Did Dimorphodon Have an External Mandibular Fenestra?

A Mandibular Fenestra in Pterosaurs?
Nesbitt and Hone (2010) and Nesbitt (2011) proposed an external mandibular fenestra for three and only three pterosaurs, recognizing that a mandibular fenestra is not found in other pterosaurs. These two workers nested pterosaurs with archosaurs to support their traditional view despite the fact that pterosaurs do not otherwise resemble any other archosaurs. To do this they excluded the heretical sisters of pterosaurs found in the large study. To be fair, Hone and Benton (2007, 2008) did include Cosesaurus in their analysis, but only a quarter of the characters were employed, despite the fact that Cosesaurus is complete and articulated.

Let’s take a closer look at each pterosaur taxon used by Nesbitt and Hone (2010) in evidence for the mandibular fenestra.

The BMNH 4212 Specimen of Dimorphodon
According to Nesbitt and Hone (2010) and Nesbitt (2011) the mandible of the BMNH 4212 specimen of Dimorphodon has a mandibular fenestra. The matrix appears within a very large hole in the posterior mandible. The surangular is apparently missing because it is not identified in figure 1 (below) from Nesbitt and Hone (2010). The BMNH 4212 specimen also sports a very deep jugal, according to  Nesbitt and Hone (2010), a trait not found in any other pterosaur or any other Dimorphodon.

The purported deep jugal and mandibular fenestra in the BMNH specimen of Dimorphodon.

Figure 1. From Nesbitt and Hone (2010), the purported deep jugal and mandibular fenestra in the BMNH specimen of Dimorphodon. emf = external mandibular fenestra. im = impression of emf. j = jugal.

The Mary Anning Specimen
In counterpoint, the Mary Anning specimen (Fig. 2) preserves the surangular in place, covering the Nesbitt and Hone (2010) “fenestra” completely. The Anning Dimorphodon has no mandibular fenestra and no deep jugal flange.

The Mary Anning Dimorphodon skull.

Figure 2. The Mary Anning Dimorphodon skull R1034a. Note the mandible has no mandibular fenestra.

Sure It Looks Like a Mandibular Fenestra…
Figure 3 portrays the BMNH 4212 specimen of Dimorphodon after DGS (digital graphic segregation). Here every bone has been color coded to improve understanding. I have not seen the fossil first hand. I reconstructed the bones in accord with other sister taxa (Fig. 3 middle). The articular bone (in gray) might be missing and replaced here with a best guess shape. If present the articular bone may be present in the area outlined in gray posterior to the mandible.

The skull of Dimorphodon macronyx BMNH 41212.

Figure 3. The skull of Dimorphodon macronyx BMNH 41212. Above: in situ. Middle: Restored. Below: Palatal view.

Reconstruction Really Helps
Evidently Nesbitt and Hone (2010) and Nesbitt (2011) did not realize the surangular had drifted dorsally in BMNH 4212. What they considered the angular was actually a displaced pterygoid (or perhaps the articular since both have the same shape, see figure 4). Such mistakes are easy to make when tracing a specimen with little attention to detail (Fig. 1).

The mandibles of Eudimorphodon with mandible elements identified.

Figure 4. The mandibles of Eudimorphodon with mandible elements identified. Note the breakage of the left dentary and displacement of the left surangular to produce the illusion of a mandibular fenestra not duplicated in the right mandible. A rod-shaped element, likely a hyoid or pterygoid, produces the illusion of a dorsal rim to the right posterior mandible.

Eudimorphodon ranzii Mandibles
The holotype of Eudimorphodon (Zambelli 1973) Upper Norian, Late Triassic, ~203 mya MCSNB 2888 provides both lateral and medial views of complete and largely articulated mandibles. Here the shapes of the posterior elements resemble those found in lizards with an articular bone extending anteriorly. There is an opening between the left dentary and surangular, but that is due to the downshifting of the surangular and the breakage of the dentary not duplicated on the right mandible. When properly reconstructed, no mandibular fenestra is present despite the fact that Eudimorphodon lived tens of millions of years earlier than Dimorphodon, closer to the origin of the Pterosauria. No genuine pterosaur precursor among the fenestrasaurs (Cosesaurus, Sharovipteryx and Longisquama) had a mandibular fenestra.

The mandible of Eudimorphodon 1994 I51

Figure 5. The mandible of Eudimorphodon 1994 I51. Top: After DGS the various points of decay and displacement appear as white spots. The jaw tip is missing. Rod-like elements are in blue and green. Displaced plate-like elements in beige. The green coronoid has been displaced posteriorly. The surangular (dark brown) has been displaced. Note the angular of Eudimorphodon ranzii (Fig. 4) is not as deep as portrayed by Nesbitt and Hone (2010, bottom).

Eudimorphdon BSP 1994 I51
The second specimen reported by Nesbitt and Hone (2010) to have a mandibular fenestra is Eudimorphodon BSP 1994 I51, an incomplete and disarticulated mandible. Here (Fig. 5) the mandible has decayed somewhat with several fenestra apparent throughout. Sure the matrix appears at the appropriate part of the mandible in which a mandibular fenestra might appear, but It’s not a convincing example due to element shifting and decay. Parts of the inside of the mandible are exposed by the flaking off of bone. The reported angular is much too deep. The coronoid has drifted posteriorly. No other pterosaur has a dorsal bump on the posterior mandible and the coronoid is otherwise missing. That posterior bump is the coronoid.

BMNH 43486
The third mandible is the BMNH 43486 specimen of Dimorphodon (Fig. 6). This mandible is more damaged than the others and so does not induce confidence that the external mandibular fenestra (emf) was real or just due to damage. The opening appears much further posteriorly than the dentary/surangular interface. If valid, one should wonder why other sister pterosaurs do not have even a small mandibular fenestra.

The BMNH 43486 specimen of Dimorphodon.

Figure 6. The BMNH 43486 specimen of Dimorphodon. Damage to the posterior portion of the mandible accounts for the apparent fenestra. What Nesbitt and Hone labeled the Meckelian groove (mg) may instead by the other mandibular rim.

It All Comes Down to Autapomorphies
If no other pterosaur sisters had a deep jugal and if no other pterosaur sisters had a mandibular fenestra, then maybe the two autapomorphies reported by Nesbitt and Hone (2010) are wrong. In BMNH 4211 re-identifying the bottom half of the “deep jugal” as the missing surangular solves the twin problems on the best example.

If anyone finds a valid external mandibular fenestra or deep jugal on any pterosaur specimen, please bring it to my attention. I’d like to see it.

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
Buckland W 1829. Proceedings of the Geological Society London, 1: 127
Owen R 1859. On a new genus (Dimorphodon) of pterodactyle, with remarks on the geological distribution of flying reptiles.” Rep. Br. Ass. Advmnt Sci., 28 (1858): 97–103.
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. online pdf
Nesbitt SJ and Hone DWE 2010. An external mandibular fenestra and other archosauriform character states in basal pterosaurs. Palaeodiversity 3: 225–233
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.
Sangster S 2001. Anatomy, functional morphology and systematics of Dimorphodon. Strata 11: 87-88
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.
Zambelli R 1973. Eudimorphodon ranzii gen.nov., sp.nov. Uno Pterosauro Triassico. Rendiconti Instituto Lombardo Accademia, (rend. sc.) 107: 27-32.

wiki/Dimorphodon

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