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How did Pterosaurs Move on the Ground?

The locomotive manner of pterosaurs is a matter of intense dispute in the scientific community. The way that these prehistoric creatures traversed the ground is continually being contested. Did pterosaurs walk how we imagine them to have; on all fours in a semi erect posture, or did they only walk on their hind legs, or on all fours and completely prone? If, in fact, they did walk on all fours, would the feet have been in line with the hands, or would the hands be splayed out much farther than the feet? Because the method of pterosaur locomotion is widely disputed, scientists rely on footprint identification in order to determine if the tracks that they are studying do actually belong to a pterosaur. Unfortunately, even this is a contested issue, as some scientists can’t even agree on which tracks belong to which prehistoric creatures. These are all issues that are disputed by scientists, but there does seem to be some common consensus when multiple seemingly conflicting scientific findings are synthesized.

The general perception of pterosaurs today is one that is possibly incredibly flawed because of theories about them from the early nineteenth century. Soemmerring confused a juvenile Pterodactylus with what he believed to be the skeleton of a deviant bat in 1817 and Cuvier’s identified pterosaurs as reptiles in 1812. These ideas have persisted until the modern day so that most people cannot shake the image of a pterosaur as a large, scaly, bat-like creature that walks around on all fours in a semi-erect fashion (Padian, 1983). While this is a possible form of locomotion, there are numerous arguments to the contrary.

One of the first writings on pterosaur locomotion on land was Wm. Lee Stokes’ “Pterodactyl Tracks from the Morrison Formation” published in 1957. Stokes found a track of nine footprints that he attributed to a pterosaur that he named “Pteraichnus Saltwashensis.” Since pterodactyls are naturally found near bodies of whatever, it’s not out of the question that these tracks would belong to one. Stokes claims that the imprint of the manus shows the unique wrist and hand of a pterodactyl and that the imprint of the pes shows the unique V-shape and sharply pointed heel that “corresponds unmistakably” to that of a pterodactyl. He says that all of this is proof that at least one pterosaur was quadrupedal.

Despite the evidence that Stoke says confirms that the creature that made these tracks was a pterosaur; this is one of the most disputed claims in the theory of the ground-level locomotion of pterosaurs. Many scientists believe that these tracks were not made by a pterosaur, but instead by a crocodilian creature. This is a distinct possibility because the tracks of pterosaurs and crocodilians are commonly confused for one another.

In “The Fossil Trackway Pteraichnus: Not Pterosaurian, but Crocodilian,” Kevin Padian and Paul Olsen claim that the tracks that Stokes attributes to Pteraichnus Saltwashensis are more likely the product of one of the four species of crocodilian known to have inhabited that area. They say that the pes is not unique to pterodactyloids, as crocodiles also exhibit four toes and a V-shaped heel. Also, they claim that the manus does not consist of four digits, but five, even though the impression of all five is not clearly preserved. Another distinguishing feature is that crocodiles are plantigrade and therefore the heel print would be visible, as it is in these tracks. This is opposed to the digitigrade nature of pterosaurs, which would cause the heel print to be excluded from the footprint.

Padian and Olsen claim that the manner of movement that the footprints represent is completely different than the kind that Stokes claims created them. Stokes suggested that the depth of the depression created by the manus was due to a large wing knuckle. Padian and Olsen argue that this is not the case, and that the depression was actually caused by the “force of the step and the incompetence of the substrate” that led to the creation of the fossilized track.

One of Padian and Olsen’s points is refuted in “Terrestrial Locomotion of the Pterosaurs: A Reconstruction Based on Pteraichnus Trackways,” published in 1997 by S. Cristopher Bennett. Padian and Olsen claim that by using Baird’s method, they were able to determine that the length of the body that made the tracks was too long to be that of a pterosaur, but long enough to be a short crocodile. Bennett’s refutation of this point rests on the idea that Baird’s method was intended to be used on tracks created simultaneously and that this was not the condition under which the tracks in contention were created. Whether this refutation is credible or not is of little importance. Padian and Olsen’s assertion that the Pteraichnus tracks were actually created by a crocodilian, and that they back it up with plausible scientific hypotheses, casts enough doubt on Stokes original claim to make these tracks inadmissible in determining how pterosaurs moved on land. The primary reason for the possible mistake that these crocodilian tracks were created by a pterosaur, according to Padian’s 1983 paper, “A Functional Analysis of Flying and Walking in Pterosaurs,” is that Owen “suspend[ed] the principles of comparative anatomy [and drew] spurious comparisons between pterosaur structures and those of lizards and crocodiles [as well as] misrepresenting the work of H. von Meyer, the German authority on pterosaurs.”

