the second alveolus, and past the third one. In 9B, the green arrows show that the lingual bar covers the first alveoli and second tooth. The red alveoli shows that the lingual bar drops at the third tooth. Abbreviations: “lb” is the lingual bar, “mg” is the Meckelian groove, and “idp” is the interdentary plate.
Therefore, young T. rex specimens didn't go through a rapid transformation as proposed by Carr (2020) (tooth loss during ontogeny, tooth morphology transformation, etc.).
4.) Larson (2013) noted that BMRP 2002.4.1 ("Jane") had skeletal fusion in some of its bone, meaning that it was nearing maturity (pp. 17-21). Longrich and Saitta said that "not all skeletal fusions are associated with maturity," the axial, scapulacoracoid, and pelvis, fusions seen in BMRP 2002.4.1 ("Jane") signify that that specimen was reaching maturity (p. 17).
5.) Rugosities on BMRP 2002.4.1 ("Jane"), KU 155809, and the SWAU specimen ("Zuri"), signify that these specimens were reaching maturity. There is "relatively weakly developed rugosity" on the juvenile T. rex specimen LACM 23845 (a 14-year old individual [Erickson et al., 2004]) (p. 18; p. 29 Table 5).
6.) The lack of an EFS (External Fundamental System) rules out that BMRP 2002.4.1 ("Jane") and BMRP 2006.4.4 ("Petey") are old adults, "but does not rule out the possibility that these animals are young adults, approaching full size." The T. rex specimen "Sue" weighed 7,930 kg before getting the EFS, but weighed only 8,223 kg when she got the EFS and then died. The animal only gained "around 300 kg." The EFS only appears when a specimen reaches full maturity, thus BMRP 2002.4.1 ("Jane") and BMRP 2006.4.4 ("Petey") were probably young adults (p. 18). Although no specimen has an EFS, a "Nanotyrannus" specimen can be found someday having one. It should be noted that only one specimen of Daspletosaurus, Albertosaurus, and T. rex, have an EFS (p. 29). Thus, more specimens need to be found.
7.) The fact that BMRP 2002.4.1 ("Jane") and BMRP 2006.4.4 ("Petey") "show low growth rates" due to the "narrow spacing of LAGs," and not "exceeding 800 kg/yr," means that these were not rapidly-growing juvenile T. rex specimens. In fact, the "Nanotyrannus" specimens didn't exhibit growth rates "exceeding 150 kg/yr for the last few years of life." Juvenile T. rex specimens "are predicted to have very high maximal growth rates as they approach 1000 kg/yr." This can be estimated from "Sue" as well. Thus, the hypothesis that BMRP 2002.4.1 ("Jane") and BMRP 2006.4.4 ("Petey") were juvenile T. rex specimens is incorrect (p. 19; p. 21 Figure 5). The growth curves for the two "Nanotyrannus" specimens do not match with the growth curves of T. rex (p. 24 Figure 7). The estimated adult body mass for BMRP 2002.4.1 ("Jane") is about 1250-1600 kg, while BMRP 2006.4.4 ("Petey") is 700-1100 kg (p. 28).
*8.) This is the best part to me. The authors did a phylogenetic analysis of "Nanotyrannus," and found it as a basal tyrannosauroid! It is not a tyrannosaurid, nor a tyrannosaurinae. This is alongside Dryptosaurus,
Appalachiosaurus, and Alioramus (pp. 26-27). I've been saying this four about two years now! Although the authors did not say that "Nanotyrannus" is the genus Dryptosaurus, they still say that "Nanotyrannus" is a basal tyrannosauroid alongside Dryptosaurus. I have also stated this. That works for me too! I'm still going to say that "Nanotyrannus" is either cf. Dryptosaurus aquilunguis, or that "Nanotyrannus" and Dryptosaurus are sister taxa within tyrannosauroidea.
