Pterosauria

Pterosauria (from the Greek pteron, "wing," and sauros, "lizard") is an extinct clade of flying archosaurian reptiles (Sauropsida) that dominated Mesozoic skies for over 160 million years from the Late Triassic (Norian, ~228 Ma) until the Cretaceous–Paleogene (K–Pg) boundary mass extinction (66.0 Ma). As the first vertebrates in Earth's history to evolve active powered flight—preceding birds (Aves) by ~75 million years and bats (Chiroptera) by ~175 million years—pterosaurs ranged in size from the sparrow-sized forest insectivore Nemicolopterus (~25 cm wingspan) to the Late Cretaceous azhdarchid giants Quetzalcoatlus northropi, Hatzegopteryx thambema, and Arambourgiania, which stood as tall as a giraffe (~5 meters at the shoulder) with 10- to 11-meter wingspans and 200–250 kg body masses, making them the largest flying animals of all time.

Crucially, pterosaurs were not dinosaurs: within the bird-line archosaur clade Avemetatarsalia (Ornithodira), Pterosauria (grouped with Triassic terrestrial cursorial lagerpetids† inside Pterosauromorpha, Ezcurra et al., 2020) is the sister clade to Dinosauromorpha (Dinosauria + Aves).

1. The Hypertrophied Fourth Finger Wing Spar and Composite Wing Membrane: Whereas a bird's wing is formed by pennaceous feathers attached to a fused arm and hand (carpometacarpus) and a bat's wing stretches a skin membrane across four elongated fingers (digits II–V), the pterosaur wing is supported along its leading edge by a single, enormously elongated fourth finger (manual digit IV) while digits I–III remain small, clawed grasping fingers at the wrist bend and digit V is lost. Rather than a simple flap of leathery skin, soft-tissue fossils from the Solnhofen Limestone (Germany), Santana/Crato Formation (Brazil), and Jehol Biota (China) show that the primary wing membrane (brachiopatagium) was a living multi-layered composite airfoil:
   • An outer epidermis reinforced internally by closely packed, parallel, high-tensile structural fibers (actinofibrils, 0.05–0.2 mm thick) arranged radiating outward in the distal wingtip to prevent aeroelastic flutter and billow-twisting without requiring extra finger bones;
   • Underlying layers of striated muscle fibers, elastic connective tissue, and a dense vascular network for flight thermoregulation and active camber adjustment; and
   • A unique wrist bone found in no other animal—the pteroid bone—projecting forward or medially from the proximal syncarpal wrist to tension a muscular leading-edge forewing membrane (propatagium) between the wrist and shoulder, acting as a movable leading-edge aerodynamic flap (slat) during slow landing and takeoff!
2. Extreme Skeletal Pneumatization and Thin-Walled Tubular Bones: Like theropod dinosaurs and birds, pterosaurs possessed a unidirectional pulmonary air-sac system whose diverticula invaded the cervical vertebrae, dorsal notarium, sternum, humerus, and wing phalanges (Claessens et al., 2009). In giant pterodactyloids (Pteranodon, Quetzalcoatlus), the hollow wing bones had cortical bone walls only 0.8 to 1.5 millimeters thick—comparable to an ostrich eggshell!—prevented from buckling under bending and torsional loads by an elliptical cross-section and an internal lattice of diagonal bony struts (trabeculae), allowing a 7-meter-wingspan Pteranodon to have a total dry skeleton mass of only a few kilograms.
3. The Quadrupedal Vaulting Launch Hypothesis: How could a 250-kilogram Quetzalcoatlus get airborne when the largest modern flying birds (wandering albatrosses, Kori bustards, and Andean condors) hit a biomechanical ceiling at ~16–22 kg? As demonstrated biomechanically by Michael B. Habib (2008) and Mark P. Witton, birds launch into flight using only their hindlimbs (meaning a heavy bird must haul dead-weight flight muscles into the air using leg muscles that do not contribute to flight once airborne). In contrast, pterosaurs walked on all fours (confirmed by fossilized quadrupedal Pteraichnus trackways) and launched via a ballistic quadrupedal vault: crouching on all four limbs, Vaulting forward onto their hyper-muscular forelimbs (whose humerus and pectoral girdle had >2× the bending strength of their femurs!), and pole-vaulting over their wrists to catapult themselves at >10–13 m/s straight into the first wing downstroke!

Did feathers evolve only in dinosaurs and birds, or even earlier in the common ancestor of all Avemetatarsalia?

