Wednesday, May 09, 2007

Guanlong wucaii = Monolophosaurus jiangi?

In paleontology, ontogeny sucks. It's not always obvious whether the animal you just dug up is a valid taxon or a juvenile of previously known animal, or the reverse--what if we find an adult Scipionyx someday? Because the type material is basically a baby, how will we know what the growth curve is for that species? For all we know, the adult is already in a museum somewhere! But I digress. Usually, knowing whether something is a juvenile or not is a straightforward process. Unfused bones, a large orbit, and, in some cases, parental dependance are good markers. But what if you find a creature that seems to be a perfectly good adult? I am talking, of course, about Guanlong wucaii, currently regarded (widely) as the most primitive known tyrannosauroid. Yes, even older than Dilong paradoxus. Anyway, I read an abstract on the PDF list of last year's Society of Vertebrate Paleontology papers, and I came across one little gem that supposed that Guanlong is a juvenile of a known species: Monolophosaurus jiangi! I apologize for not remembering who the authors of this particular abstract were, however, and I haven't read anything else about the subject since. However, after studying the skulls of the two beasties, I am finding myself agreeing with their findings.

I've reproduced, as best I can, the skulls of the animals here. Atop is Guanlong, of course, and to the right is Monolophosaurus. First, a little bit of information on the latter: Monolophosaurus is generally considered an early tetanurine, though closer to allosaurs than spinosauroids. It is a Middle Jurassic carnosaur from China whose most obvious feature is the enormous crest that runs down the length of the skull, from the orbits to the premaxilla. The crest itself is made up of the premaxilla, nasals, and a little bit of lacrimal for good measure. Two accessory fenestrae penetrate the crest above the antorbital fenestrae. There is a "rosette" of teeth at the front of the dentary, kind of like the case in spinosauroids and coelophysids. It's a big-mouthed, showy predatory dinosaur that was at the top of the food chain in its time. It probably hunted Chinese sauropods (like Mamechiosaurus) and stegosaurs (Huayangosaurus) and may have had the occassional spat with Gasosaurus, a rival Chinese theropod.
Given the great disparity between skull forms in juvenile tyrannosaurs versus old adults, it's not hard to believe that Guanlong is a juvenile form of Monolophosaurus. If we consider that Jane, once considered to be Nanotyrannus, is merely a subadult tyrannosaur, then it's not surprising that we should find juvenile animals with features that, on their own, would suggest physical maturity. Guanlong's crest is made up almost entirely by the nasals. This seems unlike Monolophosaurus until you consider that perhaps the juvenile animal grew into its large crest. I find it important that, in both animals, the prefrontal bones are incredibly small and almost form an accessory to the lacrimal, which in both animals is vaguely L-shaped, and in Monolophosaurus it extends upward to meet the crest. In both animals, the naris is extremely large and moves into the crest. The squamosal is very similar in both animals, especially in how it dips inward at the base of the lacrimal to form a small hole in Monlophosaurus. Guanlong and Monolophosaurus both have a small accessory antorbital fenestrae in roughly the same position.
It's also quite telling that both animals come from the exact same geologic age and location. Indeed, they were contemporaries in life. Also, some features of Guanlong's skull may be unknown due to the fairly crushed state of the skull, while Monolophosaurus's cranium is exquisitely preserved.
So I'm fairly convinced of this unknown author's contention. I wonder if my buddy Scott could do what he did with the Psittacosaurus to Protoceratops transitional morph and do one for these two animals, just for my own curiosity.


Scott said...

Hm. Good idea. For starters, I'll need elevation drawings of each skull--in proportion (i.e., I'll need the subadult skull to be appropriately smaller than the adultform).

Elastic Reality will do a much better job if they look realistic, so I'll paint in the appropriate look for the bones.

If you get me photoref of the original material, I can also make it look cool and fossilly.

Zach Miller said...

Well, sir, I'll see what I can do!

Darren Naish said...

The unknown author is Tom Carr, well known for his work on tyrannosaur ontogeny. I reviewed the Guanlong paper and came away pretty happy with the proposed basal tyrannosauroid identity of this taxon (sorry, I deleted my previous comment because of a silly typo).

Zach Miller said...

Wait a second, Darren. Tom Carr wrote the Guanlong = Monolophosaurus abstract? Or did he write a separate, unknown-to-me paper saying independantly that Guanlong = basal tyrannosauroid? Darn it, I've gotta find that abstract again...

王爱美 (thespian4life) said...
This comment has been removed by the author.
王爱美 (thespian4life) said...

[A.W.] (former student of T. Carr)
The abstract was by T. Carr, in 2006 for SVP, it was "Is Guanlong a Tyrannosauroid or a subadult Monolophosaurus? The question was a comparison resulting in the thesis is G. wucaii a sister species or a subadult of M. jiangi. He is at Carthage College currently, as well as the curator of the Dinosaur Discovery Museum in Kenosha, Wisconsin (my home town). I know this post is very late in coming, however, I just now stumbled across your blog (for which I am quite glad), if you have any insights, please feel free to toss me a note. There is one thing to consider, and which I would like to tell him, but do not have the guts because it is one thing for me to swear like a sailor because of the Montana heat in front of my Dr. Carr, but is quite another thing for me to go up to him and say "Nice try, it looks great, but I think you are wrong." The thing to consider is this: Guanlong's bones have been examined. The second and more complete skeleton, the smaller of the two, was found to be around 6-7 years of age, and showed signs of being in the height of its developmental growth. The first of the two, was larger, less complete, but was shown to be around 12 years old, showing signs of arrested growth. This would show that it was the adult, making Guanlong and Monolophosaurus different species. A simple explanation, but even though I saw him a few weeks ago, I am still too chicken to say it to him, he is my mentor, and as much hassle I give him (He thinks I should be more suave, but I think I should not have to change my personality to such a degree in casual times just to become an academic) I still do appreciate every lesson he has taught me and hold him in the highest of respect.