On Tyrants, Despots, and Peons
We all know and love our tyrannosaurs, and anyone who's seen Jurassic Pork -- I mean, Park -- knows these things. Indeed, Tyrannosaurus rex is the #1 ranking most favorite dinosaur in the world, and in history. Save before the 1900's, there was no other truly popular dinosaur, and before it, only scientists really cared about dinosaurs. It wasn't until the popularization in the early 1900's that children, and laypeople, and scientists of other fields, and indeed most everyone in Europe and North America, half of Asia, northern Africa, and even portions of South America, and finally Australia, fell in love with the newest and possibly one of the first tourist attractions: The gorgeous free-standing Tyrannosaurus rex mount designed by C. R. Knight and H. F. Osborn. Even after, the next most astounding skeleton mount would be another Tyrannosaurus rex, that of the infamous "Sue", largest and oldest of its species.
But these are classic tyrannosaurids. Some have been recovered in Mexico, one was found as far flung as Alberta and New Jersey from the American West from whence they first came, and they are known throughout the Plains Provinces of Canada, also in the West. Then they found them in Asia, where they are known in Mongolia and northern China and fragments from Russia. But they were all large, massive animals, over 25 feet in length and bulky. Some smaller ones have been found -- Nanotyrannus lancensis, Shanshanosaurus houyanshanensis, and Alioramus remotus -- but they may end up just as babies, teenagers on the dinosaur timeline.
Then they found one in Europe ... and the finds haven't stopped. They've gotten smaller, hairier, fluffier, and they've been found with a third finger, not the classic two-fingered "wiggler" Ray Harryhausen showed off in his movies Creation and Caveman. The arms are longer, and the head did not look the way they used to. ...
Perhaps more shocking is that these animals are also OLD, known over 80 million years before there ever was a Tyrannosaurus rex. And one was sporting headgear....
Details of the postcrania also appear to offer a substantial consideration for the anatomy of early tyrannosauroids, but they may also elucidate the origin of tyrannoraptorans, and this issue has been of more and more recent concern now that we are finding smaller, odder, and far less "tyrannosaur-like" tyrants. Last July, I had the opportunity to go over the paper reporting a new Jurassic theropod taxon Tanycolagreus topwilsoni (Carpenter, Miles & Cloward, 2005) that came with some peculiar observations of the cranial anatomy and some features of the postcrania. This included (but is not limited to) the presence of a posterior "boss" on the lachrymal projecting into the orbit, a coincident process on the postorbital also projecting into the orbit, a very short, high premaxilla, and a lachrymal with a ventral extension of the caudal margin that formed part of the orbital rim, whereas the ventral margin is V-shaped to accomodate the jugal. As was pointed out then and in private, some of these features are not exclusively known to tyrannosauroids, as abelisaurs also possess the medially inclined and deep premaxillary body. Nothing else is particularly useful, however, save the lachrymal and a referred premaxilla originally referred to Stokesosaurus clevelandi by Madsen (1976), which has the added benefits of having a tooth attached showing a lingually-rotated mesial carina and a lingual-rotated distal carina, as well as fused interdental plates. This brought me to consider these features may be tyrannosauroid in nature, and that Tanycolagreus may be a basal tyrannosauroid. But this is speculative, and there is another side to this coin.
The situation could very well be that it IS tyrannoraptoran, not tyrannosauroid, and that the features listed above were present in basal tyrannosauroids but lost in the maniraptoriform branch and retained (and exapted) by the tyrannosauroid branch. This would argue that the complex of basal coelurosaurs could all have been tyrannosauroid-like, but only the Tyrannosauroidea are Tyrannosauroidea, and those silly little dinos are just playing with their skulls.
However, there are some other considerations. Stokesosaurus from the Upper Jurassic Morrison Formation possesses a distinctive ilium with a vertical supracetabular ridge at the midlength of the iliac blade, and an anterior notch on the cranial margin of the preacetabular blade. This anatomy has been used to refer the Lower Cretaceous Wealden English theropod Iliosuchus incognitus (Galton, 1976) to a position similar to Stokesosaurus, and the referral of an ilium from Portugal's Upper Jurassic Guiamarota Formation to Stokesosaurus as a possible new species, and then to a new taxon, Aviatyrannis jurassica by Rauhut (2003). The early "tyrannosauroids" becoming expanded, the first cranial evidence to support this came from England's Early Cretaceous sediments on the Isle of Wight, Sussex, producing Eotyrannus lengi (Hutt et al., 2001). While further work on Eotyrannus elicidates it's cranial and postcranial anatomy, the strength of the morphology of the premaxillary teeth, premaxillary bone, nasal, and lachrymal is only further supported by Dilong paradoxus (Xu et al. 2003) from China's Lower Cretaceous Yixian Formation, as well as all that and the ilium in addition of the newest member of this "mini-tyrant" grade, Guanlong wucaii (Xu et al. 2006), from China's Upper Jurassic Shishugou Formation.
I've taken to calling these critters despots. Tyrants they may or may not be, they were hardly the rulers of their domains, but their relationships may speak of tyrants-to-be.
But this leads me to some curious considerations: Do all these features really add up to Tyrannosauroidea? Or are they basal to the clade as a whole? Upon further investigation, I have found cranial and pelvic features in compsognathids that closely parallel that of the tyrannosauroids, though this is hardly new and compsognaths have always been very similar, perhaps plesiomorphically so, to their huger cousins. These animals were not tyrants, or despots, but peons, whose's right is to rule over nothing but be ruled.