This paper offers an in depth analysis of the way a pterosaur would move on land based entirely on its bone structure. Padian’s primary subject of analysis is a Pteranodon, which he claims, among other things, was an active flyer, meaning that it flapped its wings. Padian claims that a pterosaur would walk upright on two legs as opposed to any form of quadripedal motion. He compares pterosaurs to both birds and bats throughout the paper. He attempts to show that the attributes that pterosaurs share with either birds or bats, or both of them, would prevent a pterosaur from being capable of quadrupedal locomotion.

Padian says that the forelimb of the pterosaur was adapted entirely for flying and not for quadrupedal walking. The humerus bone of the forearm was adapted for the type of movement that facilitated a down-and-forward flapping motion, since Padian claims that pterosaurs were active flyers, and not for the rotation of the humerus that would protract the forearm to facilitate terrestrial walking. Moving down the arm, the elbow joint in birds, bats, and pterosaurs are what allow for rotation. In bats, this allows for quadrapedal walking but, in both birds and pterosaurs, it has hindered articulation to the extent of turning their limbs into “pulley-like hinges with a wide arc of movement possible in only one plane” (Padian 1983, 227). Padian claims that the differences in the shape of humeral heads in pterosaurs were even more restrictive in the pterosaurs’ range of motion than it is in birds. The delto-pectoral crest in pterosaurs was pronounced and twisted along the axis of the shaft, which caused the deltoideus muscle to actually reverse the rotation of the humerus that was initiated by the pectoralis muscles as the pterosaur made a downward stroke while in flight. This reversal in rotation prohibited the forelimb from being moved into the proper position to facilitate its placement on the ground and therefore any form of quadrupedal locomotion.

Ultimately, Padian asserts, the structure of the forelimbs of pterosaurs prevented them from walking quadrapedally. They were incapable of parasagittal movement over the ground because of restrictions imposed on them from their bone and muscular structures.

The structure of the hind legs of pterosaurs that are included in Padian’s argument for a bipedal pterosaur is equally as important as the structure of the forelimbs. Padian bases his argument on a pterosaur that’s wing was not attached to its hind legs and the idea that despite the relatively small size of the legs of a pterosaur when compared to the rest of its body and specifically its forelimbs, it would still have been able to, and in fact been solely capable of, bipedal locomotion on its hind limbs.

Padian reconstructs the pelvis and hip joints of a pterosaur to show that certain elements of its anatomy would prevent it from bending forward in a way that would enable quadrupedal locomotion. He says that there are two girdles surrounding the point where the femur is inserted into the hip that restricts the degree of rotation of the leg so that it would be impossible to move in any way other than bipedally.

Padian uses a Pteranodon as his case study for a pterosaur with an extremely large upper body and a relatively small lower body that is still able to walk on the ground. He says that the reason the forelimbs are so large is because the mass of the Pteranodon necessitated expansive wings to provide the appropriate amount of lift while airborne. The size of the wings, though, is not correlated to the size of the legs. The surface area of the wings needed to increase at a rate of L^3/L^2 when compared to the size of the body. The disproportionate lengthening of the wing afforded a greater relative area that allowed the Pteranodon to achieve flight. On the other hand, the Pteranodon’s legs are not restricted to the same ratio. The legs can be, and are, significantly smaller than the body that they’re supporting without compromising the Pteranodon’s terrestrial locomotive abilities. In fact, the size of the Pteranodon’s hind limbs in relation to the size of the rest of its body is comparable to the relationship between the hind limbs and bodies of all other pterosaurs.

