Re: Nanotyrannus

[pharrinj@PLU.edu]

This is my best approximation of the post I sent re: albertosaurinids on 
Jan 6.

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.
> 
> 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.

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

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

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.

    Hasta luego!

     Nick Pharris
     Pacific Lutheran University
     Tacoma, WA