Nanotyrannus; Gigadispute

From: pharrinj@PLU.edu

On Sat, 6 Jan 1996 Stang1996@aol.com wrote:
> Now Dasplateosaurus torosus is recognised as an Albertosaurinid, and
> Dinotyrannus (=Albertosaurus) megagracilis and Nanotyrannus lancensisas
> Tyrannosaurinids.
>
> Is this right? Is this making any sense?

Well, I tend to think of Nanotyrannus as more primitive than any other known group of tyrannosaurs. Its wedge-shaped skull, narrow beak, large orbits, forward-pointing parasphenoid, and infratemporal fenestra without any large rostral process of the quadratojugal and squamosal make it the most troodont- or ornithomimid-like tyrannosaur known (i.e. the most primitive). That not all of these features are strictly size-related can be seen by examining the skulls of other small tyrannosaurids (Alioramus remotus, Gorgosaurus sternbergi, Maleevosaurus novojilovi), which have broad snouts and, in particular, large rostral processes across the infratemporal fenestra.

From: Dinogeorge@aol.com

Having a widely expanded occiput relative to snout width (best seen in ventral view in Gilmore's 1946 paper), resulting in orbits having a forward-pointing component, seems to be a derived feature shared by Nanotyrannus in common with Dinotyrannus and especially Tyrannosaurus. Other putative tyrannosaurinid synapomorphies include a lacrimal with no horn and a ventrally deflected occipital condyle.

From: pharrinj@PLU.edu

Well, that makes sense if you think of tyrannosaurs as "carnosaurs"; those features are indeed very advanced for that group. But what if one considers instead the more likely scenario that tyrannosaurs are protobirds? Troodonts and other protobirds tend to have binocular vision and narrow beaks like that of Nanotyrannus lancensis. Other lineages of tyrannosaurs apparently broadened their snouts to give them a more forceful head-on attack and a bigger bite. I have no doubts about the extreme breadth of the occiput of N. lancensis relative to its beak width: this is easy to see in the high-quality photographs in "Inside the Head of a Tiny T-rex" in Discover magazine; I just consider this to be a basal condition for tyrannosaurs.

From: Dinogeorge@aol.com

Which "protobirds" might have had binocular vision? The only one I can recall is Protoavis, whose skull anatomy is presently in dispute. Binocular vision is a specialization even in modern birds (e.g., owls and raptors). Most modern birds have laterally directed visual fields that require them to turn their heads sideways as an aid to seeing, with little visual-field overlap (though not absolutely none) across the beak.

From: pharrinj@PLU.edu

Sorry for the communication breakdown. I was referring to Paul-style "protobirds", i.e. troodonts, oviraptors, dromaeosaurs, archaeopterygians, ornithomimids, many of which show varying degrees of binocular vision.

From: Dinogeorge@aol.com

All the earlier tyrannosaurids of Mongolia and western North America lack any hope of binocular vision; their eyes were directed laterally with almost no forward component, and their occipita were not dramatically broadened as in Tyrannosaurus and Nanotyrannus. In the BCF scenario, tyrannosaurids were the cursorial descendants of volant dino-birds in which the number of wing digits was reduced to two--a continuation of the progressive loss of the wing digits that began with the origin of Aves in the Triassic. The earliest-known tyrannosaurian in this scenario is Compsognathus, a cursorial, secondarily flightless, didactyl-winged dino-bird. Then come Tonouchisaurus (a meter-long didactyl theropod from the Early Cretaceous of Mongolia not yet described; skull, alas, unknown), Itemirus, the shanshanosaurines, and the tyrannosaurines.

From: pharrinj@PLU.edu

I'd really like to agree with you on the two-fingered front, but, in the face of other evidence, I can't. Compsognathus, while admittedly looking very like a small tyrannosaur, lacks certain advanced features seen in tyrannosaurs, particularly arctometatarsaly. In this respect it is also more primitive than Archaeopteryx, which has three fingers and would therefore have to have split off from the lineage leading to modern birds BEFORE Compsognathus + tyrannosaurs.

Compsognathus, it is claimed, had a manual phalangeal formula of 2,2,0,0,0, more advanced than tyrannosaurs (2,3,0,0,0) and unlike any other theropod. Anyway, the manual osteology of C. longipes is in dispute. I think it pretty shaky to base this theory of tyrannosaurian origins on so fragmentary a hand.

From: Dinogeorge@aol.com

You're quite right when you say that tyrannosaurids acquired (ventrally) broadened snouts for a more forceful bite. This is a derived character for the family Tyrannosauridae. But the laterally expanded occiput and other adaptations for binocular vision (loss of the albertosaurinid lacrimal horn, which would have interfered with the overlap of the visual fields, and ventrally deflected occipital condyle, which allowed the visual fields to overlap across the top of the muzzle) are in turn more inclusive derived characters defining the tribe Tyrannosaurini.

Bakker et al. illustrate the squamosal-quadratojugal junction of Nanotyrannus as more expanded than seen in the photos in Gilmore's 1946 paper; I suspect the portion within the infratemporal fenestra might be broken off or buried in matrix in the type specimen. But even if this feature is real, it could be a derived character state. The earliest-known tyrannosaurids (well above Compsognathus) from Mongolia and western North America all have the expanded squamosal-quadratojugal junction.

From: pharrinj@PLU.edu

Actually, even the infratemporal fenestra itself is small and situated low on the cheek, as I recall, like that of an ornithomimid.

Outgroups to tyrannosaurids (troodonts, ornithomimids with the exception of Pelecanimimus, birds, archaeopterygids, dromaeosaurs) also tend not to have very strongly developed lacrimal horns.