Padian compares the bone structure of the hind limbs of pterosaurs to the bone structure of the hind limbs in modern birds. He says that, like in birds, the tibia is the main load-bearing bone in the leg. He claims that the femur had a position that ranged between slightly above horizontal to about forty-five degrees below it and that this orientation allowed for the tibia to move on a parasagittal plane. This essentially means that pterosaurs moved on the ground like most other bipedal dinosaurs.

Padian’s ultimate claim is that pterosaur hind limbs were designed for bipedal, digitigrade, locomotion when they were moving on the ground. He even suggests that it was possible for pterosaurs to achieve high rates of speed on the ground.

S. Christopher Bennett is another proponent of the upright, bipedal form of terrestrial locomotion among pterosaurs, but only among the larger pterodactyloids. He exams the acetabulum of Anhanguera, Pteranodon, and Dsungaripterus in “A Pterodectyloid Pterosaur Pelvis from the Santana Formation of Brazil: Implications for Terrestrial Locomotion,” published in 1990, and determines that its strongest part is the anterior wall. He suggests that the strength of this anterior wall allows for upright bipedal locomotion because the acetabulum becomes a downward facing component of the anatomy. This downward orientation allows for a large area of contact for the head of the femur and, therefore, the weight of the body of the pterosaur can be supported in an upright position.

Bennett refutes three arguments that are often leveled against bipedal locomotion in pterosaurs. He says that the femora bone could, in fact, have been brought underneath the body of the pterosaur, which is demonstrated by AMNH 22569. Secondly, that the hind limbs of the pterosaurs had straight femurs with distinct, inturned heads, mesotarsal ankles without a calcaneal tuber, and a foot with elongated metatarsals. Finally, he argues that the preservation of pterosaurs, in a position with their hind limbs splayed to the sides, a position that has caused many scientists to claim that they’re incapable of bipedal locomotion, is a false supposition. He notes five specimens that were preserved with their hind limbs on one side of the body, and argues that the splayed nature of the other specimens is due to greater hip flexibility in pterosaurs.

Bennett’s inclusion of the fossilized remains of pterosaurs in his argument for bipedal locomotion among the larger pterodactyloids is particularly convincing. He makes up for one of the key elements that Padian’s argument was missing, in that he actually gives concrete physical evidence to support his argument instead of supposed anatomical construction and the type of movement that that structure would or would not allow.

One thing that Padian’s analysis of the terrestrial movement of pterosaurs lack is the actual fossilized track that a pterosaur made. While the supposed structure of the bones may lead one to believe that Padian’s claims are accurate, there are numerous pterosaur tracks that indicate a quadrupedal form of locomotion among pterosaurs. The majority of the studies included in this paper have focused on larger pterosaurs, and mostly on the Pteranodon. This particular pterosaur shares a large amount of its anatomy with modern birds and it makes sense that, from an analysis paying the majority of its attention to this pterosaur, they would all seem to be bipedal. There is evidence in other fossils, though, that would say otherwise. Pterosaurs, depending on the kind they were, were either bipedal or quadrupedal.

Bennett’s refutation of Padian and Olsen’s crocodilian diagnosis of the Pteraichnus tracks contains very valid points concerning the way that pterosaurs could have possibly walked in a quadrupedal manner. He says specifically that “rhamphorhyncoids,” the earlier variation of pterosaurs, as well as small pterodactyloids were quadrupeds with plantigrade pedes (Bennett, 1997). He says that the model he constructed of Pterydactylus and that, along with other detailed studies and manipulations of other three-dimensionally preserved pterosaur specimens, the limb and girdle joints would have easily allowed for the range of motion necessary for a pterosaur to achieve a quadrupedal walking motion.

Bennett cites the Pteraichnus saltwashensis tracks as being representative of a quadrupedal pterosaur, which was disavowed earlier in this paper. He validly examines the tracks of Pteraichnus stokesi that were discovered in the Sundance Formation of Wyoming as well as pterosaur tracks from the Upper Jurassic of France. These trackways show manus and pes prints that are clearly pterosaurian. This indicates that there were certain species of pterosaur located in these areas that were quadrupedal. All of Bennett’s are based on the principle of identifying the tracks as pterosaurian based on the structure of the manus and pes.

