Re: Haha, more questions

[longrich@alumni.princeton.edu]

On Saturday, June 15, 2002, at 04:33 AM, Dino Rampage wrote:

> Many, many thanks to those kind souls who have offered me much help on 
> the evolutionary relationships * ecological niches on Mesozoic mammals. 
> I still have even more questions... you know, I never seem to run out 
> of questions :-)
>
> 1) What is Dryptosaurus' relationship to other theropods? All seem to 
> agree that it is a non- maniraptoran coelurosaur, but is it close to 
> the base of Coelurosauria, or more closely related to the Tyrannosaurs? 
> I've seen it mentioned somewhere on the list that Dryptosaurus was a 
> basal Tyrannosauroid (note the word "TyrannosaurOid")

Dryptosaurus is a very hard one to figure out. There is very little 
material, and what is there is pretty bashed up. The metatarsus has an 
interlocking structure, but it looks to me like pretty much the same 
thing you see from Torvosaurus to Ornitholestes. That, and given the 
position of the astragalus, its tetanuran. The slenderness of the tibia, 
given its size, seems to suggest coelurosauria. Maybe the most 
informative thing about it is the provenance- Maastrichtian of North 
America according to Dinosauria. We've never found any big theropods 
other than tyrannosaurids from North America in the Late Cretaceous that 
I can think of (though of course that's on the other side of the 
continental seaway, and there are still mysteries out there). Hm. I can 
see basal tyrannosaur as being perhaps the best tentative ID we can come 
up with (the humerus does look pretty straight) though I'm still 
skeptical there's enough to ID it positively.  Anyone out there (TH in 
particular) know how the teeth compare (i.e. are they any fatter than 
normal like youd expect in a tyrannosaur)?

> 2) How is Troodon related to the other coelurosaurs? They used to be 
> called Arctometatarsalia, then Bullatosaurs, and now their placement is 
> somewhat uncertain. Do they occupy a place within the maniraptoriforms, 
> like the tyrannosaurs & ornithomimids, or within the maniraptors, or as 
> Oyvind M Padron's cladogram states, within the Deionychosauria itself??

	They jump around a bit. Holtz has recovered them next to 
Ornithomimidae, one of the AMNH studies had them as the sister group to 
oviraptorosaurs+therizinosaurs, there was also a study which put them 
closer to birds than dromaeosaurs, and some just below dromaeosaurs... I 
think that the most frequently advanced hypothesis, and one we've seen 
quite a bit recently is that they are in Deinonychosauria (this is a 
return to the traditional view originally advanced by Ostrom and 
others)  and I think this is likely to be the case although a lot of the 
similarities linking them to dromaeosaurs can also be seen in Rahonavis 
which may complicate things.

> 3) Alvarezsaurs. Weird avialans, weird troodont relatives, or weird 
> ornithomimid relatives?

	I'm not sure anybody is getting them as avialans these days. The 
main change here is Confuciusornis which is pretty clearly between 
Archaeopteryx and the ornithothoracines (e.g. pygostyle, flattened 
phalanges of the major digit) but which does not show some of the 
features which were supposed to link alvarezsaurs to birds such as an 
incomplete postorbital bar or loss of a pubic symphysis. The discovery 
of basal alvarezsaurids also complicated things in that a number of the 
purportedly birdlike features of the Mononykines weren't present 
(absence of a fibular articulation, absence of hyposphene-hypantrum 
articulations). Some of the other features used to support this 
assignment weren't viewed in a larger context either- unserrated teeth 
are present in part or all of the jaw in many theropods for example, 
while short distal caudal prezygapophyses are found in troodontids, 	In my 
opinion, alvarezsaurids have nothing to do with ornithomimosaurs. The 
reduced supracetabular crest, short distal caudal prezygapophyses, 
enlarged hypapophyses, elongate fourth toe, reduced brevis fossa etc. 
all indicate that they are maniraptorans, though probably not as 
advanced as dromaeosaurs and troodontids. Some of the alleged 
alvarezsaur-ornithomimosaur synapomorphies may not even be primitive for 
ornithomimosaurs: Harpymimus has a relatively short first 
metacarpal(only a little more than 50% of MC II)  and Garudimimus, as 
illustrated in the Dinosauria, has a small prefrontal not the enlarged 
one which is supposed to link alvarezsaurids and ornithomimosaurs. Other 
features aren't unique to these two- the "unusual flattening of the 
internarial bar" I've definitely seen in a Velociraptor skull.
	Dunno. Sure they show a few similarities to ornithomimosaurs. They 
also show similarities to spinosaurs, or ornithischians, or armadillos 
and you could argue that they were related to them if you were willing 
to overlook a lot of stuff.
		In my opinion, alvarezsaurids are too primitive to represent 
troodontid relatives or Avialae, but they are advanced maniraptorans, 
about as birdlike as oviraptorids in many respects where they aren't too 
transformed to compare. The coracoid looks primitive but the whole 
forelimb is so incredibly modified... they are just really, really weird 
no matter where they go and we're still trying to figure them out. 
Troodontids on the other hand are advanced enough that with 
dromaeosaurids they may represent flightless Avialae (and Sinovenator 
really shows this) and I've recovered this in a cladistic study, 
although a few steps or revised codings could change that. I do think 
the evidence that dromaeosaurids and troodontids are flightless birds is 
much better than it is for alvarezsaurids(which is kind of funny because 
for a long time a lot of people had no problem at all with that one). 
And given that oviraptorosaurs don't seem to be avialan but could be 
secondarily flightless given their winglets etc. the same could be true 
for alvarezsaurids. Who knows, its a mess.

	Theropod systematics is constantly changing as we get new data. Ten 
years ago there was disagreement as to whether therizinosaurs were even 
saurischians or ornithischians; these days they are universally 
understood to be coelurosaurs. Its only very recently that we've even 
recognized groups like alvarezsaurs or abelisaurs, either. While there 
is a lot of well-founded consensus and agreement to be sure, I think 
there are plenty of surprises in store. It's absolutely chilling (in a 
good way) to think just how *little* time we've actually spent digging 
up the dinosaur-bearing Liaoning strata for example...

4) Is Meagraptor believed to be an aberrant giant Gondwanan dromaeosaur?
	You'd sure think so looking at the claw. However the ulna doesn't 
look even remotely like a maniraptoran, let alone a dromaeosaurid, in 
its general outline and the shape of the proximal end. Of course, some 
parts of the dromaeosaur Achillobator don't look remotely maniraptoran 
either, so maybe one shouldn't be too hasty.