The Deltoid in Pterosaurs

Short post today.

The anterior head of the deltoid inserts on the lateral clavicle. Since the clavicle is incorporated into the sternal complex of fenestrasaurs (including pterosaurs, Fig. 1, Wild 1993), the anterior head of the deltoid would have inserted at the lateral edge of the sternal complex, close to the rib 10 (dorsal rib 2) articulation, creating even more ventral power to the pterosaur flight stroke.

The evolution of the pterosaur pectoral girdle and sternal complex

Figure 1. The evolution of the pterosaur pectoral girdle and sternal complex featuring Huehuecuetzpalli, Cosesaurus, Longisquama, and the basal pterosaur, MPUM 6009. If the anterior head of the deltoid maintained its anchor on the lateral head of the clavicle, then it would have migrated ventrally and posteriorly, adding to the musculature of the downstroke.

Different but logical.
In early fenestrasaurs and basal pterosaurs the clavicle extends to the very end of the sternal complex. Only in later Triassic forms, like Eudimorphodon, and beyond, does the sternal complex ossify posteriorly beyond the clavicle, enabling more pectoral contribution to the anchoring of flapping muscles.


Figure 2. Eudimorphodon with an enlarged sternal complex, extended with more ossified sternum posteriorly. Click for more info.

Step by step.
Little steps. That’s the way evolution works.

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.

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.