Some pterosaurs (like Rhamphorhynchus and the new Painten pterosaur) had 4 or 5 tarsals. Others had only two (like Pteranodon, Figs. 1-3).
Figure 1. Pteranodon tarsals (in color). Blue = astragalus. Yellow + calcaneum. YPM = Yale Peabody
The question is:
In those pterosaurs with two tarsals is it more parsimonious that the 1) distal tarsals disappeared? or 2) the distal tarsals fused to the proximal tarsals? or 3) converging with birds, did the proximal tarsals fuse seamlessly to the tibia/fibula?
What does the evidence indicate?
There are pterosaur workers (Padian 1983, Bennett 2001, Nesbitt 2011, Witton 2013) who consider the tibia + fibula of pterosaurs a “tibiotarsus” because they say the proximal tarsals (astragalus + calcaneum) fused seamlessly to the distal tibia/fibula (Fig. 1). (We looked at this earlier here.) Birds have this sort of tibiotarsus. Padian 1983 compared bird tibiotarsi to Dimorphodon (Fig. 2) and the case looked pretty good back then.
It’s important to remember that birds had a long ancestry as dinosaurs with distinct ascending processes of the astragalus that ultimately fused seamlessly to the tibia after the miniaturization that preceded and succeeded Archaeopteryx. Pterosaurs don’t have that long history, nor do they have ancestors with an ascending processes, nor did they undergo phylogenetic miniaturization prior to getting their wings. Even Archaeopteryx has a distinct ascending process — not seamless.
Under the Padian 1983 hypothesis
the two tarsals found with Dimorphodon are distal tarsals. Likewise, Bennett (2001) proposed a tibiotarsus for Pteranodon. Eaton (1913, Fig. 1) called them podials, a general name form carpals or tarsals. We don’t see the same long ancestry progress in pterosaur ankles. In fact, there’s no ancestry for this type of ankle at all.
Figure 2. Pterosaur distal tibia. Left: Dimorphodon. Right Pteranodon in anterior (above) and posterior (below) views. Padian (1983) and Bennett (2001) consider the bulbous parts to be the fused proximal tarsals. They are not. The proximal tarsals, astragalus (blue) and calcaneum (yellow) are distinct. Missing here are any distal tarsals. Padian identified this view of Dimorphodon as the anterior, because it looked so much like the anterior of the distal bird tibiotarsus (not shown here). But look again. It looks more like the posterior of the distal tibia of Pteranodon identified by Bennett.
Figure 3. Foot and tarsus of Pteranodon, FHSM-P-2062 and restored and relabeled on top, from original online mislabeled image found at OceansofKansas.com (below). Note, the distal tibia bulge is posterior in Pteranodon, but bulges both ways in Dimorphodon and other pterosaurs, like the Painten pterosaur.
Rather, when you look at basal pterosaurs like Peteinosaurus (Fig. 4), you find four distinct tarsals.
Figure 4. Peteinosaurus and Dimorphodon BMNH4212 pedes. Four tarsals are present on both. Yes the tarsals have moved in Dimorphodon with the distal tarsals rising to the level of the proximal tarsals.
Same with the classic specimen of Dimorphodon. The engraving (Fig. 5) shows four tarsals.
Figure 5. Click to enlarge. The four tarsals identified on the the classic BMNH 41212 specimen of Dimorphodon. Non-foot bones are ghosted out. Calcaneum = yellow. Astragalus = blue. Distal tarsal 4 = pink. Centrale = magenta. Yes, they have moved during taphonomy, If you count four tarsals, that’s all I’m asking for now.
This is in contrast to Padian’s (1983) interpretation of BMNH 41212 (Fig. 6) where he adds a cylindrical joint to the distal tibia with a circumference smaller than in the other tibia at left.
Figure 6. Tarsals of Dimorphodon BMNH 41212 specimen according to Padian 1983. Figure 5 matches in most regards — except for the tarsals.
