“Protopterosaurus” – still the official poster child for pterosaur ancestry

Wild 1984 had just finished describing the first Triassic pterosaurs when he proposed a hypothetical pterosaur ancestor (Fig. 1) that was re-illustrated by Wellnhofer (1991) in his famous pterosaur encyclopedia. Today, nearly 30 years later, most pterosaur experts are still crossing their fingers, hoping for just such a creature to appear in the fossil record, having dismissed and rejected (Hone and Benton 2007, 2008) several actual candidates proposed by Peters (2000), now some 12 years ago, and supported by a greatly expanded phylogenetic analysis here.

"Protopterosaurus" t

Figure 1. “Protopterosaurus” the hopeful hypothetical ancestor of pterosaurs created by Wild (1984) and supported by Wellnhofer (1991). Not sure why the tail was so thick and the hind limbs were so short. Cosesaurus demonstrates that the flying membrane aft of the forelimb originated distally, not medially, with a narrow-gauge occasionally bipedal trackway in which pedal digit 5 impressed far behind the other pedal digits, known as Rotodactylus. So, this should be a runner and flapper first, then a tree climber.

So the question is, how well does the hypothetical ancestor stack up against the real McCoys, as determined by phylogenetic analysis? [Think Cosesaurus.] 

And what  would “Protopterosaurus?” be? A lizard or an archosaur?

What is “Protopterosaurus”?
Wild (1984) and Wellnhofer (1991) both illustrated “Protopterosaurus” as an unarmored arboreal quadrupedal reptile with laterally extended limbs and digit 4 the longest on all four digits. Thus it can’t be an archosaur! Hmm. With that morphology it would have nested in the lizard or protorosaur clades with a strong lean toward the former. In several regards “Protopterosaurus” resembled something between Huehuecuetzpalli and Macrocnemus, (the latter Wild was quite familiar with), but with a much thicker tail.

Unfortunately we can’t peer beneath the skin here. “Protopterosaurus” was illustrated ‘in vivo.’

The Head
The naris was displaced from the tip of the snout in “Protopterosaurus” as in fenestrasaurs, not archosaurs. The naris preceded a depression, presumably an antorbital fenestra, as in Cosesaurus and archosaurs. The snout was pointed in both dorsal and lateral views (more like Cosesaurus). The eyes were raised above the midline of the back of the skull, probably due to a deep jugal. That’s not duplicated in any candidate, all of which had a larger orbit expanded nearly to the jawline, with a very shallow jugal.  The retroarticular process extended far behind the cranium, producing an anteriorly leaning quadrate. Likewise, that is not found in any candidate except, perhaps, Jesairosaurus.

The Neck, Torso and Tail
The cervical series in “Protopterosaurus” was as long as the skull and the neck was robust, as in most candidates. No S-curve was given to the dorsal series. No armor was present either. The caudals must have been provided with wide transverse processes and deep chevrons as the tail is illustrated to be thick and meaty, like that of most archosaurs and lizards — and unlike the tritosaurs and fenestrasaurs, including pterosaurs, all of which had a very thin bony tail.

The Appendices
The forelimbs of “Protopterosaurus” extended laterally with fingers of increasing length laterally, especially digit 4, which was as long as the humerus+ulna. Cosesaurus had fingers 3 and 4 virtually subequal when it developed a prepubis, pteroid, a locked down coracoid, sternal complex, membranes and an antorbital fenestra. So the flight digit came later, in Longisquama. Finger 5 was a vestige in “Protopterosaurus“. Finger 5 in reality was always short, but did not become a vestige until the pterosaur stage. The metacarpals of “Protopterosaurus” were illustrated longer medially than laterally, which is not duplicated in any candidate.

The femur was no longer than the humerus in “Protopterosaurus” and the tibia was no longer than the femur. These proportions are not found in present candidates, all of which had a longer hind limb than forelimb and relatively larger hind limbs compared to the torso. All candidates, from Scleromochlus to Cosesaurus, have been illustrated as bipeds and narrow-gauge tracks are known for a sister to Cosesaurus. Thus the lateral limbs and belly down configuration are not duplicated in any candidate. The foot in “Protopterosaurus” was about the length of the tibia, which is correct. Pedal digit 5 extended no further than p4.1 or p4.2, which is also correct with regard to Cosesaurus. Scleromochlus and its sisters had a vestige to no pedal digit 5, which essentially removes them from pterosaur sisterhood candidacy.

Soft Tissue
Flying squirrel-like membranes in “Protopterosaurus” extended from finger 4 to the knee, remaining shallow until reaching the elbow. Current evidence (Peters 2009) indicates that membranes developed distally first, just the opposite of the hypothetical traditional model. Current evidence indicates that flapping as a secondary sexual behavior also preceded gliding, so “Protopterosaurus” had it backwards. Minor membranes appear in “Protopterosaurus” behind the acetabulum and a propatagium (strangely without a pteroid and preaxial carpal) appeared in front of the forelimb. We know Cosesaurus had a pteroid and preaxial carpal (Peters 2009), but no propatagium is known in relation to it.

Wellnhofer (1991) considered the development of wing membranes as an aid in falling, parachuting and surviving, then in gliding to expand the range of escape and exploitation. Last to develop would be the enlarged sternum and pectoral girdle to add thrust to gliding. This is the opposite of what actually happened, as reported earlier and demonstrated in Cosesaurus, which used flapping as a secondary sexual behavior after becoming bipedal.

So, the imagined creature, “Protopterosaurus,” had many traits later found in the actual ancestors of pterosaurs, but missed several others. Not sure why current studies in pterosaur ancestry are not at least considering the possibility that fenestrasaurs and tritosaurs are good candidates for this honor. The first PhD to do so will be much less embarrassed by the last twelve years than the last PhD to do so will be.

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
Peters D 2000. 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 1984.  
A new pterosaur (Reptilia, Pterosauria) from the Upper Triassic (Norian) of Friuli, Italy. Gortania Mus Friulano Stor Nat 5: 45-62.
Wellnhoffer P 1991.  The Illustrated Encyclopedia of Pterosaurs. London: Salamander. 192 pp.

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