The Origin of the Pterosaur Sternal Complex

It is a common and mistaken paradigm that pterosaurs appeared out of nowhere, seemingly unrelated to other prehistoric reptiles. Those who say this (and the list is long) also judiciously avoid any discussions of pterosaurs as lizards or fenestrasaurs. Their investments in the outmoded and unsupportable archosaur hypothesis have not provided answers — and never will. Here we will take a look at the development of the sternal complex of pterosaurs evolving from the most parsimonious sister taxa yet discovered (Peters 2000a, 2007).

The Pectoral Girdle in Huehuecuetzpalli
The story begins with Huehuecuetzpalli (Reynoso 1998), a basal tritosaurid lizard with a fairly typical pectoral girdle (Figure 1). A T-shaped interclavicle and sinuous tapered clavicles anteriorly framed the short scapula and fenestrated but otherwise discoidal coracoid. A broad sternum was located at the posterior tip of the interclavicle. The coracoid was free to rotate between the clavicles, interclavicle and sternum, increasing the range of motion of the humerus.

The Pectoral Girdle in Cosesaurus
Several changes to this pattern can be seen in the basal fenestrasaur and tritosaur, Cosesaurus (Figure 1). The interclavicle developed an anterior process. The sternum moved anteriorly, now dorsal to the transverse processes of the interclavicle. The clavicles were shorter, no wider than the sternum and aligned with the anterior rim of the sternum. The coracoids were relatively larger and considerably narrower as the anterior fenestrations expanded until just the quadrant-shaped posterior rim remained. The scapula was strap-shaped with a long posterior process extending over several more dorsal ribs. With the sternum leading edge now anterior to the interclavicle trailing edge, the coracoids had no room to move and their ventral stems became socketed and essentially immobile, resembling the configuration in birds and serving as a precursor to the configuration in pterosaurs.

 

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. Figure 1. The evolution of the pterosaur pectoral girdle and sternal complex featuring Huehuecuetzpalli, Cosesaurus, Longisquama, and the basal pterosaur, MPUM 6009.

The Pectoral Girdle in Longisquama
In Longsiquama (Figure 1) the interclavicle, clavicles and sternum are closely integrated, as in pterosaurs. Distinct from all other tetrapods, the clavicles curved posteriorly, extending to the posterior rim of the crescent-shaped sternum, which they frame. The cruciform interclavicle extended ventrally to form a small keel. Taphonomically displaced to beneath the throat, the overlapping clavicles were mistaken by Jones et al. (2000) for a bird-like furcula (fused clavicles in birds).

The Pectoral Girdle in Pterosaurs
In basal pterosaurs (Figure 1) there were few changes from the Longisquama pattern. So the sternal complex (Wild 1994), like many other aspects of pterosaur morphology, had evolved before the advent of large pterosaurian wings (Peters 2002, contra Bennett 2008).

Summary
All these changes could never have taken place if Cosesaurus was restricted to a typical quadrupedal configuration. The forelimbs had to become elevated from the substrate in a bipedal configuration, as imagined (based on morphology) in its phylogenetic predecessors, Lacertulus (Carroll and Thompson 1982) and Huehuecuetzpalli — and as evidence by matching Cosesaurus pedes to Rotodactylus tracks (Peters 2000b) which were ocassionally bipedal. Cosesaurus had a pectoral complex essentially and mechanically identical to that of pterosaurs (and broadly similar to that of birds). So it seems likely that it was also flapping, probably in some sort of territorial or mating ritual, long before gliding and flying were possible in its descendant taxa, Sharovipteryx, Longisquama and pterosaurs.

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:
Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. Pp. 127–141 in E. Buffetaut & D.W.E. Hone (eds.), Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. Zitteliana, B28.
Carroll and Thompson 1982. A bipedal lizardlike reptile fro the Karroo. Journal of Palaeontology 56:1-10.
Elgin RA, Hone DWE and Frey E 2011. The extent of the pterosaur flight membrane. Acta Palaeontologica Polonica doi: 10.4202/app.2009.0145 online pdf
Jones TD et al 2000. Nonavian Feathers in a Late Triassic Archosaur. Science 288 (5474): 2202–2205. doi:10.1126/science.288.5474.2202. PMID 10864867.
Peters D 2000a. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2000b. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. Historical Biology 15: 277-301.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Reynoso V-H 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate (Reptilia) from the Early Cretaceous of Tepexi de Rodríguez, Central México. Philosophical Transactions of the Royal Society, London B 353:477-500.
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.

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