Earlier here, here and here we had a critical look at the hypotheses regarding various aspects of pterosaur phylogeny and morphology. Today we’ll look at the muscles of the pterosaur hind limb and how Witton (2013) emaciated them.
Make sure those hind limbs look emaciated
if you want to convince others that pterosaur hind limbs were not capable of providing bipedal locomotion (in step with quadrupedal locomotion for most) or hindlimb leaping/launching/takeoff. Witton 2013 emaciates his pterosaur femoral muscles and reduces their points of origin on the ilium and prepubis. Why? He supports the forelimb launch hypothesis for pterosaurs big time.
Real lizard femoral muscles are robust and meaty (Figs. 2,3 ). The muscles get thicker at mid thigh. This even happens in birds and crocs! Why would Witton emaciate them?
No Prepubis Anchor
Pterosaurs extend their ventral muscle anchorage by adding a prepubis, which can be very long indeed in Rhamphorhynchus (Fig. 2) and Campylognathoides (Fig. 3). No muscles attach to the prepubis in Witton’s version (Fig. 1). One wonders why not, especially when the prepubes and femora are aligned during normal locomotion (Figs. 2-4).
Instead Witton 2013 follows Claessens et al. (2009) mistake when he reports the prepubes “were capable of moving up and down with each breath taken by their owner.” This “rotating prepubis” hypothesis was falsified earlier based on the Claessen et al. use of a flipped and partial prepubis to support their hypothesis. They got the bone upside down!! No other prepubes in any other pterosaurs support the Claessen et al. hypothesis. The pubis/prepubis joint is a butt joint in all pterosaurs. So it basically cannot move. The prepubis acted as an extension to the pubis. Pubofemoralis muscles probably extended down the prepubis as if it were an elongated pubis. Respiration occurred by expansion of the ribs, as in all tetrapods, not by the rotation of the prepubes. Correctly configuration shown below (Figs. 2-4).
And why would the all pterosaur ilia extend so far anterior (especially so in Sos 2428), framing so many sacrals (Fig. 1), without bringing a few muscles with them? After all, that’s what mammals and dinosaurs do. And the muscles arising from the ilium in lizards concentrate anteriorly. Finding homologies and analogies is how we find the most parsimonious answer.
The missing caudofemoralis
Lizards and most dinosaurs have a robust tail with elongate transverse processes and deep chevrons. These are muscle anchors for the caudofemoralis, tail muscles that pull the femur posteriorly, contributing to the step cycle. In birds and pterosaurs these muscle anchors are largely, but not completely missing. The pelvis (and prepubis) have taken over those duties. The caudofemoralis is largely, but not completely missing in birds and probably pterosaurs. As in birds, pterosaur the anchoring transverse processes are vestigial or missing and their chevrons, where present, extend parallel to the caudal centra, not ventrally. In pterosaurs, chevrons are not caudofemoralis anchors, but secondarily adapted as tail stiffeners. They are essentially absent in basal pterosaurs, like MPUM6009. They redevelop in several taxa. These same caudal patterns (attenuated tails) are found in pterosaur precursors, the fenestrasaurs, evolving from less attenuated tails in tritosaur lepidosaurs, a key trait that ties them all together.
It’s important to examine living animals to see their muscle patterns in order to reconstruct them in prehistoric animals. It’s important to know what new bones, like the prepubis, are used for (not respiration). It’s important to note the details in a skeleton, establishing articular surfaces and creating accurate reconstructions.
Claessens, LPAM, O’Connor PM and Unwin DM 2009. Respiratory Evolution Facilitated the Origin of Pterosaur Flight and Aerial Gigantism. PLoS ONE 4(2):e4497.http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004497
Romer AS 1971. The Vertebrate Body (shorter version). WB Saunders Co. 452 pp.
Witton M. 2013. Pterosaurs. Princeton University Press. 291 pages.x