Where are the internal nares in plesiosaurs?

Simosaurus and Anningasaura. Somewhere between these two the internal and external nares of these protoplesiosaurs became much smaller, almost useless vestiges. Apparently breathing continued through the mouth alone.

Figure 1. Simosaurus and Anningasaura. Somewhere between these two the internal and external nares of these protoplesiosaurs became much smaller, almost useless vestiges. With such a tiny nostril in Anningsaura, apparently breathing continued through the mouth alone. The placement of the internal nares did not shift much. Is that a secondary choana (internal naris) between the pterygoids? Probably not because it’s not much larger than the real choana, so no advantage. There is no medial extension of the maxilla here as in other reptiles with a secondary palate (see below).

The recent paper on pliosaur palates by Schumacher et al. (2013) considered the placement of the internal nares in plesiosaurs (Figs. 1, 3). They noted Buchy et al. (2006) questioned the various placements and offered evidence for a functional secondary palate in plesiosaurs, shifting the internal nares to the back of the palate, posterior to the pterygoids, similar to the situation in crocodilians (Fig. 2). However in crocs, as you can see, the maxillae and palatines contact medially producing a standard sort of secondary palate.

The palate of Alligator. Note the posterior placement of the internal nares with paired maxillae, palatines and pterygoids forming the secondary palate.

Figure 2. The palate of Alligator. Note the posterior placement of the internal nares with paired maxillae, palatines and pterygoids forming the secondary palate.

The traditional view
holds that a pair of very small openings in the anterior half of the plesiosaur palate (Figs. 1,3), anterior to the palatines, represent the internal nares through which respiration took place.

Another traditional view
Williston (1903) accepted such a position for Dolichorhynchops, but not for Brachauchenius. Instead, despite the retention of internal nares in the traditional place, Williston placed the internal nares between the pterygoids separated by the parasphenoid (see Plesiosaurus in figure 3) as in crocodilians.

More recently, Schumacher et al. (2013) reported, “We have independently concluded that the posterior interpterygoid vacuity…should be called the internal nares.”

The Schumacher Hypothesis
Such a posterior position of the internal naris would be due to the development of a secondary palate in plesiosaurs, according to Schumacher et al. (2013). Benefits: Shifting the internal nares posteriorly, as in crocodiles, separates nasal respiration from oral functions (like biting, chewing, reorienting and swallowing). Problems: Small nares restrict air passage and ventilation. Since the posterior openings are typically slightly larger than the anterior ones Schumacher et al. (2013) suggest that the shift was made by soft tissue within the skull and the tiny anterior openings would have been covered with mechanosensory or chemosensory tissue, thereby completely blocking respiration there. According to Schumacher et al. (2013) respiration would then have taken place at the back of the palate where the slightly larger interpterygoid opening is.

Enaliosaur palates

Figure 3. Click to enlarge. Enaliosaur palates beginning with Claudiosaurus (upper left). The internal nares shrink in Anningasaurus, Pisotosaurus and Plesiosaurus compared to Simosaurus and Pachypleurosaurus. The palate is open posteriorly only in Plesiosaurus, but that does not appear to be a channel for respiration.

Sea turtle with secondary palate. Here the palatines join medially to shelve the respiratory tract and shifting the internal nares to mid palate.

Figure 4. Sea turtle with secondary palate. Here the palatines join medially to shelve the respiratory tract and shifting the internal nares to mid palate. From Brown and Madara 2000. See Proganochelys for a turtle without a secondary palate.

Of course
All that presupposes that plesiosaurs actually breathed through those tiny nostrils and internal nares. Maybe they didn’t. Maybe, once the nares became sufficiently tiny (vestigial), plesiosaurs began to breathe through their mouth, like the sea turtle in figure 5. This makes all the more sense in hyper-long-necked elasmosaurs, as the mouth offers no respiratory restrictions while elevating the skull above the water surface (even slightly above as in fig. 5). This also makes sense in giant-jawed pliosaurs where breathing might have taken place with the jaws just barely open and air respiring between the slightly gaped giant teeth, perhaps while “spy-hopping,” or during less obvious maneuvers.

