Pterosaurs: Bipedal? Quadrupedal? or Both?

Figure 1. Quadrupedal Dimorphodon based on Seeley 1901

How Did Pterosaurs Walk?
Good question. Over a hundred years ago Harry G. Seeley (1901) wondered about that, so he posed Dimorphodon both as a quadruped (above, Figure 1) and as a biped (below, Figure 2). Lacking any pterosaur tracks, the question stood unanswerable for the next 50 years. It’s interesting that Seeley bent back manual digit 3, which matches pterosaur manus tracks that would not be discovered until long after his death. Apparently Seeley configured pedal digit 5 as an analog to the bat calcar, not knowing what else to do with it, despite the fact that the digit was bent back on itself at an interphalangeal joint in the specimen.

Figure 2. Dimorphodon as a digitigrade biped based on Seeley 1901.

A Short History of Recent Pterosaur Walking Hypotheses
The long-standing question, whether pterosaurs walked bipedally or quadrupedally, plantigrade or digitigrade, was finally answered over 50 years ago by W.L. Stokes (1957) who found pterosaur tracks that were plantigrade and quadrupedal. The toes pointed anteriorly, but the three small fingers (no trace at all of the wing finger) all pointed laterally to posteriorly. That was key to several insights.

Twenty four years later, Padian and Olsen (1984) dismissed these claims by attributing the tracks to crocodylians. Unwin (1988) agreed and illustrated his predictions for genuine pterosaur ichnite shapes should they ever be found.

Figure 3. Dimorphodon in a more bird-like pose by Padian 1983.

The dismissal of Stokes (1957) began a few years earlier when young Dimorphodon, a primitive pterosaur, was more bird-like than bat-like. Padian reconstructed Dimorphodon as a bipedal pterosaur, running on digitigrade feet (Figure 3). He based his assessment on its anatomy and the close relationship he figured for pterosaurs (without employing any fenestrasaurs) to both Scleromochlus and basal dinosaurs, recently shown to be unsupportable.

Over a decade later Mazin et al. (1995) and Lockley et al. (1995) confirmed the earlier Stokes (1957) study when both teams found many examples of pterosaur tracks that could not have come from crocodylians, but must have come from pterosaurs. From then on all the experts (Bennett 1997a, b; Unwin 1997; Clark et al. 1998) said ALL pterosaurs were plantigrade and quadrupedal. But, without testing all of them, how could the experts be so sure?

Figure 4. Click to animate. Walking pterosaur according to Bennett. Note the forelimbs provide no forward thrust, but merely act as props. In fact they would provide some sort of braking effect as they would bend/compress on contact in their step cycle, unless they were somehow pulling the pterosaur in its journey, rather than pushing it along, as in all other tetrapods. But then the fingers would have to be pointing anteriorly, and they don't. Rather finger 3 is oriented posteriorly. See below for a more upright animation.

Bennett’s 1997 Reconstruction of a Pterosaur Walking
S. Chris Bennett (1997) illustrated several steps in the step cycle of pterosaurs (Figure 4). They have been animated here. Most experts agree with this bent-over configuration (see Clark et al. 1998, below), but the forelimbs here do not produce thrust and over extend the humerus.

Figure 5. Dimorphodon as a plantigrade quadruped by Clark et al. 1998. They were not sure what to do with pedal digit 5, but left it folded. Then they impossibly elevated metatarsal 5 and with it the digit. Probably to match then known tracks, none of which preserved pedal digit 5 because they were all made by other pterosaurs with a vestigial pedal digit 5. No Dimorphodon tracks are known but digitigrade tracks of anurognathids related to D. weintraubi have been reported (Peters 2011). According to Clark et al. 1998, the elevation of the heel would have been negligible during the step cycle.

Clark et al. (1998) noted that “Dimorphodon” weintraubi could not have elevated its metatarsals like a bird, as Padian (1988) had reported for Dimorphodon, because a butt joint at the metatatarsophalangeal interface prevented any movement there. And that was an important point to be made. Unfortunately Clark et al. did not present a reconstruction with elevated proximal phalanges, which is an option that permits pedal digit 5 to operate in a fashion duplicated in ichnites (Peters 2011). A reconstruction of D. weintraubi and its foot matched to a digitigrade ichnite is here.

Figure 6. Dimorphodon in a bipedal and digitigrade configuration according to Peters 2000, 2010, 2011

Peters (2000, 2010, 2011) provided a fresh look at pterosaur feet by by creating reconstructions and matching them to tracks rather than making unsupported pronouncements as others had. Peters (2000) matched Cosesaurus feet to Rotodactylus (Peabody 1948) tracks, distinguished by pedal digit 5 making an impression far behind the other toes and elevating the proximal phalanges in line with the metatarsals. That solved the problem raised by Clark et al. (1988) because Cosesaurus also had a butt joint at the metatarsophalangeal interface. Peters (2000) also reported on PILs, parallel interphalangeal lines that could be drawn between the joints of all tetrapods. In certain pterosaurs the lines were complete in a plantigrade configuration. In others  they were complete in a digitigrade configuration. Peters (2011) matched digitigrade individual footprints to certain anurognathid pterosaurs. No hand prints were found nearby and no other footprints were found in association as a trackway.

So the bipedal question goes unanswered and without hard evidence at present — except for noting that ALL pterosaurs are able to balance bipedally with their shoulder glenoid over their toes. Elevating the forelimbs in preparation for flight does not upset the pterosaur’s balance.

