Witton and Naish (2013) proposed a terrestrial stalking mode of operation for azhdarchid pterosaurs (Fig. 1). We looked at various aspects of that earlier here and here. Today, a few more details need to be considered.
Figure 1. Click to enlarge. On right from Witton and Naish 2013. On left reconstruction based on data from Cai and Wei 1994 of Zhejiangopterus. Compare right stalking image with figure 3 wading image. Consider the great weight of that big skull on the end of that long skinny neck supported by those tiny fingers. All those problems are solved when wading (Fig. 3).
The following notes are retrieved from the boxed captions surrounding the Witton and Naish image (Fig. 1), which you can enlarge to read. 1. Reclined occipital face – Head perpetually angled towards ground when neck is lowered. – True of all wading pterosaurs and most pterosaurs in general. 2. Neck anatomy and arthrology – Long neck reduces effort to produce large movements; range of motion allows easy access to the ground. – True of all wading pterosaurs and most pterosaurs in general. 3. Skull shape and hypertrophied jaw tips – Skull morphology most similar to terrestrial feeding generalists, such as ground hornbills and modern storks; jaw elongation reduces neck action required to reach ground level – True of all wading pterosaurs and most pterosaurs in general.
Figure 2. Zhejiangopterus fingers. On left based on Cai and Wei 1994. On the right, according to Witton and Naish who want you to believe that these three fragile fingers on three spaghetti-thin metacarpals are suitable weight-bearing bones – OR that mc4 is a weight-bearing bone. Neither is true. Metacarpal 4 NEVER makes an impression. The wing finger NEVER makes an impression. They were both held above the substrate in ALL pterosaurs. The Witton Naish metacarpal is over rotated in order to allow fingers 1-3 to hyper-extend laterally, but that means the wing finger also opens laterally, not in the plane of the wing! Their mc 1-3 are pasted against mc 4, dorsal sides to dorsal side following the false Bennett model. Their fingers don’t match ichnites.
4a. Large coracoid flanges: distally displaced crests. Enlarged anchorage and increased lever arm for flight muscle; powerful takeoff ability. – Actually azhdarchids have relatively small pectoral complexes and small humeri. Witton and Naish employed a juvenile sample for their humerus. 4b. Enlarged medial wing length, decreased wing finger length. Increased forelimb stride; enlarged medial wing region and greatest lift; reduced risk of snagging wingtips on vegetation. – This is also true of all wading bottom feeders, and most pterodactyloid-grade pterosaurs in general. Also note that these traits are present in tiny Solnhofen pterosaurs (Fig. 3). Decreased wing finger length reaches a nadir in JME SOS 2482, a flightless pterosaur with a big belly and definitely NOT a stalker of terrestrial vertebrates. 5. Robust digit bones. Adaptations to weight bearing. – Obviously not true. The free fingers of Zhejiangopterus are both small and gracile and have no obvious adaptation to weight bearing. So, why are they bearing nearly all the weight of Zhejiangopterus (Fig. 1) in the Witton and Naish reconstruction? Instead, think of pterosaur forelimbs like ski poles, good for steadying (Fig. 3), especially while feeding in moving waters (Fig. 4. All weight bearing runs through the hind limb. Here is a Zhejiangopterus matched to tracks (Fig. 3). Witton and Naish make no effort to match a manus and pes to the tracks they use.
Figure 3. The large azhdarchid pterosaur, Zhejiangppterus. is shown walking over large pterosaur tracks matched to its feet from Korea (CNUPH.p9. Haenamichnus. (Hwang et al. 2002.)
6. Elongate femur (>1.6 humeral length). Increases stride efficiency; decreases attitude of axial column during feeding. – In azhdarchids the femora is not relatively longer than in precursor taxa. The humerus is relatively shorter than in most pterosaurs and shorter than azhdarchid precursors like n42 in particular (Fig. 4). The torso is also relatively shorter, but this is also true of tiny precursor azhdarchids, like n42. l7. Narrow-gauge trackways (Haenamichnus). Sub-vertical limbs providing efficient carriage when walking. – The Witton and Naish drawing overlooks the shallow angle of the femoral head relative to the shaft that would have produced a relatively sprawling, lizard-like femoral angle, as preserved in situ. Even so, the ankles would have remained below the body so long as the knees were below the acetabulum. It is also clear that pterosaur knees were bent during terrestrial locomotion, as in virtually all tetrapods. 8. Compact, padded pes and manus. Maximizes outleaver forces during step cycle; cushioning and increased traction on firm ground. – Witton and Naish based this claim on such loose and sloppy ichnites that individual toes were not distinct. Pads are also not distinct other than in the original drawing. When you look at the actual pes of Zhejiangopterus (Fig. 1) the metatarsus is indeed compact, narrower than in all other pterosaurs.
