(News) Actual Tyrannosaurus hatchlings discovered? (Longrich et al., 2026).

Tyrannosaurus hatchlings (Walking with Dinosaurs, 1999, Episode 6):

It seems that, after Nanotyrannus has been vindicated as its own genus, some paleontologists have been trying to find actual young specimens of Tyrannosaurus. Recently, Longrich et al., (2026) has recently published a paper on what they believe to be hatchling specimens of the "tyrant lizard." I must say that these specimens are fragmentary, but there is one bone that I am very keen on describing to you because it vindicates something that I discovered a couple of years ago. 

The paper describes both Tyrannosaurus, Gorgosaurus, and "nanotyrannidae" hatchling specimens. I believe that the nanotyrannidae specimens should be called just Nanotyrannus, but I'll stay consistent with the name since it is used in the paper. I'm going to focus on the Tyrannosaurus specimens because the old boy needs to have some actual young specimens assigned to its genus (I wish it was "Baby Bob," but after what my friend "Luke" showed me on Instagram recently, I've decided that "Baby Bob" is a whole can of worms that I would like to ignore for now). The Tyrannosaurus hatchlings described in the paper are a "partial dentary," a bunch of "incisors" and "lateral teeth," and a "metatarsal III." All of these specimens were recovered from the "latest Maastrichtian" of the Frenchman Formation in Canada (3. Results: 3.1. Systematic Paleontology: Tyrannosaurus rex, Material). 

Metatarsal 3 (RSKM P2416.82) (Figure 1):

Estimated sizes of RSKM P2416.82 (Figure 2):

Partial dentary (FMNH PR 4920) (Figure 3):

Teeth (Figure 4):

The specimen that I want to focus on is YPM 55604, which is a premaxillary tooth. It has the typical characteristics seen in adult specimens of Tyrannosaurus: the carinae are on the lateral and lingual (or mesial and distal) sides of the tooth and not directly on the posterior side, albeit the mesial carina are moving onto the lingual/posterior side of the tooth. The distal carina are serrated, despite the mesial carina not being so. Typically, nanotyrannidae do not have serrated premaxillary teeth. However, Longrich has noted that some of the premaxillary teeth in the specimen BMRP 2002.4.1 ("Jane") contain "weakly developed" serrations/denticles at "the base." There is a wide mesial ridge/"apicobasal ridge" on the posterior side of the tooth, but it is very wide unlike in the albertosaurinae and nanotyrannidae (3. Results: 3.1. Systematic Paleontology: Tyrannosaurus rex, Teeth, para. 2-3).

Thankfully, Longrich et al. described a nanotyrannid hatchling premaxillary tooth for comparison. The authors didn't know what taxonomic name to call it because they believe, or Longrich does at least, that Stygivenator is possibly valid. The specimen is catalogued as YPM 55587. The "carinae" are "lying on the posterior surface" of the tooth, and "are unserrated." Only a few minute serrations are present "basally" on the "distal carina." This hatchling "tooth closely resembles that of the adults." The authors stated that any teeth previously labelled as "Aublysodon" belonged in the nanotyrannidae clade (3. Results: 3.1. Systematic Paleontology:  Nanotyrannidae indet., Tooth, para. 2-4). I would place YPM 55587 in the genus Nanotyrannus sp., but it's unclear if it belongs in the species N. lancensis or N. lethaeus. 

YPM 55604 (Figure 4a-e):

YPM 55587 (Figure 7):

If you didn't know yet, I wrote a paper for Academia a couple of years ago that said that baby specimens of Tyrannosaurus had premaxillary teeth identical to the adults. The same goes for Nanotyrannnus. The Tyrannosaurus tooth in question is UCMP 119853, and the Nanotyrannus baby specimen is UCMP 124406. I also stated that on this blog before I wrote the paper. I remember "Luke" telling me that people blew my paper off, and said that the Tyrannosaurus tooth in my paper probably came from an adult instead of a juvenile.

Figure 2 from Brunson (2023): 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 8 from Brunson (2023): 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:

Now, it seems that my idea about tooth morphology being consistent throughout ontogeny for both genera is valid. Thus, tooth morphology is a good strategy for differentiating between Tyrannosaurus and Nanotyrannus, no matter the size or age of the specimens in each taxa. 

Link:
Tyrannosaurus hatchlings (Walking with Dinosaurs, 1999, Episode 6) photo: 
https://share.google/6QmnCYWXMiXrkysST
Longrich et al., (2026):
https://www.mdpi.com/2079-7737/15/13/1090
My blogpost on tyrannosauroid teeth:
https://psdinosaurs.blogspot.com/2022/05/tyrannosauroids-did-not-lose-teeth.html
My Academia paper on a Tyrannosaurus hatchling premaxillary tooth (Brunson, 2023):
https://www.academia.edu/96575022/A_Baby_Tyrannosaurus_rex_Premaxillary_or_First_Maxillary_Tooth