Xianglong – A Glider, But Not an Agamid Lizard

Updated August 27, 2015 with new reconstructions of the skull. 

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.

Xianglong zhaoi (Li et al. 2007, Fig. 1) 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 (Fig. 2). Well those aren’t exactly transverse processes. They’re fused ribs. That makes the rib-like frames for the gliding membranes actually dermal ossifications, as in Coelurosauravus and other Triassic rib gliders. Xianglong shares a suite of traits with Kuehneosaurus and Icarosaurus, but it had fewer membrane supports. Xianglong was a Triassic rib-gliding kuehneosaur that survived into the Cretaceous.

Key distinctions include: Xianglong had what appear to be elongated transverse processes, but no agamid nor Triassic rib glider has elongated transverse processes. These are actually ribs fused to the neural spines and centra, as in Icarosaurus and Kuehneosaurus. This is not the pattern seen in Draco (Fig. 2). Xianglong had a pes in which metatarsal 2 was longer than mt 4, as in the Triassic rib gliders, not lizards, in which metatarsal 4 (or 3 and 4) is generally the longest.

Draco volans

Figure 2. Draco volans in dorsal view based on an X-ray. Note the lack of transverse processes. Metatarsal 3 is subequal to mt 4 and mt 2 is shorter. Click for more info.

Distinct from Icarosaurus, the skull of Xianglong had a larger lacrimal and a more robust jugal and postorbital. The anterior cervicals were taller. The fused ribs were relatively shorter. As in Icarosaurus, posterior ribs did not carry pseudoribs. Like Kuehneosaurus, the tail was longer than the presacral series. The forelimb was relatively short, especially in the forearm. The carpus was poorly ossified, a trait shared with Kuehneosaurus. Metacarpal 2 was reduced relative to mc 3. The hind limbs were gracile, as in Kuehneosaurus. Metatarsal 2 was  longer than mt 3 and mt 4 was short as in Icarosaurus and Kuehneosaurus. Xianglong was a sister to Icarosaurus. Moving it to Kuehneosaurus adds 5 steps. Moving it to a sisterhood with Draco adds 36 steps.

Original Xianglong tree

Figure 3. Original Xianglong tree (modified with color). Click to enlarge. Taking a look at this tree makes one think that kuehneosaurids were not given a “fair shake” because individual and outgroup taxa were not provided for them. Note the lack of resolution within the Iguania, which raises red flags.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

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
Robinson PL 1962.
 Gliding lizards from the Upper Keuper of Great Britain. Proceedings of the Geological Society London 1601:137–146.
Stein K, Palmer C, Gill PG and Benton MJ 2008. The aerodynamics of the British Late Triassic Kuehneosauridae. Palaeontology, 51(4): 967-981. DOI: 10.1111/j.1475-4983.2008.00783.x

wiki/Kuehneosaurus
wiki/Xianglong

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