Did azhdarchids wade or stalk?

The following was added as a comment to a PlosOne paper on azhdarchid ecology.

Poor waders?
Witton and Naish (2008) reexamined the probable feeding niche employed by the largest pterosaurs, the azhdarchids. I agree with their hypothesis that azhdarchid pterosaurs did not feed while skimming, as other pterosaurs probably did. Instead they argued that azhdarchids were stork- or ground hornbill-like generalists, foraging in diverse environments for small animals and carrion because they were poorly suited for all proposed lifestyles bar terrestrial foraging. They dismissed wading as a niche because footprints show that their feet were relatively small, padded and slender, and thus not well suited for wading.

A black-necked stilt is an example of a wading bird with long legs and slender feet without webbing.

Figure 1. A black-necked stilt is an example of a wading bird with long legs and slender feet without webbing.

Good waders!
Unfortunately, modern long-legged waders, like the black-necked stilt (Himantopus mexicanus) have relatively small, slender toes with virtually no webbing. So such toes and long legs are indeed perfectly suited to wading.

Witton and Naish (2008) reported, “Moreover, other pterodactyloids with larger pedal surface areas (most notably ctenochasmatoids) were almost certainly better adapted waders than azhdarchids.”

The ctenochasmatid clade does have relatively larger feet, but much shorter legs, better suited to shallower waters with shifting sands and surf like those surrounding Solnhofen islands. This is distinctly different from the long-legged wading azhdarchids adapted for deeper quieter inland lake waters.

Other problems are phylogenetic in nature
1. Witton and Naish (2008) mistakenly include Bakonydraco with the azhdarchids. Bakonydraco phylogenetically nests with eopteranodontids/eoazhdarchids (Peters 2007).

2. Witton and Naish (2008) mistakenly nest tupuxuarids with azhdarchids because they share a few convergent traits. Phylogenetic analysis indicates that tupuxuarids are derived from a distinct clade of tapejarids and germanodactylids. Azhdarchids are derived from dorygnathids (Peters 2007).

3. Witton and Naish (2008) also incorrectly nest toothless Pteranodon with toothy Anhanguera (Peters 2007).

Still other problems are morphological in nature
Witton and Naish (2008) report, “Langston [17 (bracketed numbers refer to PlosOne references)], Wellnhofer [24] and Chatterjee and Templin [16] reconstructed azhdarchids with narrow brachiopatagia extending to the top of the hindlimbs, whereas Frey et al. [74] suggested that the membrane extended to the ankle, forming a much broader wing. No fossilised azhdarchid wing membranes are known, but evidence from anurognathids, campylognathoidids, rhamphorhynchids, ctenochasmatoids and non-azhdarchid azhdarchoids [86]–[91] indicates that ankle-attached wing configurations are more accurate.”

My independent examination of the data (Peters 2001, not cited by Witton and Naish 2008) agrees with the Langston, etc. narrow-chord interpretation for all pterosaurs. No pterosaur shows an attachment to the ankle. All pertinent examples (e.g. the Zittel wing of Rhamphorhynchus (BSP 1880 II 8), the Vienna wing of Pterodactylus (NHMW 1975/1756), the holotype of Jeholopterus (IVPP V 12705) (Peters 2001)) show an attachment to the top of the hindlimbs following a fuselage fillet. [not in the PlosOne comments: Even Sordes, the prime example among deep chord devotees has a narrow chord wing, as shown here.] Thus little confidence can be placed with any arguments advanced by Witton and Naish (2008) for hypothetical capabilities based on their fictional wing shape.

Witton and Naish (2008) report, “The size of these forms also dictates that they would need to process enormous amounts of probed invertebrates to sustain themselves.” Unfortunately they failed to note that azhdarchids had the relatively smallest torso of all pterosaurs and that implies they had the smallest stomach as well. Furthermore, it’s not uncommon to see dozens of wading bird (click for image) from several different genera picking at a small patch beneath a shallow body of water. Now, simply exchange dozens of wading birds for a few wading azhdarchids in waters too deep for other waders and there’s undoubtedly enough food to go around, in place and not running away or fighting back like terrestrial tetrapods do.

In summary,
the difference between terrestrial foraging and shallow water wading probably cannot be determined by the factors brought up by Witton and Naish (2008). Deeper water wading seeking buried invertebrates remains a possible niche environment for long-legged giant azhdarchids.

References
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. Historical Biology 15: 277-301.
Peters D 2007. The origin and radiation of the Pterosauria. Flugsaurier. The Wellnhofer Pterosaur Meeting, Munich 27. This pterosaur clade phylogenetic analysis has been expanded at www.reptileevolution.com/MPUM6009-3.htm.
Witton MP and Naish D 2008. A Reappraisal of Azhdarchid Pterosaur Functional Morphology and Paleoecology. PLoS ONE 3(5): e2271. doi:10.1371/journal.pone.0002271

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