From: Dinogeorge@aol.com

I'd say the closest outgroup to Tyrannosauridae is Compsognathidae.

From: pharrinj@PLU.edu

And I'd say that is incorrect. Tyrannosaurs share many features with (at least basal) ornithomimids not shared with Compsognathus, including (but not limited to):

1. arctometatarsaly
2. rugose nasals
3. lacrimal eminences
4. expansion of the quadratojugal-squamosal junction
5. D-cross-sectioned premaxillary teeth.

From: Dinogeorge@aol.com

The earliest tyrannosaurians also have no lacrimal horns, which is the character state right up through tribe Tarbosaurini. The lacrimal horns appear only in tribe Albertosaurini, the genera

Albertosaurus, Gorgosaurus, Daspletosaurus, and the unnamed genus for FMNH PR308. Then they were secondarily subdued in tribe Tyrannosaurini.

From: pharrinj@PLU.edu

The long, sloping head is also seen in tyrannosaurids universally seen as basal (Alioramus, Alectrosaurus, Aublysodon).

From: Dinogeorge@aol.com

Aublysodon is a nomen dubium based on a single tooth. You're probably thinking of Stygivenator, the "Jordan theropod."

In tarbosaurinids and albertosaurinids, the occipital condyle is not deflected ventrally nearly as much as in the tyrannosaurinids, so I think tyrannosaurines of the former two tribes carried their skulls directed more horizontally than did the tyrannosaurinids. But the orientation of the condyle is difficult to confirm in tarbosaurinids because that part of the skull is seldom illustrated. I had to examine photos, and I'm still not quite satisfied about my knowledge of the distribution of this character in the family.

From: pharrinj@PLU.edu

I stand by my contention that the near lack of a rostral quadratojugal-squamosal process across the infratemporal fenestra (seen in ALL other tyrannosaurid skulls I've seen illustrated but more weakly developed in troodonts and ornithomimids) is a very primitive feature.

From: Dinogeorge@aol.com

Your points are very well taken. It is entirely possible that Nanotyrannus represents a "primitive" tyrannosaurid lineage separate from all the other tyrannosaurids, but I have less trouble presently believing it is a small member of the same tyrannosaurine clade as Tyrannosaurus and Dinotyrannus and not particularly closely related to Troodon, etc.

From: pharrinj@PLU.edu

No one is saying that N. lancensis is particularly closely related to Troodon. I'm saying that all tyrannosaurs are, and that Nanotyrannus retains some troodontlike features. :-)

From: Dinogeorge@aol.com

I've always thought of the narrow muzzle and downwardly deflected skull relative to the occipital condyle of the tyrannosaurinids as derived characters, because they occur only in the latest Late Cretaceous tyrannosaurines (Lance/Hell Creek). The top and back of the skull of Troodon are markedly different from those of any tyrannosaurid. Troodon frontals are huge, and the postorbital and squamosal articulate in a wildly different fashion from the way they articulate in tyrannosaurids. The occiput of Troodon is not expanded laterally and in fact is even narrower than the skull width at the orbits. I really don't see a particularly close relationship between Troodon and the tyrannosaurids, and I think it likelier that the narrowing of the muzzle in troodontids and tyrannosaurids was convergent.

The rugose dorsal surface of the nasals of Nanotyrannus makes it a tyrannosaurine. In your scenario, however, this character would have had to develop convergently in Nanotyrannus and Tyrannosaurinae, because the nasals are smooth in all shanshanosaurines in which the nasals are at least partially known (Shanshanosaurus, Alectrosaurus, Stygivenator).

From: pharrinj@PLU.edu

Not at all. I think the nasal rugosity was lost in shanshanosaurs, because the primitive tyrannosaur Alioramus has EXTREMELY rugose nasals and because nasal rugosity is even found in the ornithomimidae, the closest outgroup to tyrannosaurs in my scheme.

From: GSP1954@aol.com

The "Nanotyrannus" skull is definitely crushed and distorted, and is probably a baby T. rex.

From: Adam Yates [zooamy@zoo.latrobe.edu.au]

So what about the advanced state of fusion between the bones of the skull? Is this just an artifact resulting from the sutures being plastered over by the restorer?

From: pharrinj@PLU.edu

Paul makes a similar (and similarly unconvincing) case in _Predatory Dinosaurs of the World_. He says the back of the skull is crushed from top to bottom, while the front is crushed from side to side. Neat trick!

In the pictures from the _Discover_ article, the skull looks nicely symmetrical and quite 3-dimensional. Also, the dentaries look like they articulated (maybe even fused) at about midlength, indicating that the beak really was very narrow.

From: Jeff Poling

Bakker did a CAT scan of the Nanotyrannus skull which was able to distinguish between fossil bone and plaster. The fusion is definitely not from the sutures being plastered over.

From: Dinogeorge@aol.com

I have the feeling that skull-bone fusion may not be as good an indicator of juvenility or subadulthood as we've thought. If Nanotyrannus had a bimodal growth pattern, say rapid growth from hatchling to subadult, then slower indeterminate growth from subadult to large adult, it could well have had skull-bone fusion already at the subadult stage. I'm just waving my hands here, however.

From: pharrinj@PLU.edu

How could a Nanotyrannus skull get from the known size to the enormous size of Tyrannosaurus skull after the skull bones had fused?!!

From: Dinogeorge@aol.com

Although the teeth are plastered in, it still seems to have the wrong tooth count to be a T. rex; it may be a juvenile of a different species of Tyrannosaurus.

From: pharrinj@PLU.edu

Again, what about the fused skull elements, the forward-pointing parasphenoid (platelike even in juvenile "Albertogorgon"), and the ENORMOUS transverse crest atop the braincase?