While pterosaurs were experimenting with flapping flight in the Late Triassic, several arboreal lepidosauriforms were gliding with hyper-elongated, rib-like, dermal extensions anchored to their reduced and modified ribs. Welcome to the world of the Triassic gliders, their Permian precursors and their one and only known successor in the Early Cretaceous, Xianglong.
Figure 1. Coelurosauravus reconstructions from Carroll, Frey et al and Peters.
Traditional and Published Views
Carroll (1978, 1988) separated Coelurosauravus from Icarosaurus + Kuehneosaurus. The former was considered a primitive diapsid and the latter two were considered lizards. Both were reported to extend lateral gliding membranes framed by elongated ribs, as in the modern gliding lizard, Draco. Like Draco, no transverse processes were reported in Coelurosauravus (Figure 1), but large transverse processes were reported in Icarosaurus + Kuehneosaurus. Then Frey et al. (2007, Figure 1) found short ribs in Coelurosauravus, which meant the gliding membrane extensors were ossified dermal rods. They reported, “The rods are independent of the ribcage and arranged in distinct bundles to form a cambered wing.” Finally, the Early Cretaceous glider, Xianglong, was reported (Li et al. 2007) to be an agamid lizard, like Draco.
Figure 1. Click to enlarge. The Triassic gliders and their non-gliding precursors.
The Heretical View
Here sets of anterior dermal rods of Coelurosauravus were bundled and anchored to the tips of the anterior two ribs while the posterior rods were associated one-to-one with individual dorsal ribs. Here the purported transverse processes of Icarosaurus and Kuehneosaurus are short, straight ribs fused to their centra and the purported “ribs” are dermal rods, as in Coelurosauravus. Here Coelurosauravus is a sister to Icarosaurus + Kuehneosaurus and all three are non-lepidosaur lepidosauriforms. Finally, Xianglong also had short, straight ribs fused to their centra and so was related to Icarosaurus + Kuehneosaurus, not Draco.
There are as many origins and nesting for the “rib” gliders as there are studies that include them. Laurin 1991 nested Coelurosauravus between the diapsid Petrolacosaurus and the synapsid Apsisaurus. Evans 1988 nested Coelurosauravus between Mesenosaurus and Claudiosaurus. Kuehneosaurs nested in two places, between Choristodera and rhynchosaurs and also between Saurosternon and Gephyrosaurus + Squamata. Evans 2003 nested kuehneosaurs between archosauromorpha (prolacertiforms, rhynchosaurs, archosauriforms) and Marmoretta. Motani (1998) neste kuehneosaurs between lizards and sauropterygians. Müller (2003) nested kuehneosaurs and Coelurosauravus together between Claudiosaurus and Ichthyosaurs + thalattosaurs. The latter seems especially unlikely, nesting aerial reptiles with marine taxa.
Nesting Within the Larger Study
The larger study nested the gliders together with Saurosternon and Palaegama as outgroup taxa.
Let’s Begin with Palaegama
Palaegama was a Late Permian lepidosauriform blessed with elongated arms and legs. These would have been useful living in trees, or perhaps sprinting on the ground bipedally. Palaegama has been recognized as a basal lepidosauriform along with Saurosternon and Paliguana. Estes, Pregill and Camp (1988 ) reported, “they share more features of modern lizards than do any other reptiles of the lat Paleozoic and early Mesozoic.” Yet they were not lizards. They were lizard predecessors. In particular, the skull shape and naris placement of Palaegama indicate a close relationship with Coelurosauravus.
(Latest Permian/Earliest Triassic) Saurosternon was smaller, but with relatively larger feet. Twin sternae appear posterior to the coracoids. These likely indicated an increase in humerus adduction, as in tree clinging. The shorter body shape indicates a closer relationship to Icarosaurus than to Coelurosauravus.
(Late Permian) Coelurosauravus was longer, leaner, with a more exotic skull, shorter ribs and more gracile limbs. Elongated dermal ossicles anchored on the rib tips, were able to fold and extend huge lateral membranes, probably for gliding, but also useful as secondary sexual characters (again, check out that skull for exotic extremes).
(Late Triassic) Mecistotrachelos was a coelurosauravian with a longer neck, shorter tail and a much more slender (almost stick-like) torso in which the ribs were fused to the centra, making them appear to be transverse processes. Fewer dermal “pseudo-ribs” were used to frame the gliding membrane. The cranial crest remained, but was reduced.
(Late Permian) only the skull has been published (Modesto and Reisz 2003), and it was originally considered an enigma, but its affinities are with Icarosaurus and the gliders. In a recent abstract, Reisz and Modesto (2011) reported, “The skeletal anatomy of Lanthanolania provides evidence of limb proportions that suggest that this small reptile is the oldest known bipedal diapsid.” Unfortunately, Lanthanolania was not a diapsid. Nor was it as old as Eudibamus, another diapsid biped. Apparently it also does not have extended pseudoribs, otherwise, they would have been mentioned.
(Late Triassic) Icarosaurus transformed the short ribs of Saurosternon into short “transverse processes” fused to the centra. This transformation has been overlooked by other paleontologists, who report that Icarosaurus had extended ribs, like Draco, the living rib glider. The problem is, no sister taxa have transverse processes, Draco doesn’t have transverse processes, several unfused ribs appear between the scapulae in Icarosaurus and the phylogenetic precursors have not been identified as they are here. In any case, a short tail, deep pelvis and short torso characterize this genus.
(Late Triassic) The biggest of the gliders, Kuehneosaurus was most similar to Icarosaurus but had feet much larger than the hands. Certain posterior (fused) ribs angled anteriorly.
(Early Cretaceous) Xianglong was considered an agamid lizard by Li et al. (2007), but it clearly had short “transverse processes” (actually ribs fused to centra) not found in agamids like Draco. Xianglong demonstrates the survival of the PermoTriassic gliders into the Cretaceous. A poorly ossified carpus may indicate immaturity in the one known specimen.
The PermoCretaceous gliders reduced the dorsal ribs, fused these to the centra and developed elongated dermal extensions to extend lateral gliding membranes. Coelurosauravus and its membranes were considered distinct and convergent, but here they were homologous with those of kuehneosaurids. Xianglong was a late-surviving non- lepidosaur lepidosauriform.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
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