Figure 9 (phylogenetic chart) from Longrich and Saitta (2023) (p. 27):
Figure 9A:
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In both charts from Figure 9, "Nanotyrannus" is situated outside of tyannosauridae.
2. Problems:
1.) I will say that there are a couple of problems that I had with the paper. First, I am surprised that there weren't any figures of the bones aside from the skull of CMNH 7541.
2.) The authors state that only one juvenile T. rex has been discovered, which is LACM 23845 (pp. 18, 26, and 29). There are actually many baby and juvenile T. rex specimens that do not exhibit "Nanotyrannus" traits. Here's my own growth series for T. rex:
*UCMP 119853: 7 feet (2.1 meters):
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*UCRC-PV 1: 26 feet (8.0 meters);
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BHI 3033 ("Stan"): 40 feet (12.2 meters) (18 years old).
CM 9380: 42 feet (12.9 meters) (22 years old).
FMNH 2081 ("Sue"): 45 feet (13.6 meters) (28-33 years old).
The three specimens with asterisks next to them are specimens that are smaller in, or are comparable in, size to the "Nanotyrannus" specimens "Jane" and "Petey." LACM 23845 is only 14 years old while BMRP 2006.4.4 ("Petey") is 15 (Woodward et al., 2020). In fact, UCMP 119853 is smaller than other more complete "Nanotyrannus" specimens, which means that it was younger as well. There are other juvenile T. rex specimens as well, like TCM 2001.90.1 ("Bucky") which is 16 years old.
In the middle:
1.) This next remark is not really a problem, nor is it really a highlight. The authors say that some of the "Nanotyrannus" specimens ("Dueling Dinosaurs," and LACM 28471) might actually be a third genus of tyrannosauroid: Stygivenator (pp. 29-30). I'm not entirely against that idea, but to keep things simple for now, I'm going to keep the "Nanotyrannus" specimens as either cf. Dryptosaurus aquilunguis or "Nanotyrannus lancensis." Heck, Dryptosaurus lancensis might even work.
1st Conclusion:
In summation, Longrich and Saitta (2023) confirm the majority of my hypothesis that "Nanotyrannus" is a basal tyrannosauroid, not a tyrannosaurid or tyrannosaurinae. T. rex doesn't go through a major transformation during ontogeny, and that two tyrannosauroids coexisted in North America during the late Maastrichtian.
Other sources used:
Larson (2013):
https://www.researchgate.net/publication/289687970_The_case_for_Nanotyrannus
Woodward et al., (2020):
https://advances.sciencemag.org/content/6/1/eaax6250
Update (1/3/24):
Longrich's and Saitta's paper has been officially published:
https://www.mdpi.com/2813-6284/2/1/1
They have figures, and more data, in the paper. Of particular note are:
1.) They stated that "Nanotyrannus'" lingual bar on the medial side of the dentary covers the first alveolus, while T. rex's lingual bar covers the first two alveoli (Figure 12):
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2.) No amniote (any animal that has an egg during its embryonic stage of life) has their hands shrink, or reabsorbed, during ontogeny (p. 31):
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Description of UCMP V84133 part 1 (p. 43):
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Link:
Holtz's Tweet (1/4/24):
https://x.com/tomholtzpaleo/status/1742968130995945476?s=61&t=DgZNoZU2b-DFLtG5TI6cbQ
"Nanotyrannus" alongside Dryptosaurus, Appalachiosaurus, Alioramus, and other basal tyrannosauroidea taxa (p. 47 Figure 32; p. 49 Figure 34A).
Nanotyrannus is a basal tyrannosauroid (p. 46):
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Figure 33B:
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As I have been saying for almost three years now, "Nanotyrannus" is a basal tyrannosauroid. I'm still certain that it is either Dryptosaurus (as D. aquilunguis or even D. lancensis), or it is a sister taxon to Dryptosaurus. I will be writing more about that in the future. Always keep an open mind and do your own research, ladies and gentlemen!
