In part 1 of this post we reviewed a new paper on pterosaur pelves by Hyder et al. (2012). Yesterday we discussed part 2. Today we’ll finish.
Hyder et al. (2012) include toothless, sword-jawed Pteranodon and Nyctosaurus with the toothy, spoon-tipped ornithocheirids. This is a mistake created by not including enough taxa as demonstrated by the large pterosaur family tree. Hyder et al. (2012) report, “The unfused scapulocoracoids and skull bones of these remains suggest they represent immature individuals, indicating that only the oldest ornithocheiroids have completely fused, imperforate pelves.” Actually these are all mature individuals. Lack of fusion in the sacral and pelvic elements are phylogenetic in pattern, not ontogenetic. The pelves of pteranodontids (closer to eopteranodontids and germanodactylids) bear little resemblance to those of ornithocheirds (closer to cycnorhamphids and scaphognathids), except for the Field Museum specimen of Nyctosaurus, which is the only specimen from this clade illustrated by Hyder et al. (2012).
Hyder et al. (2012) include pterodactylids, ctenochasmatids and cycnorhamphids in this clade, but the larger study separates these taxa widely. Here again, Hyder et al. (2012) stumble when they report, “Unfortunately, the majority of these specimens represent immature individuals (Bennett 1996).” Fusion follows phylogenetic pattern in pterosaurs. As lizards they don’t follow archosaur fusion and maturation rules, but may fuse early in ontogeny and keep growing, or they may never fuse certain bones, no matter how old they get (Maisano 2002). This reminds of the joke, “Where are all the baby pigeons? – Those ARE the babies.” It’s not a funny joke, it just came to mind.
Hyder et al. (2012) correctly include germanodactylids and dsungaripterids in this clade, but others (e.g. tapejarids, pteranodontids) were left out. The 3-D pelves in this clade are wonders to behold. Note that all fuse or meet ventrally. None of this open bottom baloney.
Hyder et al. (2012) incorrectly include azhdarchids and tapejarids in this clade. To their credit they report, “…adult remains of the tapejarid Sinopterus also seem to lack notaria and supraneural plates (e.g. Lü et al. 2006b), suggesting this group never attained neural spine fusion in their trunk region.” Supraneural plate development occurs occasionally in this clade by convergence, as in other large pterosaur specimens from other clades.
Functional Evolution of the Pterosaur Pelvis
Once again, Hyder et al. (2012) avoid discussing the origin of pterosaurian pelvic traits from plesiomorphic taxa, a subject covered by Peters (2000, 2002). Hyder et al. (2012) report, ichnological and biomechanical evidence indicates,”all pterosaurs were primarily plantigrade quadrupeds when walking and running.” This ignores recent literature on bipedal pterosaurs and fenestrasaurs featuring bipedal tracks. Hyder et al. (2012) report, “we follow hypotheses that all pterosaurs, including the earliest forms, held their legs in an erect stance.” This is correct for basal forms, but ignores derived pterosaurs in which the femoral head was more or less aligned with the femoral shaft, creating a very sprawling posture (Fig. 1).
Figure 1. Terrestrial pterosaurs – two configurations. Above the configuration promoted by Hyder et al. (2012). The short red arrows point to 1. an overextended elbow; 2. too large of a pes; and 3. femoral head not aligned with acetabular cup. The long red arrow shows the power vector running from the hand to the shoulder. Note: it is braking the animal with every step, not contributing a forward vector during locomotion. Below, the configuration promoted here, in which the hind limbs provide all of the propulsion with the forelimbs merely acting to steady the animal. Note the center of balance remains beneath the shoulder glenoid when standing still. This changes with forward motion, as it does in humans who also lean forward when they walk. Ornithocheirids were not good walkers. Their spindly legs merely kept their rumpus off the tarmac.
Hyder et al. (2012) compare the proportions of the Dimorphodon pelvis and hind limb with those of Scleromochlus and dinosauromorphs, ignoring the morphological differences. They report, “pterosaurs were capable of bursts of speed,” but impeded by the uropatagia (uropatagium) stretching between the hind limbs. The hypothesis of a single uropatatium between the hind limbs is a false precept covered here. Hyder et al. (2012) support a bounding, hopping gait to work around this problem, but no pterosaur tracks show this. Could be. Might be. So far no evidence.
Lengthening of the Preacetabular Process
Hyder et al. (2012) report, “The broadening of the attachment site for epaxial musculature may reflect stiffening and strengthening of the torso, a trait that could be interpreted as a precursor to the development of supraneural plates over the pectoral and pelvic girdles in many forms.” Unfortunately this ignores the fact that Cosesaurus, expressing the genesis of this elongated ilium trait, had an otherwise tiny pelvis. Hyder et al. (2012) also report, “the elongate preacetabular process provides larger attachment sites or greater lengths for the hindlimb extensor muscles, thereby increasing their strength or endurance.” This is obvious. Hyder et al. (2012) report, “The condition of the preacetabular process does not seem to change relative to hindlimb morphology, which may suggest its development is largely independent of hindlimb mechanics.” Nothing could be further from the truth. Bones, muscles, they all interact.
Noteworthy, Hyder et al. (2012) completely ignore the great reduction in caudofemoralis anchors on the attenuated tail, largely without transverse processes or descending chevrons. The loss of posterior muscles and the enlargement of anterior muscles directed at the hind limb shift the center of balance forward and made possible flight, as in birds and, along a different path, bats.
Hyder et al. report, “The pelves of ornithocheiroids have undergone greater changes than in any other pterosaur group, presumably reflecting distinct hindlimb mechanics in this clade. The dorsal inclination of the preacetabular process is their most distinctive pelvic feature and would have been detrimental to leverage of the limb extensors.” Some of the problems they see are due to an incorrect reconstruction. It’s important to match the acetabular cup with the femoral head, to keep the prepubis between the femora, to keep the knees well bent and to keep the toes below the center of balance, below the armpits. Elevating the torso does this and Hyder et al. recognized this to their credit.
By homology (according to Hyder et al.) and by convergence (according to the large pterosaur family tree) the pelves of tapejarids and azhdarchids developed a large, fan-shaped post-acetabular process. Hyder et al. report, “it may represent a unique solution to increasing hindlimb retractor power among archosauriforms. Most archosauriforms use their robust tails to anchor a powerful, femur retracting M. caudofemoralis, but the slender pterosaur tail was unable to support such powerful musculature (Persons 2010). Instead, azhdarchoids appear to have increased the size and leverage of M. flexor tibialiis and M. iliofemoralis musculature, and may have given them larger, more superficially mammalian-like haunches, than reptilian.” Good point. However, the one thing both have in common is a large skull, sometimes crested, sometimes at the end of a long neck. In both cases additional opposite leverage would have been helpful and this may be why the post-acetabular process became expanded in these taxa. But really, who knows?
Why there’s something very wrong with our pterosaurs.
Giving credit where credit is due, the following pterosaur experts all had a hand in the report by Hyder et al. (2012) according to the acknowledgements: Darren Naish, Steve Vidovic, Dave Unwin, Dino Frey, Ross Elgin, Lorna Steel, Mike Habib, David Hone and Michael Benton.
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.
Hyder ES, Witton MP and Martill DM 201X. Evolution of the pterosaur pelvis. Acta Palaeontologica Polonica 5X (X): xxx-xxx. http://dx.doi.org/10.4202/app.2011.1109
Maisano JA 2002. Terminal fusions of skeletal elements as indicators of maturity in squamates. Journal of Vertebrate Paleontology 22:268-275.