A Reappraisal of Azhdarchid Pterosaur Functional Morphology and Paleoecology

Abstract

Azhdarchid pterosaurs were among the most widespread and successful of pterosaur clades, but their paleoecology remains controversial. Morphological features common to all azhdarchids include a long, shallow rostrum; elongate, cylindrical cervical vertebrae that formed a long and unusually inflexible neck; and proportionally short wings with an abbreviated fourth phalanx. While azhdarchids have been imagined as vulture-like scavengers, sediment probers, swimmers, waders, aerial predators, or stork-like generalists, most recent authors have regarded them as skim-feeders, trawling their lower jaws through water during flight and seizing aquatic prey from the water's surface. Although apparently widely accepted, the skim-feeding model lacks critical support from anatomy and functional morphology. Azhdarchids lack the many cranial specialisations exhibited by extant skim-feeding birds, most notably the laterally compressed lower jaw and shock absorbing apparatus required for this feeding style. Well-preserved azhdarchid skulls are rare, but their rostra and lower jaws appear to have been sub-triangular in cross-section, and thus dissimilar to those of skim-feeders and sediment probers. Taphonomic data indicates that azhdarchids predominately inhabited inland settings, and azhdarchid morphology indicates that they were poorly suited for all proposed lifestyles bar wading and terrestrial foraging. However, azhdarchid footprints show that their feet were relatively small, padded and slender, and thus not well suited for wading. We argue that azhdarchids were stork- or ground hornbill-like generalists, foraging in diverse environments for small animals and carrion. Proficient terrestrial abilities and a relatively inflexible neck are in agreement with this interpretation.

Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull

“…Key among these were shortening of the tail, elongation of the metacarpus and reduction of the fifth toe, changes that appear to have significantly improved the locomotory abilities of pterodactyloids (Unwin 2005). Critically, these permitted a much greater degree of agility on the ground (Witton & Naish 2008), facilitating the invasion of a variety of terrestrial habitats that were inaccessible to basal pterosaurs and Darwinopterus because of their more limited terrestrial ability, owing, in part, to the large cruropatagium which linked the hind limbs (Unwin & Bakhurina 1994;Unwin 2005;Bennett 2007). Assuming that these events postdated Darwinopterus, but predated the first appearance of pterodactyloids, which are certainly known from the mid-Upper Jurassic (Wellnhofer 1978(Wellnhofer , 1991Unwin 2005), and possibly even slightly earlier (Buffetaut & Guibert 2001), then this second phase must have occurred in the late Middle to early Late Jurassic (figure 4c).…”

Pterosaur body mass estimates from three-dimensional mathematical slicing

“…Extensive fossil trackways attributed to extremely large azhdarchid pterosaurs (i.e., forms that must have been similar to Q. northropi in size) are known from the Late Cretaceous of Korea (Hwang et al, 2002). These trackways show that the limbs were moved with a parasagittal gait, demonstrating that these large azhdarchids had an effective mode of terrestrial locomotion (Witton and Naish, 2008). Lastly, the remains of Q. northropi have been recovered from rocks that record inland, terrestrial environments (Lawson, 1975), very different from the marine and lacustrine environments that other large pterosaurs are typically associated with (Wellnhofer, 1991a).…”

“…Extant flightless birds (ostriches, rheas, cassowaries, emus), and many of the volant birds that forage on the ground (shoebills, storks, bustards, secretary birds, vultures), are all much larger than other birds. An analysis of the limb proportions in azhdarchids reveals that they had long hind limbs, and the expectation is that they would have been better at terrestrial locomotion than other pterosaurs (Witton and Naish, 2008). Extensive fossil trackways attributed to extremely large azhdarchid pterosaurs (i.e., forms that must have been similar to Q. northropi in size) are known from the Late Cretaceous of Korea (Hwang et al, 2002).…”

A Summary of Current Research on the Functional Morphology of Flight in Azhdarchid Pterosaurs

“…According to Naish, all of those depictions are inconsistent with the anatomical features of these pterosaurs. Naish and Witton have hypothesized that these giants were actually terrestrial stalkers and were capable of covering large distances by soaring, could forage on land by walking quadrupedally using their long necks to reach down and grab small animals, parts of dead tissues, and possibly fruits [1], [2]. Evidence to support this theory is that there is fossil evidence of tracks of the Azhdarchid's showing a "parasagittal gait" [3], a winged planform (meaning the shape of the wing, which would help with flying over land rather than water), and from data indicating that all Azhdarchid fossils are found in "continental depositional setting."…”

“…Current theories due to the terrestrial stalker hypothesis would be modern ground hornbills or the Marabou Stork [1]. This is also based on their anatomy of "robust hindlimb elements" [2]. Others think that they are more closely related to pelicans or vultures due to the evidence of the neck anatomy found in the fossil record [4].…”