Why Dimorphodon weintraubi is not a Dimorphodon

Dimorphodon weintraubi made the cover of Nature

Figure 1. Click to enlarge. Dimorphodon weintraubi made the cover of Nature

Dr. Jim Clark et al. (1994, 1998) described in Nature an uncrushed partial skeleton, IGM 3494, of a medium-sized Early to Middle Jurassic pterosaur from Mexico and attributed it to the genus Dimorphodon, but reported it was a distinct species, D. weintraubi (Fig. 1). D. weintraubi was closer in size to D. macryonx than to any known anurognathid. It wasn’t until the discovery of the IVPP embryo that another anurognathid similar in size to D. weintraubi was known.

Figure 1. Dimorphodon macronyx and D. weintraubi, an anurognathid larger than its namesake.

Figure 2. Dimorphodon macronyx and D. weintraubi, an anurognathid larger than its namesake, both to scale. No phylogenetic analysis was undertaken by Clark et al. (1994, 1998) perhaps because no anurognathids of this size were known at that time.

Basal pterosaurs, including Dimorphodon macryonx and Dimorphodon weintraubi

Figure 3. Basal pterosaurs, including Dimorphodon macryonx and Dimorphodon weintraubi, separated by several genera. Click to see entire pterosaur tree.

The Phylogenetic Question
The problem is, there are several other pterosaur genera between the two when both are included in a large phylogenetic analysis (Fig. 2) not considered by Clark et al. (1994, 1998).

The Visible Distinctions
D. weintraubi was overall more robust. The coracoid did not have an expanded ventral process. The humerus was straighter. Metacarpal 4 was relatively closer in width to the other three. Manual 4.1 was relatively longer, closer to the elbow when folded. Manual 4.4 was longer. The metatarsus was relatively longer. Pedal digit 4 was shorter than 3. Pedal 5.1 did not extend beyond metatarsal 4.

The Invisible Distinctions
Because D. weintraubi nests (Fig. 2) after the more complete IVPP embryo (which would have grown up to be larger than D. weintraubi assuming a standard 8x increase, Fig. 3), phylogenetic bracketing permits us to assume certain traits in D. weintraubi that were not preserved in the fossil. The skull was likely shorter and wider, more like that of sister anurognathids, including Dendrorhynchoides, and the tail was likely much shorter and more gracile than in the holotype of Dimorphodon.

The Anurognathidae to scale.

Figure 4. Click to enlarge. The Anurognathidae to scale.

The Digitigrade Question
This uncrushed specimen helped Clark, et al. (1998) determine that the basal pterosaur pes was incapable of a digitigrade configuration because the flat metatarsophalangeal joints did not permit the sort of extension seen in birds, as Padian (1983) had promoted for Dimorphodon macryonx. This is true. However, sufficient elevation of the posterior pes was permitted by the sort of extension of the interphalangeal joints seen in bipedal lizards, including the tritosaur fenestrasaur lizard, Cosesaurus,  (Peters 2000). The ichnite Rotodactylus preserves just this sort of proximal phalangeal elevation. Clark et al. (1994) did not consider this possibility. They folded digit V OVER the foot, which is bizarre but preserved (probably by taphonomic accident) in the specimen.  Their interpretation of a horizontal backbone while quadrupedal gait produces a forelimb in which the hand plants and lifts anterior to the elbow and shoulder, which produces no thrust, only braking. Digitigrady in some pterosaurs, plantigrady in others with an elevated backbone was discussed earlier.

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
Casamiquela RM 1962. Sobre la pisada de un presunto sauria aberrante en el Liassico del Neuquen (Patagonia). Ameghiniana 2, 10:183-186.
Clark J, Montellano M, Hopson J and Fastovsky D. 1994. In: Fraser, N. & H.-D Sues, Eds. 1994. In the Shadows of Dinosaurs. New York, Cambridge: 295-302.
Clark J, Hopson J, Hernandez R, Fastovsk D and Montellano M. 1998. Foot posture in a primitive pterosaur. Nature 391:886-889.
Padian K 1983. Osteology and functional morphology of Dimorphodon macronyx (Buckland) (Pterosauria: Rhamphorhynchoidea) based on new material in the Yale Peabody Museum, Postilla, 189: 1-44.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods. – Ichnos 7(1): 11-41.

wiki/Dimorphodon

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