Journal of Paleontology, 92(2), 2018, p. 254–271 Copyright © 2017, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.84
New smallest specimen of the pterosaur Pteranodon and ontogenetic niches in pterosaurs
S. Christopher Bennett Department of Biological Sciences, Fort Hays State University, Hays, Kansas 67601-4099, USA ⟨
cbennett@fhsu.edu⟩
Abstract.—A new juvenile specimen of Pteranodon from the Smoky Hill Chalk Member of the Niobrara Formation of western Kansas had an estimated wingspan in life of 1.76 m, ~45% smaller than the smallest previously known specimens, but does not differ in morphology from larger specimens. Its presence indicates that juveniles were capable of flying long distances, so it falsifies the interpretation of Pteranodon as growing rapidly to adult size under parental care before flying. Instead juveniles were precocial, growing more slowly to adult size while flying and feeding independently for several years before going to sea. Because juveniles are otherwise unknown in the Smoky Hill Chalk Member, they must have occupied different environments and ecological niches than adults; thus Pteranodon exhibited ontogenetic niches. Evidence is presented that most other pterosaurs (e.g., Rhamphorhynchus, Pterodactylus, Anhanguera) also exhibited various ontogenetic niches, which, along with their large body size, suggests that pterosaur taxonomic diversity was rather low, like that of crocodilians.
Introduction
The first specimens of the pterosaur Pteranodon, two isolated distal ends of wing metacarpals, were found in 1870 by O.C. Marsh and the Yale College Scientific Expedition in the Smoky Hill ChalkMember of the Niobrara Formation of western Kansas (Marsh, 1871). Over the next ten years, hundreds of additional specimens of Pteranodon plus ten or so specimens of the smaller Nyctosaurus were collectedinthe Smoky Hill ChalkMember by Marsh and various parties working for him. Less-intensive collecting has continued to the present day such that there are over 1000 specimens of Pteranodon in museum collections. Bennett (1992, 1994a, 2001) studied all available
Pteranodon specimens from the Smoky Hill Chalk Member and found two species, P. longiceps Marsh, 1876 and P. sternbergi Harksen, 1966, which differed in jaw and cranial crest shape, but no evidence that more that one species was present at any time, so they seem to have formed a single anagenetically evolving lineage with P. sternbergi from the lower parts of the Smoky Hill Chalk Member evolving into P. longiceps from the upper parts of the Smoky Hill Chalk Member and the Sharon Springs Member of the Pierre Shale. Specimens were bimodally distributed with the small size-class with estimated wingspans in life of 3.0–4.8m roughly twice as abundant as the large size-class with wingspans of 4.8–6.7 m. The ontogenetic ages of specimens were determined using size- independent criteria reflecting the extent of ossification and fusion of bones, and immature individuals were found to make up ~15% of the sample and to be present in both size-classes such that the size-classes could not be age-classes (Bennett, 1993). The size-classes differed only in cranial crest size and pelvic morphology, and were interpreted as dimorphic
sexes: the small class consisting of females with small crests and large pelvic canals to allow the passage of relatively large eggs and the large class consisting of males with small pelvic canals and large crests to attract mates and/or intimidate rivals in a polygynous mating system (Bennett, 1992). The absence of specimens of Pteranodon with wingspans of <3m from the Smoky Hill Chalk Member was not attributed to a bias against the preservation of small specimens because the Smoky Hill Chalk Member also preserves specimens of the pterosaur Nyctosaurus with wingspans of 1.6–3.3 m. Therefore, based on the absence of small individuals and the presence of large subadults with immature skeletons, it was argued that Pteranodon exhibited rapid growth to adult size while under parental care before flying and feeding independently (Bennett, 1993). Similarly, based on the absence of small juveniles from the collections of Nyctosaurus from the Smoky Hill Chalk Member and ornithocheiroids from the Cambridge Greensand of England and the Romualdo Formation of Brazil, it was argued that the pattern of rapid growth while under parental care was shared with other large pterodactyloids. A new juvenile specimen of Pteranodon collected from
the Smoky Hill Chalk Member is so small that it challenges the interpretation of rapid growth to large size before flying and feeding (Bennett, 2014a). This paper describes the specimen and discusses its implications for Pteranodon’s growth and behavior. The interpretation of rapid growth while under parental care is rejected and it is concluded that Pteranodon exhibited ontogenetic niches, flying and feeding independently from hatching and growing to adult size over several years while occupying different environments and ecological niches at different stages of its life history. Evidence is also presented that most other pterosaurs exhibited ontogenetic niches.
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