What is Orientognathus? Nest it with Changchengopterus and Sordes.

Earlier on January 24, 2015 we looked at a description of Orientognathus chaoyngensis (Lü et al. 2015. Fig.1) a new Late Jurassic rhamphorhynchoid pterosaur known then from a series of comparative descriptions only. No images. Now that I’ve seen a low-resolution photograph (Fig. 1), let’s review the data we had to work with on the 24th of January.

Figure 1. Orientognathus in situ, tracing moved to live position, comparisons to sisters Changchengopterus and Sordes.

Figure 1. Orientognathus in situ, tracing moved to live position, comparisons to sisters Changchengopterus and Sordes. Click to enlarge. More resolution is needed to get more out of this. The wings and tail are not very long here. The sternum is a small triangle.

Data provided:

  1. toothless tip of dentary, slightly pointed – True
  2. mc4/humerus ratio = 0.38 – True and precise
  3. ulna < each individual wing phalanx – False, the ulna is quite long
  4. tibia subequal to femur – False, tibia is longer, both tibia are broken.
  5. deltopectoral crest more developed than in Qinlongopterus – True, but not as much as in Sordes.
  6. anterior teeth stouter and longer than in Pterorhynchus – True, but Pterorhynchus has pretty short anterior teeth.
  7. teeth are straight and longer than in Jianchangnathus Subequal actually. 
  8. pteroid/humerus ratio = 0.21; pteroid has expanded distal end True enough (0.23)
  9. larger than other rhamphorhynchine pterosaurs from Late Jurassic NE China (measurements not indicated). Did not check, but seems pretty big (Fig. 1).

The specimen is nearly complete and partly disarticulated. 
The antebrachium (forearm) is broken. Put them back together and that’s a long forearm.

It turns out the the tibia is not equal to the femur in length
So all of the prior candidates become rejects.

Step two: ulna is not smaller than each individual wing phalanx
Orientognathus has a large antebrachium, subequal to m4.2 and longer than m4.1.

Figure 2. The original nesting of Orientognathus from Lü et al. 2014. Note the lack of resolution in that clade. Changchengopterus is not listed, but should be.

Figure 2. The original nesting of Orientognathus from Lü et al. 2014. Note the lack of resolution in that clade. Changchengopterus is not listed, but should be.

Step three: Phylogenetic Analysis
The prior cladogram lost resolution when Orientognathus was added (Fig. 2) .

However
A new analysis of the large pterosaur tree (not yet updated) nests Orientognathus between Changchengopterus and the primitive specimen of Sordes (Fig. 1) with no loss of resolution.

These three (Sordes, Orientognathus and Changchengopterus) are the metaphorical ‘plain brown sparrows’ among pterosaurs from which all later spectacular specimens are derived. It’s that lack of any ‘distinctive’ traits that is actually their own distinctive trait.

Thus Orientognathus is basal to the several specimens of Sordes and the many specimens of Dorygnathus, from which arise darwinopterids (wukongopterids), scaphognathids and ultimately all the pterodactyloids. Orientognathus may have been hard to nest and caused so little stir because it is indeed plesiomorphic. Getting that antebrachium and tibia right would have helped. If anyone has access to high resolution imagery of the skull and foot, that would be very helpful.

References
Lü J, P H-Y, We X-F, Chan H-L and Kundrat GM 2014. A new rhamphorhynchid pterosaur (Pterosauria) from Jurassic deposits of Liaoning Province, China. http://www.biotaxa.org/Zootaxa/article/view/zootaxa.3911.1.7/0

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