For those who missed it, Neil Brocklehurst was kind enough to remind me that, “It is IMPOSSIBLE for evolution to occur without changes in ontogeny. Not unlikely, not rare, impossible.”
My answer: The changes you refer to occur in the egg, not after hatching. As I’ve posted before, the fact that Pterodaustro came into sexual maturity at half of its final old age size (Chinsamy et al. 2008) is the mechanism by which pterosaurs could rapidly produce half-size progeny (embryo can’t be larger than the egg > egg can’t be larger than the pelvic opening). Evidently this can be applied across the pterosaur board, based on the many tiny transitional series that phylogenetically follow larger specimens and then beget larger specimens that live longer, get bigger, have bigger eggs, etc.
References
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
The Pterosaur Heresies 