Bennett—Smallest Pteranodon
wing loading until sustained flight is attained at ~29 days (first flight ~22 days; Isaac andMarimuthu, 1997). Those of larger fruit bats Cynopterus sphinx (Vahl, 1797) and Rousettus leschenaulti (Desmarest, 1820) similarly exhibit a linear increase in wing length and area until attaining wingspans of 70-80%that of adults and first flight at ~50 days (sustained flight ~60 days) whereas wing loading decreases linearly for ~35 and ~50 days, respec- tively, before beginning to increase with increasing body mass (Elangovan et al., 2004, 2007). Birds are more variable than bats in this regard, but many hatchling birds lack flight feathers and grow considerably before flying, increasing wing span and area (Starck and Ricklefs, 1998). In addition, the long bones of wings of hatchling California Gulls, Larus californicus (Lawrence, 1854), remain relatively weak as they grow to flight size and only attain the necessary strength for flight when wing flapping begins shortly before fledging at ~35 days (Pugesek, 1983; Carrier and Leon, 1990). If pterosaurs delayed the onset of flight until they were half grown, then they probably would have hatched with small wings that only reached the necessary length, strength, area, and wing loading for flight as the individual approached half size and the onset of flight, but that is not the case. One last problem is that Prondvai et al. (2012, p. 2) did not
Pipistrellus mimus (Wroughton, 1899), neonates are small and there is a linear increase in wing length and area and decrease in
discuss how long Rhamphorhynchus would have been restricted to the ground and/or trees, receiving parental feedings or feeding themselves, and stated only that the presence of fibrolamellar bone suggested that “growth did not protract over several years.” The fact that fibrolamellar bone is correlated with ‘rapid’ deposition in
extant endotherms has been used to argue that the presence of fibrolamellar bone in extinct organisms indicates ‘rapid’ growth; however, just as with the argument that flying vertebratesmust be endothermic because extant vertebrate fliers are endotherms, correlation does not imply causation, and ‘rapid’ in pterosaurs may not be the same as ‘rapid’ in extant birds. Fibrolamellar bone has been reported in free living Alligator (Tumarkin-Deratzian, 2007) and in temnospondyls (Woodward et al., 2014), so the presence of fibrolamellar bone in Rhamphorhynchus is not necessarily inconsistent with an Alligator-like growth rate, and any bone tissue will be deposited more slowly in an animal that allows its metabolic rate and body temperature to decrease significantly at rest and at night resulting in a low mean body temperature (e.g., endotherms exhibiting torpor, ectotherms) than in an animal that maintains a constant high metabolic rate and body temperature. Without temporal information to support it, Prondvai et al.’s (2012) suggestion that fibrolamellar bone in Rhamphorhynchus indicates that growth did not take several years is unreliable. Bennett’s (1995, 1996) interpretation that the discontinuous distribution of small, medium, and large size- classes in the samples of Rhamphorhynchus and other pterosaurs and fishes from the Solnhofen Limestone are year-classes resulting from seasonal mortality and/or preservation of speci- mens provides the needed temporal information to evaluate the growth rate of Rhamphorhynchus. The time represented by the gap between the small andmedium size-classes is ~1 year, which contradicts Prondvai et al.’s (2012) suggestion that growth did not take several years. Prondvai et al. (2012) made no attempt to counter the interpretation of year-classes or explain away the discontinuous distributions.
263 The histological transition to slower-growing bone in
Rhamphorhynchus seems to have occurred in the larger individuals in the medium year-class, so if juveniles delayed the onset of flight until the histological transition, then they would have delayed flight for ~1 year. That would be a long time to be grounded and receiving parental feedings, and the abundance of fossils of hatchlings and juveniles in the
Solnhofen Limestones, significantly greater than that of non- flying lizards (Barthel et al., 1990), suggests that they spent much time in the lagoons, which would be unlikely if they received parental feeding. It is also unlikely that juveniles could have fed themselves adequately on the ground or in the trees with their fish-grab dentition and their very long wingfingers and very short hindlimbs hampering both walking and climbing. If the onset of flight was correlated with the histological transition to a slower-growing bone in other pterosaurs, then Pterodaustro would not have flown for 2–3 years until half grown. Similarly, Pteranodon subadults that were not significantly smaller than mature adults would not have attained flight until after they arrived in the Western Interior Seaway because they exhibit well-vascularized plexi- form bone without evidence of lamellar bone or LAGs (Bennett, 1993). FHSM 17956 with 1.76m of well-developed wings are no different structurally or histologically from those of subadults, and would not have been able to fly until it almost doubled its size. In short, there is no evidence that Rhamphor- hynchus and Pterodaustro could not have flown and sustained growth to half-size over the course of ~1 and 3–4 years, respectively, and the well-developed wings of hatchling Rhamphorhynchus and Pterodactylus, plus the fact that FHSM 17956 must have flown to the middle of the Western Interior Seaway, demonstrate that hatchlings and juveniles with plexi- form and fibrolamellar bone were capable of flight. Therefore, Prondvai et al.’s (2012) interpretation of delayed onset of flight must be rejected. In the end, we cannot know how old Pteranodon subadults
were when they went to sea. However, if their growth rate was comparable to that of Rhamphorhynchus, Pterodactylus, and Pterodaustro, then it would have taken at least three years to reach the size of FHSM 17956 and another 3–4 years to reach ~3m wingspan. In growing to modal wingspans of 3.8 and 5.6m for females and males (Bennett, 1992), respectively, Pteranodon presumably prolonged the period of rapid growth even further than Pterodaustro did by delaying the transition from plexiform to lamellar bone until their wingspan was 3.0m or more. Indeed, based on the assumption that a 3m wingspan subadult would grow into a modal 3.8m mature adult female, the transition would occur at ~80% of maximum size rather than the ~50% seen in Pterodaustro. Pteranodon also seems to have delayed the development of large cranial crests by males until after they went to sea (Bennett and Penkalski, 2017), which suggests that sexual maturation in both sexes was roughly coincident with skeletal maturation, as indicated by well- ossified epiphyses, mature grain on limb bone shafts, and fusion of bones some time after going to sea. That would be consistent with the interpretation of Chinsamy et al. (2008, 2009) that the histological transition from fast- to slower-growing bone reflects the onset of sexual maturation and the diversion of nutrients and energy from rapid growth to reproduction.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204