Compsognathus longipes (Wagner, 1859), from Germany's Upper Jurassic Solnhofen Limestone [Formation], unfortuntely, is poorly preserved. The holotype is missing large portions of the pelvis, and manus is scattered, and the skull is lacking the premaxilla in much detail, aside from being crushed and partially disarticulated. Furthermore, the referred specimen from France's Upper Jurassic, C. "corallestris" ( Bidar, Demay and Thomel, 1972), has an even worse preservation of the pelvis, the manus is almost entirely missing, and the skull split down the middle of the bones, making details difficult to assess. Though Karen Peyer in her thesis and has upcoming publication on these critters, our use of these forms is ... problematic.
Sinosauropteryx prima (Ji & Ji, 1998), also from China's Lower Cretaceous Yixian Formation, is known from three specimens, one which has been alleged as distinct from the holotype and was not part of Currie and Chen's 2001 revision of the taxon. Nonetheless, the specimens are complete in all details we might consider save the skull has been split so that surface bone is hardly preserved, and only shapes are apparent. Nonetheless, distinctive features such as the arched lachrymal and short premaxilla with elongated caudal ramus overlying the maxilla can be compared to tyrannosauroids exclusively.
Compsognathids also bear pelvises with a large ischiadic obturator process, a little ischiadic boot, and a long pubic boot. Nearly all tyrannosauroids also have these features, but oddly, the earliest ones appear to have lacked some of them. Dilong, for example, possesses a long pubic boot, lacks an anterior projection of the boot, and even the ischiadic boot and obturator, though Guanlong, amazingly, does not. Rather, the ischiadic obturator is enclosed in a plate forming an ischiadic fenestra, there is a slight ischiadic boot, smaller than in Dilong, and the pubic boot has a small anterior projection, unlike in Dilong. Most interesting, however, is Mirischia asymmetrica (Naish, Martill & Frey, 2004) from the Brazilian Lower Cretaceous Santana Formation, a place also known for its fish and pterosaur fossils. This specimen exposes a particularly odd pelvis, which on one side shows an open pubic fenestra and a close ischiadic fenestra, and on the other a closed pubic fenestra and an open ischiadic one, forming an ischiadic obturator process. It seems that this "compsognathid" may in fact be a tyrannosauroid, albeit a small one, and likely the most tyrannosauroid-like "peon".
There are other animals to consider, however.
There is the Wealden Aristosuchus pusillus (Owen, 1876; Seeley, 1887), based on a pubis and sacrum, similar to both Dilong and Compsognathus, currently treated as a compsognathid (Naish, 2002).
And there is the Yixian Huaxiagnathus orientalis (Hwang et al., 2004), a very large (1m, 3ft.+) specimen with a peculiar skull and generalized postcranium, including a long process of the maxilla as in Guanlong with the premaxilla extending a process caudally atop it.
And Proceratosaurus bradleyi (Woodward, 1910; von Huene, 1917) from England's Great Oolite, Upper Jurassic of Gloustershire, which is known from a skull lacking the braincase and roofing bones, save for one tiny, enigmatic, and scintillating fragment of a nasal, which has been used, as in Paul, 1988a and 1988b, to give it a nasal crest (which he also reconstructed for Ornitholestes, though Carpenter et al. 2005 have rejected this hypothesis).
Carpenter et al. referred Tanycolagreus to the "Coeluridae", a group of possibly paraphyletic taxa that has been used to lump many disparate (and primitive) coelurosaurs. I do not think their plesiomorphic nature warrants recognition of this grouping, but then, I am loathe to prevent it based on the possibility of these "plesiomorphies" being consistent and apomorphic for a group named "Coeluridae". Other coelurids, however, include the eponymous Coelurus fragilis (Marsh, 1886) and Ornitholestes hermanni (Osborn, 1906), both from the Upper Jurassic Morrison Formation, and both substantially well known from vertebral, limb, and at least in Ornitholestes, a complete skull. Even so, Ornitholestes bears several "tyrannosauroid" apomorphies: Both lachrymal and postorbital bear these "suborbital" projections noted for Tanycolagreus, as present in Dilong but absent in Guanlong; the premaxilla is short and deep, though relatively large compared to the skull, which is relatively short for the skeleton, but this shortness of skull seems to be present in a referred specimen of Guanlong with a skull length shorter than the ilium, whereas the holotype adult skull is 250% longer, and the ilium only 20%; the short skull and the bound, but distinct neurocentral sutures in dorsal vertebrae in Ornitholestes suggest that it may not have been fully mature. The pelvis is well-preserved, and the ischium bears a nearly complete, Mirischia-like obturator process, such that it is likely to have been closed by cartilage in life. However, the pubis lacks a long boot, and a break in the anterior margin of the ilium, which lacks a vertical crest, does not help us resolve whether the pelvis is "tyrannosauroid," and it's like the animal wasn't.
This might then lead us to speculate that the so-called Tyrannosauroidea features of Tanycolagreus may not, in fact, be such, though it doesn't remove the considerations involved in the premaxilla and ventral lachrymal morphology, which optimize in cladistic analyses as tyrannosauroid autapomorphies, as well as the tooth morphology, which is shared by Dilong and Guanlong to the exclusion of all "coelurids" save Tanycolagreus.
So in the end, this leads us to the original speculation:
Are these coelurids tyrannosauroids? Or are some basal tyrannosauroids ... not tyrannosauroids? It seems that Tyrannosauroidea may be almost indistinguishable from within these animals, and Guanlong supports the origins of Tyrannosauroidea within the Jurassic Period. We may find the remainder of coelurosaur origins a large "mess," but it gives us plenty of work ahead to consider all the differences involved and also the similarities, and their implications for phylogeny, biogeography, and paleoecology.