The skull of Xianglong – Early Cretaceous kuehneosaur

Xianglong zhaoi (Li et al. 2007) Yixian Formation, Early Cretaceous, 15.5 cm in length was originally considered an agamid lizard with elongated transverse processes and hyperelongated ribs, like the extant Draco volans. However Xianglong has a larger suite of traits shared with Kuehneosaurus and Icarosaurus. Not a lizard, Xianglong was a kuehneosaur that survived into the Cretaceous. That clade nests outside of the Lepidosauria in nearly all cladograms including the large reptile tree.

The skull was complete, but thoroughly crushed (Fig. 1).

Figure 1. Xianglong animated GIF file. Here DGS (digital graphic segregation) is the technique used to pull bone shapes out of this apparent chaos. Many of the bones overlap others and many long bones are broken. See figure 2 for the reconstruction. The light green vomers here are gold in figure 2.

Figure 1. Xianglong animated GIF file. Here DGS (digital graphic segregation) is the technique used to pull bone shapes out of this apparent chaos. Many of the bones overlap others and many long bones are broken. See figure 2 for the reconstruction. The light green vomers here are gold in figure 2.

The animated GIF (Fig. 1)
shows skull and mandible/hyoid elements on segregated layers using digital graphic segregation (DGS). Below (Fig.2) in the second half of any DGS process, those elements are reset to reproduce the skull in dorsal, palatal and lateral views.

Figure 1. Xianglong zhaoi, a late-surviving sister to Kuehneosaurus and Icarosaurus. What appear to be ribs framing the gliding membrane are in fact dermal ossifications as in Coelurosauravus.

Figure 1. Xianglong zhaoi, a late-surviving sister to Kuehneosaurus and Icarosaurus. What appear to be ribs framing the gliding membrane are in fact dermal ossifications as in Coelurosauravus.

Figure 2. Skull elements of Xianglong reconstructed in several views. Some soft tissue is also shown (light green). Elements pulled from figure 1. If you find any errors here, please call them to my attention.

Figure 2. Skull elements of Xianglong reconstructed in several views. Some soft tissue is also shown (light green). Elements pulled from figure 1. If you find any errors here, please call them to my attention.

The reconstruction
is rather straightforward, moving elements back into their in vivo positions. Some elements seen edge-on, like the skull roof in lateral view, are either freehanded to the correct length and curved to fit, or reduced in one direction using the scaling tool of Photoshop. Note the great resemblance of this skull to that of a sister taxon, Kuehneosaurus (Fig. 4).

Figure 3. Xianglong overall. Note the detail recovered in the tracing of the skull here. These authors had the original in their hands, yet DGS was able to pull more data out using published photos.

Figure 3. Xianglong overall. Note the detail recovered in the tracing of the skull here. These authors had the original in their hands, yet DGS was able to pull more data out using published photos.

The skull of Xianglong
was originally traced with little regard to details (Fig. 3). DGS (Fig. 1) was able to pull those details out in a matter of hours from the published literature. Despite the large number of current detractors, DGS has value. This is just one of many such demonstrations.

The Triassic kuehneosaur gliders and their non-gliding precursors.

Figure 4. Click to enlarge. The Permian, Triassic and Early Cretaceous kuehneosaur gliders and their non-gliding precursors. Included are Coelurosauravus, Mecistotrachelos, Kuehneosaurus, Icarosaurus and Xianglong, all with extended dermal processes mimicking ribs. Palaegama and Saurosternon do not have these gliding/display elements.

Draco volans (Fig. 5) is an extant iguanian squamate lepidosaur with genuine elongate ribs framings its gliding membrane. Note the distinct skull shape. Also note the complete lack of elongate transverse processes on the dorsal vertebrae. Those elongate so-called transverse processes on kuehneosaurs are often, but not always the actual ribs, fused to the vertebrae (the proportion of rib to transverse process changes along each spinal column), as discussed earlier here in yet another heretical observation at odds with current paleontological conventions and paradigms.

Figure 6. Draco volans a living true rib glider. Note the distinct skull morphology, closer to that of Iguana than to Xianglong.

Figure 5.  Draco volans a living true rib glider. Note the distinct skull morphology, closer to that of Iguana than to Xianglong.

References
Li P-P, Gao K-Q, Hou L-H and Xu X. 2007. A gliding lizard from the Early Cretaceous of China. PNAS 104(13): 5507-5509. doi: 10.1073/pnas.0609552104 online pdf

wiki/Xianglong

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