Today we’ll take a look at the pterosaur femur, how it evolved and it’s range of motion in various pterosaurs.
Traditional Views of Basal Pterosaurs
Dr. Mark Witton reported, “… the orientation of the femoral head in basal pterosaurs means that the femur is projected forward, upward and laterally from the acetabulum, thereby causing the sprawling gait for the hindlimbs that acted in concert with the relatively short metacarpals to bring the bodies of these pterosaurs close to any surface they happened to be climbing over.”
Dr. Dave Hone reported, “On the ground the ‘rhamphorhynchoids’ were probably pretty poor. Their large rear membrane would have shackled their hindlegs together making walking difficult, and the shape of their hips and upper legs meant that could only really sprawl and not walk upright.”
Let’s See What the Fossils Tell Us
Contra the above assertions, the fossils tell us just the opposite. Basal pterosaurs, like Austriadactylus and Eudimorphodon, had a right angle femoral head, as in dinosaurs (Figs. 1 and 2) by convergence. Derived pterosaurs, like Pteranodon and Anhanguera, had a much more obliquely angled femoral head (Fig. 3). The false notion of a “large rear membrane” that purportedly shackled the hind limbs mentioned by Dr. Hone (above) was dealt with earlier here.
Figure 1. Eudimorphodon ranzii femur in medial view, the head is circular because it is pointed toward the Z axis, at right angles to the plane of the matrix and the rest of the femur.
Figure 2. Femur of Eudimorphodon cromptonellus illustrating the right angle femoral head. This tiny specimen may be juvenile, hence the incomplete ossification at each end. When the axes of the femoral neck and laterally-oriented acetabulum lined up, an upright configuration was produced.
Figure 3. Derived pterosaur femur samples. Above, Pteranodon. Below, Anhanguera. Note the oblique angle of the femoral head. When the axes of the femoral neck and laterally-oriented acetabulum lined up in these pterosaurs, a sprawling configuration was produced.
Range of Motion
The range of motion in a pterosaur femur has been of some interest (Padian 1983; Wellnhofer 1988, 1991), impacting the quadrupedal/bipedal debate, among other topics. The discovery of uncrushed pterosaurs has been helpful, but those can’t discount the fossil record of crushed pterosaurs. Preservation in various angles and exposures paints a complete picture confirmed by comparing several sisters.
Padian (1983) proposed that pterosaurs tucked their hind limbs into the body while flying. While possible in basal pterosaurs, and likely while resting, such a configuration would have proven difficult in Anhanguera and Pteranodon with their sprawling femora. Plus reducing the distance between the femur and tibia in flight would have made the uropatagia disappear at a time when they would have been most useful. A more recent hypothesis (Peters 2002) proposed a more widespread femur acting as a horizontal stabilizer, a second wing (Fig. 5), as in the pterosaur sister Sharovipteryx. After all, lizards like Draco take to the air with outspread limbs and pterosaurs were flying lizards.
The Example of Austriadactylus
Austriadactylus was another basal pterosaur with a right angle femoral head (Fig. 4). Seen in various views it becomes clear that an upright stance with knees slightly beyond the ankles would have been appropriate given the shapes of the various elements. Contra Witton’s assertions (see above), Austriadactylus was not forced to crouch close to any surface it happened to be climbing over.
Figure 4. Austriadactylus femur. Range of motion. The foot was rotated posteriorly out of the airstream. The toes were likely webbed in all pterosaurs. When spread the webbed foot turned into a sail with lateral lift, helping to keep the knee extended even while facing the airstream.
Potential Problems with a Right Angle Femoral Head
While a derived sprawling femur would have had no problem assuming an outstretched flying configuration, I always wondered how a primitive right angle femur would handle it. Seemingly an inverted V-shape would have been all a basal pterosaur could muster. After closer examination (Fig. 4) that turned out to be true, but to less of an extent than I thought earlier. The femur could not rise to the horizontal, but it could come to within 20 degrees. The fact that the femoral head was more spherical in basal pterosaurs enabled this.
Lift from the Pelvis?
In Sharovipteryx and most pterosaurs the ilium does not rise much above the acetabulum. However in Longisquama and basal pterosaurs the anterior and posterior processes of the ilium rise as much as 45 degrees to the horizon or 90 degrees to each other (Fig. 4). Such a pelvis raises the torso and tail, which is ideal for bipedal leaping, as in Longisquama and living lemurs. Not so great though, to have an upright tail while flying. However, such an upright ilium also provides the leverage for the thigh muscles to lift (abduct) the hind limbs for flight. Later pterosaurs evidently did not need so much leverage with sprawling femora more aerodynamically shaped to provide their own lift in the airstream.
Figure 5. Arthurdactylus in dorsal view while flying. Note the sprawling hind limbs acting like horizontal stabilizers.
More About Derived Pterosaurs
Dr. Hone also reported, “The pterodactyloids had no such problems, as can be seen by their extensive fossil record of footprints. They had split their rear membrane in two freeing the legs which were brought under the body to allow them to walk far more effectively than their predecessors.” Let’s put more data on these assertions. While true that the feet were brought under the body (as in ALL pterosaurs, see Fig. 4), the knees in Pteranodon were actually configured further laterally than in basal pterosaurs. No problem. As you can see by the illustration below (Fig. 6), based on a 3D reconstruction of a complete skeleton, no matter the sprawl of the knees, as long as the knees were bent at right angles and the knees were below the plane of the acetabulum the feet remained beneath the body. This is simple engineering and standard operating procedure for all pterosaurs.
Figure 6. Standing Pteranodon. Note the sprawling femora do not hinder the bipdal stance. And, yes, Pteranodon likely placed its hands on the ground while walking, but so far anterior they could provide no forward thrust, only support.
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
Kellner AWA and Tomida Y 2000. Description of a New Species of Anhangueridae (Pterodactyloidea) with Comments on the Pterosaur Fauna from the Santana formation (Aptian-Albian), Northeastern Brazil. National Science Museum, Tokyo, Monographs, 17: 1-135.
Jenkins FA Jr, Shubin NH, Gatesy SM and Padian K 1999. A primitive pterosaur of Late Triassic age from Greenland. Journal of the Society of Vertebrate Paleontology 19(3): 56A.
Jenkins FA Jr, Shubin NH, Gatesy SM and Padian K 1999. A diminutive pterosaur (Pterosauria: Eudimorphodontidae) from the Greenlandic Triassic. Bulletin of the Museum of Comparative Zoology, Harvard University 155(9): 487-506.
Padian K 1983. A functional analysis of flying and walking in pterosaurs. Paleobiology 9:218.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. Historical Biology 15: 277-301.
Wellnhofer P 1988. Terrestrial locomotion in pterosaurs. Historical Biology 1: 3.
Wellnhofer P 1991. The Illustrated Encyclopedia of Pterosaurs. (Salamander Books, London).
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.
The Pterosaur Heresies 