For a long time now, the debate over the identity of Nanotyrannus has, to my understanding, leaned towards it being a juvenile Tyrannosaurus rex rather than a separate species.
Now, a new study looking at the famous Duelling Dinosaurs fossil from Hell Creek determined that the supposed juvenile Tyrannosaurus involved is in fact a fully grown adult, based on bone growth patterns. Further, they did a detailed analysis of the skulls of the dinosaurs identified as potential Nanotyrannus, and they found differences in things like the placement of blood vessels between these animals and definite adult Tyrannosaurus - and found differences that don’t seem to be possible to explain ontogenetically, like the placement of particular blood vessels relative to bone.
They seem to make a pretty compelling case for Nanotyrannus being a separate genus (potentially with multiple distinct species). But I think the most interesting implications are ecological. For a long time, there has been the idea that there simply were no medium sized carnivores in the latest Cretaceous of North America, with juvenile Tyrannosaurus filling the niches that medium predators would normally fill. If Nanotyrannus is truly a distinct species, this means that places like Hell Creek had a more typical ecosystem than is sometimes portrayed.
Hm, I knew of the debate (there’s a Nanotyrannus fossil at the Cleveland Museum of Natural History), but I hadn’t heard that there was ever a point where “juvenile Tyrannosaurus” was the consensus.
I’ve also heard speculation (at the Museum of the Rockies, which has extensive collections of both ceratopsians and tyrannosaurs) that some of the ceratopsians that are typically described as different species might be just different ages of the same species. Presumably investigations like this could also be applied there.
Maybe consensus is overstating things; that was the impression I had from the PopSci I consumed growing up in the 90s and 2000s.
Here’s an old tweet from Darren Naish (whose paleontology blogs I’ve been reading on and off since I was in high school) that supports this, I think:
Jack Horner at the Museum of the Rockies is a big proponent of these growth stage ideas. Aside from Nanotyrannus, there are a few other dinosaurs he’s cast doubt on.
He’s suggested that Dracorex and Stygimoloch are juvenile Pachycephalosaurus:
Funnily enough, I didn’t realize that Dracorex was named by Bakker. Bakker and Horner have a bit of a friendly rivalry - Horner advised on the Jurassic Park movies, and in the second movie there’s a scene where the T Rex hits the Bakker analogue palentologist of that movie. Bakker supposedly sent a still of that shot to Horner and wrote, “I told you Tyrannosaurus was an active hunter!”. I guess it’s understandable they’d be rivals if one is potentially invalidating species named by the other.
The other big dino I heard Horner make this claim about is Torosaurus being an older Triceratops. Torosaurus has frill holes but Triceratops does not, which makes this one especially dubious, but the idea was that maybe the frill thins out as the Triceratops ages until it opens completely.
My understanding was that the Torosaurus theory was pretty widely rejected, the Nanotyrannus theory was pretty much accepted, and the Pachycephalosaurus theory was somewhere in between, leaning towards acceptance. (Although I’ve also heard rumors that there’s a very well preserved juvenile Pachycephalosaurus in private hands and it doesn’t look anything like Dracorex, so hopefully we get more evidence soon).
