The Case Against Bipedal Pterosaurs

Two Camps
The subject of pterosaur bipedality remains contentious. In the modern era, Stokes (1957 attributed odd quadrupedal tracks to pterosaurs. For the next 25 years no one argued against it. Then Padian and Olsen (1983) determined that Pteraichnus tracks were crocodilian in origin. This convinced several other paleontologists along the way (Unwin 1989). In 1995 two papers independently determined that Pteraichnus tracks could only be pterosaurian in origin (Mazin et al. 1995; Lockley et al. 1995), which turned attitudes around universally. No one else bought into the bipedal story except Bennett (1997, 2003) who illustrated the oddly proportioned and fingerless Nyctosaurus as a biped, but otherwise fell in with the quadrupedal folks.

The Bipedal Ancestor Irony
Virtually all the quadrupedal theorists also insist on a close relationship with the obligate biped, Scleromochlus and a close relationship to basal bipedal dinosaurs. Hmm…

The Heretic
My own phylogenetic work (Peters 2000, 2011) shows that basal fenestrasaurs (including pterosaurs) were occasionally to obligatorily bipedal and that several clades of derived pterosaurs were quadrupeds. All pterosaurs would have been capable of bipedal locomotion in the manner of lizards that can attain a bipedal configuration (Fig. 1, that’s how they spread their wings in preparation for a take-off). However many clades preferred quadrupedal locomotion during beachcombing/feeding/ordinary walking as demonstrated by their quadrupedal ichnites and the relative length of the fore and hind limbs. This form of locomotion was secondarily derived, as demonstrated by the orientation of the manual fingers. Manual digit 3 often is oriented posteriorly, the opposite of all other terrestrial tetrapods, and digits 4 and 5 were elevated off the substrate.

BAHH!
The quad proponents dispute virtually all suggestions of bipedality (see below). Hone and Benton (2007) went so far as to say, Cosesaurus is treated as a biped by Peters (2000) with characters coded based on this assumption.” Not sure how that could possibly affect bone traits and ratios, but that’s the attitude and paradigm out there. The authors were aware that Peters (2000) stated Cosesaurus was an occasional biped based on matching its feet to Rotodactylus ichnites, which are occasionally bipedal.

The Evidence from Ichnites
So far we have quadrupedal ichnites for pre-germanodactylids, ornithocheirids, pterodactylids, ctenochasmatids and azhdarchids. We have pedal ichnites without manus impressions for anurognathids. We have pedal ichnites with occasional manus ichnites for cosesaurids that are called Rotodactylus (Peters 2011).

A 2003 Argument Against Bipedal Pterosaurs
Darren Naish is a brilliant paleontologist with many discoveries to his credit, but on this subject he was in the “all quadruped virtually all the time” camp. In 2003, Darren discussed on the DInosaur Mailing List (DML) bipedality in lizards compared to that of pterosaurs in response to the publications of Peters (2000 and 2002) and several posts I had made to the DML. I have abridged his arguments (in yellow below, but you can read his full post here).

Darren: …here I state more clearly why I think the lizard/pterosaur analogy is flawed. then he quotes an earlier post I made to the DML , “If lizards can do it, irrespective of the math, pterosaurs could do it because they have superior equipment (increased sacral number, anteriorly hypertrophied ilium = bigger thigh muscles). As in birds or bipedal lizards, the CoG can be easily manipulated to be either head heavy or tail heavy by moving the tail, head, femur, tibia or angle of the back. Nothing out of the ordinary is required to balance a pterosaur. And the forelimbs are always within a whisker of touching the substrate to deal with momentary lapses.”

Then Darren reports, “– Why do certain lizard species run bipedally? Is it just so that they are faster? Probably not: the fastest lizards are quadrupedal runners.”

Indeed. Speed is not the reason.

Darren: “What appears most likely is that bipedal running in lizards has evolved to circumvent Carrier’s constraint: by relying on the hindlimb complex alone, bipedal running lizards are not compressing the thorax as they run, and they are therefore able to maintain breathing while sprinting (in contrast to quadrupedal running lizards).

Unfortunately not true. According to Christopher Clemente“When you see a lizard running bipedally, it’s just a consequence of its acceleration.”  Clemente found that lizards trotting on two legs ran out of steam quicker, indicating that bipedalism does not serve to conserve energy.

Bipedal lizard video marker

Figure 1. 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.

If you want to see a video of a quadrupedal lizard screaming along at a breakneck pace, then, for reasons known only to itself, lifts up its forelimbs and continue running, click here. This is not an acceleration situation. Some lizards run more upright than this one.

Then Why Go Bipedal?
Clemente said, “If you’re not using your front legs anymore, they can develop for something else.” Pterosaur predecessors, like Cosesauruswere flapping their forelimbs according to their bird-like/ pterosaur-like, strap-like scapula and locked down stem-like coracoids (both very un-lizard-like).

Sharovipteryx, another pterosaur sister, could not have contacted the substrate with such short forelimbs and long hindlimbs. It too flapped forelimbs anchored with a stem-like coracoid.

Longisquama was similar, but with much larger, more pterosaur-like hands. Certain pterosaurs were secondarily quadrupedal, as demonstrated by phylogeny and the lack of pronation in the manus. Certain pterosaurs left only pedal impressions.

Darren: Given that pterosaurs would clearly not have needed to avoid Carrier’s constraint (because, even if they were to run quadrupedally, they would not be compressing the thorax with each stride), why run bipedally?

