Pterodactylus? kochi? B St 1967 I 276 (No. 6 of Wellnhofer 1970) was the smallest all of all known pterosaurs (with the possible exception of the embryo Ornithocephalus) and smaller than three embryo pterosaurs, yet it was probably an adult. Derived from a sister to SMNS 81775, No. 6 phylogenetically preceded No. 12 at the base of the Germanodactylia.
Overall smaller than and distinct from SMNS 81775 and No. 31, the skull of No. 31 had a longer antorbital fenestra and taller orbit. The premaxilla was slightly larger. The jugal descended.
Distinct from No. 31, the cervicals and torso were longer.
The deltopectoral crest was squared off. The humerus was straight. The metacarpus and ulna were longer.
The hind limb was more robust. The unguals were shorter. Digit V was straight and shorter.
Size comparisons between No. 23, No. 12 and No. 6 can be found here and here.
The surface/volume ratio is maximized in this species and its hatchlings. Considering that living lizards with a snout/vent length under 18mm (Hedges and Thomas 2001) dry up and die when removed from their moist leaf litter environment, the younger, smaller juveniles of this pterosaur were likewise destined to always fend off desiccation. The adults had a snout/vent length of just over 40 mm, so hatchlings would have been in the range of 5mm, perhaps too small to fly in dry air, unless they had a novel method for conserving moisture with their wings outstretched. They would have been the size of house flies.
Hone and Benton (2006) reported, "The remarkable extinct flying reptiles, the pterosaurs, show increasing body
size over 100 million years of the Late Jurassic and Cretaceous, and this seems
to be a rare example of a driven trend to large size (Cope’s Rule)." They arrived at this "result" by drawing a straight line from early pterosaurs, like Anurognathus, to the Late Cretaceous pterosaur, Quetzalcoatlus over time and by deleting all purported juveniles. They did not realize that 1) there were four pterodactyloid-grade lineages; 2) the purported juveniles were actually adults; and 3) any sort of a roller-coaster effect of size increase/decrease/increase/decrease over time would be negated by drawing a straight line.