The Locked Down Coracoid and the Origin of Flapping

The ancestors of bats, birds and pterosaurs (carnivores, theropods and fenestrasaurs respectively) have been established by cladistic analysis, despite the fact that certain detractors continue to hold on to their pet “mysteries.” Today we’re going to talk about the key trait all three flyers share: an elongated locked down anchor for the scapula. In bats that’s an elongated clavicle. In birds and fenestrasaurs (including pterosaurs) that’s a stem-like coracoid.

Bats, birds and pterosaurs are united by the convergent ability to flap their forelimbs/wings. In this way they generate thrust for true flying. Gliders don’t flap and they don’t have elongated scapula anchors.

Figure 1. Archaeopteryx and several other basal birds. Here Archaeopteryx is a relative giant. This is an older illustration, predating all of the recent and now, not so recent, finds from China. The wings, sternum and tail are derived in the smaller birds. That’s the smallest Archaeopteryx, the Eichstätt specimen.

In birds
The skeleton of Archaeopteryx is distinct from more primitive sisters by the elongation of the coracoid, which no longer slides between the clavices and the interclavicle, but is locked down onto the sternum. Descendants of Archaeopteryx typically had a longer, narrower coracoid. The clavicles fuse medially, becoming a single bone, the furcula, which further increases the rigidity of the pectoral girdle. Observing the length of the coracoid we can tell whether or not a particular theropod was a predecessor or a descendant of basal birds like Archaeopteryx. Oviraptorids and dromaeosaurids had longer coracoids.

Figure 2. Click to enlarge. Cosesaurus aviceps displaying its many pterosaurian characters. Note the large, quadrant-shaped coracoids (Co) the posterior border remnants of coracoid fenestration and erosion that is typical of lizards.

In fenestrasaurs (and pterosaurs)
The skeleton of Cosesaurus is distinct from more primitive sisters by the erosion of the anterior coracoid leaving only the quadrant-shaped posterior margin. The tip of this coracoid stem was locked down, inserting into a glenoid of the sternal complex. Both Sharovipteryx and Longisquama had a stem-like locked down coracoid. In Sharovipteryx and most pterosaurs it was straighter, but in Longisquama and basal pterosaurs, the coracoid retained a quadrant shape.

Figure 3. Bat clavicles acting as coracoid substitutes anchoring the scapulae and providing muscle anchors for the humeri. Clavicles highlighted in green. In most mammals the clavicles are neither this long nor this upright. Photo courtesy of Wesleyan College, Macon GA. Click to see more of their osteological collection.

In bats
The skeletons of  Icaronycteris and Onychonycteris are distinct from more primitive sisters by the elongation of the clavicles, which places the shoulder glenoid higher than the spinal column. Close to a bat ancestor, the basal dermopteran and living tree shrew, Ptilocercus had an elongate coracoid that pushes the scapula and shoulder glenoid higher toward the vertebral spines. The common tree shrew and rabbit ancestor, Tupaia, also preserves this feature, but descendant taxa, like Plesiadapis, do not.

Figure 4. The coracoids (in pink) slide along the sternum (in aqua) behind the interclavicle (in yellow).

The basal coracoid
In non-flappers either the clavicle is shorter or the coracoid is disc-like, sliding between the clavicles and interclavicle to elongate each stride (Fig. 4).

Crocs
Not all tetrapods in which the coracoid becomes strap-like are flappers. Crocodilians have an elongate coracoid. Strangely the elongation did not occur in bipedal primitive taxa, but in Sphenosuchus and its descendants, the first of the quadrupedal crocs. Crocs don’t have a clavicle, and the coracoid in this case is flexible with an elongated, curved medial rim, enhancing shoulder mobility.

The soft trailing edge
From Cosesaurus through pterosaurs, from Archaeopteryx through Passer, from Onychnycteris to Myotis, these flappers all include a thin trailing edge of either feathers or muscular skin. These soft morphologies flex both on the up-stroke and the down-stroke to push air backwards.

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.