Bipedal lizards do not avoid Carrier’s constraint.
Following Clemente’s comments (above), bipedal lizards “run out of gas” sooner rather than later. So, they’re still holding their breath while running bipedally. In pterosaurs a short stiff torso was one trait to help avoid Carrier’s constraint. Combine that with a lack of large caudofemoral muscles. Lizards use these to propel their sprawling hind limbs with alternate lateral pulls of the femur. Instead pterosaurs and their fenestrasaur antecedents had large, dinosaur-like thighs as long as the extent of their elongated ilia. They ran using fundamentally different muscles than lizards do (thigh muscles vs. tail muscles), and basal pterosaurs had more erect hind limbs than more derived pterosaurs.

Darren: Furthermore, given that pterosaurs exhibit features associated with leaping (see, e.g., Bennett 1997) and/or scansoriality, it is probable that they wouldn’t need to sprint in order to take off.

Not sure why Darren discounts running in creatures capable of leaping. I can think of several mammals that are great leapers AND runners (rabbits, deer, roadrunners, squirrels, big cats, Michael Jordan. With birds some can take off with a simple leap. Others require a runway.

Darren: Bottom line: there is no inherent ‘need’ for good bipedal running abilities in pterosaurs, in contrast to the situation in lizards.

Unfortunately, Darren did not take into account the possibility of a secondary sexual characteristic, flapping, that pterosaur predecessors practiced (based on their stem-like coracoids). Flapping to show off is how pterosaurs developed the necessary ‘equipment’ to flap to fly. Ironically, Darren’s remarks seem to be indicating there IS a need for bipedality in lizards, after arguing there was no good reason for it in lizards either (see above) – yet they do it! (but not for sex~). Evolution does not proceed based on “need” in any case, but on random changes, some of which prove to enhance survivability.

Darren asks, “What allows certain lizards to run bipedally? Dave is fond of stating that pterosaurs could run bipedally because they exhibit an increased number of sacrals relative to their probable outgroups, and a large preacetabular process on the ilium. As has been pointed out several times in the literature, it?s relatively easy for a lizard to run bipedally IF it combines these two features WITH (1) hindlimbs that are proportionally longer than its forelimbs (and consequently the animal has proportionally short forelimbs), (2) a proportionally short thorax and (3) short neck*, and (4) a long muscular tail (see Synder 1954, 1962, Bellairs 1969, Rieppel 1989 etc). Note that most of these features are to do with reducing the mass of the foreparts and thus shifting the CoG caudally. On point (4), as shown by Russell and Bauer (1992), the most important anatomical correlate of bipedality in lizards is the presence of a large m. caudofemoralis longus that inserts relatively distally on the tail (thus explaining why the lizards that run bipedally are the same ones that don?t practise caudal autotomy). *Apparently _Chlamydosaurus_ has a longer neck than most other agamids. Its neck is still not as proportionally long as that of a pterosaur though. Pterosaurs obviously don’t have proportionally short forelimbs, but more importantly they don’t have the short neck seen in bipedal lizards, nor do they have a tail that would have supported a large m. caudofemoralis longus: even in basal long-tailed forms, transverse processes (and hence a reliance on m. caudofemoralis longus) are extremely reduced (and, incidentally, there is no indication that pterosaurs switched to the knee-based retraction system seen in birds). On the relevance of this reduction in caudofemoral musculature to bipedal locomotion, Synder (1954) writes “while a long, heavy tail does not necessarily indicate bipedal habits, a short, lighter tail precludes the possibility of this type of locomotion? (p. 9). Given then the profound differences evident here between pterosaurs and bipedal lizards, I think the analogy is seriously suspect.”

Unfortunately Darren (like so many others) completely ignores or overlooks the origin of bipedality in pterosaur ancestors that I described in 2000. Bipedality appeared with Cosesaurus, which had feet which matched occasionally bipedal and always narrow-gauge, digitigrade tracks. Cosesaurus, Longisquama and basal pterosaurs (including anurognathids) all had short necks. Longer necks evolved later. Sharovipteryx, an obvious biped, had a long neck. So that’s not an issue. Basal pterosaurs with short forelimbs and long hindlimbs, like MPUM6009 would have been awkward quadrupeds. The longer forelimbs developed AFTER pterosaur ancestors were already flapping, leaping and running about bipedally.

Darren continues, “– So what of the alleged correlates of bipedality present in pterosaurs? Dave suggests that an increased number of sacral verts and a hypertrophied preacetabular process on the ilium are indicative of “improved: bipedality in pterosaurs. The problem is that, firstly, the features discussed above are needed as well (viz, proportionally short neck, big m. caudofemoralis longus etc), and, secondly, when the sacral and iliac features are present without these others, they may not be indicative of bipedality but of quadrupedality. Look at (e.g.) ceratopsians. Relative to basal ceratopsians, ceratopsids have a longer preacetabular process and an increased number of sacrals (10-11 compared to 6), so according to your criteria ceratopsids might be better suited to bipedality than psittacosaurs. Parareptiles come to mind too: in nycteroleterids, nyctiphruretids, procolophonoids and sclerosaurs there are (usually) 3 sacrals and a short or absent preacetabular process, but in pareiasaurs – most notably in big derived forms like _Scutosaurus_ – there are 4-6 sacrals and the preacetabular process may be so hypertrophied that the pelvis looks much like that of a pterosaur (see Fig. 14E in Lee 1997). As in ceratopsians, these sacral and iliac trends are to do with improved quadrupedal abilities.”

Well, when you start comparing pterosaurs to pareiasaurs and ceratopsians, I think we can all agree, Darren is really reaching here. Certainly a high sacral number is a convergent feature here, but for different reasons. Note, he avoids mention of Sharovipteryx and Cosesaurus, both with an increased sacral count and much closer to pterosaurs than pareiasaurs and ceratopsians.

