Guanlong wucaii

Brusatte, Stephen L., Benson, Roger B. J., Currie, Philip J. & Xijin, Zhao, 2010, The skull of Monolophosaurus jiangi (Dinosauria: Theropoda) and its implications for early theropod phylogeny and evolution, Zoological Journal of the Linnean Society 158 (3), pp. 573-607 : 601-602

GUANLONG WUCAII : BASAL TYRANNOSAUROID, JUVENILE MONOLOPHOSAURUS OR NEITHER ?

Xu et al. (2006) described a mid-sized theropod taxon, Guanlong wucaii , from a level of the Shishugou Formation (Oxfordian: Eberth et al., 2001) slightly higher than the type locality of Monolophosaurus . Guanlong was interpreted as the oldest known tyrannosauroid, and a member of a ‘specialized lineage in the early evolution of tyrannosauroids’ that possesses a mosaic of primitive tetanuran features and derived coelurosaurian characters ( Xu et al., 2006: 717). The most notable feature of this taxon is an enlarged, thin, fenestrated midline crest that resembles the crest of Monolophosaurus . Noting this similarity, Carr (2006) suggested that the smaller Guanlong may represent a subadult Monolophosaurus , or that the two theropods are sister taxa. Histological analysis of the holotype of Guanlong , outlined in the supplementary appendix of Xu et al. (2006), clearly demonstrates that the specimen pertains to an adult, ruling out the first hypothesis of Carr (2006). The presence of a number of autapomorphies in each taxon (reviewed above and in Xu et al., 2006) also argues against this suggestion. However, the second hypothesis deserves further consideration.

The crests of Monolophosaurus and Guanlong are strikingly similar, especially in lateral view. Both are single midline crests, comprising primarily the nasals and excavated by large fenestrae, features unknown among other basal theropods. Homologizing features of the crest is difficult, as these structures differ in detail. Most notably, that of Guanlong is larger, thinner, excavated by four fenestrae (as opposed to two) and reinforced by several thin laminae ( Xu et al., 2006). However, it is possible that a single, fenestrated crest is a synapomorphy uniting a clade of Monolophosaurus and Guanlong . Less equivocal are two synapomorphies unrelated to the crest. First, both taxa share a large, ovoid external naris that is 25% or more longer than the length of the skull ( Table 1). This derived state is unknown in other basal theropods, and contrasts with the much smaller nares of tyrannosauroids ( Brochu, 2002; Currie, 2003; Xu et al., 2004), basal tetanurans ( Table 1) and basal coelurosaurs ( Compsognathus: Ostrom, 1978 ; Peyer, 2006; Ornitholestes: Carpenter et al., 2005 ; Pelecanimimus: Perez-Moreno et al., 1994 ; Scipionyx: Dal Sasso & Signore, 1998 ; Sinosauropteryx: Currie & Chen, 2001 ). Second, both taxa share a weak to nonexistent lateral shelf on the surangular, a feature otherwise only known in an isolated surangular from the Middle Jurassic of England (OUMNH J.29813). In contrast, tyrannosauroids ( Carr, 1999; Currie, 2003; Holtz, 2004; Xu et al., 2004) and basal coelurosaurs ( Compsognathus: Peyer, 2006 ; Sinocalliopteryx: Ji et al., 2007 ) have a robust shelf that strongly overhangs the surangular foramen dorsally, a condition that characterizes theropods in general (see theropod chapters in Weishampel et al., 2004).

In addition, several features of Guanlong , cited as tyrannosauroid apomorphies by Xu et al. (2006), are more widely distributed. Many of these are also present in Monolophosaurus , and include: fused nasals (also in Ceratosaurus , spinosaurids and some abelisaurids, and which may be related to the development of nasal ornamentation in these taxa); a large frontal contribution to the supratemporal fossa; a pneumatic foramen in the antorbital fossa on the jugal (also in allosauroids); a short retroarticular process; and a median vertical crest on the ilium. Similarly, the elongate anterior ramus of the maxilla and ischial foramen of Guanlong are unknown in other tyrannosauroids, but are present in Monolophosaurus .

At the same time, however, Guanlong possesses several characters diagnostic of Coelurosauria and Tyrannosauroidea, which prompted testing by cladistic methods to resolve this homoplasy. Xu et al. (2006) inserted Guanlong into the basal theropod cladistic analysis of Rauhut (2003), which found both strong tree support and character support for placing Guanlong as a basal coelurosaur (a tyrannosauroid) and distant from the more basal tetanuran taxon Monolophosaurus . In particular, 22 unambiguous synapomorphies place Guanlong within Coelurosauria, and seven place it within Tyrannosauroidea. Coelurosaurian characters include clear synapomorphies, such as an elongate antorbita fossa (character 14), medially inclined iliac blades (character 171), an anteroposteriorly elongate and narrow pubic peduncle of the ilium (character 175) and a concave anterior margin of the pubic peduncle (character 179). Clear tyrannosauroid characters include the sharp and narrow vertical crest on the ilium (character 172) and a concave anterodorsal region of the preacetabular process of the ilium (character 173). Constraining Guanlong and Monolophosaurus as sister taxa in Benson’s (2008) updated version of the dataset of Xu et al. (2006) requires an additional 19 steps, or 3% of tree length (693 versus 674 steps). Thus, there is a strong phylogenetic signal linking Guanlong and tyrannosauroids, despite the homoplasy identified above.

We consider the coelurosaurian and basal tyrannosauroid position of Guanlong as a well-supported hypothesis based on current datasets. Our suggestion that Monolophosaurus is a much more basal tetanuran (see above) strengthens this hypothesis, as it increases the phylogenetic distance between the two taxa (as opposed to their separation by only two nodes in the Rauhut/Xu/Benson dataset), and would invoke additional homoplasy if the two formed a clade of crested basal tetanurans. However, a close affinity between Guanlong and Monolophosaurus , as suggested by Carr (2006), should be tested further. Most importantly, the two taxa have never been included in an analysis that recovers Monolophosaurus as a more basal tetanuran, and thus it is unclear what cost would be invoked by pulling Guanlong into this part of the tree. In addition, the two putative synapomorphies of Guanlong and Monolophosaurus identified above, as well as some of the homoplastic tyrannosauroid ‘apomorphies’ identified by Xu et al. (2006), have yet to be included in an analysis. Ultimately, a large phylogenetic analysis of basal tetanurans and basal coelurosaurs is needed, but this is outside the scope of this paper.

As a final note, the fragmentary basal coelurosaur Proceratosaurus from the Bathonian of England (BMNH R 4860) possesses several unique characters of Monolophosaurus and Guanlong . Most notably, the external naris is enlarged (greater than 20% of the skull length) and some form of thin cranial crest was present (although only the anterior region is preserved), features seen in both Monolophosaurus and Guanlong . In addition, the form of the squamosal and quadratojugal is similar in Monolophosaurus and Proceratosaurus , as both taxa have a squamosal ventral ramus that is kinked and projects strongly forward into the lateral temporal fenestra. A close relationship between Monolophosaurus and Proceratosaurus is unlikely for the same reason as discussed above for Guanlong : Proceratosaurus possesses a number of coelurosaurian characters that place it in a more derived position among theropods than Monolophosaurus (for example, Holtz et al., 2004). However, it appears as if Middle Jurassic basal coelurosaurs ( Guanlong , Proceratosaurus ) retained a number of more primitive tetanuran characters, and may have generally resembled basal tetanurans more so than closer coelurosaurian relatives. As Proceratosaurus is currently under study by O. Rauhut and A. Milner, it will not be discussed further here.