Another piece of research that proves that quadrupedal locomotion was present in pterosaurs is “First Record of a Pterosaur Landing Trackway,” published in 2009 by Jean-Michel Mazin, Jean-Paul Billon-Bruyat, and Kevin Padian. The tracks that they are examining are clearly the landing pattern of a pterosaur because they originate with two pes prints and then continue on in a walking pattern. They claim that the stance of the pterosaur was erect because the pedes tracks are placed almost directly below the hip joints. After the first impression of the pedes, the second impression includes both pedes and manus impressions. The impressions of the manus are splayed out wider than pes, which is the generally accepted idea of how a pterosaur would ambulate. The tracks, not only created by a flying creature, are located in the Late Jurassic site known as “Pterosaur Beach,” which is a place universally agreed upon to contain unquestionable pterosaur tracks. Therefore, the landing pattern and subsequent tracks, that clearly show quadrupedal locomotion, prove that at least some pterosaurs moved in this way on the ground.

The debate surrounding the terrestrial locomotion of pterosaurs can be broken down into many categories and subdivisions. It’s clearly that one form of locomotion, whether it be bipedal, quadrupedal, plantigrade, digitigrade, erect, semi-erect, or any other distinction, cannot be used to define every single pterosaur that ever existed. This is similar to the idea that one form of locomotion cannot be used to describe every dinosaur that ever existed. There is variation between species of pterosaurs.

The larger pterosaurs seem to be generally more bird-like and more inclined to quadrupedal locomotion. The Pteranodon, specifically, almost definitely was a biped. Its wing structure was the most similar to that of a bird (Padian, 1983), and its two hind limbs were more than sufficient for bothing locomotion and launching off from the ground in order to initiate flight.

Smaller pterosaurs, on the other hand seem to be more similar in locomotive method to the general perception of how pterosaurs moved, in a kind of quadrupedal crawl. The tracks found at Pterosaur Beach, and if actually created by a pterosaur, those of Pteraichnus saltwashensis, prove that at least some smaller pterosaurs were quadrupedal.

In conclusion, any attempt to define the entire order of Pterosauria as quadrupedal or bipedal is an exercise in futility. The larger pterosaurs, such as Pteranodon, were bipedal, which is apparent based on their wing and leg structures. They would have been unable to move their forelimbs into a position that would allow them to place them on the ground and the mobility of their hind limbs allowed only for bipedal locomotion. Smaller pterosaurs, such as Pteraichnus stokes, were definitely quadrupedal in terms of locomotion since there have been numerous tracks found confirming this idea.

Works Cited

Bennett, S. (1990). A Pterodactyloid Pterosaur Pelvis from the Santana Formation of Brazil: Implications for Terrestrial Locomotion. Journal of Vertebrate Paleontology, 10(1), 80-85. Retrieved from http://www.jstor.org/stable/4523298

Bennett, S. (1997). Terrestrial Locomotion of Pterosaurs: A Reconstruction Base on Pteraichnus Trackways. Journal of Vertebrate Paleontology, 17(1), 104-113. Retrieved from http://www.jstor.org/stable/4523790

Mazin, J., Billon-Bruyat, J., & Padian, K. (2009). First Record of a Pterosaur Landing Trackway. Proceedings: Biological Sciences, 276(1674), 3881-3886. Retrieved from http://www.jstor.org/stable/30245351

Padian, K. (1983). A Functional Analysis of Flying and Walking in Pterosaurs. Paleobiology, 9(3), 218-239. Retrieved from http://www.jstor.org/stable/2400656

Padian, K., & Olsen, P. (1984). The Fossil Trackway Pteraichnus: Not Pterosaurian, but Crocodilian. Journal of Paleontology, 58(1), 178-184. Retrieved from http://www.jstor.org/stable/1304743

Sánchez-Hernández, B., Przewieslik, A., & Benton, M. (2009). A Reassessment of the Pteraichnus Ichnospecies from the Early Cretaceous of Soria Province, Spain. Journal of Vertebrate Paleontology, 29(2), 487-497. Retrieved from http://www.jstor.org/stable/20627057

Wm. Lee Stokes. (1957). Pterodactyl Tracks from the Morrison Formation. Journal of Paleontology, 31(5), 952-954. Retrieved from http://www.jstor.org/stable/1300563

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