The Evolution of the Pterosaur Palate – part 1

Not much attention has been paid to the pterosaur palate.
Ever since Williston (1902) and continuing through Huene 1914, Wellnhofer (1978, 1991) and Bennett (1991, 2001a,b), the solid palatal plate in pterosaurs has been considered the palatine (Fig. 1). That’s the tradition and that’s the paradigm – but that’s wrong.

Scaphognathus palate

Figure 1. Click to enlarge. (Left) The traditional reconstruction of the palate of the pterosaur Scaphognathus (Wellnhofer 1978) in which the palatal shelf is considered the palatine bone. (Right) The heretical and recently vindicated reconstruction of the palate in which the palatine is fused to the ectopterygoid creating a L-shaped bone, the ectopalatine. The medial extent of the maxillary palate shelf is unknown. 

Not the Palatine, the Maxilla!
Virtually ignored, Newton (1888), Seeley (1901 and Woodward (1902) reported that the solid palatal plate was an outgrowth of the maxilla, not the palatine. Unfortunately, I did not know that when I reported (in Peters 2000) that the palatal plate actually originated from the maxilla. (Thought I had discovered something!) The much smaller palatine and ectopterygoid were fused to form a single L-shaped element, the ectopalatine. This was due to an ancestry among fenestrasaurs and Macrocnemus, rather than archosaurs. Even that was largely ignored until…

Reinterpretation and Vindication
Osi et al. (2010) reported, “The hard palate is formed by the extensive palatal plate of the maxilla and not by the palatine as has been generally reconstructed.”  This was well demonstrated by a 3D Dorygnathus skull (Fig. 2).

Palate of the WDC specimen of Dorygnathus

Figure 2. The Osi et al. (2010) WDC 3-D Dorygnathus palate. Click to enlarge. The non-fusion of the palatine and ectopterygoid might have signaled ontogenetic immaturity, but the specimen is half again larger than other Dorygnathus specimens. The original interpretation of the premaxilla was based on the break at mid maxilla, not the sutures, which are revised at right. The purported foramen incisivum is not found in other pterosaurs. It represents missing vomer bone. 

The Origin and Evolution of the Palatal Elements in Basal Pterosaurs
To trace the origin of the palatal elements in pterosaurs we go back to Lacertulus, a basal tritosaur from the Late Permian (Fig. 3. Lacertulus had round wide jaws with large wide elements with no maxillary component.

In Huehuecuetzpalli (Fig. 3) the snout was narrower and so were all the palate elements. In Macrocnemus, narrower still. In Tanystropheus, not so narrow, but the interpterygoid vacuity was narrow. In both of these the palatine barely contacts the ectopterygoid.

In Cosesaurus (Fig. 3) the vomer teeth were probably absent (hidden in the fossil). In Sharovipteryx the ectopterygoid and palatine were both straight and fused. The rostrum in Longisquama was narrowed and the palatal elements were reduced.

The evolution of the pterosaur palate

Figure 3. The evolution of the pterosaur palate from basal tritosaur ancestors. Note the fusion of the palatine and ectopterygoid creating the ectopalatine in adults. The non-fusion of these elements in figure 1 may represent an ontogenetic character. Not sure. Vomers in green. Palatines in blue. Pterygoids in orange. Ectopterygoids in magenta. The latter two elements are fused in Longisquama and most pterosaurs. The medial extent of the maxillary palate processes and the width of the rostral portion of the skull is unknown in specimens exposed in lateral view.

Figure 3. The evolution of the pterosaur palate from basal tritosaur ancestors. Note the fusion of the palatine and ectopterygoid creating the ectopalatine in adults. The non-fusion of these elements in figure 1 may represent an ontogenetic character. Not sure. Vomers in green. Palatines in blue. Pterygoids in orange. Ectopterygoids in magenta. The latter two elements are fused in Longisquama and most pterosaurs. The medial extent of the maxillary palate processes and the width of the rostral portion of the skull is unknown in specimens exposed in lateral view.

In the basal pterosaur, MPUM 6009, the palate did not become so narrow. Like Sharovipteryx, the anterior pterygoid was more robust, forming a “head” and the posterior portion was quite gracile. Other pterosaurs had more gracile palatal elements. A maxilla palatal process leading from the base of the ascending process connects to the palatine portion of the ectopalatine.

The solid portion of the palate
The evolution of the maxillary process of the palate would probably come later in the evolution of pterosaurs (Fig. 1), appearing in Rhamphorhynchus and, by convergence, in Dorygnathus and other pterosaurs derived from this genus or by convergence after Scaphognathus. Due to crushing, etc. the medial extent of the maxilla toward the vomers is difficult to gauge in most Triassic and early to mid-Jurassic pterosaurs, other than anurognathids) which we’ll cover in a future blog.

New Openings?
The new premaxillary openings discovered by Osi et al. (2010) were indeed novel with regard to Dorygnathus, but were also found in Rhamphorhynchus muensteri by convergence (Fig 4). They do not represent vomernasal organs, but are new structures. Only higher scleroglossan lizards have vomernasal openings. No other such openings have been reported in pterosaur ancestors or other pterosaurs. Longisquama (Fig. 2) may have had something similar my convergence (this is educated guesswork). In any case they were secondarily developed in all cases and not related to or pinched off from the choanae.

