Ontogeny: Pteranodon vs. Diomedea

The Albatross (Diomedea exulans) and Pteranodon ingens (Fig. 1) were two large soaring reptiles, the former a bird, the latter a pterosaur. Insight into the ontogeny, maturity and lifespan of the extant Diomedea may shed some light on the extinct Pteranodon.

Pteranodon and the albatross

Figure 1. Left: Pteranodon. Right: Diomedea (albatross).

Widest Wingspan Among Birds
The albatross has a wingspan that reaches an extreme of 12 feet and averages 9 feet. The pterosaur Pteranodon had a similar wing plan with an enlarged wingspan up to 20 feet. Thus, and for little other reason, the albatross is the closest living analog to Pteranodon.

Albatross: Latest Maturation Among Birds
Most birds reach maturity in less than a year and many (crow, ostrich) do so in two years. By contrast, albatross males begin breeding at age 7, females at age 10. Some wait until 13. The life expectancy of an albatross is 30 years. According to Couzens (2008) it takes years for the albatross to become proficient at finding enough food for itself and more to take on the extra task of feeding a chick. The albatross also takes a long time to establish a pair bond.

Lizards vs Birds: Traditional Views
Most workers follow the paradigm that cold-blooded lizards mature more slowly than warm-blooded birds and mammals. Often, but not always, this is the case. However, more than mere physiology, size is typically an overriding factor. As everyone knows, mice mature faster than dogs, which mature faster than elephants and humans. The blue whale, which matures at 5 (females) or 8+ (males), does not follow this pattern. Among cold-blooded reptiles, iguanas are sexually mature at 50% of maximum size before the end of the second year (Kaplan 2007). Varanus hatchlings triple in size to sexual maturity and reach maximum size by the end of the first year (Pianka 1971). However some may continue growing thereafter, reaching up to 50% longer.

Pterodaustro Ontogeny
The only pterosaur for which a complete record of growth is known is the filter-feeder Pterodaustro. Chinsamy et al. (2008) reported: “…upon hatching, Pterodaustro juveniles grew rapidly for approximately 2 years until they reached approximately 53% of their mature body size, whereupon they attained sexual maturity. Thereafter, growth continued for at least another 3–4 years at comparatively slower rates until larger adult body sizes were attained.” So, this pterosaur’s growth rate, despite an apparent warm-blooded metabolism and active lifestyle, was not dissimilar to that of other lizards, reaching sexual maturity at 50% of the ultimate size. However, Pterodaustro took twice as long as Varanus, a cold-blooded lizard. Apparently, growth was not so rapid in pterosaurs — more along the lines of Iguana.

Pteranodon
If we add in the factor of increased size to what we know of Pterodaustro, we can imagine that Pteranodon might have had a maturation rate similar to that of the albatross (sexually mature at 7 to 10) along with a similar lifespan (30 years). However, if Pteranodon was more like Pterodaustro we get 2 years until half-grown, 5 to 6 years until fully grown.

Where Are the Juvenile Pterosaurs?
A long maturation brings up a problem. Where are the juveniles and immature forms (ages 0 to 6)? The Pterodaustro bone beds (nesting sites) provide the only evidence. There we find all sizes of Pterodaustro.

Ptweety the Only Juvenile Pteranodon
We know of only one juvenile Pteranodon. All others are adults that fit neatly into a phylogenetic framework of increasing size and crest size originating with a specimen of Germanodactylus (SMNK-PAL 6592) as an outgroup. This falsifies the current paradigm presented by Bennett (1991, 2001) and followed by others (Hone et al. 2011) of gender and maturation variation in most known specimens of Pteranodon. Here there is evidence of speciation leading to the largest crested forms (that in one clade only preceded a continuing clade of smaller crested, smaller forms.)

Bone Histology
The age of sexual maturity in Pteranodon has not yet been determined. Neither has the lifespan. Bone histology in Pteranodon has not provided the data needed due to crushing and resorption of the inner walls of the extremely thin long bones. At present we can only guess using extant analogs, like the albatross, and extinct analogs, like Pterodaustro.

Then There’s the Tiny Pterosaur Hypothesis
Tiny pterosaurs giving birth to fly-sized hatchlings were likely terrestrial until reaching adult-size due to desiccation problems, as discussed earlier. Larger pterosaur hatchlings, like Pteranodon (and all known, apparently flight ready pterosaur embryos), did not have a problem with desiccation — but they may have retained some sort of non-flying lifestyle living in environments not conducive to fossilization. This may explain the lack of immature pterosaurs in the fossil record (contra all traditional studies that considered tiny adults to be juveniles and embryos to be flight ready).

Not ready to jump on the flightless hatchling hypotheses quite yet, but it’s something to consider when faced with current and future evidence.

Just a Reminder
Maisano (2002) provides guidance on lizard ontogeny that can be applied to pterosaurs. That is: fusion can precede maturation and ultimate size or fusion may never take place in the oldest individuals, depending on their phylogeny. Recent work by Lü et al. (2012) show that the archosaur model continues to be wrongly applied to pterosaur studies.

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
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
Couzens D 2008. Extreme Birds: The world’s most extreme and bizarre birds. Firefly Books.
Hone DWE Naish D and Cuthill IC 2011. Does mutual sexual selection explain the evolution of head crests in pterosaurs and dinosaurs? Lethaia, DOI: 10.1111/j.1502-3931.2011.00300.x
Kaplan M 2007. Iguana Age and Expected Size. iguana/agesize online
Lü J, Unwin DM, Zhao B, Gao C and Shen C 2012. A new rhamphorhynchid (Pterosauria: Rhamphorhynchidae) from the Middle/Upper Jurassic of Qinglong, Hebei Province, China. Zootaxa 3158:1-19. online first page
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.