Re: Nanotyrannus

[Dinogeorge@aol.com]

>On Sat, 6 Jan 1996 Dinogeorge@aol.com wrote:

>In a message dated 96-01-06 19:30:27 EST, pharrinj@PLU.edu
>writes:

>>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.

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_).

>>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.

>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.

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.

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.

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.

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

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

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.

>The long, sloping head is also seen in tyrannosaurids
>universally seen as basal (_Alioramus_, _Alectrosaurus_,
>_Aublysodon_).

_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.

>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.

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.