In bipeds, like pterosaurs and dinosaurs (both bipeds and descendants of bipeds), the sacrum is put under greater stress as the fulcrum balancing all the weight anterior to the sacrum and all the weight aft. Elongating the ilium, adding to the muscle mass of the thigh, along with strengthening the fulcrum is the reason for adding sacrals (often coosified) in pterosaurs. No one championing the quadrupedal configuration has ever proposed another reason for increasing the sacral count and ilium length in pterosaurs. Sure cynodonts added sacrals. The also reduced their caudofemoralis muscles and their tails while elongating their ilia, all convergent with pterosaurs – without going bipedal. Anyone can take parts and make arguments any way they want to, but they can’t take the whole suite of characters and make the same argument. And, as Darren and most paleontologists would agree, parsimony only rules when you look at the sum of a taxon’s traits, not just a few of its convergent parts.

Darren continued, “A better way of testing for bipedality in Pterosauria might be to look at intermembral indices (viz, forelimb:hindlimb ratios), at the CoG (as I mentioned, Sangster has been working on this), at unambiguous soft tissue evidence (e.g., the Crato azhdarchoid with its preserved brachiopatagium), or at trackway evidence? and right now the evidence from all of these areas shows that quadrupedality is better supported, or in other words that pterosaurs were more likely quadrupedal.

Here Darren is talking about most pterosaurs, not basal taxa or their ancestors. As above, all pterosaurs more derived than MPUM6009 were capable of placing their hands on the substrate – without bending over an iota! (a fact typically overlooked in most other quadrupedal reconstructions of pterosaurs). Even so, the most primitive pterosaurs for which we have ichnites, the anurognathids, preserve no manus impressions.

Darren asked, “– Why be bipedal anyway when the forelimbs are plenty
long enough? Dave notes that, in bipedal pterosaurs, “the forelimbs are always within a whisker of touching the substrate”. Well, if that;s so, it seems more likely to me that the animals would have employed the forelimbs in locomotion. I can’t think of a group of living animals in which the forelimbs are close to the substrate, and are not then employed in locomotion (think monkeys and apes). Again, the hard evidence we have (trackways) shows that the forelimbs were deployed in quadrupedal locomotion.

Darren was not aware of the other hard evidence of pedal impressions of pterosaurs without manus impressions.  In virtually all pterosaurs the forelimbs were plenty long enough to touch the substrate and the tracks show that many clades were quadrupedal. I never argued against that. Some pterosaurs reverted to quadrupedal locomotion, always retaining the ability to walk or run bipedally, simply by lifting the forelimbs off the substrate. Their toes were already planted beneath their centers of gravity, the shoulder glenoid, as ALL of my pterosaur reconstructions demonstrate. It’s that simple. As in Darren’s example, think lemurs, monkey and apes, all of which can go bipedal whenever they want to.

Darren concludes with, “The presence of a well-developed iliopubic ligament (=ligamentum inguinale) might indicate that pterosaurs were good at elevating the thorax, but given that everyone agrees that pterosaurs must have been bipedal when opening or closing their wings (and they thus would have needed to elevate the thorax at least occasionally), this doesn’t necessarily indicate bipedal running. Incidentally, see Hutchinson (2001, pp. 156-8) for a discussion of iliopubic ligament distribution in Reptilia. Because reptiles including ceratopsids, pareiasaurs and turtles appear to have had a hypertrophied iliopubic ligament as well, the correlation between this structure and an enhanced bipedal ability is not immediately clear.

I appreciate the half-hearted concession Darren made regarding opening the wings. Nice to hear. Yes, pterosaurs had to stand bipedally to open the wings. (Forelimb launch was not a consideration when Darren wrote this, and it has major flaws in any case.) The anterior extension of the ilium does not always signal a bipedal configuration (e.g. basal mammals). I never argued that it did. But in the case of pterosaur antecedents, a long ilium is one of a long list or suite of traits  shared with pterosaurs. In Peters (2002), I was moved to report that other than the twist of the wing finger, nearly every pterosaurian trait could be found in fenestrasaur antecedents. No has ever argued against that hypothesis and presented a more parsimonious series of antecedents from some other distant clade. And it has been ten years!

Darren further concluded, “The long forelimbs of pterosaurs, combined with the morphology of the patagia and the evidence from trackways, show that the interpretation of pterosaurs as predominantly quadrupedal is better supported and less speculative than interpretation of them as bipedal.”

Cosesaurus and Rotodactylus, a perfect match.

Figure 2. Click to enlarge. Cosesaurus and Rotodactylus, a perfect match. Elevate the proximal phalanges along with the metatarsus, bend back digit 5 and Cosesaurus (left) fits perfectly into Rotodactylus (right).

Predominate? Yes. However, Darren was not, at the time, aware of certain pterosaur pedes imprints attributed to anurognathids that were not attended by manus imprints. Those were described in Peters (2011), but “Sauria aberrante” (Casamiquela 1962) and Rotodactylus (Peabody 1948, Fig. 2) have been known for decades. Even though they are in the minority at present, they still count.

In summary, looking for reasons to go bipedal is probably not the way to go. Looking at bits and pieces by themselves is also not the way to go. Cladistic analysis and judging a taxon as a whole are the ways to go. And I like the example of quadrupedal/bipedal primates. That’s a good analogy!

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.

Darren Naish, School of Earth & Environmental Sciences University of Portsmouth UK, PO1 3QL

References
Click to read Darren Naish’s complete comment to the DML
Bennett SC 1997. Terrestrial locomotion of pterosaurs: a reconstruction based on Pteraichnus trackways. Journal of Vertebrate Paleontology, 17: 104–113.
Bennett SC 2003. New crested specimens of the Late Cretaceous pterosaur Nyctosaurus.Paläontologische Zeitschrift 77: 61-75.
Casamiquela RM 1962.
 
