Scleromochlus nested with basal bipedal crocodylomorphs,
(Fig. 1) close to the origin of dinosaurs. Note the tiny hands on Scleromochlus. Note the lack of pedal digit 5 on Scleromochlus. By contrast, pterosaurs had large hands and a specialized pedal digit 5 that had two large phalanges that folded together such that the distal phalanx was dorsal side down, making an impression behind pedal digits 1–4 (Figs. 10, 11). More on this below.
Figure 2. The origin of pterosaurs now includes Kyrgyzsaurus, nesting between Cosesaurus and Sharovipteryx. Click to enlarge.
Pterosaurs didn’t fossilize very well?
False. Look at all the excellent pterosaur fossils we know of, some with soft tissue.
Pterosaurs are not archosaurs.
Peters 2000 introduced the clade Fenestrasauria for pterosaurs + their above named ancestors. These in turn were part of a new clade of lepidosaurs, named Tritosauria
, nesting between Rhynchocephalians and Protosquamates published in Peters 2007.
Ornithodirans are a junior synonym
for Reptilia (=Amniota, see cladogram link below). Not wise to bring up this invalidated clade name.
Figure 3. Scaphognathians to scale. Click to enlarge.
The pterodactyloid grade of pterosaur
was attained four times by convergence (two from the genus Dorygnathus,
two more from the genus Scaphognathus
, Fig. 3). Transitional taxa were all tiny Solnhofen forms (Fig. 3). As in many other clades, phylogenetic miniaturization attended the genesis of derived pterosaurs.
As in giant birds,
(Fig. 4) grew so large because it was flightless
. All azhdarchids over six-feet-tall had clipped wings (vestigial distal wing phalanges) good for flapping and walking on, not for flying.
Figure 4. Estimating giant azhdarchid weight from estimated height and comparables with similar smaller taxa.
Figure 5. Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex.
How did pterosaurs get their wings?
Convergent with theropods ancestral to birds, Cosesaurus
reorganized its pectoral girdle to flap (Fig. 5). The scapula became immobile and strap-like. The coracoid became immobile and stalk-like. The clavicles, interclavicle and single sternum migrated together, then fused together. The forelimbs of Cosesaurus were too short for flight, but fully capable of flapping, probably as a mating ritual. Likewise the pectoral girdles of Sharovipteryx
and Longisquama were similarly built. Of the three, Longisquama
had the largest hands, but still could not fly. Bergamodactylus
was the basalmost pterosaur and it could fly. See links below.
Why guess how a hypothetical ancestor learned to fly
when we have excellent samples of every stage? (see links below)
The arboreal leaping model
does not require flapping — and gliders do not evolve into flappers (e.g. colugos, squirrels, sugar gliders, etc.)
The arboreal parachute model
worked for bats
, but they were seeking prey beneath their perches as fingers 3-5 then 2-5 elongated. Pterosaurs only elongated one digit: #4. It made a better wing than bug-in-the-leaf-litter trap.
The terrestrial model
is Lamarckian, growing bigger wings to catch insects just out of reach for most is not good science.
Figure 6. Cosesaurus forelimb with pro to-aktinofibrils trailing the ulna.
The valid hypothesis for bird and pterosaur wing evolution is competitive attractiveness during mate selection (think birds-of-paradise) with cosesaur-like creatures flapping and displaying. BTW, both Cosesaurus and Longisquama are preserved with membranes trailing finger 4, (Fig. 6) which folds in the plane of the wing in Longisquama (Fig. 7).
Figure 7. Click to enlarge. The origin of the pterosaur wing and the migration of the pteroid and preaxial carpal. A. Sphenodon. B. Huehuecuetzpalli. C. Cosesaurus. D. Sharovipteryx. E. Longisquama. F-H. The Milan specimen MPUM 6009, a basal pterosaur.
Not to be outdone,
Sharovipteryx (Fig. 8) had membranes (uropatagia) trailing each hind limb. These are reduced in pterosaurs, which continue to use their hind limbs as horizontal stabilizers, their feet as twin rudders, as the flapping forelimbs, closer to the center of gravity, become ever larger, better for display, then for short flapping hops, then for flight.
Figure 8. Sharovipteryx reconstructed. Note the flattened torso.
Another false statement corrected here:
The scapula of Scleromochlus (Fig. 1) was tiny. It only had to support a tiny forelimb with vestigial fingers.
Scleromochlus had a ‘square pelvis’
because it, too was a biped. But that was nothing compared to the larger pelvis of Cosesaurus (Fig. 9), which also had a prepubis, a pterosaurian trait not found on Scleromochlus. The pelvis of Sharovipteryx was larger still.
Figure 9. Cosesaurus flapping. Tere should be some bounce in the tail and neck, but that would involve more effort and physics.
Scleromochlus had a long muscular tail.
As in crocs and dinos, and most reptiles, the caudofemoral muscles were pulling the femur. Compare that with the attenuated tail of pterosaurs, Cosesaurus and Sharovipteryx. Only pelvic muscles were pulling the femur.
Back legs longer than front legs in Scleromochlus?
That’s what we also see in Cosesaurus, Sharovipteryx and Longisquama.
Figure 10. Cosesaurus and Rotodactylus, a perfect match. Elevate the proximal phalanges along with the metatarsus, bend back digit 5 and Cosesaurus (left) fits perfectly into Rotodactylus (right).
Walking on its toes?
We have Rotodactylus ichnites
(hand and footprints, Figs. 10, 11) that match Middle Triassic Cosesaurus
in the Early Triassic. These include the impression of pedal digit 5 behind toes 1-4. Nothing else like them in the fossil record.
was like the modern jerboa
, with its tiny vestigial hands, totally inappropriate as a pterosaur ancestor.
Not all pterosaur tracks are quadrupedal. Only derived pterosaurs, those that frequented beaches were. We have bipedal pterosaur tracks (Fig. 12). See references below.
Figure 11. Cosesaurus foot in lateral view matches Rotodactylus tracks.
Quadrupedality in pterosaurs is secondary.
Note the backward pointing manual digit 3 in quad tracks. Note the fusion of four to thirteen sacrals into a sacrum and the elongation of the ilium to anchor large femoral muscles and anchor the increasingly larger sacrum in all pterosaurs. In order to flap, you have to be a biped.
Figure 12. Pteraichnus nipponensis, a pterosaur manus and pes trackway, matched to n23, ?Pterodactylus kochi (the holotype), a basal Germanodactylus.
All quad pterosaurs can be attributed to pterodactyloid-grade pterosaurs,
those that underwent phylogenetic miniaturization during the Jurassic. At that time, the fly-size hatchlings of the hummingbird-sized adults (Fig. 13) could not leave the moist leaf litter or risk desiccation until growing to a sufficient size. So they walked around on all fours until attaining flight size.
Figure 2. A hypothetical hatchling No. 6 alongside a fly, a flea and the world’s smallest insect, a fairy fly (fairy wasp). The fairy wasp is shown enlarged here (scaled in red) and in figure 1.
The extinction of pterosaurs can be attributed to their great size at the end of the Cretaceous. They had no tiny representatives, like they did at the end of the Jurassic, to weather the rapid climate changes and/or seek shelter.
For fossils and reconstructions of pterosaur ancestors, see:
And here are all the peer-reviewed academic publications
that some pterosaur experts don’t want to talk about:
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.
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141.