Cosesaurus is the “Archaeopteryx” of pterosaurs, the “ur-flugsaurier,” the one that has so many pterosaur traits, yet does not have the key trait, wings. The large reptile tree indicates that Cosesaurus is closer to the origin of pterosaurs than is any archosaur in traditional studies.
Earlier we talked about how the pes of Cosesaurus matches narrow-gauge, digital, occasionally bipedal tracks with a hyper-flexed pedal digit 5 attributed to Rotodactylus. Those tracks document this sort of locomotion in this sort of reptile.
Earlier we also talked about the stem-like coronoid and strap-like scapula and sternal complex of Cosesaurus, all of which contributed to its ability to flap, based on similarities to birds and pterosaurs.
Earlier we talked about the important contributions of Dr. Paul Ellenberger, who, unfortunately, insisted that Cosesaurus was a pro-avian and was thus blinded to the possibility that Cosesaurus was a pro-pterosaur. Similarly modern paleontologist keep insisting that pterosaurs were archosaurs, blinding them to the possibility that pterosaurs were lizards.
Added a day later
And just so the point is not lost, no predecessor to the basal pterosaur, MPUM6009, could fly. They could leap, flap and glide, but the wings and pectoral girdle were not relatively large enough to sustain climbing flight.
So, without further ado, I present a simple animation of Cosesaurus running and flapping based on Dr. Bruce Jayne’s treadmill lizards (Jayne’s video here).
Figure 1. Click to enlarge and animate. Cosesaurus flapping – fast. Dr. Jayne’s treadmill lizards run extremely fast and the legs here, if just as fast, should appear as blurs. There should be some bounce in the tail and neck here, but the effort to produce that was not attempted.
Figure 2. Click to enlarge. Cosesaurus running and flapping in slower motion. Pedal digit 5 would have impressed only while walking. Cosesaurus was also capable of quadrupedal locomotion, according to Rotodactylus tracks and the hands impressed in a digitigrade fashion.
In Summary
The elongate ilium and the addition of two sacrals for a total of four in Cosesaurus indicates a bipedal configuration, as is often the case with terrestrial reptiles. The prepubis may have contributed to this ability. The simple hinge (mesotarsal) ankle joint supports this. Unlike most lizards, members of the Tritosauria, like Cosesaurus and Huehuecuetzpalli, did not fuse the astragalus and calcaneum. The attenuated tail is a tritosaur/pterosaur trait. The pectoral girdle was pterosaurian, able to flap, but the arms were too short to fly. Even so a pteroid, preaxial carpal and trailing fibers were also pterosaurian traits (the fibers support the wing membrane in pterosaurs). Flapping was likely a secondary sexual behavior, designed to attract mates or drive off enemies, analogous, perhaps, to the frilled neck of the similar, but unrelated Australian frillneck lizard, which is also a capable biped, which does not flap its arms.
As in Dr. Jayne’s lizards
and despite a sprawling femur, the footfalls of Cosesaurus (Rotodactylus) were narrow-gauge and digitigrade, countering decades of traditional thinking regarding running lizards.
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.
References
Ellenberger P and de Villalta JF 1974. Sur la presence d’un ancêtre probable des oiseaux dans le Muschelkalk supérieure de Catalogne (Espagne). Note preliminaire. Acta Geologica Hispanica 9, 162-168.
Ellenberger P 1978. L’Origine des Oiseaux. Historique et méthodes nouvelles. Les problémes des Archaeornithes. La venue au jour de Cosesaurus aviceps (Muschelkalk supérieur) in Aspects Modernes des Recherches sur l’Evolution. In Bons, J. (ed.) Compt Ren. Coll. Montpellier12-16 Sept. 1977. Vol. 1. Montpellier, Mém. Trav. Ecole Prat. Hautes Etudes, De l’Institut de Montpellier 4: 89-117.
Ellenberger P 1993. Cosesaurus aviceps . Vertébré aviforme du Trias Moyen de Catalogne. Étude descriptive et comparative. Mémoire Avec le concours de l’École Pratique des Hautes Etudes. Laboratorie de Paléontologie des Vertébrés. Univ. Sci. Tech. Languedoc, Montpellier (France). Pp. 1-664.
Peabody FE 1948. Reptile and amphibian trackways from the Lower Triassic Moenkopi formation of Arizona and Utah. University of California Publications, Bulletin of the Department of Geological Sciences 27: 295-468.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330
Wild R 1993. A juvenile specimen of Eudimorphodon ranzii Zambelli (Reptilia, Pterosauria) from the upper Triassic (Norian) of Bergamo. Rivisita Museo Civico di Scienze Naturali “E. Caffi” Bergamo 16: 95-120.
The Pterosaur Heresies 
Running plus flapping does not lead to powered flight. I suggest you take a look at The Physics of Leaping Animals and the Evolution of Preflight (Caple, Balda, and Willis, 1983). Hopping and harvesting — now that’s the ticket.
True, but it CAN, however, become flight as the evolution moves forward. Running and flapping can become gliding, which can become flight.
Chickens never glide.
Good point. And I noted that Cosesaurus was capable of flapping, but not flight. Two successors, Sharovipteryx and Longisquama, were capable of gliding, probably from a hind limb leap, and flapping, but not flight. Not until we get to pterosaurs is flight possible, probably from a hind limb leap as MPUM6009 has long hind limbs, longer than in any subsequent pterosaur. Flapping was a secondary sexual characteristic that goes back to Langobardisaurus. Flying was co-opted as leaps became more daring. Thanks for the comment.