Re: Nanotyrannus

[pharrinj@PLU.edu]

On Sun, 7 Jan 1996 Dinogeorge@aol.com wrote:

> >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.

Points taken.  An intelligent response will have to wait until I can get 
my hands on the _Discover_ article on _N. lancensis_, which includes some 
nice photographs of dorsal and lateral views of the skul.


> 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_).

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.


> >>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. 

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.

> 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.
> 

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.

> 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.

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

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.

> 
> 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.

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. :-)

I like the BCF theory, to a point.  I do not like it in that very few 
candidates for "dino-bird" fossils have yet been found.  I also do not 
like the way Dinogeorge insists on every large theropod lineage being 
descended from separate "dino-bird" ancestors.  I see clear connections 
among known lineages yielding a tree along the following lines:

Theropoda (or Aves, or whatever)
   Coelophysidae
   Neotheropoda
      Ceratosauria
         Ceratosauridae
         Neoceratosauria
      Tetanurae
         Megalosauria
            Eustreptospondylidae
            Megalosauridae
            Spinosauridae
         Neotetanurae
            Carnosauria
               Sinraptoridae
               Allosauridae
               Carcharodontosauridae
            Coelurosauria (although I don't like the name)
               _Ornitholestes_ (an almost perfect link between 
                        allosauroids and dromaeosaurs)
               _Compsognathus_
               Dromaeosauroidea
                  Dromaeosauridae
                  Oviraptoridae
               Avialae
                  Archaeopterygidae
                  Paraves (new; "beside birds")
                     Tyrannosauroidea
                        Ornithomimidae
                        ?Dryptosauridae
                        ?Deinocheiridae
                        ?Labocaniidae
                        Tyrannosauridae
                     Troodontoidea
                        ?Elmisauridae
                        Archaeornithoididae
                        Troodontidae
                  Euaves ("true birds")
                     _Sinornis_ to modern birds

I have cheated in leaving out some problematic taxa such as _Segisaurus_ 
and _Xuanhanosaurus_, but you get the idea.  Troodontoids may be closer 
to euavians than tyrannosauroids, and _Avimimus_ might be either a 
paravian or a euavian.

     As always, comments and questions encouraged!

     Nick Pharris
     Pacific Lutheran University
     Tacoma, WA