Figure 4. Quetzalcoatlus scraping bottom while standing in shallow water. Note the attempt here to shift weight posteriorly while the neck is extended anteriorly. Keeping the wing finger close to the forelimb reduces the exposed wing area, important for underwater stability. The air-filled skull is weightless when in water. Not so when terrestrial stalking.
If, on the other hand, azhdarchids were waders, as were their tiny ancestors, like n42 (Fig. 5), then we can see not only their original tall, thin, morphology and their gradual evolution to great size while maintaining their wading niche (Fig. 5), but also a reason for getting bigger; gradually deeper water access. Unfortunately, Witton and Naish make no attempt to nest azhdarchids phylogenetically and certainly make no reference to their tiny ancestors.
Figure 5. Sisters to Microtuban include No. 42 (more primitive) and Jidapterus (more derived).
The actual trackmaker of Haenamichnus had fingers (digits 1-3) as long as its foot. That is not found in Zhejiangopterus, but is found in Jidapterus (Fig. 5), a precursor azhdarchid.
Figure 6. Click to enlarge. Here’s the 6 foot 1 inch President of the USA alongside several azhdarchids and their predecessors. Most were knee high. The earliest examples were cuff high. The tallest was twice as tall as our President. This image replaces an earlier one in which a smaller specimen of Zhejiangopterus was used.
We already have pterosaurs that could have been terrestrial stalkers, like ground hornbills. We call them germanodactylids (Fig. 7). And THEY have horny/bony crests and a sharp, dangerous beak like a hornbill!
Figure 7. Germanodactylus and the Dsungaripteridae. Click to enlarge. If any pterosaurs were like ground hornbills, these even had horn bills!
The beak tip of azhdarchids is a better pick-up tweezers than a stabbing knife. Better for picking up defenseless invertebrates than for stabbing terrestrial prey capable of fighting back or running away. Remember, when you go back further in azhdarchid phylogeny, you come to dorygnathids, a clade that also gave rise to wading ctenochasmatids. The devil is in the details Witton and Naish give us a pterosaur metacarpus with the false Bennett configuration (Fig. 8) in which metacarpals 1-3 are rotated as a set, like a closed draw bridge, against the anterior (formerly dorsal) surface of mc 4. That provides no space for all four extensor tendons. Now to get those fingers to hyper-extend laterally, Witton and Naish over rotate mc4 by another 90 degrees (Fig. 2). But now their wing opens laterally, no longer in the plane of the wing, as all fossils indicate.
Figure 8. Pterosaur finger orientation in lateral view, the two hypotheses. On the left the Bennett hypothesis. On the right the Peters model that is supported by all fossil pterosaurs. These images graphically show how gracile metacarpals 1-3 were and why they could not support the weight of the pterosaur during terrestrial locomotion. The Bennett migration of the metacarpals is another problem. Witton and Naish take the Bennett mc4 one step further by rotating it another 90 degrees in order to produce lateral finger impressions. during hyperextension.
Witton and Naish give us a metacarpus and wing finger that should impress the substrate, but no pterosaur ichnite ever shows an impression of mc4 or the wing finger. So we know those two elements were held aloft during terrestrial locomotion, no matter how much Witton and Naish (and others see figure 9) wish otherwise. Witton and Naish give us a pteroid (Fig. 2) articulated to the preaxial carpal (another Bennett mistake) when the pteroid actually articulates with the radiale. Only soft tissue connects the pteroid and preaxial carpal. Witton and Naish give us pterosaur free fingers that don’t match tracks and don’t match bones. Witton and Naish illustrated from their imagination, both in shape and orientation. Witton and Naish currently hold court on pterosaur morphology, but I think you’ll agree they do so with false reconstructions. These two need to adopt strict and precise standards in which the bones agree with the ichnites and vice versa. Witton and Naish support the forelimb launch in all pterosaurs including giant Quetzalcoatlus. Considering the strain that would run through the three tiny fingers and three slender metacarpals, why do so many smart people take this idea seriously? Earlier we noted the morphological falsehoods artists added to the hand of an anhangueird pterosaur (Fig. 9) to make their forelimb launch hypothesis more logical and appealing by reducing the three free fingers and hoping the giant mc4 and wing finger made an impression in the substrate — but they don’t.
Figure 9. Errors in the Habib/Molnar reconstruction of the pterosaur manus. This manus uses the false Bennett reconstruction adopted by Witton and Naish and shortens the fingers. Corrections are provided in the lower images.
BTW I’m not blackwashing ALL of the output of Witton and Naish, just the above dozen or so problems. References Witton M and Naish D 2013. Azhdarchid pterosaurs: water-trawling pelican mimics or “terrestrial stalkers”? Acta Palaeontologica Polonica. available online 28 Oct 2013 doi:http://dx.doi.org/10.4202/app.00005.2013