Various pterosaur palates

Figure 4. Comparing pterosaur palates. Left to right: the new WDC Dorygnathus, Rhamphorhynchus muensteri, Cacibupteryx and Dorygnathus purdoni. The pterygoid extends a process to the jugal in three of these taxa, but such a process is otherwise rare in pterosaurs.

The Fragile Stem-like Portion of the Palate
As the maxillary portion of the palate expanded, the posterior elements (palatine and ectopterygoid) become more or less smaller in all pterosaurs (Fig. 4). More later on these derived taxa.

Problems with the Supplementary Material in Osi et al. (2010)
1. Comparisons were only made to archosaurs when in fact no line up of archosaurs documents a gradual increase in pterosaurian characters. They report: “Hone and Benton (2007, 2008) provided new evidence to support the origin of the Pterosauria within Archosauria and we follow their definition here.” Unfortunately, Hone and Benton made their test by omitting the data from Peters, 2000, omitting the two fenestrasaurs closest to pterosaurs and scoring the other two for only a quarter of their available characters. Their so-called ‘test’ was no test at all, but elimination of the competition and a foregone conclusion with only one possible outcome.

Reinterpretation of purported premaxilla in Dorygnathus

Figure 5. Here I reinterpret more bones in this purported premaxilla of the WDC 3-D Doryganthus of Osi et al. (2010). Purple = maxilla. Green = vomer. Pink = nasal.

2. Osi et al. (2010 fig. 3) is a multiview photo of the premaxilla of the new specimen  (Fig. 5). Unfortunately the authors failed to recognize the pmx/mx suture essentially extending from the antorbital fenestra not quite to tooth #4. So a healthy portion of the maxilla is present here laminated to the pmx. The break occurred at the narrowest portion below the naris.

3. The sagittal rdge (SI) in the pmx is the paired, fused, anterior vomers.

4. Considering the proportions of the very elongate premaxilla, longer than in any other Dorygnathus of which I am aware, and as long as the maxilla, this is probably a derivation of Dorygnathus and sufficiently distinct to merit its own genus. I would encourage the authors to erect a new genus and perform a phylogenetic analysis of all known Dorygnathus specimens to confirm the new specimen’s closest relations. There is an unrecognized variety in Dorygnathus that needs to be explored. I don’t think this specimen is basal to any known taxa, which all have a relatively shorter premaxilla.

5. Their figure 9 misidentified the entire broken rostrum of Gnathosaurus as the premaxilla, when it should have included only the first four teeth and a long premaxillary ascending process.

6. Paradoxically, and contra the evidence of their specimen, the hypothetical  reconstructions in their figure 11 separate the palatine from the ectopterygoid, giving each a separate articulation on the pterygoid for no apparent reason, other than following the pattern seen in crocodilians. These two elements were in direct contact and fused in many taxa.

7. Figure 11 also purports to extend the premaxilla in “(B) pterodactyloid pterosaurs” to include over a dozen teeth. This is also wrong. The premaxilla in all pterosaurs includes up to 4 teeth, no more.

8. In their figure 11 the authors should have reconstructed the posterior vomers lateral to the anterior pterygoids, not medial to them. They had it right in their figure 8.

9. The authors ascribe a lateral pterygoid process contacting the jugal to all basal pterosaurs, but this process is restricted to certain Rhamphorhynchus and certain Dorygnathus by convergence.

We’ll look at anurognathid palate tomorrow…

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:
Bennett SC 1991. Morphology of the Late Cretaceous Pterosaur Pteranodonand Systematics of the Pterodactyloidea. [Volumes I and II]. – Ph.D. thesis, University of Kansas [Published by University Microfilms International/ProQuest].
Bennett SC 2001a, b. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I and 2. General description of osteology. – Palaeontographica, Abteilung A, 260: 1-153.
Newton ET 1888. On the skull, brain and auditory organ of a new species of pterosaurian (Scaphognathus Purdoni) from the Upper Lias near Whitby, Yorkshire. Philosphoical Transaction of the Royal Society, London 179: 503-537.
Osi A, Prondvai E, Frey E and Pohl B 2010. New Interpretation of the Palate of Pterosaurs. The Anatomical Record 293: 243-258.
Peters D 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Seeley HG 1901. Dragons of the air. An account of extinct flying reptiles. – London, Methuen: 1-240.
Wellnhofer P 1978. Pterosauria. Handbuch der Paläoherpetologie, Teil 19.– Stuttgart, Gustav Fischer Verlag: 1-82.
Wellnhofer P 1991. The Illustrated Encyclopedia of Pterosaurs. London, Salamander Books, Limited: 1-192.
Williston SW 1902. On the skull of Nyctodactylus, an Upper Cretaceous pterodactyl. Journal of Geology 10:520–531.
Woodward AS 1902. On two skulls of Ornithosaurian Rhamphorhynchus. Annals of the Magazine Natural History 9:1.

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