A new paper on pterosaur breathing has arrived.
Unfortunately, Geist et al. (2013) follow Claessens et al. (2009) in hoping that the prepubis was mobile in order to drive a thoracic lung pump. It’s not. (BTW, we looked at Claessens et al earlier here).
Geist et al. report, “we note that many of the large pterodactyloid taxa had relatively rigid trunks due to a high degree of fusion of the thoracic skeleton, a condition we describe as a synthorax… but also appear to have severely constrained ribcage movement for respiration.”
How is this shown?
Geist et al. report, “Although their morphology is variable, they [pterosaurs] always have a narrow, rod-like caudoproximal peduncle, but widen and flatten cranially. Their general appearance and orientation is often strikingly similar to that of the pubic bones of extant crocodilians.”
This is an error.
Prepubes are oriented ventrally with a non-moving butt joint, and only sometime slightly cranially. Thus they are not similar to extant crocodilians, which rotate their pubes uniquely.
After just reporting that some pterosaurs fuse their prepubes medially, Geist et al. report, “The hinge-like [medial] articulation with the pubic bones indicates that the prepubic bones were highly mobile in the vertical plane, but were restricted in transverse movements.”
This is also bad reporting, based on bad drawings in the literature, (like Claessen 2009), not based on manipulating 3D prepubes on pubes, as I have done over several dozen taxa.
The prepubes act as immobile pubic extensions. And in that capacity they anchor adduction muscles to the femur, like the long booted pubis of T-rex. They have nothing to do with respiration. Nor were they co-oped for respiration from their original function.
Sure prepubes had large muscle scars. They were attached to large leg muscles!
The most mobile parts of the pterosaur thorax were the posterior dorsal ribs. They were slender and single-headed to retain their mobility. Lizards breathe with their ribs. Here’s a short video and another video for anyone who can’t picture when a lizard holds still, their rib cage keeps pumping. Holding still, btw, is the same as having a fused backbone, in terms of methodology.
Geist et al report, “Claessens et al. (2009) presented a detailed model for an avian-like mechanism of sternal excursion as the primary lung ventilation mechanism for both small and large pterosaurs—the “skeletal breathing pump” model.” In essence the sternal complex rotated on the coracoid joint with some movement from the scapulocoracoid. This pulled the ribs and gastralia forward increasing thorax volume.”
Geist et al were critical of Claessens noting, “In pterosaurs the orientation of the rib articulations on the thoracic vertebrae are very unlike those of birds (Fig. 3), and likely would not have permitted the degree of fore-aft movement of the caudal ribs as proposed by Claessens et al. (2009).”
Geist et al continue, “Three dimensionally preserved specimens indicate that pterosaur ribs project more or less perpendicular to the long axis of the body (e.g., see Fig. 4A–E). A caudal inclination of these ribs similar to the orientation of those in extant birds cannot be confirmed here.”
That would be odd. Pterosaurs are now flying pancakes? No. Perhaps that’s just an odd choice of words because their illustrations don’t follow that morphology. Sharovipteryx was the flying pancake!
Geist et al. report, “the morphology of the synthorax of large pterosaurs is permissive of extracostal mechanisms that resembled the visceral pumps [diaphragms] found in mammals and crocodilians.” After considering turtle, mammal, bird and croc respiration, they never once mentioned lizards. Unfortunate.
Ultimately, Geist et al. report, “Accordingly, we propose a model for large pterosaurs in which a more or less crocodilian-like visceral displacement pump drove the inhalation phase of the respiratory cycle, and contraction of flank muscles acting on the gastralia and prepubic bones restored the viscera to their initial positions to drive exhalation (Fig. 9). In our model, sheets of diaphragmaticus-like skeletal muscle originated on the pubis, prepubic bones, and/or caudal-most gastralia and inserted on a transverse septum or the fascia of the liver.”
Of course, as they admit, there is no evidence for this. And I’ll add some serious evidence against it.
Some pterosaur pubes are much shorter than their ischia. What would this mean for a cranially directed prepubis? Take a look at Figure 1 and put it together for yourself. Unfortunately, Geist et al. looked at only those pelves that had a long pubis.
If pterosaurs had a avian-like or monitor-like air-sac system associated with their lungs, then the posterior ribs could have pumped air in and out of those sacs, driving air through the lungs. Very typical of lizards, not crocs or birds.
To their credit, Geist et al. touch on the fact that sternal ribs are rarely ossified, noting they were likely often cartilage-based.
Claessens LPAM, O’Connor PM, Unwin DM 2009. Respiratory Evolution Facilitated the Origin of Pterosaur Flight and Aerial Gigantism. PLoS ONE 4(2): e4497. doi:10.1371/journal.pone.000449
Geist NR, Hillenius WJ, Frey E, Jones TD and Elgin RA 2013. Breathing in a box: Constraints on lung ventilation in giant pterosaurs. The Anatomical Record. 013 Dec 10.