Sobre la pisada de un presunto sauria aberrante en el Liassico del Neuquen (Patagonia). Ameghiniana, 2(10): 183–186.
Lockley MG, Logue TJ, Moratalla JJ, Hunt AP, Schultz RJ and Robinson JW 1995. The fossil trackway Pteraichnus is pterosaurian, not crocodilian; implications for the global distribution of pterosaur tracks. Ichnos, 4: 7–20.
Mazin J-M, Hantzpergue P, Lafaurie G and Vignaud P 1995. Des pistes de pterosaures dans le Tithonien de Crayssac (Quercy, France). Comptes rendus de l’Academie des Sciences de Paris, 321: 417–424.
Peabody FE 1948.  Reptile and amphibian trackways from the Lower Triassic Moenkopi formation of Arizona and Utah.  University of California Publications, Bulletin of the  Department of Geological Sciences 27: 295-468.
Peters D 2000.
 Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7(1): 11­-41.
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. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Peters D 2010. In defence of parallel interphalangeal lines. Historical Biology 22:437-442.
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos, 18: 2, 114 —141.
Stokes WL1957. Pterodactyl tracks from the Morrison Formation. Journal of Palaeontology, 31: 952–954.
Unwin DM 1997. Pterosaur tracks and the terrestrial ability of pterosaurs. Lethaia, 29: 373–386.

Darren’s References:
Bellairs, A d’A 1969. The Life of Reptiles, Vol. 1_ Weidenfeld & Nicolson (London), pp. 282.
Bennett SC 1997. The arboreal leaping theory of the origin of pterosaur flight. Historical Biology 12, 265-290.
Hutchinson JR 2001. The evolution of pelvic osteology and soft tissues on the line to extant birds (Neornithes). Zoological Journal of the Linnean Society 131, 123-168.
Lee MSY 1997. Pareiasaur phylogeny and the origin of turtles. Zoological Journal of the Linnean Society 120, 197-280.
Reeder TW, Cole CJ and Dessauer HC 2002. Phylogenetic relationships of whiptail lizards of the genus_Cnemidophorus_ (Squamata: Teiidae): a test of monophyly, reevaluation of karyotypic evolution, and review of hybrid origins. American Museum Novitates 3365, 1-61.
Rieppel O 1989. The hind limb of Macrocnemus bassanii (Nopcsa) (Reptilia, Diapsida): development and functional anatomy. Journal of Vertebrate Paleontology 9, 373-387.
Russell AP and Bauer AM 1992. The m. caudifemoralis longus and its relationship to caudal autotomy and  locomotion in lizards (Reptilia: Sauria). Journal of Zoology 227, 127-143.
Synder RC 1954. The anatomy and function of the pelvic girdle and hind limb in lizard locomotion. American Journal of Anatomy 95, 1-45.
Synder RC 1962. Adaptations for bipedal locomotion of lizards. American Zoologist 2, 191-203.

A 4th Finger Imprint in the Haenamichnus Pterosaur Track?

Tracks attributed to pterosaurs are now known worldwide. The largest such tracks have been rightly attributed to giant azhdarchid pterosaurs like Quetzalcoatlus. The  Haenamichnus trackway (Hwang et al. 2002, Fig. 1) leaves no doubt as to the identity of the trackmaker, but the individual impressions differ greatly from one another and are indistinct at best. It must have been a wet, muddy day when these were produced. Sometimes the manus and pes impressions are separate, but often they glob together.

haenamichnus-track

Figure 1. Yellow arrow points to a possible 4th digit imprint in the trackway of Haenamichnus. Note how messy these muddy tracks are. Not much detail in each one. Only the walking pattern (on right) is precise. Note that Step 7 is the only one without a pes imprint, unless that medial shape IS the pes imprint.

Step 7
Among the many Haenamichnus ichnites (Fig. 1), step 7 is interesting because it appears to impress an oddball fourth impression medially between #1 and #3. What could it be?  In other pterosaurs or, for that matter, in other ichnites within this trackway digit 4, the wing finger, did not leave impressions. The wing finger was carried vertically, folded against the arm during terrestrial locomotion and the knuckle was elevated above the substrate. If digit 4 DID make that impression, why was the impression bent and so short? After all, digit 4 was a long straight bone that terminated in a large knuckle.

The first guess: The odd shape is the (otherwise missing) pes impression, blended into the manus impression. And with that, perhaps I need go no further… but I will.

The second guess:  The odd shape is the first impression of manual digit 3 before it was lifted and repositioned, rotated on the axis of the impression of digit 1. If so, the pes did not make an impression. In this case the pes might have hit a dry spot or was later obliterated.

The third guess:  Whatever geological thing that discoloration or dip was, it was there before the pterosaur touched it and remained afterwards.

Don’t be fooled by those who say: four impressions = four digits. Critical thinking, and the lack of similar traces in other pterosaurs argue against such snap judgements.

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
Hwang KG, Huh M, Lockley MG, Unwin DM and Wright JL 2002. New pterosaur tracks (Pteraichnidae) from the Late Cretaceous Uhangri Formation, southwestern Korea. Geology Magazine 139(4): 421-435.

An Obligate Bipedal Basal Pterosaur

The Traditional View
All pterosaurs were quadrupedal, based on trackway evidence.

