Several traits indicate pterosaurs were aerobic and endothermic (warm-blooded)

Pterosaurs, by all accounts, were not your ordinary saurians.
Pterosaurs arose from a previously unreported clade of extinct lepidosaurs, the Tritosauria, not from any living squamates. They could fly and some were fantastically adorned with crests and soft tissues that enabled flight. Moreover, many, if not all, had hair/fibers/fur. The origin of these fibers appears to be in non-volant Middle Triassic fenestrasaurs at the level of Sharovipteryx and Cosesaurus, long before dinosaurs and birds developed protofeathers.

Living lizards are ectothermic (cold-blooded). Pterosaurs are widely considered to be endothermic (warm-blooded) due to their fur-covering, but that’s not the complete story. There’s more:

Jeholopterus2013-588

Figure 1. Jeholopterus in lateral view. Note the extreme length of the dermal fibers, unmatched by other pterosaurs, likely to keep biting insects away from its sensitive skin as it exploited and made wounds on dinosaurs.

1. Ptero fur — aka: pycnofibers, covered pterosaur bodies according to several well-preserved fossils of small pterosaurs. Most of our evidence for a pelage comes from small German and Chinese pterosaurs, but at least some specimens of Zhejiangopterus, a large azhdarchid, had ptero-fur. Preservation of hair apparently depends on subtle differences in substrate geochemistry. Like feathers, ptero-fur could have had uses other than trapping body heat, like keeping flying insects (mosquitoes and flies) from biting sensitive skin, as in Jeholopterus (Fig. 1) and, or course, could be considered decoration or camouflage if striped, spotted or colored.

2. Tiny adult size and even tinier hatchlings — We’ve seen in phylogenetic analysis that tiny pterosaurs succeeded fading larger clades and preceded expanding larger clades. Thus reducing adult size was a survival mechanism for the gene pool. Since moisture loss and heat loss would have been more stressful for tiny pterosaurs and especially the hatchlings of tiny pterosaurs, a pelage might have been useful to keep the wee ones warm, but mostly moist. “Endothermy originated in smaller, active eurythermal ectotherms living in a cool but variable thermal environment,” according to Clarke and Pörtner 2010. Desiccation is the main problem facing today’s tiniest reptiles, all of whom are restricted to moist leaf litter environs (Hedges and Thomas 2001). Unfortunately, we have no examples of tiny pterosaurs with ptero-fur.

Pterodactylus with hair in life pose, preparing to take off.

Figure 1. Pterodactylus with hair in life pose, preparing to take off.

Conversely large pterosaurs with soft and hard crests and extremely long necks and wings increased their surface-to-volume ratios, expanding these natural passive heat radiators when deploying their wings, evidently reducing the need for insulation and fur. We don’t see the body diameter length ptero-hair on large pterosaurs, like we do on Jeholopterus. Rather large pterosaurs, like Zhejiangopterus, appear to have had a short pelage.

3. Flying — Active muscle rapidly gets warm and steady activity due to flying gets a boost from an endothermic aerobic metabolism. The most widely accepted explanation for the evolution of endothermy has been selection for enhanced aerobic capacity.

On the flip side, flying by its very nature, requires a constant airstream and with it, heat loss by convection — if the ambient air is cooler than the body. This is emphasized in pterosaurs with their long wings laced with blood vessels, perhaps acting like giant gills, if not in oxygenation, then in heat exchange.

4. Short, laterally stiff torso — Most lizards cannot breathe while running quadrupedally. Undulating lizards experience Carrier’s constraint because their lungs cannot fill with air while laterally undulating (one lung compresses as the other expands then vice versa beneath the expanding and contracting ribcage). Short torso pterosaurs (and Sharovipteryx) did not undulate. Like birds, they don’t use their tail muscles to retract their hind limbs. Femora retractors have shifted to the enlarged hips. Pterosaurs breathed like we do and like birds do, by expanding both sides of their ribcage at once. (Not but rotating their prepubes back and forth! Gaak!)

5. Hollow bones –- Like warm-blooded birds, many pterosaurs had hollow bones that probably contained air sacs that inflated and cooled the bones with air from their advanced lungs.

6. Erect hind limbs —  Like warm-blooded birds, pterosaurs walked with more or less erect hind limbs that elevated their bellies far above the substrate. Maintaining this configuration required more energy than belly-floppers typically muster.

Clarke and Pörtner (2010) declared the metabolic status of pterosaurs remains unresolved. They reported, “Endothermy has evolved at least twice, in the therapsid-mammal and theropod-bird lineages. The benefits of endothermy are clear: a high and relatively constant internal body temperature allows a fine tuning of metabolism, high muscular power output, fast growth, and a significant degree of independence from environmental temperature. The costs are also well understood: the high rate of metabolism needed to sustain endothermy requires a great deal of food. Undoubtedly the most successful hypothesis, however, has been the suggestion of Bennett & Ruben (1979) that the key factor in the evolution of endothermy was selection for an enhanced aerobic capacity to allow increasingly sustained locomotor activity. The evolution of a higher body temperature and endothermy followed as secondary events. This proposal, the aerobic scope hypothesis, has withstood two decades of further research, and it remains the most widely accepted theory for the evolution of endothermy.”

Benefits of a warmer body

  1. Processing of food proceeds more rapidly
  2. Speed of nervous conduction is temperature dependent
  3. Higher growth rates in the young
  4. Improved food-gathering capability by adults for provisioning developing young. [This is likely not important for pterosaurs, who were likely independent from the moment of hatching because they could fly.]

Embryo development
Like other lizards, pterosaur mothers held eggs within their bodies until just before hatching. This warmth likely decreased the in-utero period by accelerating the embryo’s development, enabling flying shortly after hatching. Since pterosaurs likely laid only one egg at a time, (none have been found in clutches), accelerating embryo development would have increased the reproductive rate, especially among tiny pterosaur adults, which reached adulthood more rapidly.

References
Clarke A and Pörtner H-O 2010. Temperature, metabolic power and the evolution of endothermy. Biological Reviews online.
Hedges SB and Thomas R 2001. At the Lower Size Limit in Amniote Vertebrates: A New Diminutive Lizard from the West Indies. Caribbean Journal of Science 37:168–173.