Also the forelimbs provide absolutely no anterior thrust vectors as they do in normal tetrapods. The fingers are never posterior to the shoulders or elbows. This means the hands of quadrupedal pterosaurs were used more like crutches or assisted walking devices as shown in the video of a pterosaur walking below.

Backtracking in the face of growing evidence Padian (2003) suggested that “trackways considered for attribution to pterosaurs should show (1) manus prints up to three interpedal widths from midline of body, and always lateral to pes prints, (2) pes prints four times longer than wide at the metatarsophalangeal joint, and (3) penultimate phalanges longest among those of the pes.” Strangely, requirement number one forbids bipedalism in pterosaurs, a configuration that Padian (1983a, b) had earlier championed. Requirement number two forbids a digitigrade configuration in pterosaurs, another configuration that Padian (1983a, b) had championed. Requirement number three forbids most pterosaurs from making tracks as only a minority have elongated penultimate phalanges on every digit.

Figure 7. Anhanguera taking off in a plantigrade bipedal configuration according to Chatterjee and Templin 2003. This illustration has many problems.

Chatterjee and Templin (2004) reported, “Pterosaurs adopted both modes of locomotion: quadrupedal during slow walking, but bipedal for a short burst during take-off and landing.” They agreed with Clark et al. (1998), supporting a plantigrade stance in all pterosaurs, but during the transition from walking to running they thought pterosaurs would have become digitigrade, “making less contact with the surface to provide rapid footfall and increased stride.”

Figure 3. Click to play video. Just how fast can quadrupedal/bipedal lizards run? This video documents 11 meters/second in a Callisaurus at the Bruce Jayne lab.

How Living Lizards Run Bipedally
The Bruce Jayne Lab in Cincinnati, Ohio, has produced a video of a zebra-tailed lizard (Callisaurus) in fast quadrupedal and bipedal locomotion filmed on a treadmill. When the fore limbs are elevated the hind limbs go digitigrade. The speed is an incredible 11 meters per second.

Putting It All Together: Digitigrady vs. Plantigrady
Peters (2000, 2010, 2011) noted that some pterosaurs were plantigrade and others were digitigrade based on the continuity of PILs. Basal forms, like Dimorphodon, were digitigrade with proximal phalanges elevated in line with the metatarsals in accord with the example of Cosesaurus and several digitigrade pterosaur tracks attributable to anurognathids (Peters 2011) including the so-called “sauria aberrante” footprint (Casamiquela 1962). These tracks indicate that pedal digit 5 was not held elevated, but contributed to making tracks far behind the other toes, typically beneath the elevated heel. Later digitigrade pterosaurs developed rounder metatarsophalangeal joints and thus were able to perform minor extension there which enabled the proximal phalanges to make impressions.

Figure 8. Click to animate. Plantigrade and quadrupedal Pterodactylus walk matched to tracks according to Peters. Note even in this more upright position, with the toes planting beneath the shoulder glenoid, the forelimbs produce no forward thrust. At no time do the fingers fall behind the elbow or shoulder.

Putting It All Together: Biped vs. Quadruped
Basal pterosaurs descended from obligate bipeds, like Sharovipteryx and Longisquama, so basal pterosaurs were also bipeds. With the second most basal pterosaurs the forelimbs were long enough to touch the ground without bending over or shifting the center of balance. So that’s why we get quadrupedal pterosaur tracks. The fact that the fingers point laterally to posteriorly and digit 4 folds away without ever leaving an impression indicates that quadrupedalism was secondarily acquired, after the appearance of wings. Peters (2000, 2011) matched specific pterosaurs to specific tracks and noted that plantigrady appeared to be restricted to certain clades, those with a reduced pedal digit 5. However a primitively digitigrade genus, Pteranodon, did ultimately produce some derived plantigrade species.

Figure 9. Dimorphodon pes with shadows. Pedal digit 5 can swing beneath the metatarsus. Note elevated proximal phalanges and low elevation of heel.

Putting It All Together: Pedal Digit 5, its Use and Ultimate Disappearance
Prior to Peters (2000) no one had any idea what pedal digit 5 was used for. It was not used to control the uropatagium because there was no uropatagium, only uropatagia (plural). Peters (2000) matched Cosesaurus to Rotodactylus ichnites. Peters (2011) matched anurognathids to “Sauria aberrante” ichnites. In both cases pedal digit 5 folds upon itself and may leave a small circular impression beneath the heel. It does not carry the weight, which is balanced over the toes. Pedal digit 5 acted as a prop and as a universal wrench to aid in perching (Peters 2000). The beachcombing pterosaurs responsible for the majority of pterosaur tracks were flat-footed. Flat footed pterosaurs don’t need a pedal digit 5 prop, so pedal digit 5 shrank to become a vestige in these several lineages.

Figure 10. Just having fun, showing what a familiar friend might look like as a pterosaur.

So there you have it. The answer is: Both.
Pterosaurs were both plantigrade and digitigrade. Pterosaurs were both bipedal and quadrupedal. Pedal digit 5 was useful for basal pterosaurs, but not for derived flatfoots. All of these traits are like those of living lizards, the ones capable of standing, walking and running bipedally. At such times, these lizards turn from plantigrady to digitigrady without overextending the metatarsophalangeal joints, without having symmetrical pedes and without having all of the various morphological advantages that pterosaurs enjoyed, such as an anteriorly elongated ilium, an expanded sacral series for balance and prepubes to help elevate their femora. Pterosaurs likely took off bipedally, NOT with their forelimbs as described here. They certainly had to land bipedally.

Figure 11. Click to animate. Quetzalcoatlus running like a facultatively bipedal lizard prior to takeoff.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References:
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