A key distinction between basal pterosaurs and derived “pterodactyloid”-grade pterosaurs is the apparent disappearance of the naris. Typically the naris is said to be “confluent” with the antorbital fenestra. Unfortunately the steps or stages in this disappearance or confluence have never been documented. Here, in this heretical blog, we’ll attempt to remedy that by observation, phylogenetic analysis and comparison.
As discussed earlier, there were at least four origins of the “pterodactyloid”-grade, but there were five disappearances of the pterosaur naris. The fifth occurred in the Pterorhynchus/Darwinopterus clade which, otherwise, did not achieve the “pterodactyloid” grade postcranially. Here we’ll document the naris reduction three (Fig 1) by two (Fig 2).
Sordes > Pterorhynchus
Between Sordes and Pterorhynchus the naris was drastically reduced (Fig. 1) as the antorbital fenestra increased its length. The antorbital fenestra extended anteriorly beneath the naris for half of its length in both cases. It is difficult to determine if a naris was present or not in Kunpengopterus through Wukongopterus, sisters to Darwinopterus.
Scaphognathus > Germanodactylus and Pterodactylus
Between the n109 specimen of Scaphognathus and Germanodactylus rhamphastinus there appears to be a reduction and splitting of the naris into a primary naris (posteriorly) and a secondary naris (anteriorly). The nasal and jugal appear to extend to the anterior (secondary) naris.
Scaphognathus > Cycnorhamphus and Zhenyuanopterus
Between the n110 specimen of Scaphognathus and Cycnorhamphus and Zhenyuanopterus the pattern of narial reduction duplicates the previous pattern. Often in ornithocheirids the tiny anterior naris is completely eliminated by bone growth sealing the opening.
Dorygnathus > Zhejiangopterus
In Dorygnathus through Zhejiangopterus the naris became smaller and moved anteriorly. Several holes appeared in derived skulls, but these could be artifacts and/or erosion.
Naris? Or Skull Erosion?
The difficulty in searching for the naris lies in trying to determine what is a tiny naris and what is a tiny gap or erosion of the bone. The nearby anterior processes of the nasal and jugal provide some guide, but the evidence is often equivocal. Comparisons to sister taxa are also helpful. In any case, I see no evidence for erosion of the maxillary process meeting the nasal that separated the naris from the antorbital fenestra. Thus there was no confluence.
The Reduction of the Naris in Birds
Several birds, like the diving gannett (Sula bassana), pelican (Pelicanus) and the sifting spoonbill (Ajaia ajaia), have reduced their naris to a slit or eliminated it entirely. Most other birds had a large naris separated from the antorbital fenestra. Others, such as the cassowary (Casuarius casuarius) had a large confluent naris and antorbital fenestra. These bird skulls can be seen here.
MacDonald (2009) reported, “The gannet has well-developed secondary external nares to compensate for the primary ones which are occluded. It is questionable if this occlusion is a direct consequence of the habit of high diving. Occlusion of the external nares apparently limits the function of the nasal glands as salt excreting organs.” According to Hieronyomous (2009) inspired air does not enter the naris in certain birds with sealed nares, rather air enters the throat directly.
The Reduction of the Naris in Pterosaurs
Rather than confluence, the naris in many pterosaurs simply became so small that it became difficult to segregate from bone destruction and crushing. If the reduction of the naris in pterosaurs followed (by convergence) the pattern in birds, it would appear that taxa that dipped or drove their beaks into water reduced the naris. That seems reasonable when looking at the gannett-like germanodactylids. However the initiation of naris reduction in four out of five clades occurred during phylogenetic size reductions at the bases of four “pterodactyloid”-grade clades. So tiny pterosaurs are the key to naris reduction. Subsequent longer-rostrum and overall larger taxa simply inherited a small naris from their smaller and shorter rostrum ancestors. Pterorhynchus is the exception. Its predeccessor, Sordes, was smaller and had a large naris.
Did Pterosaurs Breathe Through Their Nostrils?
If they had big nostrils, yes. Tiny nostrils, hmm, probably not. Along with a reduction in naris size and elongation of the jaws there is a general trend toward filling in the palate. That may have been more of structural innovation than a way to separate air from water and food, considering the reduced size of the naris in these pterosaurs.
Rather than confluence, the naris in derived pterosaurs was reduced five separate times by convergence. The narial reduction occurred during phylogenetic size reductions and was retained by their larger descendants.
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.
MacDonald JD 2009. Secondary external nares of the gannet. Journal of Zoology online 20 Aug 2009. DOI: 10.1111/j.1469-7998.1960.tb05852.x
Hieronymous TL 2009. Comparative Anatomy and Physiology of Chemical Senses in Aquatic Birds in: Sensory Evolution on the Threshold – Adaptations in Secondarily Aquatic Vertebrates. Edited by Thewissen JGM and Nummela S. Berkeley: University of California Press. 2008. 351 pp., $75 (cloth). ISBN 978-0-520-252783.