266


Journal of Paleontology 92(2):254–271 The sample of Aurorazhdarcho micronyx (Meyer, 1856)


consists of 12 specimens after the holotype of A. primordius Frey et al., 2011 was synonymized with Pterodactylus micronyx and the four specimens mentioned above that were referred to Ctenochasma elegans (Bennett, 2013a) are excluded, but only ten specimens after the two specimens that have different skeletal proportions and may represent a distinct taxon (Bennett, 2013a) are excluded. All of them, except the holotype of Aurorazhdarcho primordius, form what is interpreted as a single year-class of hatchlings and young-of-the-year with wingspans of 23–46cm (Fig. 7.4). The single large specimen had a wingspan of 1.06 m. The sample of Germanodactylus cristatus (Wiman, 1925)


is too small to reveal the presence or absence of year-classes, but consists of two hatchlings with wingspans of 21 and 22 cm, one subadult, and one adult with a wingspan of 96 cm. Bennett (2006) noted that the two hatchlings came from a single quarry, Workerszell near Eichstätt, that seems not to have produced any other pterosaur specimens, which suggests that that part of the basin was near the Germanodactylus nesting grounds and provided conditions preferred by the hatchlings, and that juveniles occupied other habitats. Comparison of the ontogenetic compositions of the


samples of Solnhofen pterosaurs reveals three different patterns of abundance. Rhamphorhynchus and Pterodactylus share a pattern with two year-classes, the second more than twice as abundant as the first, and subadults and adults rare to absent. It is surprising that yearlings are more abundant than hatchlings. The abundance of hatchlings suggests that the nesting grounds were close to the Solnhofen lagoons, and the abundance of juveniles indicates that they remained near the lagoons feeding and growing for up to two years. If one assumes that hatchlings and yearlings had equivalent mortality and preservation rates, this could have resulted if hatchlings were present near the lagoons for only part of the depositional season whereas yearlings were present for the entire season. Alternatively, yearlings might have had a higher mortality rate than hatchlings because they were transitioning to a different food source. If the histological transition that occurred in the larger individuals in the medium year-class correlates with the onset of sexual maturity, then Rhamphorhynchus probably would have first bred at two years old. Although specimens of ~1.8mwingspan are expected to be rare, the relative rarity of specimens between the upper limit of the medium year-class and the largest specimens, which would represent two or more additional year-classes, suggests that sexually mature breeding individuals spent most of their time elsewhere. Comparable histological information is not available for Pterodactylus, but it is possible that Pterodactylus was similar to Rhamphorhynchus in age at first breeding and preferred habitat of adults. Ctenochasma exhibits a second pattern with a single year-


class of hatchlings and young-of-the-year and low numbers of older juveniles through adults. This suggests that Ctenochasma individuals remained near the lagoons feeding on small aquatic prey throughout their life, and is consistent with a high hatchling mortality followed by reduced mortality of juveniles through adults well adapted to their environment. Aurorazhdarcho exhibits a third pattern with a single year- class of hatchlings and young-of-the-year and a few large adults,


whereas yearlings and larger juveniles are absent. This suggests that the nesting grounds were close to the Solnhofen lagoons and that hatchlings spent only their first year near the lagoons, whereas older juveniles and adults left the lagoons and fed elsewhere, perhaps returning to the area only to lay their eggs. This might have been because the dentition of Aurorazhdarcho was suited to feeding on considerably larger prey than Ctenochasma, such that the preferred foods of older juveniles and adults were not available in the Solnhofen lagoons.


Bissekty Formation.—The Bissekty Formation consists of ~67m of medium- to coarse-grained sands with occasional cross-bedding and abundant fossils reflecting a fluvial environ- ment with a very diverse vertebrate fauna, including the pterosaur Azhdarcho lancicollis Nesov, 1984 (Archibald et al., 1998; Averianov, 2010). The sample of Azhdarcho consists of one hatchling, five small juveniles, four large juveniles, two immature subadults, and one mature adult based on a count of proximal humeral fragments (A. Averianov, personal commu- nication, 2015). That pattern of abundance is consistent with progressively decreasing mortality of small juveniles through adults occupying the same riverine habitat. If not attributable to a preservational bias, the relative rarity of hatchlings may sug- gest that the depositional area of the Bissekty Formation was some distance from the nesting grounds and protected habitats where hatchlings presumably spent their first months.


Lagarcito Formation.—The “Loma del Pterodaustro” Lager- stätte within the Lagarcito Formation consists of ~8m of lami- nated claystones and massive siltstones with a few sandstone beds deposited in the middle of a large, shallow perennial freshwater lake that supported abundant algae and invertebrates, a somewhat restricted fish fauna, and the pterosaur Pterodaustro guinazi Bonaparte, 1970 (Chiappe et al., 1998; Prámparo et al., 2005, Arcucci et al., 2015). The vast majority of Pterodaustro specimens came from ~1m of laminated claystones and the sample consists of one egg with embryo, an incomplete egg- shell, two hatchlings little bigger than the embryo, no small juveniles, two catalogued and many other large juveniles, two immature subadults, and many mature adults (L. Codorniú, personal communication, 2015). The data in Table 2 are based on the arbitrary assumption that ‘many’ equates to 10. The pattern of abundance suggests that large juveniles through adults occupied the same lacustrine habitat, whereas the rarity of small juveniles suggests that they occupied other habitats. Pterodaustro has been interpreted as feeding while


standing in shallow waters, dipping its curving jaws into the water to sieve out small organisms (Wellnhofer, 1991a; Chiappe et al., 1998); however, it does not exhibit any wading adaptations such as elongate limbs. Whereas sieving on the wing is precluded because of the excessive drag the mandible


would have produced, it is possible that Pterodaustro fed while floating, duck-like and sieving surface waters where prey might congregate to feed on phytoplankton. If that were the case, they could have fed on a much greater proportion of the lake’s surface area and the lake could have supported much larger populations of Pterodaustro than if they were restricted to shallow waters. In addition, if small juveniles remained in shallow protected waters near shore and large juveniles and


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