Longisquama Pectoral Girdle and Sternal Complex Documented

In my efforts to help explain the relationship of the lepidosaur tritosaur fenestrasaur, Longisquama (Sharov 1970) to basal pterosaurs I present a pair of images (Figs. 1, 2) designed to illustrate just where the pectoral elements are. Admittedly these elements are difficult to pick out at first glance. So these are for those naysayers who often say, “I can’t see what you’re seeing!!” Here’s why:

Longisquama in situ. See if you can find the sternal complex, scapula and coracoid before looking at figure 2 where they are highlighted.

Figure 1. Click to enlarge. Longisquama in situ. See if you can find the sternal complex, scapula and coracoid before looking at figure 2 where they are highlighted.

The trouble
with seeing the bones of Longisquama is they are largely covered with scales and skin. That’s just a fact we need to embrace. Longisquama has more dermal and extra dermal tissue preserved than most other Mesozoic fossils. In such cases DGS (digital graphic segregation) really becomes a powerful tool, finding bones beneath the skin.

 

Figure 2. Click to enlarge. Longisquama ghosted out with DGS, other than the sternal complex (interclavicle+clavicles + sternum), scapula and coracoid.

Figure 2. Click to enlarge. Longisquama ghosted out with DGS, other than the sternal complex (interclavicle+clavicles + sternum), scapula and coracoid.

Here the affinities with pterosaurs are quite clear
Everyone has seen the horse-shoe-shaped clavicles, but few have recognized that they overlap medially and frame the sternum AND are surmounted by keeled interclavicle with crossbars that accommodate the quadrant-shaped ventral coracoids. (Whew!) This is the standard morphology of the classic pterosaur sternal complex (Wild 1993). Even the overall shape is identical to that in the basal pterosaur, MPUM6009 (Fig. 3). Only the fenestrasaurs, Cosesaurus, Sharovipteryx and pterosaurs share all these traits. In most derived pterosaurs the stem of the coracoid does indeed straighten out more, if you’ve noticed the difference.

Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex.

Figure 3. Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex. The quadrant-shaped coracoid is the remnant and result of the anterior fenestra of the coracoid enlarging to the posterior rim, which is the only part that remains.

Still finding it difficult to spot some of these elements?
Then I encourage you to more closely examine the specimen itself or high resolution photos and become acquainted with sister taxa that will prepare you for what you should be looking for. I know that sounds self-fulfilling, but if you know that most tetrapods have four limbs you don’t stop at three. It’s the same thing at this point. Also see this page on ReptileEvolution.com for the same images of Longisquama (Figs. 1, 2) presented on a mouseover.

Flapping
Earlier we discussed the evolution of the sternal complex and pectoral girdle in fenestrasaurs (Fig. 3) and its role in flapping on a bipedal frame. That’s why these elements are so distinctly shaped, approaching the morphology of birds by convergence. The coracoid no longer slides against the interclavicle and sternum as in more primitive quadrupedal lizards.

Front Half Only
Even if only the front half of Longisquama was known (Sharov, 1970; Fig. 1), this would still be enough to cement Longisquama to pterosaurs. Adding the back half and fingers just puts frosting on the cake.

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
Sharov AG 1970. A peculiar reptile from the lower Triassic of Fergana. Paleontologiceskij Zurnal (1): 127–130.
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.

wiki/Longisquama

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