Padian 1983 removed tarsals from the matrix of two far less complete specimens attributed to Dimorphodon, YPM 350 and YPM 9182 (Figs. 7-9). Oddly, the smaller of the two specimens (YPM 9182) fused the two large tarsals to one another (the only such event I am aware of). The larger specimen (YPM 350) did not.
Figure 7. The YPM 9182 specimen attributed to Dimorphodon. Note the tarsals fuse to one another despite the smaller size. The femora do not match. The ventral maxilla is straighter. The jugal is deeper. M4.2 is shorter. So there is some doubt that this is indeed congeneric with Dimorphodon. The question here is: did the calcaneum fuse to the fourth distal tarsal? And if so, did Padian get his tarsal backwards? With Padian’s orientation the tarsal has a posterior tuber. But no pterosaur ever developed a tuber, certainly not on any distal tarsals. And not on any calcaneum either. Let’s keep an eye out for further examples of this.
Figure 8. About the size of the classic Dimorphodon, the YPM 350 specimen has unfused tarsals. Note the very few bones. The specimen is extremely disarticulated. The other two tarsals could have been easily scattered. This specimen appears to be closer to the classic Dimorphodon in all regards.
Figure 9. Location of the tarsals (red circles) on the YPM 350 and YPM 9182 specimens attributed to Dimorphodon by Padian 1983. Do you think some other tarsals could have escaped?
Padian 1983 noted the cylindrical shape of the distal tarsals and their convergence with the bird tibiotarsus. But there are pterosaurs, like the Painten pterosaur (Fig. 10), that have a cylindrical distal tibia AND four tarsals.
Figure 10. The Painten pterosaur with tarsals colorized. There are four of them. Note the cylindrical shape of the distal tibia/fibula.
So, the evidence for Dimorphodon having only two tarsals is fading. The evidence for cylindrical distal tarsals is strong. Pteranodon has only two tarsals. Whether they were created by fusion or reduction awaits further evidence. There is no evidence for a gradual evolution of fusion in the tarsals and tibia/fibula. Rather, there is plenty of evidence for the retention of paired distal and paired proximal tarsals. There is also evidence in YPM 9182 for the fusion of the proximal tarsals in certain pterosaurs.
Nesbitt 2011 fell prey to the idea of a fused tibiotarsus in pterosaurs when he wrote: “a few peculiar features in the hind limb of lagerpetids merit discussion and suggest that they may be more closely related to pterosaurs than to dinosaurs. Specifically, the ankle of lagerpetids is more similar to that of basal pterosaurs (in particular, Dimorphodon) than to basal dinosauriforms and early dinosaurs. The calcaneum and astragalus are coossified, the ventral surface of the calcaneum is rounded like that of the astragalus, there is no posterior groove of the astragalus, and the calcaneum lacks any sort of calcaneal tuber in both Dimorphodon and lagerpetids. These four character states shared between lagerpetids and Dimorphodon are absent in basal dinosauriforms (e.g., Marasuchus, Asilisaurus). Basal dinosauriforms have a separate calcaneum and astragalus, the ventral surface of the calcaneum, although rounded, is different from the ventral surface of the astragalus, they have a posterior groove of the astragalus, and the calcaneum bears a small calcaneal tuber. It is possible that pterosaurs and lagerpetids share additional ankle characters or differences; however, the ankle of Dimorphodon is heavily ossified, thus concealing the distal end of the tibia and the proximal surface of the astragalus.”
The large reptile tree demonstrates that pterosaurs have no relationship with Lagerpeton and neither do basal dinosaurs, which are distinct from both.
Bennett SC 2001. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260: 1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260: 113–153.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Padian K 1983. Osteology and Functional Morphology of Dimorphodon macronyx (Buckland) (Pterosauria: Rhamphorhynchoidesa) Based on New Material in the Yale Peabody Museum. Postilla 189 44pp.