Does a Big Crest Make a Nyctosaurus Mature?

Bennett (2003) reported on two Nyctosaurus discoveries made by fossil collector Kenneth Jenkins. Both (Fig. 1) had the largest crests, relative to skull length and body size, of any known pterosaur. This came as even more of a surprise to paleontologists because no other Nyctosaurus specimens show any hint of a crest.

Late in Ontogeny?
Bennett (2003) reported in his abstract, “Despite the large crest, the specimens do not differ significantly in morphology from previously known specimens of Nyctosaurus, and do not represent a new species of Nyctosaurus. The specimens suggest that the cranial crest was developed late in ontogeny, which is consistent with the interpretation of pterosaur cranial crests as intraspecific display structures.” Unfortunately, these statements have been taken as gospel and have been uncritically repeated. For instance, here’s an online pdf of an article by Greg Paul from the Prehistoric Times.

Nyctosaurus clade

Figure 1. The clade of Nyctosaurus and kin. Click to enlarge.

Actually the Variations is Easy to See
Bennett (2003) did not make reconstructions of the clade for comparison. One look at reconstructions of several known Nyctosaurus specimens shows that none are conspecific (Fig. 1). There ARE many significant differences in morphology (contra Bennett 2003, details in reptileevolution.com starting here). Even the two crested Nyctosaurus specimens have distinct differences in crest shape and wing length.

In Pterosaurs More Mature = Larger
If the crested specimens were indeed more mature, then one would expect them to be larger, following the study by Chinsamy et al. (2008) on the growth series documented in Pterodaustro, the only pterosaur with a varifiable growth series. That study found that sexual maturity occurs at half the largest size attained by individuals, a pattern also found in certain lizards like Iguana (Kaplan 2007)  and Varanus (Pianka 1971). The crested specimens are actually smaller than some, similar in size to other Nyctosaurus (not counting the largest known Nyctosaurus specimens known from a pelvis and disassociated scraps.) Nyctosaurus nanus (known from a humerus and pectoral girdle) is the only Nyctosaurus that is genuinely smaller than the others pictured here.

Is the Crest a Sexual Signal?
Sure. It appears that the crest is a secondary sexual characteristic. If so one would expect a crest to appear at sexual maturity (half the final size). There is only one pair of crested pterosaurs that I am aware of that appear to be conspecific and those are a pair of tupuxuarids that have identical crests, identical rostral lengths and identical orbit sizes relative to their overall size. The smaller specimen is less than half the size of the larger one, so it was prepubescent, which falsifies the notion of a sexual signal. No, the crests appear to have identified species, not gender, maturity or sexual fitness (mutual selection). Other sorts of secondary sexual characters must have been present in crested and crestless specimens, such as wattles, coloration or behavior.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Bennett SC 2003. New crested specimens of the Late Cretaceous pterosaur Nyctosaurus.Paläontologische Zeitschrift 77: 61-75.
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
Kaplan M 2007. Iguana Age and Expected Size. iguana/agesize online
Maisano JA 2002. Terminal fusions of skeletal elements as indicators of maturity in squamates. Journal of Vertebrae Paleontology 22: 268–275.
Pianka E 1971.
Notes on the Biology of Varanus tristis. West. Aust, Natur, 11(8):80-183.

wiki/Nyctosaurus