Figure 6. Sea turtle breathing at the surface. Both the nares and the mouth are open. (Photo by Joe Raedle/Getty Images)

Figure 5. Sea turtle breathing at the surface. Both the nares and the mouth are open. (Photo by Joe Raedle/Getty Images)

Phylogeny Clues
If we look at a series of plesiosaurs and their ancestors (Fig. 6) we see that Claudiosaurus, with its tiny skull, had the relatively largest internal nares. The nothosaurs (Pachypleurosaurus, Lariosaurus, Simosaurus (Fig. 1) had a smaller, but still substantial internal naris. In contrast, Anningasaura and Pistosaurus had vestigial internal nares, obviously unusable for respiration. Other marine taxa (Fig. 3) also had tiny internal nares.

Something changed.
Between Simosaurus and Anningasaura both the internal and external nares shrank to vestiges (Fig. 1). I think it’s likely that Anningasaura and its plesiosaur and pliosaur descendants were breathing through their mouth.

Secondary palate development in a series of synapsid cynodonts leading to mammals.

Figure 6. Secondary palate development in a series of synapsid cynodonts leading to mammals. In  the eutheriodont and Procynosuchus the secondary palate is incomplete. In Thrinaxodon and all subsequent cynodonts, including all mammals, the secondary palate is complete when the maxillary palatal processes meet each other along with the palatine palatal processes. Note the gradual posterior shift of the internal naris from Thrinaxodon to Morganucodon. Image from Hopson 1991.

In a real secondary palate,
as in synapsids (Fig. 6), the development of a secondary palate (maxillae and palatines meet medially) can be traced through a series of fossils. The same holds true for crocodilians (Fig. 2, see Scleromochlus and Terrestrisuchus for taxa without a palate), sea turtles (see Proganochelys for a taxon without a palate) and pterosaurs (Fig. 7, see Cosesaurus for the primitive condition). Champsosaurus doesn’t have a secondary palate, just a longer snout and the internal nares stayed put.

Evolution of the pterosaur palate from Eudimorphodon to Pterodaustro.

Figure 7. Click to enlarge. Evolution of the pterosaur palate from Eudimorphodon to Pterodaustro. The secondary palate formed by maxillary palatal processes meeting at the midline shift the internal nares posteriorly. A similar expansion of the maxillae and migration of the internal nares is not documented in sauropterygians.

Sauropterygians do not document a similar gradual shift of the internal nares. Rather the nares simply shrinks reflecting a lack of usage while shifting to respiration through the mouth. There is no medial extension of the maxilla or palatine. Various valves would have been present to open and shut the esophagus (to the stomach) and epiglottis (to the lungs). Otherwise, no bony changes document the development of a secondary palate as in other reptiles (contra Schumacher et al. 2013). And there’s no real benefit to that marginally larger interpterygoid opening. The presupposition that the nares were used for breathing is at the heart of the problem. The problem goes away when that supposition goes away.

References
Buchy M-C, Frey E and Salisbury  2006. Internal cranial anatomy of Plesiosauria (Reptilia, Sauropterygia): evidence for a functional secondary palate. Lethaia 39:290-303.
Hopson JA 1991. Systematics of the nonmammalian Synapsida and implications for patterns of evolution in synapsids, in H-P Schultze & L Trueb [eds], Origins of the Higher Groups of Tetrapods: Controversy and Consensus. Comstock, pp. 635-693.  Schumacher BA, Carpenter K and Everhart MJ 2013. A new Cretaceous Pliosaurid (Reptilia, Plesiosauria) from the Carlile Shale (middle Turonian) of Russell County, Kansas. Journal of Vertebrate Paleontology 33(3):613-628.

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