Nesbitt and Hone (2010) broke with tradition to propose that certain pterosaurs had a mandibular fenestra. We discussed this mistake earlier. Now we are going to look at another one of their other futile grasps at the archosaur straw, a purported antorbital fossa in Dimorphodon (Figs. 1, 2) and Raeticodactylus (Fig. 3). An antorbital fossa is not found in ANY other pterosaur. And the two examples they propose don’t match each other in any way or fashion. So, ironically, Nesbitt and Hone (2010) were acting as heretics and I am here to hold the traditional line.
Figure 1. From Nesbitt and Hone 2010, a purported antorbital fossa in Dimorphodon. Note where it is compared to figure 3 (below). This triangular strut support is a little thinner and therefore a little deeper than the rest of the cylindrical ascending process. Dimorphodon depresses this area more than other pterosaurs, like Eudimorphodon. This also may be due to crushing, similar to the crushing surrounding each tooth. Oops. Yeah, there it is…
Figure 2. The jugal of Dimorphodon adds depth to the tooth-bearing portion of the maxilla, adding to the impression of an antorbital fossa, a fact overlooked by Nesbitt and Hone (2010).
Dimorphodon has one of the largest and lightest skulls of any early Jurassic or Triassic pterosaur. The nasal, antorbital and orbital fenestra made up the vast majority of the skull separated by the thinnest struts of bone in the Pterosauria. Like any good engineer Dimorphodon supported its grid-like struts with small triangles of bone, like the one at the base of the slender ascending process of the maxilla. Paper thin, this triangular support at the base of the cylindrical ascending process was identified as an antorbital fossa by Nesbitt and Hone (2010). No other pterosaur depresses, or thins this area, which may be thinner due to crushing. Note the areas between the maxillary teeth, which exhibit similar crushing. Nesbitt and Hone (2010) also failed to note the presence of the laminated jugal (Fig. 2), which adds depth to the tooth-bearing portion of the maxilla.
Figure 2. Raeticodactylus skull. According to Nesbitt and Hone (2010) the red areas represent the antorbital fossa. Here these areas are interpreted as the transverse width of the girder-like ascending process (stronger to support that rhino-like horn when it’s called into action), and otherwise typically buried in the matrix. At the top the transverse lacrimal is equally wide in the Z-axis. Note the ventral view of the skull (in blue, twisted during crushing) that confirms we’re seeing the ventral aspect of the maxilla/lacrimal portion of the antorbital fenestra. Also note this purported antorbital fossa is not the same as that seen in Dimorphodon (Fig. 1). No homology here.
Raeticodactylus was also promoted by Nesbitt and Hone (2010) as having an antorbital fossa, but there’s no basal triangular support for the maxillary ascending process here. So the two do not reflect homologous morphologies (which should have raised a red flag, except they were so hell-bent on providing “evidence” for an archosaur connection they ignored or overlooked this key fact). Instead what we’re seeing is the crushed transverse width of the girder-like ascending process of the maxilla and the ventral aspect of the lacrimal and skull roof. The skull had to be stronger than a typical pterosaur skull. After all it was doing something with that rhino-like horn and this reinforcement tells us it wasn’t just for display~!
Bottom line: No mandibular fenestra. No antorbital fossa. Pterosaurs are not archosaurs.
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
The Pterosaur Heresies 
Dimorphodon seems more ambiguous, but I think Raeticodactylus clearly has a fossa. Just because some bones are in ventral view doesn’t mean the dorsal lacrimal is, and many theropods have a similar laterally exposed fossa on the dorsal lacrimal. The better argument is that the morphology of one pterosaur doesn’t mean that was primitive for Pterosauria. Maybe Raeticodactylus is convergent with archosauriforms, and other pterosaurs lacked the fossa.
Oh, and the argument from non-similarity is weak. Take Byronosaurus, for instance. Its maxillary bar between the maxillary fenestra and antorbital fenestra is not depressed. But most theropods have a depressed antorbital fossa between fenestrae and in front of the maxillary fenestra. Still, the structures in Byronosaurus and e.g. Tyrannosaurus are no doubt homologous. Different areas can be depressed or not due to sinus medial extent fairly easily.
I like the convergent argument for Raeticodactylus, but can’t buy into the fossa idea. To me it just has more transverse width or in situ depth. Byronosaurus appears to be crushed exceedingly, as I just saw in Digimorph. Even so, it is still the exception.