Bennett—Smallest Pteranodon
inter-LAG spacing decreasing outward demonstrating that the growth rate decreased progressively until full size was attained in ~7 years. If Pteranodon grew as Pterodaustro did, then LAGs would not be expected in FHSM 17956 because it is only ~46% the size of a fully grown modal female with ~3.8m wingspan. Bennett (1992) proposed that Pteranodon’s marked sexual
dimorphism and female biased sex ratio was indicative of a lek- type mating system with large males competing for access to more abundant females. In addition, the presence of immature specimens with poorly ossified epiphyses and limb bone shafts with virtually all bones unfused combined with the absence of specimens of <3m wingspan was interpreted as evidence of rapid growth to adult size. I assumed that juveniles could not have sustained rapid growth while flying and feeding indepen- dently, and so thought that Pteranodon hatchlings would have required parental feeding before flying and feeding indepen- dently, much as many extant seabirds do (Bennett, 1993). The discovery of FHSM 17956 does not affect the interpretation of sexual dimorphism, but falsifies the assumption of rapid growth and parental feeding before flying and feeding independently. If small juveniles were capable of flying out to sea, then
why are they so rare in the Smoky Hill Chalk Member? It is conceivable that there was a bias against the preservation of small specimens. The sample of Nyctosaurus from the Smoky Hill Chalk Member consists of 30 specimens, most of which have estimated wingspans of 1.6–2.0 m. If one were to assume that Pteranodon and Nyctosaurus were equally abundant in the Western Interior Seaway, then the relative rarity of small Nyctosaurus specimens (30 Nyctosaurus versus >1000 Pteranodon) might suggest a bias against the preservation of small individuals, but if that were the case then small juveniles of Pteranodon would still be ~30 times rarer than similarly sized specimens of Nyctosaurus. However, there is no evidence that Nyctosaurus was as abundant as Pteranodon and no evidence of a bias against the preservation of small pterosaurs. Therefore, the extreme rarity of small juveniles of Pteranodon in the Smoky Hill Chalk Member is interpreted as reflecting an extreme rarity in life because they did not normally go to sea until their wingspans were ~3 m. Juveniles between ~1.5 and ~3m wingspan must have been occupying different environ- ments and ecological niches than adults did. Thus, Pteranodon exhibited ontogenetic niches. Ontogenetic niches have been defined as varying patterns
of resource use as individuals increase in size from hatching or birth to adulthood and beyond (Werner and Gilliam, 1984), such that individuals in different stages of their life history occupy different environments and/or ecological niches in sequence.
Ontogenetic niches are found in a wide variety of animals. Most fishes exhibit several shifts of food type and often habitat as they grow; most amphibians transition from aquatic, often herbi- vorous larvae to carnivorous terrestrial adults, and newts return to water after a terrestrial eft stage with associated shifts in food type (Tomašević Kolarov et al., 2011); and many reptiles switch from carnivory to herbivory with increasing size. There can also be morphological changes associated with habitat and dietary changes; for example, hatchling Alligator mississippiensis (Daudin, 1802) occupy well-vegetated swamps and marshes, have short blunt snouts and needle-like teeth suited
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to piercing, and feed primarily on insects; juvenile A. mississippiensis have longer snouts and transition to crustaceans and small vertebrates; and adults prefer deeper open waters of rivers and lakes, have long and broad snouts and robust blunt teeth suited to crushing, and feed on turtles and large mammals (Subalusky et al., 2009; Gignac and Erickson, 2015). However, many birds and mammals do not exhibit ontogenetic niches other than a period of parental feeding before transitioning to feeding independently in the same environment and niche as their parents. Ontogenetic niches are often simply a necessary con-
sequence of the great increases in body length and mass that some organisms exhibit, e.g., a few mm to 2.5m body length in Atlantic Goliath Grouper Epinephalus itajara (Lichtenstein, 1822) (Artero et al., 2015) or a 7000-fold increase in body mass in Alligator (Gignac and Erickson, 2015), with the animal switching to larger and larger foods and the environments where those foods can be obtained as it grows. However, selection will favor those environments and niches that provide an optimum balance of predation risk and growth rate (Dahlgren and Eggleston, 2000). Ontogenetic niches have been recently described in dino-
saurs (Codron et al., 2012, 2013; Fowler et al., 2013), which exhibited great increases in body length and mass as they grew to adulthood. Therefore, it should not be surprising that Pteranodon exhibited ontogenetic niches as hatchlings with wingspans of ≤1m that grew into adults with wingspans up to ~6 m. With the exception of FHSM 17956, the sample of Pteranodon from the Smoky Hill Chalk Member consists of ~85% mature adults and ~15% immature subadults of similar size (Bennett, 1993). The fossils are so abundant in the Smoky Hill Chalk Member of the Niobrara Formation and the Sharon Springs Member of the Pierre Shale Formation, making up ~10% of tetrapod fossils in both the Smoky Hill Chalk Member (SCB, personal observation) and the Sharon Springs Member (Nicholls and Russell, 1990), that adults and subadults must have spent much time in the Western Interior Seaway. Pteranodon may also have been long-lived because a thin section of wing bone of an adult (Bennett, 1993, fig. 2D; Fig. 5) exhibits three closely spaced circumferential bands ~22 μm apart, which compare well to the closely spaced LAGs in the peripheral cortex of the dinosaur Syntarsus (Chinsamy, 1990, fig. 1C). Spaced ~22 μm apart and ~480 μm below the external surface, if interpreted as LAGs and annual lines, the thickness of bone between the firstLAGand the external surface would represent ~22 years if the growth rate was constant. Thus some individuals may have survived many years after reaching skeletal maturity, which would be similar to long-lived alba- trosses that can live 40 years or more (Bird Banding Laboratory, 2016).
The Smoky Hill Chalk Member in western Kansas consists
of ~180m of impure chalk deposited in open sea 150–300m deep in the middle of the Western Interior Seaway (Hattin, 1982), which supported a diverse and abundant planktonic and pelagic biota, but had a depauperate benthos because of poorly oxygenated bottom waters (Stewart, 1990; Everhart, 2005). Pteranodon at sea fed on fish and perhaps invertebrates as well. An isolated mandible of a female Pteranodon, AMNH 5098, is preserved in dorsal view with ~3cm diameter mass of fish
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