MPUM 6009, the Milan specimen, the most primitive known pterosaur

Figure 1. The most primitive known pterosaur, the Milan specimen, MPUM 6009. The long hind limbs and relatively short fore limbs were homologous with those in Sharovipteryx and Longisquama. The extremely slender tail is most like that of Sharovipteryx, not later pterosaurs which thickened the tail with elongated chevrons and zygapophyses. Gray tones represent possible soft tissues, homologous with those in Cosesaurus and Longisquama.

The Heretical View
One basal pterosaur, MPUM 6009 (Wild 1978), was an obligate biped, retaining the long-legged morphology of its ancestral sisters, Sharovipteryx and Longisquama. All pterosaurs following MPUM 6009 (such as Raeticodactylus and Eudimorphodon) had shorter hind limbs and longer forelimbs, a combination that enabled quadrupedal locomotion.

MPUM 6009 was considered a small Carniadactylus by Dalla Vecchia (2009), but the differences are many.

MPUM 6009 in situ.

Figure 2. MPUM 6009 in situ. Click to enlarge and portray the Wild (1978) interpretation. Bones, impressions of bones and some soft tissue complete this articulated skeleton at the very base of the Pterosauria. The crushed skull required reconstruction. Here, using the DGS method, the bones have been colorized. This permits subtle impressions to be identified. Sister taxa share many of these traits, confirming their identity.

Longer Legs, Shorter Forelimbs
Here the reconstruction tells the tale. Question is, is the reconstruction accurate? The clues are, admittedly ephemeral, yet even without such long legs, MPUM 6009 nests at the base of the Pterosauria. So long legs are not beyond the realm of possibility. The relatively short neck allies this basal pterosaur with Longisquama, the outgroup sister taxon. The laterally increasing toe length and deep pelvis also ally this taxon with Longisquama. The sternal complex is also essentially identical.

Ironically…
Such long legs and short forelimbs “ally” this pterosaur with Scleromochlus, and basal dinosaurs, but — really, seriously — hardly at all. It’s convergence!! So if anyone from the traditional camp wants to bitch about this reconstruction, think twice. You’ll only be shooting yourself in the foot. Things happen when the forelimbs are elevated off the substrate, as we humans all can attest.

Bipedal lizard video marker

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.

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
Dalla Vecchia FM 2009. Anatomy and systematics of the pterosaur Carniadactylus (gen. n.)rosenfeldi (Dalla Vecchia, 1995). Rivista Italiana de Paleontologia e Stratigrafia 115 (2): 159-188.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Wild R 1978. Die Flugsaurier (Reptilia, Pterosauria) aus der Oberen Trias von Cene bei Bergamo, Italien. Bolletino della Societa Paleontologica Italiana 17(2): 176–256.

wiki/Eudimorphodon

Lacertulus and the Origin of the Squamata in the Permian

This post was updated December 04, 2014 with a revision of the nesting of Lacertulus as a basalmost protosquamate, close to the origin of the Tritosauria and Rhynchocephalia. This was due to the inclusion of taxa to the large reptile tree. 

Updated July 7, 2020
the LRT moves Meyasaurus, Indrasaurus and Hoyalacerta to the base of the Yabeinosaurus + Sakurasaurus clade within the Scleroglossa and Squamata.

Traditional Hypotheses
The paradigm in paleontology holds that lepidosaurs (lizards and sphenodontians) and archosaurs (dinosaurs and crocs) both descended from a sister to Youngina. The present reptile family tree tested this traditional hypothesis with many more taxa and found a new tree that kept Youngina with protorosaurs and archosaurs, but moved lepidosaurs over to another major branch, the lepidosauromorpha.

Lacertulus

Figure 1. Lacertulus, a basal protosquamate.

Paliguana, at the base of the Lepidosauriformes
A skull-only taxon, Paliguana, nested at the base of the Lepidosauriformes. It lived 250 mya during the latest Permian or earliest Triassic. It gave rise to a clade of pre-lepidosaurs including Sophineta, Palaegama, Saurosternon and the Triassic gliders.

Paleagama gave rise to the Lepidosauria: including the Rhynchocephalia (Sphenodontia), the Tritosauria and the Protosquamata (including the Squamata).

Lacertulus at the base of the Protosquamata (basal Lepidosauria)
Lepidosauria is now restricted to the last common ancestor of Squamata (lizards, snakes and amphisbaenians) and Rhynchocephalia (or Sphenodontia, represented by Sphenodon), and all descendants of that ancestor (e.g., Gauthier et al., 1988). Unfortunately there are many lepidosaurs that nest in neither of these clades.

Lacertulus, at the base of the Squamata
The living and extinct lizards all descended from a sister to Lacertulus from the Late Permian. It was described as a potential biped, but the nonfusion of the astragalus and calcaneum removed it from the Squamata in the eyes of Carroll and Thompson (1978). Since then, a few more squamates without a fused astragalus and calcaneum have been discovered including Meyasaurus and Huehuecuetzpalli. Along with Lacertulus, some of these nested at the base of a third, previously unidentified, squamate clade, the Tritosauria, which included tanystropheids and pterosaurs. The others nested with the Protosquamata, which includes the Squamata.

The Longevity of Individual Squamate Taxa
Many of these tritosaurs and protosquamates survived into the Jurassic and Cretaceous, but not beyond. All are now extinct. Given the longevity of Sphenodon into the modern era and the long-lived examples of Huehuecuetzpalli and Homoeosaurus, perhaps other lizards known from more recent times will also be found closer to the Permo-Triassic origin of squamates.

Prior “Oldest Known Lizards”
When Protorosaurus was discovered, it was described as the “oldest known lizard.” Here it nested far from the lizards.  Paliguana was also described as the “oldest known lizard.” Currently the oldest known fossil lizard is Tikiguania from the Late Triassic, 220 mya. Most other old fossil lizards are known from the Jurassic and Cretaceous.

