Longisquama insignis (Sharov 1970) Norian, Late Triassic, ~210 mya was originally considered a pseudosuchian. Since then it has taken center stage in debates on bird origins (Jones et al. 2000) and pterosaur origins (Peters 2000). Longisquama has even been nested with the Permian glider, Coelurosauravus (Senter 2003, 2004), but that was the result of many misidentifications, a common problem that has plagued Longisquama since its discovery. No previous workers, including myself (Peters 2000) were able to report on the posterior half of Longisquama because we all considered it missing. As it turns out, we all saw certain elements without realizing it. The lowermost plume traced by Sharov (1970) and others was actually a femur and tibia aligned in a straight line (see below). The subdivided plume shafts observed by Jones et al. (2000) are actually phalanges in the displaced pedes. The big fourth fingers and pelvis (see below) were missed by everybody until now.
Longisquama was derived from a sister to Cosesaurus and Sharovipteryx and was a phylogenetic predecessor to the basal pterosaur, MPUM6009.
Distinct from Cosesaurus, the skull of Longsiquama had a more constricted snout, which enhanced binocular vision. The orbits were larger. The teeth had larger cusps. The naris was probably larger. With increased bipedalism and active flapping, Longiquama probably experimented with aerobic metabolism.
The cervicals were shorter and the dorsal series was longer, especially so near the hips and between the ilia. The sacrum curved dorsally 90 degrees, which elevated the attenuated tail. These vertebral modifications made Longsiquama similar to a lemur, which also leaps from tree to tree. Such a long torso provided more room for plumes, gave the back great flexibility, and provided more room for egg production.
The pectoral girdle was little changed from Cosesaurus. The clavicles curved around the sternal complex and the sternal keel was deeper. Fused together the interclavicle, clavicles and sternum form a sternal complex, as in pterosaurs. During taphonomy the sternal complex of Longisquama drifted to beneath the cervicals, exactly where the clavicles are found in non-fenestrasaur tetrapods, including birds. This has led to confusion because the clavicles overlapped giving the appearance of a bird-like furcula.
It is difficult to determine if metacarpal IV was torsioned or not and difficult to ascertain the ability and degree of finger IV to fold. The fingers were all greatly enlarged, especially finger IV. As in Cosesaurus, the pterosaur-like pectoral girdle and socketed coracoids enabled Longisquama to flap and generate thrust during leaps.
The pelvis was greatly elongated anteriorly and posteriorly with a posterior ilium rising along with the dorsally curved sacrum of seven vertebrae. The pubis and ischium were much deeper, which provided a much larger pelvic aperture to pass a much larger egg. The distal femur was concave and the proximal tibia convex, as in Sharovipteryx. Both the femur and tibia/fibula were more robust. The foot was relatively large with digits of increasing length laterally. Pedal digit V had a curved proximal phalanx.
Longisquama is famous for, and was named for, its dorsal plumes. Another set of plumes arose from its skull and neck. Former caudal hairs (in Cosesaurus) formed a tail vane in Longisquama. As in Sharovipteryx and pterosaurs, Longsiquama had a uropatagium trailing each of its hind limbs. Like Cosesaurus, Sharovipteryx and pterosaurs membranes trailed the forelimbs, too. This documents the origin of the pterosaur wing and proves that it developed distally on a flapping wing (Peters 2002) rather than proximally as a gliding membrane (contra Elgin, Hone and Frey in press) and certainly without wing pronation, loss of digit V, loss of ungual 4 and migration of metacarpals I-III to the anterior face of metacarpal IV (contra Bennett 2008).
Longisquama was overloaded with secondary sexual characteristics. From plumes to flapping arms, Longisquama was all about creating an exciting presentation unrivaled until the present-day bird-of-paradise. Longisquama had everything Cosesaurus had, only wildly exaggerated. With increased bipedalism and active flapping, Longiquama probably experienced the genesis of aerobic metabolism.