Updated May 23, 2016 with a deeper maxilla posterior to the antorbital fenestra. This was needed, as pointed out by M. Mortimer, to house the tooth roots. I missed the splinter that made the difference and someday may try to trace the palatal elements, which I have avoided at present.
(Hwang et al. 2004, Fig. 1) was originally considered a large compsognathid. The Hwang et al tree (now 12 years old) nested Huaxiagnathus with Compsognathus and Sinosauropteryx in the clade Compsognathidae, derived from a sister to Ornitholestes, and basal to therizinosaurs, alvarezsaurs, oviraptors, birds, and deinonychosaurs.
Figure 1. Huaxiagnathus in situ with reconstructed skull, pes, manus and pelvis. Note the relatively large pedal digit 3, the large hyoid, and the twisty lacrimal. Hwang et al. did not provide a reconstruction.
in the large reptile tree Huaxiagnathus nests at the base of the tyrannosauroids, between Tianyuraptor + Fukuivenator and Zhenyuanlong. Yet, another heresy…
Hwang et al. reported the absence of a sternum.
That’s odd because all current sisters have a sternum. The fossil was collected by farmers, but no preparator was mentioned. Perhaps there was a village preparator. After many tests conducted by AMNH personnel, the fossil was determined to be genuine, singular and not a chimaera. Given the presence of both humeri where they are, the sternum should be between them. It is not, so one wonders if the sternum was removed by the preparators to expose the underlying humerus. A DGS tracing appears to show the remains of a posterior sternum (Fig. 2, magenta, contra Hwang et al.).
Figure 2. Pectoral region of Huaxiagnathus with various elements colored for clarity. The magenta bone appears to be posterior rim of a sternum, overlooked or considered an elbow by Hwang et al. A second overlay colorizes bits and pieces of the possible sternum extending toward the coracoids.
The Hwang et al. diagnosis reports:
“Differs from other known compsognathids in having
- a very long posterior process of the premaxilla that overlaps the antorbital fossa,
- a manus as long as the lengths of the humerus and radius combined,
- large manual unguals I and II that are subequal in length and 167% the length of manual ungual III,
- a first metacarpal that has a smaller proximal transverse width ( i.e. “narrower”) than the second metacarpal and
- a reduced olecranon process on the ulna.”
- The premaxilla doesn’t overlap the maxillary fossa, but tyrannosaurs have a similar long posterior process
- true! and no related taxa share this trait, even those with more bird-like morphologies
- okay… but that’s a pretty exact percentage for ungual three! (similar to Zhenyuanlong, though)
- if so, then just barely a smaller transverse width
- as in several basal tyrannosauroid sisters
- Not mentioned above, but those pedal proportions seem unique, with a dominant pedal digit 3. The hyoid is enormous. So few and so large are the maxillary teeth that they seem to be unusual, especially compared to the tiny teeth of Compsognathus. There seem to be many ossified stiffening element scattered throughout the vertebral column. Higher resolution should solve this problem.
Huaxinagnathus had a short neck and large skull longer than the cervicals and just about as long as half the presacral length. The convex maxilla orients the premaxilla into an ‘up’ orientation. The quadratojugal, here broken into several parts, has a mushroom dorsal process that meets a squamosal ‘lid’. The lacrimal has the familiar tyrannosaur-ish in and out twist. The the maxillary teeth are BIG and few.
Figure 3. Huaxiagnathus skull with elements colorized and reconstructed in figure 4. Orignal tracing is in black outline. Many of the bones are broken.
A reconstruction puts the elements
back into their in vivo positions (Fig. 4). Many of the bones are broken and had to be repaired. The scleral elements are scattered.
Figure 4. Huaxiagnathus skull and hyoid reconstructed. See figure 4b for other clade member skulls.
Basal theropod subset of the large reptile tree
shows the nesting of Huaxiagnathus in the basal tyrannosauroids (Fig. 5). Both Compsognathus specimens have a most recent common ancestor, with no intervening taxa. Huaxiagnathus, originally considered a compsognathid is one if the whole clade is considered the Compsognathidae. Otherwise, Only Struthiomimus and the Compsognathus holotype form a clade and are sisters. The CNJ79 specimen of Compsognathus is not the adult form of the holotype (contra Peyer 2006), but deserves a new generic name.
Figure 5. Basal theropod subset of the large reptile tree showing the two Compsognathus specimens. Hauxiagnathus is a basal tyrannosauroid derived from a sister to Compsognathus.
with every new taxon repairs do get made to the large reptile tree, but the tree topology does not change very often. The theropod subset just keeps growing without shifting around. You would think that if there were enough scoring mistakes the tree topology would change. The key thought here is that some repairs actually cement relationships. The repairs typically, but not always, remove misinterpreted ‘autapomorpies.’ For instance, the ilium of Zhenyuanlong was earlier misinterpreted as having a longer anterior process, which would be an autapomorphy for the clade. A reexamination revealed the relatively longer posterior process (Fig. 6). So, it’s true what they say about me, I don’t get it right the first time all the time.
Figure 6. Zhenyuanlong has a new ilium with a shorter anterior process that was earlier misinterpreted.
Huaxiagnathus further cements
the relationships of Zhenyuanlong, Tianyuraptor and Fukuivenator to the tyrannosaurs (contra Hone 2016) and Brusatte (2015). For its size, it looks like one (Fig. 7) with robust lower limbs, large teeth on a curved maxilla, a large head relative to the neck and torso. And don’t forget to picture this skeleton with lots of feathers as in Zhenyuanlong (Fig. 6).
Figure 7. Huaxiagnathus reconstructed in lateral view, sans feathers.
Brusatte S 2015. Rise of the Tyrannosaurs. Scientific American 312:34-41. doi:10.1038/scientificamerican0515-34
Hwang SN. Norell MA, ji Q and Gao K-Q 2004. A large compsognathid from the Early Cretaceous Yixian Formation of China. Journal of Systematic Palaeontology 2(1):13-30.