Bipedal lizards & pterosaurs

[darren.naish@port.ac.uk]

Re: bipedality in lizards compared to that of pterosaurs.

I said the previous email was going to be my last one for a 
while? I lied, this one is. In previous emails on the 
pterosaur palaeobiology thread, I noted that the bipedal 
lizard/pterosaur analogy might be flawed. Dave Peters 
(thanks for your response Dave, and for your kind words) 
has responded to this, and here I state more clearly (I hope) 
why I think the lizard/pterosaur analogy is flawed. 
Incidentally, Dave has previously published comments on 
the lizard/pterosaur analogy in?.

Peters, D. 2000. A reexamination of four prolacertiforms 
with implications for pterosaur phylogenesis. _Rivista 
Italiana di Paleontologia e Stratigrafia_ 106, 293-336.

On pterosaur bipedality, Dave wrote?

--------------------
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.
--------------------

I think the assertions made by Dave here are erroneous, and 
in the following rambling spiel I?ll attempt to show why. 
Well done if you make it to the end?

-- Why do certain lizard species run bipedally? 

Is it just so that they are faster? Probably not: the fastest 
lizards are quadrupedal runners, not bipedal runners 
(_Ctenosaura_ has been reliably clocked at 34.9 km/h and 
_Cnemidophorus_ [or, if you follow Reeder et al. (2002), 
_Aspidoscelis_] at 29 km/h? these are the fastest lizard 
speeds recorded according to Carwardine?s _Guinness Book 
of Animal Records_, and both genera are quadrupedal 
runners). 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). 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? If speed is what it?s about, it strikes me 
as unlikely that animals like pterosaurs would rely on 
terrestrial sprinting to escape predators or to get from A to 
B. 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. Bottom line: there is no inherent ?need? 
for good bipedal running abilities in pterosaurs, in contrast 
to the situation in lizards.

-- 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. 
Read on?

-- 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. 

Similarly then, the features you cite in pterosaurs might 
therefore indicate improved _quadrupedal_ abilities relative 
to their (?) more bipedal ancestors. 

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.

-- 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.

So, in conclusion?.

-- Bipedal lizards are not bipedal so that they can be faster 
or better runners, but so that they can avoid Carrier?s 
constraint. However pterosaurs ran, they would not have 
been under the same pressure to avoid Carrier?s constraint, 
so just because some lizards _can_ run bipedally it does not 
follow that pterosaurs would have ?wanted? to do the same.

-- Bipedality in lizards relies on a robust tail with a large m. 
caudofemoralis longus, yet pterosaurs lacked this. 
Consequently I feel that an analogy between bipedality in 
lizards and pterosaurs is inherently flawed.

-- A long preacetabular iliac process and increased number 
of sacrals are not correlates of bipedality unless present 
alongside proportionally short forelimbs and a short neck, 
and the presence of a long preacetabular iliac process and 
increased number of sacrals in pterosaurs may instead 
indicate improved _quadrupedal_ abilities. 

-- 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.
 
-- 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.

Ok that?s it. That really was the last email. Bye?

Refs --

Bellairs, A. d?A. 1969. _The Life of Reptiles, Vol. 1_ 
Weidenfeld & Nicolson (London), pp. 282.

Bennett, S. C. 1997. The arboreal leaping theory of the 
origin of pterosaur flight. _Historical Biology_ 12, 265-290.

Hutchinson, J. R. 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, M. S. Y. 1997. Pareiasaur phylogeny and the origin of 
turtles. _Zoological Journal of the Linnean Society_ 120, 
197-280.

Reeder, T. W., Cole, C. J. & Dessauer, H. C. 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, A. P. & Bauer, A. M. 1992. The m. caudifemoralis 
longus and its relationship to caudal autotomy and 
locomotion in lizards (Reptilia: Sauria). _Journal of 
Zoology_ 227, 127-143.

Synder, R. C. 1954. The anatomy and function of the pelvic 
girdle and hind limb in lizard locomotion. _American 
Journal of Anatomy_ 95, 1-45.

- . 1962. Adaptations for bipedal locomotion of lizards. 
_American Zoologist_ 2, 191-203.

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

email: darren.naish@port.ac.uk
tel: 023 92846045