Today’s paper on Rhamphorhynchus bone histology (Prondvai et al. 2012) reported, “Whereas morphological studies suggested a slow crocodile-like growth strategy and superprecocial volant hatchlings, the only histological study hitherto conducted on Rhamphorhynchus concluded a relatively high growth rate for the genus. These controversial conclusions can be tested by a bone histological survey of an ontogenetic series of Rhamphorhynchus.”
Rhamphorhynchus presents a wide variety of adult sizes with the largest specimens 8x as tall as the smallest (Fig. 1). Unfortunately, the authors gave no indication that the five specimens in their study were from the same single species (of several) within the genus which would have presented an ontogenetic series. This can be determined only by a cladistic analysis of several dozen Rhamphorhynchus specimens, including their five study specimens, but that first step was not done. Figure 1 demonstrates a phylogenetic series (as determined by cladistic analysis) of the genus Rhamporhynchus. The morphological differences are easy to see. Even the pedal phalanges differ among species (Peters 2009), as noted earlier.
Substantial morphological change during growth does NOT occur in pterosaurs, as already demonstrated by embryos (especially Pterodaustro). What we’re looking for are tiny versions of the adults already known in the fossil record, and to my knowledge, no juveniles have yet been found that morphologically match 2x to 8x larger adults (the latter being the standard difference between hatchlings and adults as determined by hypothetical egg sizes matched to pelvic openings and the example of Pterodaustro). Ptweety is the only juvenile pterosaur I know of and it also has the proportions of an adult.
What Happens During a Phylogenetic Size Squeeze in Pterosaurs?
“Teens” start having babies. Sexual maturity comes sooner and sooner. Longevity decreases. Adult size decreases. Chinsamy et al. (2008) reported that sexual maturity in Pterodaustro occurred at half the largest size attained. Smaller hips produce smaller eggs. The bone histology of a small specimen with a shorter lifespan mimics the expected histology of a juvenile. Scapulocoracoid fusion likewise disappears during serial phylogenetic size reductions.
The limiting factor in superprecocial flight still appears to be dermal evaporation and desiccation due to a large surface of volume ratio in tiny pterosaurs. Only those pterosaurs hatching from eggs the size of the three published embryos appear to create a threshold size for flight shortly after hatching. Smaller hatchlings than this had to be terrestrial, hiding in damp leaf litter, rather than taking to the skies, so the lack of flying in hatchling pterosaurs reported by Prondvai et al. (2012) is likely correct, though not for the same reason.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
Prondvai E, Stein K, Ősi A. and Sander MP (2012) Life History of Rhamphorhynchus Inferred from Bone Histology and the Diversity of Pterosaurian Growth Strategies.
PLoS ONE 7(2): e31392. doi:10.1371/journal.pone.0031392