Icarosaurus siefkeri (Colbert 1966) Carnian, Late Triassic ~228 mya, ~10 cm in snout to vent length was a much smaller and more derived glider. Derived from a sister to Coelurosauravus, Icarosaurus was a sister to Kuehneosaurus and Xianglong.
Distinct from Coelurosauravus, the skull of Icarosaurus did not have a posteriorly displaced naris, but may have evolved from this condition because the nares were raised relative to the jawline as if phylogenetically rotated from the displaced position during rostrum shortening. The nares were not separated medially. There is no trace of any decorative enlargements to the squamosal and supratemporal, so in this area the skull of Icarosaurus was more like that of Paliguana. It is also possible that skull decorations only developed with maturity. Since Icarosaurus was so much smaller than Coelurosauravus, Icarosaurus may have matured precociously before the onset of crest development. On that note, fitting those wings into an egg would have been difficult, so hatchlings probably grew them as they matured.
The cervicals were shorter than in Owenetta. The dorsal series was short, as in Saurosternon, but probably shortened from Coelurosauravus, which had more dorsal vertebrae. Most of the dorsal ribs were straight and laterally oriented, fused to their centra. A few anterior ribs were not fused. No sister taxa have elongated transverse processes. The tail was shorter than the dorsal series.
The scapula was tiny and the coracoid and interclavicle were shortened. The forelimb was more robust. Metacarpals II-IV were subequal.
The ventral pelvis was extremely deep, perhaps for delivering larger eggs. The hind limbs were more robust and relatively longer. Pedal proportions were similar to those in Coelurosauravus, but digit III extended slightly further than IV due to different metatarsal lengths.