The myth of the pterosaur tibiotarsus

Earlier and over several posts we looked at the various misconceptions promoted by Mark Witton (2013) in his new book, “Pterosaurs.” Today we’ll look at Witton’s misconstrued view of the so-called “tibiotarsus”, a purported fusion of the tibia and proximal tarsals (astragalus + calcaneum) that never actually happened.

Figure 1. Right: Witton's view of the pterosaur foot and ankle. He does not identify the individual tarsal bones but assumes the tibia and proximal tarsals are fused. At left a corrected pes of the same specimen with ankle bones identified.

Figure 1. Right: Witton’s view of the pterosaur (Peteinosaurus) foot and ankle. He does not identify the individual tarsal bones, but assumes the tibia and proximal tarsals are fused. At left a corrected pes of the same specimen with ankle bones identified. Compare these identities to those in figure 2. The metatarsus was actually appressed, not spreading as Witton presumes in this sister to Dimorphodon and anurognathids. For clarity in both views the tibia/fibula has been slightly dislocated and would, in vivo, articulate with the dorsal/anterior faces of the astragalus and calcaneum.

Wellnhofer (1991) had it right.
There’s a tibia and a tarsus. Not a tibiotarsus. Witton did not identify the two rows of individual bones of his pterosaur tarsus, but was sure that the astragalus and calcaneum were fused to the tibia. The first demonstrates a lack of knowledge and testing. The second shows a reliance on tradition without evidence.

Witton’s mistakes were fourfold.
1. The tibiotarsus misidentification goes back at least as far as Galton 1980 and Bennett 1991, who noted Pteranodon had a tibiotarsus and just two distal tarsals. The purported “distal tarsals” in Pteranodon are actually the astragalus and calcaneum fused to the centrale and distal tarsal 4 respectively. The outgroup to Pteranodon, the SMNK 6597 specimen of Germanodactylus, retains four tightly packed tarsals of subequal size indicating none were vestigial.

2. Witton’s error was a direct result of being unable to determine the ancestry of pterosaurs. Once that is known we can see the homologies and identify the remaining four major tarsal bones (Fig. 2) in pterosaurs.

3. His third problem was his refusal to reference the one paper in the literature that actually traced the ancestry of pterosaurs and the homologies of the tarsal bones: Peters 2000. Odd that. Witton shares a trait with Hone and Benton in that regard.

4. According to the traditional paradigm, the proximal tarsals were supposed to fuse to the tibia during ontogeny, but no one has showed that yet. No pterosaur embryo has additional unfused tarsals. Furthermore, no tiny pterosaur, formerly considered a juvenile, has additional unfused tarsals. This myth of fusion evidently comes from the false paradigm associating pterosaurs with dinosaurs (which do have a tibiotarsus) and from the finished, curved articulating surfaces of the distal tibia/fibula in pterosaurs. Such a large pulley-like surface gives pterosaurs a widely swinging ankle for walking on top of the tarsus and then for flying with feet trailing to roll/flex the foot ~90 degrees back for aerodynamic reasons.

Here we’ll rectify all of Witton’s problems with phylogeny
We can identify the individual bones of the pterosaur tarsus by comparing them to ancestral nonvolant taxa with a larger number and more readily identified tarsals first reported by Peters (2000). Here’s a nice morphological sequence if you’ll ignore the autapomorphic short digit 5 of Macrocnemus. Huehuecuetzpalli, its phylogenetic ancestor, had a much longer pedal digit 5.

Figure 2. The tarsi of Macrocnemus, Langobardisaurus and Cosesaurus demonstrating the reduction of ankle bones. None of these fuse to the tibia.

Figure 2. The tarsi of Macrocnemus, Langobardisaurus and Cosesaurus demonstrating the reduction of ankle bones (Peters 2000). None of these tarsals fuse to the tibia. The reduction of pedal digit 5 in Macrocnemus is restricted to that genus. Cosesaurus has the same number of large ankle bones as pterosaurs. Occasionally smaller distal tarsals also ossify.

Tarsal homologies in fenestrasaur tritosaurs
The homologies of pterosaur ankle bones can be traced from Macrocnemus to Langobardisaurus and Cosesaurus. Distal tarsals 1 and 3 generally diminish (but sometimes ossify in various pterosaurs). This leaves the astragalus, calcaneum, centrale and distal tarsal 4 in Cosesaurus and pterosaurs to form the ankle bones and create a mesotarsal ankle joint (Peters 2000). Pterosaurs share with Langobardisaurus and Cosesaurus that short metatarsal 5 and long m5.1. Note the continued contact of the distal fibula with the calcaneum. The apparent loss of various tarsals can come about either by way of 1.) fusion or 2.) reduction to absence or by 3.) lack of ossification.

Hopefully someday
this foolish and unfounded reliance on false traditional paradigms (a genuine ‘house of cards’ constructed by Witton (2013) as we have seen over and over again) will fall by the wayside and both the large reptile tree and the large pterosaur tree will become accepted, or at least tested under an academic banner.

References
Bennett SC 1991. Morphology of the Late Cretaceous Pterosaur Pteranodon and Systematics of the Pterodactyloidea. [Volumes I & II]. Ph.D. thesis, University of Kansas, University Microfilms International/ProQuest.
Galton PM 1980. Avian−like tibiotarsi of pterodactyloids (Reptilia: Pterosauria) from the Upper Jurassic of East Africa. Pal.ontologische Zeitschrift 54 (3/4): 331–342.
Peters D 2000. A redescription of four prolacertiform genera and implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106: 293-336.
Wellnhofer P 1991. The Encyclopedia of Pterosaurs. Salamander Books.
Witton M. 2013. Pterosaurs. Princeton University Press. 291 pages.

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