The most primitive squamate and the oldest squamate
The basal most squamate, Scandensia, lived in the Early Cretaceous, far from its origins in the Permian. A phylogenetic descendant, the TA1045 specimen is an anguimorph scleroglossan that lived in the Early Permian. It is the oldest known squamate.

References
Carroll and Thompson 1982. A bipedal lizardlike reptile from the Karroo. Journal of Palaeontology 56:1-10.

What is Eudibamus?

Updated July 21, with new skull images based on higher resolution pix.

Eudibamus cursoris (Berman et al. 2000) was a long-legged reptile from the Early Permian of Germany described as the earliest biped and originally considered a bolosaurid. The discovery of Eudibamus prompted a certain amount of excitement because previously bolosaurids (Bolosaurus and Belebey) were known principally by skulls only.

Figure 1. Eudibamus reconstruted.

Figure 1. Eudibamus reconstruted.

Why Bolosaurids?
Berman et al. (2000) originally considered Eudibamus a bolosaurid based on the rounded shape of its very small teeth. They may have also seen the lateral temporal fenestra, but they never published a skull reconstruction, perhaps because it was badly crushed. In overall shape, the skull of Eudibamus does indeed resemble that of Bolosaurus.

Figure 2. Compare Eudibamus to bolosaurids on the left and diapsids on the right.

Figure 2. Compare Eudibamus to bolosaurids on the left and diapsids on the right.

Why Not Diapsids?
Even without the skull, Eudibamus nested here close to the diapsids, but the skull has a diapsid temporal region, quite similar in configuration to Spinoaequalis and Petrolacosaurus (Figure 2), both of which predated Eudibamus by less than 10 million years. Moreover, the rest of the anatomy also closely matches that of other diapsids. There is no large coronoid process, only a gentle rise in the posterior jaw. The teeth did not resemble the strange crushers of bolosaurids so much. Here they appear to be merely short and blunt.

Eudibamus foot

Figure 3. Eudibamus foot compared to diapsids (in pink) and bolosaurid sister taxa (in yellow).

If we take a look at the feet, for instance (Figure 3), there is a closer match to Petrolacosaurus and Spinoaequalis than to bolosaurid sister taxa, Casea and Eunotosaurus. Digit 1 is very tiny, unlike the robust digit 1 of bolosaurid sisters. Digit 4 included at least two phalanges beyond the ungual of digit 3, unlike digit 4 in bolosaurid sisters. Digit 5 did not extend beyond p4.1, unlike digit 5 in bolosaurid sisters.

Unfortunately few to no other sister taxa of bolosaurids are known that preserve the post-crania and feet. From the list of sister taxa, such as Milleretta, we can surmise that bolosaurids had a short neck, bulky body (perhaps with expanded dorsal ribs), short legs and short robust toes, like those of Casea and Eunotosaurus. The skull of Eudibamus was preserved curled back, facing posteriorly, which means the neck was long enough to do this. Some cervicals are missing, but the elongated ones that remain resemble those in Petrolacosaurus more the shorter ones in Milleretta.

References
Berman, DS, Reisz RR, Scott D, Henrici AC, Sumida SS and Martens T 2000. Early Permian bipedal reptile. Science 290: 969-972.

wiki/Eudibamus

Which Way Did Pterosaur Fingers Flex?

Pterosaur fossils have been known for over 200 years and yet we still argue about which way the fingers flexed. Undisturbed or minimally disturbed fossils give some clues. Fossil handprints do too.

Most pterosaur fossils are found crushed and the finger claws (unguals) are crushed along with the rest of the body (Figure 1). Tall and thin, like the raptorial, tree-clinging claws they were, pterosaur claws were most often preserved broad side down, tips pointing anteriorly, medial faces exposed. And that’s the way they are traditionally reconstructed, paying little attention to the cylinder-shaped interphalangeal joints or evolutionary precedent.

Right hand of Shenzhoupterus, dorsal view.

Figure 1. Click to enlarge. Right hand of Shenzhoupterus, dorsal view. Note the crushing of the ungual broadside down and disarticulation of the various finger joints. If you ignored these facts you might want to orient the claws anteriorly as shown.

There are Two Hypotheses at Work Here
In the traditional model (Bennett 2008) metacarpals 1-3 were stacked with #1 on top and all three were bound to metacarpal 4. See Figures 2 and 3. The fingers flexed anteriorly (in flight). Any specimens in which metacarpals 1-3 were found lined up anterior to metacarpal 4, Bennett (2008) ascribed to the effects of gravity on the bones after death.

Pterosaur hand dorsal view

Figure 2. Pterosaur hands, dorsal view, the two opposing hypotheses. See Figure 3 for anterior views to see how Bennett (2008) intended the fingers to stack with #1 on top. The Bennett configuration orients the fingers facing up when the hands are adducted (brought together), which would be unsuitable for tree climbing/clinging. The Peters configuration, like clapping hands, points the fingertips together, ideal for tree climbing.

In the heretical yet more conservative model (Peters 2002) metacarpals 1-3 lined up side-by-side anterior to metacarpal 4 and the fingers flexed ventrally (as in all other tetrapods). See Figures 2 and 3. In this configuration only metacarpal 4 twisted 90 degrees axially so the palmar side faced posteriorly (in flight) to facilitate wing folding. The three small fingers did not change their configuration or orientation. The palmar side of fingers 1-3 continued to point ventrally in flight.

