A while ago, I blogged about “despots,” my term for a grade of small to medium-sized theropods that predate the evolution of the large to gigantic tyrannosaurids, or “tyrants.” In it, I projected that a small group of theropods would form a grade within the larger group of theropods that would lead directly to Tyrannosauridae; this group, generally the Tyrannosauroidea, would arise from small predaceous cursors, long limbed and long armed, large skulled and generally unremarkable otherwise, save that they would have features of their bigger descendants.
A month ago, one of these taxa has been given the spot-light with a century-long-awaited redescription (and an immensely detailed and thourough one at that):
Rauhut, O. W. M., A. C. Milner and S. Moore-Fay. 2010. Cranial osteology and phylogenetic position of the theropod dinosaur Proceratosaurus bradleyi (Woodward, 1910) from the Middle Jurassic of England. Zoological Journal of the Linnean Society 158(1):155-195. (DOI:10.1111/j.1096-3642.2009.00591.x)
Proceratosaurus is one of the earliest English theropods described, and is represented by a nearly complete skull, including lower jaw and hyoid aparatus, but lacking the cranial roofing bones and partial braincase bones associated with it (most of the nasal, and all of the frontals, parietals, and portions of the lachrymals, squamosals, prefrontals, postorbitals, and so on). Detailed study of the cranial morphology was carried out with computed tomgraphy imaging, which permited investigation of the pneumatic nature of several bones including the lachrymal, jugal, and nasal, features previous unknown. It confirmed that the strange triangular snout “horn” was correctly identified, and being hollow corresponded well with what is now known for Dilong, Guanlong, Tyrannosaurus, etc.
Many features of the skull imply that successive evolutionary changes that are associated with the size of the taxa were present at small size early on, and I touched on several of these in that previous post, including:
- Fusion of the nasal (almost certainly prior to inflation and the presence of internal chambers);
- An anteroposteriorly short premaxilla (or ventrodorsally tall) due in part to rotation of the premaxillary bone medially;
- Premaxillary teeth much, much smaller than the maxillary teeth;
- Lachrymal with a posterior crest or ridge that projects into the orbit and likely supported a tendon or musculature associated with the anterior process of many a postorbital in this group but which does not always appear in smaller tyrannosauroides (some have argued it is an adult or subadult ontogenetic feature).
Rauhut et al. further postulate the nature of Proceratosaurus by analyzing it in a phylogenetic matrix of 38 taxa and >300 characters, drawn from Rauhut (2003) and other papers. This analysis placed Proceratosaurus next to Guanlong, an early despot that i also discussed briefly, and the foundation set for a monophyletic Proceratosauridae, diagnosed by the presence of a very elongated external bony naris, the anterior maxilla producing a small “step” at the position of the premaxilla and ascending process for the nasal, and of a sagittal cranial crest. Moreover, further characters from above supported my previous arguments on cranial similarity, including
- Pneumatization of the jugal;
- Presence of a ventral jugal “boss” or “horn;”
- Ventral margin of the basioccipital faces posteroventrally (ordinarily, this faces further posteriorly);
- Anteriormost premaxillary teeth with a mesial and distal carina projection from the lingual surface of the crown;
- Denticulations of the premaxillary teeth;
- and, finally, a distinct difference in size between mesial and distal denticulations of the maxillary and dentary tooth carinae (although this is also diagnostic of dormaeosaurids, it is unlikely to be confused based on the other chaarcters, and is reversed later on in tyrannosaurid evolution).
Distinct cranial morphology of the skull reveals that specialization of the dentition and jaw were adapted with increased pneumatization of the skull and the arrangement of the basiocciptial likely corresponded with a revision of the cranial jaw musculature. These features reveal that, among other things, early tyrannosauroids had set themselves apart from their contemporaries, but as the middle and latest Jurassic samples of medium to large-bodied theropods are undersampled, it is difficult to gauge specialization in the context of evolution as a competitive strategy.
It is important to note that this all occured at a relatively small body size, as the theropods Guanlong and Proceratosaurus were no larger than 4-5m, and based on the relative gracility of taxa that I have also projected into this group (such as Tanycolagreus) would have weight less than 100kg. It would be easy to propose, therefore, that the distinct dental and cranial morphology would have permitted these taxa to exploit a different feeding strategy. Unlike their largest descendants, this would likely have not included bone processing, megaherbivore predation, etc.
So the best effective solution is to assess the mechanical functionality of this unique cranial morphology (a rounded snout, smaller premaxillary “nipping” teeth but also larger, very ziphodont lateral teeth, robust bite force generation) but as of yet, this has not been done (largely due to the lack of an easily examinable and complete skull for a despot). At this time, Proceratosaurus bradleyi (Woodward, 1910) and Guanlong wucaii (Xu, Clark, Forster, Norell, Erickson, Eberth, Jia and Zhao, 2006) possess the most ideal skulls for finite-element analysis, but the incompleteness of some or the crushing in others renders a full analysis improbable.