Bennett—Smallest Pteranodon


and their fragile cortical bone was lost when prepared with a rotating wire brush, revealing the internal septate bone. The bones are otherwise unremarkable.


Ontogenetic status.—The articular surfaces of the radius, ulna, pteroid, McIV, and WP1 are pitted and rough, indicating incomplete ossification, and similar pitting is visible on the carpal elements as well. The diaphyses of the radius, ulna, pteroid, McIV, and WP1 exhibit immature grain with many vascular canals opening onto the external surface of the bone. The proximal and distal rows of carpals are not fused into proximal and distal syncarpals, and the extensor tendon process is not fused to WP1. Thus, the specimen exhibits immature states of all three size-independent criteria that Bennett (1993) used to identify immature specimens of Pteranodon and does not exhibit any evidence of osteological maturity, so it is immature.


Thin section.—Cortical bone was sampled from the posterior side of the neck of McIV, just proximal to the distal condyles where it was thought that the bone would be thickest and pro- vide the best chance of finding a line of arrested growth (LAG), if present. The resulting thin section (Fig. 3.1) is 540 µm thick, and exhibits well-vascularized plexiform bone (sensu Francillon-Vieillot et al., 1990) with vascular canals opening onto the external surface. The internal surface is uneven and appears to be resorptive. No LAG is evident, but a second section ground much thinner exhibits multiple discontinuous narrow circumferential bands of blue against a background of magenta when examined under polarized light with a first-order retardation (=λ) plate (Fig. 3.2). The spacing of the bands (~80 µm) suggests some sort of regular variation in fiber orien- tation. The pattern of bone and vascularization is similar to that previously found in the immature bone of a subadult Pteranodon (Bennett, 1993, fig. 2C), although the bone is thinner and the spacing between vascular canals is somewhat less than in the subadult.


Discussion


Only two pterosaur genera, Pteranodon and Nyctosaurus, are known from the Smoky Hill Chalk Member (Bennett, 1994a). FHSM 17956 is referred to Pteranodon because the morpho- logy of its distal syncarpal with prominent preaxial carpal process and ice hockey-stick-shaped pteroid (Bennett, 2001, and unpublished data) are as found in Pteranodon and unlike Nyctosaurus, in which the syncarpal has a short preaxial carpal process and the pteroid has a pointed lateral end extending distally beyond the articulation (Williston, 1903; Bennett, 2003). In addition, the proportions of FHSM 17956 (i.e., ratio of lengths of McIV and WP1, ratios of proximal width and diameters of distal condyles to McIV length, and ratios of proximal, midshaft, and distal widths to WP1 length, Bennett, 2001, and unpublished data) are consistent with those of Pteranodon and inconsistent with those of Nyctosaurus (Fig. 4). There is no evidence that the specimen represents a new species, but it is not clear which Pteranodon species it does represent. Bennett (1994a) thought that the transition between P. sternbergi and P. longiceps occurred in the middle of the


257


Figure 3. Pteranodon sp. indet. (FHSM 17956). (1) Thin section of cortical bone from the distal end of metacarpal IV photographed under visible light; (2) thinner section photographed under polarized light and first order retardation (= lambda) plate showing narrow discontinuous circumferential bands of blue (light gray) against a background of magenta (medium gray); dark band at top is chalk residue on external surface of bone. Abbreviations: cb, circumferential band; ch, chalk; and vc, vascular canal. Scale bar is 300μm.


Smoky Hill Chalk Member and arbitrarily chose the non- fossiliferous caprock (Marker Unit 10, Hattin, 1982) as the dividing line between the species. Because the specimen was collected from just below the caprock and does not exhibit any characters used to differentiate the species of Pteranodon,itis considered to be Pteranodon sp. indet. The lengths of the missing elements of FHSM 17956 were


calculated using Bennett’s (2001) regression equations, and the wingspan in life estimated as 1.76m (Table 1). The smallest relatively complete previously known specimen of Pteranodon, AMNH 4908, consists of a partial trunk skeleton and tail, scapulocoracoid, humerus through WP2, both femora and tibiae, and a disarticulated foot (Bennett, 2001; Table 1), had an estimated wingspan in life of 3.33 m. Just slightly larger is YPM VP 2345, an incomplete wing of an immature individual, and YPM VP 2350, an isolated WP1 (352mm long). The greatest diameters of the dorsal condyles of McIV of AMNH 4908 and YPM VP 2345 are 20 and 19.9mm, respectively. There are eight fragmentary specimens with greatest dorsal condyle


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