• Since the discovery of Sordes pilosus ("hairy devil") in Kazakhstan in 1971, paleontologists have known that pterosaurs were covered in a dense pelt of insulating filaments called pycnofibers—confirming alongside fibrolamellar bone histology that pterosaurs were endothermic (warm-blooded) vertebrates.
• In 2018 and 2022, Zixiao Yang, Baoyu Jiang, Maria E. McNamara, Michael J. Benton, and colleagues analyzed Jurassic anurognathid pterosaurs from China and the Early Cretaceous Brazilian tapejarid Tupandactylus imperator using scanning electron microscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS). They discovered four distinct morphological types of branched feathers (including monofilaments, tufted down-like bundles, and central shafts with laterally branching barbs) packed with color-producing melanosomes of distinct geometries (rod-shaped eumelanosomes vs. ovoid phaeomelanosomes in different feather bands!). This discovery indicates that branched feathers and melanosome color patterning evolved ~245 million years ago in the Early Triassic common ancestor of Pterosauria and Dinosauria.

Could hatchling pterosaurs ("flaplings") fly immediately after hatching from the egg?

Because birds provide intensive parental care to helpless naked nestlings (or ground-running precocial chicks whose wing feathers take weeks to grow), paleontologists debated whether pterosaurs cared for their young. The discovery of hundreds of fossilized parchment-shelled pterosaur eggs and 3D embryos of Hamipterus tianshanensis in Xinjiang, China (Wang et al., 2017) and X-ray CT biomechanical modeling of hatchling humerus strength and wing aspect ratio by David M. Unwin and Charles Deeming (2019) and Darren Naish et al. (2021) demonstrated that hatchling pterosaurs ("flaplings") emerged from buried eggs with fully ossified wing spars, ossified sternal attachments, and functional airfoils capable of powered, agile forest flight within hours or days of hatching—occupying a completely different ecological niche (small forest insectivores) from their giant open-plain or pelagic parents (ontogenetic niche partitioning)!

Why did pterosaurs go extinct at the K–Pg boundary (66 Ma) while smaller birds survived?

Throughout the Early and Mid-Cretaceous, small- and medium-bodied pterosaurs were progressively replaced in forested and coastal habitats by the radiating clade of toothed stem-birds (Enantiornithes and Ornithuromorpha), except as ontogenetic "flapling" juveniles of giant pterodactyloids (Benson et al., 2014). By the latest Maastrichtian (66.0 Ma), remaining pterosaur lineages (Azhdarchidae, Nyctosauridae, and Pteranodontidae) were medium-to-giant soaring and terrestrial specialists (3 to 11 meters wingspan) dependent on warm thermal updrafts, surface marine food webs, or large terrestrial prey—all of which collapsed abruptly during the dark, freezing impact winter triggered by the Chicxulub asteroid strike, whereas only small (<1 kg), ground-foraging, seed- and detritus-eating toothless crown birds (Neornithes) survived.

1. Witton, M. P., & Naish, D. (2008). A reappraisal of azhdarchid pterosaur functional morphology and paleoecology. PLOS ONE, 3(5), e2271. doi:10.1371/journal.pone.0002271
2. Claessens, L. P. A. M., O'Connor, P. M., & Unwin, D. M. (2009). Respiratory evolution facilitated the origin of pterosaur flight and aerial gigantism. PLOS ONE, 4(2), e4497. doi:10.1371/journal.pone.0004497
3. Lü, J., Unwin, D. M., Jin, X., Liu, Y., & Ji, Q. (2010). Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull. Proceedings of the Royal Society B, 277(1680), 383–389. doi:10.1098/rspb.2009.1603
4. Witton, M. P. (2013). Pterosaurs: Natural History, Evolution, Anatomy. Princeton University Press. doi:10.1515/9781400847655
5. Ezcurra, M. D., Nesbitt, S. J., Bronzati, M., Dalla Vecchia, F. M., Agnolin, F. L., Benson, R. B. J., ... & Langer, M. C. (2020). Enigmatic dinosaur precursors bridge the gap to the origin of Pterosauria. Nature, 588(7838), 445–449. doi:10.1038/s41586-020-3011-4
6. Cincotta, A., Nicolaï, M., Campos, H. B. N., McNamara, M., D'Alba, L., Shawkey, M. D., ... & Godefroit, P. (2022). Pterosaur melanosomes support signalling functions for early feathers. Nature, 604(7907), 684–688. doi:10.1038/s41586-022-04622-3