Peters (2002) reported: “…in certain Cretaceous forms [the medial three digits] rotated into the vertical plane and became closely appressed to the much larger metacarpal IV (Bennett, 1991; 2000b). From this configuration, they flexed anteriorly in a subhorizontal plane when the wing was extended.” I apologize for this. Unfortunately I was influenced by Dr. Bennett at the time. Subsequent studies helped me realize the error of this statement.

Bennett and Peters pterosaur finger orientation configurations

Figure 3. Bennett (2008) and Peters (2002) pterosaur finger orientation configurations. See Figure 2 for dorsal views. Note: Bennett wanted digit 1 dorsal as shown here, not as in Figure 2.

The Wellnhofer (1991) Twist
Wellnhofer (1991) lined up the metacarpals anteriorly, but also twisted the unguals anteriorly. Of course, this could be a problem in pterosaurs with fingers of similar lengths and does not take into account the various disarticulations at several finger joints.

The Evolution of the Bennett (2008) Configuration
Bennett (2008) imagined the evolution of the pterosaur hand (Figure 4) based on an imaginary taxon. He started with the supination of the entire arm, which rotated all the palmar surfaces anteriorly. Metacarpal 4 became thicker than the others as it supported a lengthening wing finger. Overlooked by Bennett (2008), but implicit in his arguments, the next step involved migration of the metacarpals 1-3 as a unit down the anterior face of a much larger metacarpal 4. The supination of the hand envisioned by Bennett (2008) ultimately included reversing the flexion and extension of digit 4 such that hyperextension folded the wing in his view. No other tetrapod has ever done this. Also note there is no space for the large wing extensor between the attached metacarpals (1-3 back-to-front with 4) in the Bennett (2008) configuration. Bennett (2008) also envisioned the early disappearance of ungual 5 on the wing, which, due to supination, also faced anteriorly in this configuration. However, the wing ungual was retained. Bennett (2008) also imagined the loss of manual digit 5, but manual digit 5 was retained. He did not envision an origin for the preaxial carpal and pteroid, which occurred as far back as Cosesaurus (Peters 2009).

Pterosaur finger orientation in lateral view

Figure 4. Pterosaur finger orientation in lateral view, the two hypotheses. There are several problems with the Bennett (2008) hypothesis, least of all it leaves no room for the big wing extensor tendon.


The Evolution of the Peters (2002) Configuration
Peters (2002) discussed the evolution of his pterosaur hand configuration (Figure 4) based on actual taxa, including Longisquama insignis. Between Longisquama and the first pterosaur manual digit 4 was rotated axially so that the palmar (flexor) surface became the new posterior surface to facilitate wing folding in the plane of the wing with hyperflexion. Digit 5, already reduced, became a vestige. It revolved, along with metacarpal 4, to the new dorsal side of the metacarpus. During the rotation, metacarpals 1-3 shifted to the ventral rim of metacarpal 4 while retaining their configuration. This left plenty of room for a large extensor tendon (Figure 4). Contra Bennett (2008), flexion remained flexion in all the fingers. There was no reversal of function. The preaxial carpal and pteroid first appeared in the fenestrasaur and pterosaur precursor, Cosesaurus (Peters 2009) having migrated to the medial wrist from the central carpus where they were identified as the two centralia seen in Sphenodon.

Here’s Where the Trouble Started
In many crushed fossils, like  Shenzhoupterus (Figure 1) and the left hand of Eudimorphodon (Figure 5), the claws point anteriorly because they are crushed broadside down. In order to do this, some finger joints must disarticulate and this is always observable.

But look what happens in the right hand of Eudimorphodon
In the same Eudimorphodon (Figure 5) the right hand has the palmar surface exposed. The metacarpals were lifted and flipped over the palmar surface of metacarpal 4, but metacarpal 3 remained attached to metacarpal 4. Moreover the unguals pointed posteromedially. According to Bennett (2008) this should not have been possible if metacarpals 1-3 were bound to the anterior face of metacarpal 4 and pointed anteriorly. Digit 1 should have been buried first and deepest, but it was not.

Eudimorphodon hands.

Figure 5. Eudimorphodon hands. The right hand preserves the metacarpals lined up anteriorly with only metacarpal 3 attached to metacarpal 4. The claws are disarticulated due to crushing. The right hand, preserved with its palmar side exposed shows what happens when the lighter digits 1-3 drift as a unit with metacarpal 3 moving the least because it was attached to metacarpal 4.


Santanadactylus hand and fingers

Figure 6. Click to enlarge. Santanadactylus hand with metacarpals preserved at a 45 degree angle to the anterior face of metacarpal 4.

Even in 3D Fossils There Can be Some Confusion
Here, in this 3D Santanadactylus hand, the metacarpals have been raised like a drawbridge, far from their original orientation (palmar side down) and close to being pressed against the large metacarpal 4, palmar side anterior. Such a configuration permits no space for the big tendon between the appressed surfaces of metacarpal 4 and the three small metacarpals. This is an excellent example of taphonomic lifting on the hinge at the metacarpal 3-4 interface from an origin with the palmar side down for metacarpals 1-3. It is the only configuration that permits the big extensor tendon of metacarpal 4 to run unimpeded dorsal to the three small metacarpals and their extensor tendons (Figure 4). When the extensor tendon rots or pops, there is nothing to prevent metacarpals 1-3 from rising and falling like an airplane elevator in the drifting sea currents.

The left manus of Pteranodon KUVP 49400

Figure 7. The left manus of Pteranodon KUVP 49400 in a rare anterior burial. Here the unguals were crushed in their natural orientation, palmar side down. Thanks to M. Everhart at OceansofKansas.com for this image.

