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.