Updated August 27, 2015 with new reconstructions of the skull.
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.
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.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
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