Here is an accumulation of info about mandibular fenestra. Since this character varies within the various groups we cannot establish ancestry based on it.
Right, so back to the [mandibular] fenestra itself, what’s going on? Given that the evolution of pterosaurs is essentially one of ever lighter constructions and better flight capabilities, one would expect a hole to be kept as long as a possible. Getting rid of any excess bone, no matter how small will make a bit of a difference to the mass, and lowering it is always good so one would expect the fenestra not just to be retained in pterosaurs, but in fact to get bigger, not disappear.
The obvious answer to this is that pterosaurs actually reduced their jaws as a whole making the bones relatively low and thin. Sticking a hole in a very thin set of bony plates might make them incredibly weak, and so if you close up that fenestra you can reduce the weight of the jaw overall and keep the jaw relatively strong, than keeping a normal jaw and putting a hole in it. So it makes sense for pterosaurs to close it up, but in that case how? There is absolutely no evidence of it anywhere.
In 2003 pterosaur supremo Peter Wellnhofer described an isolated jaw of the basal (ish) pterosaur Eudimorphodon from the Late Triassic of Austria that had, yes, wait for…wait a bit longer…a mandibular fenestra.https://archosaurmusings.wordpress.com/2010/12/16/the-pterosaur-mandibular-fenestra-part-2/
This [mandibular fenestra] at least suggests that this really is a hang over from the ancestral archosaurian condition and that it was then lost several times in various pterosaur clades, though probably quite quickly given how rare it seems to be.
All taxa recovered as basal avialans by our analysis, such as the scansoriopterygids, Sapeornis and Jeholornis, resemble oviraptorosaurs and to a lesser degree therizinosaurs4 but differ from deinonychosaurs including archaeopterygids in having such cranial and dental characteristics as a dorsoventrally high premaxilla that is significantly larger than the maxilla, a dorsally positioned external naris, a dorsoventrally tall antorbital fossa, a jugal with a relatively vertical postorbital process and a long quadratojugal process, a quadrate with a large pterygoid ramus, a relatively long parietal, an anteriorly downturned and strongly dorsally convex mandible, a large external mandibular fenestra, and enlarged anterior teeth. Some of these features are optimized by our analysis as synapomorphies of a clade containing the Oviraptorosauria, the Therizinosauroidea, the Avialae and the Deinonychosauria, but are lost in the last group.
367. External mandibular fenestra, size: small (0) or large (1).
It [Archaeopteryx] does not appear to have had a mandibular fenestrahttp://www.ncbi.nlm.nih.gov/pmc/articles/PMC3259976/
Elsewhere within Theropoda, mandibular fenestrae are absent in compsognathids but are otherwise ubiquitous in non-avian theropods and only absent in certain avian lineages [9–11]http://www.palaeodiversity.org/pdf/03/Palaeodiversity_Bd3_Nesbitt.pdf
Pterosauria, a successful clade of extinct flying vertebrates, possesses a radical body plan that offers few clues about their origin and closest relatives. Whereas most researchers hypothesize an origin within Archosauria as the sister-group to Dinosauromorpha, others favor a position among non-archosauriform archosauromorphs. Here we present evidence that supports a placement within Archosauriformes: the presence of an external mandibular fenestra in two basal pterosaur taxa, Dimorphodon macronyx and a specimen referred to Eudimorphodon cf. ranzii (= ‘Seefeld Eudimorphodon’; BSP 1994 I 51). Furthermore, the arrangement of the mandibular bones surrounding the mandibular fenestra and the presence of a posterior process of the dentary that laterally overlaps the angular in the mandible of Dimorphodon and BSP 1994 I 51 are identical to those of Erythrosuchus, Euparkeria, and Archosauria.
When mapped on a cladogram, presence or absence of an external mandibular fenestra in basal pterosaurs possibly indicates that the feature is primitive for Pterosauria but later lost. The presence of an external mandibular fenestra, along with morphological evidence elsewhere in the body of pterosaurs (serrated teeth, antorbital fossa present, fourth trochanter on the femur present), supports a placement of Pterosauria within Archosauriformes and is consistent with a position within Archosauria.
Pterosaurs have been cited as lacking a lateral (or external) mandibular fenestra (Bennett, 1996). A lateral mandibular fenestra is clearly absent in the holotype of Eudimorphodon (Wild 1978). However, a mandibular fenestra is clearly present in a specimen referred to Eudimorphodon sp. (BPS 1994 I 51; Wild, 1993) and Dimorphodon (BMNH R1034) (S.J.N., personal obs.).
Unlike most archosaurs, which have several openings in the skull in front of the eyes, in pterodactyloid pterosaurs the antorbital opening and the nasal opening was merged into a single large opening, called the nasoantorbital fenestra. This likely evolved as a weight-saving feature to lighten the skull for flight.https://en.wikipedia.org/wiki/Darwinopterus
Darwinopterus, like its closest relatives, is characterized by its unique combination of basal and derived pterosaurian features. While it had a long tail and other features characteristic of the 'rhamphorhynchoids', it also had distinct pterodactyloid features, such as long vertebrae in the neck and a single skull opening in front of the eyes, thenasoantorbital fenestra (in most 'rhamphorhynchoids', the antorbital fenestra and the nasal opening are separate).
In most 'rhamphorhynchoids', the antorbital fenestra and the nasal opening are separate.
In the Monofenestra pterosaurs the nasal opening was merged into a single large opening, called the nasoantorbital fenestra.
The Monofenestrata are an unranked group of pterosaurs that includes the family Wukongopteridae and the suborder Pterodactyloidea.
The clade Monofenestrata was in 2009/2010 defined as the group consisting of Pterodactylus and all species sharing with Pterodactylus the synapomorphy, shared derived trait, of an external nostril confluent with the antorbital fenestra, the major skull opening on the side of the snout.
An antorbital fenestra (plural: fenestrae) is an opening in the skull that is in front of the eye sockets. This skull character is largely associated with archosaurs, first appearing during the Triassic Period. Among extant archosaurs, birds still possess antorbital fenestrae, whereas crocodylians have lost them.