YPM 49400 – a Rare Anterior Burial and Posterior Exposure in Pteranodon
Here in a Pteranodon specimen YPM 49400 metacarpus was preserved anterior face down. This is a very rare burial. The manus was preserved intact and in its natural orientation, fingers 1-3 palmar side down and metacarpal 4 palmar (flexor) side now posterior for wing folding. The claws here pointed ventrally as in other tetrapods. The metacarpals lined up as in other tetrapods. Buried like this there was no chance for them to wave around or become disarticulated in the bottom currents. The proximal wing phalanx would have stood vertically erect (essentially the Z-axis) in this configuration, but it rotted, disarticulated and fell on its dorsum, exposing its ventral face.

Pteraichnus nipponensis

Figure 8. Pteraichnus nipponensis (Lee et al. 2009) along with a matching trackmaker, n23 of Wellnhofer 1970.

The Evidence of Ichnites
Pterosaur handprints (Figure 8) commonly preserve digits 1 and 2 laterally and digit 3 posteriorly. Sometimes digit 1 extends anteriorly (Lee et al. 2009). In the Bennett (2008) configuration, all three fingers would have extended posteriorly when quadrupedal because the arms were supinated. In the Peters (2002) configuration, all three fingers would have extended laterally when quadrupedal because the arms were neither supinated nor pronated, but in the neutral position.

Digit 3 Goes the Opposite Way
The key to orienting digit 3 posteriorly (and sometimes digit 1 anteriorly) goes back to the lizard ancestry of pterosaurs. The metacarpophalangeal joint of digit 3 is different than digit 2. Shaped more like a hemisphere than a cylinder, it permits digit orientation in several directions. With digit 3 directed posteriorly, digit 4 never touched the substrate. This evidence is in direct contrast with the configuration envisioned for the difficult to support wing launch hypothesis currently in favor also illustrated here.

On a Side Note
Bennett (2008) connects the extensor and flexors tendons to the proximal tips of the first wing phalanx. This is wrong. In lizards these tendons split in half, bypassing the nearest points and inserting further down the bone. As shown here, this permits complete wing folding, something the Bennett (2008) attachment is unable to do. We just learned about the evolution of the manus hand and pteroid from a Sphenodon-like ancestor here.

Bennett (2008) reported, “The reconstruction of the long extensor and flexor of thewingfinger suggests that there was no rotation of Mc IV about its long axis. If there had been a rotation, the tendon of m. flexor digiti quarti would have had to spiral posteriorly under the metacarpus to insert on the posterior process of wing phalanx 1 and the tendon of m. extensor digiti quarti longus to digit.” As mentioned above, it is a mistake to attach the flexor to the posterior process of wing phalanx 1, as Bennett (2008) proposes. Rather the insertion is further down the phalanx shaft, as in lizards, and in order to complete wing folding. Thus there would have been no problematic spiraling if metacarpal 4 rotated axially.

In Summary
All the evidence points to a configuration in which the hand of pterosaurs was configured the same as in all other tetrapods with the exception that the big wing finger was axially rotated so that the old palmar surface became the new posterior surface. This configuration is based on actual predecessor taxa, not figments of Bennett’s imagination. The new configuration creates a large channel for the massive extensor tendon that is missing from the Bennett configuration. The new configuration does not require the flexor side of the wing finger to become the extensor side and vice versa.

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:
Bennett SC 2008. Morphological evolution of the wing of pterosaurs: myology and function. Zitteliana B28:127-141.
Lee YN, Azuma Y, Lee H-J, Shibata M, Lu J 2009. The first pterosaur trackways from Japan. Cretaceous Research 31, 263–267.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29:1327-1330.
Wellnhofer P 1991. The Illustrated Encyclopedia of Pterosaurs. Salamander Books, Limited, London, 192 pp.

Pterosaurs: Bipedal? Quadrupedal? or Both?

Dimorphodon Seeley

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.

Dimorphdon Padian

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?

Walking pterosaur according to Bennett

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.

Dimorphodon as a plantigrade quadruped

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.

Dimorphodon in a bipedal and digitigrade configuration

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.

Anhanguera taking off

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.”

Bipedal lizard video marker

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.

Pterodactylus walk matched to tracks according to Peters

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.

Dimorphodon pes with shadows.

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 x. Just having fun.

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.

Quetzalcoatlus running like a lizard prior to takeoff.

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:
Bennett SC 1997a. The arboreal leaping theory of the origin of  pterosaur flight. Historical Biology, 123(4): 265–290.
Bennett SC 1997b. Terrestrial locomotion of pterosaurs: a reconstruction based on Pteraichnus trackways. Journal of Vertebrate Paleontology, 17: 104–113.
Casamiquela RM 1962. Sobre la pisada de un presunto sauria aberrante en el Liassico del Neuquen (Patagonia). Ameghiniana, 2(10): 183–186.
Chatterjee S and Templin RJ 2004. Posture, locomotion, and paleoecology of pterosaurs. The Geological Society of America Special Paper 376:1-63.
Clark, J, Hopson J, Hernandez R, Fastovsky D and Montellano M 1998. Foot posture in a primitive pterosaur. Nature 391: 886-889.
Lockley, MG, Logue TJ, Moratalla JJ, Hunt AJ, Schultz RJ. and Robinson JW 1995. The fossil trackway Pteraichnus is pterosaurian, not crocodilian; implications for the global distribution of pterosaur tracks. Ichnos 4: 7-20.
Mazin J-M, Hantzpergue P, Lafaurie G, and Vignaud P 1995. Des pistes de ptérosaures dans le Tithonien de Crayssac (Quercy, France). Comptes rendus de l’Academie des Sciences de Paris 321: 417-424.
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