Following remarks from fellow paleontologists asking for my study to include more Nesbitt (2011) characters in the large reptile study, I thought we should dive right into them, taking a few days to digest them all — a bite at a time. Earlier we considered more basal clades in parts 1, 2, 3 and 4.
Nesbitt Characters for Archosauria
Sterling Nesbitt (SN) reported, (1) Palatal processes of the maxilla meet at the midline (32-1). Not known in basal pterosaurs.
The palate is known in basal pterosaurs and the maxillae do not meet at the palatal midline in dimorphodontids, but do meet at the midline in eudimorphodontids.
(2) Lagenar/cochlear recess present and elongated and tubular (118-1). Not known in basal pterosaurs.
Such a process is absent in lepidosaurs including fenestrasaurs and pterosaurs.
(3) External foramen for abducens nerve within prootic only (122-1). Not known in basal pterosaurs.
Such a foramen is absent in lepidosaurs including fenestrasaurs and pterosaurs.
(4) Antorbital fossa present on the lacrimal, dorsal process of the maxilla, and the dorsolateral margin of the posterior process of the maxilla (the ventral border of the antorbital fenestra) (137-2). This character is difficult to score for any pterosaur as also observed by Bennett (1996). However, a slight fossa in Dimorphodon (BMNH 41212) suggests that an antorbital fossa surrounded much of the border of the antorbital fenestra.
Note: There is no antorbital fossa on any pterosaurs antorbital fenestra. Dimorphodon was unusual, even among pterosaurs for its extremely large fenestra and extremely narrow skull bones. What Nesbitt (2011) considered an antorbital fossa is a paper-thin structural flange supporting a very weak bone intersection.
(5) Posteroventral portion of the coracoid possesses a ‘‘swollen’’ tuber (225-1). Difficult to score in the highly modified coracoids of pterosaurs.
This is bogus. The coracoid of pterosaurs is no more “highly modified” than a bird coracoid and is shared with basal fenestrasaurs. The coracoid of fenestrasaurs may be expanded ventrally or not. In basal forms it is not.
(6) Lateral tuber (5 radial tuber) on the proximal portion of the ulna present (237-1). Not known in basal pterosaurs.
Note: This tuber is absent on all lepidsaurs, including pterosaurs.
(7) Ratio of longest metacarpal to longest metatarsal, less than 0.5 (245-1). The apomorphically elongated metacarpal IV in pterosaurs nullifies the scoring of this character.
This is bogus. Metacarpal IV is not elongated in basal pterosaurs (relative to the ulna), it is simply more robust and axially rotated. Taxa from Huehuecuetzpalli to Sharovipteryx match this trait. Longisquama and the basal pterosaur MPUM6009 do not.
(8) Anteromedial tuber of the proximal portion of the femur present (300-1). Clearly present in Dimorphodon (fig. 39).
Note: This trait has not been identified on other basal fenestrasaurs and pterosaurs, most of which have a crushed femur. A similar structure can be seen in the basal tritosaur, Huehuecuetzpalli.
(9) Tibial facet of the astragalus divided into posteromedial and anterolateral basins (366-1). Not known in basal pterosaurs.
This is bogus. The astragalus tibial facet is not divided in pterosaurs.
(10) Calcaneal tuber orientation, relative to the transverse plane, between 50 degrees and 90 degrees posteriorly (377-2). Pterosaurs lack a tuber; therefore, this character could not be scored.
Note: Not only do pterosaurs lack such a tuber, so do all tritosaurs, including fenestrasaurs.
Note: The large reptile tree does not recover a monophyletic Archosauria, but finds pterosaurs and traditional archosaurs separated, evolving several traits by convergence.
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.
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.