The snakebird lacks external nares, breathes through its mouth

Figure 1. Skull of Anhinga rufa, an Old World relative of the New World Anhinga anhinga. Note the expansion of the maxilla (or overlying horny tissue) nearly obscuring the naris and antorbital fenestra. Compare to the loon in figure 3.

Figure 1. Skull of Anhinga rufa, an Old World relative of the New World Anhinga anhinga. Note the expansion of the maxilla (or overlying horny tissue) nearly obscuring the naris and antorbital fenestra. Compare to the loon in figure 2.

Anhinga anhinga (Linneaus 1766; 89cm) is the extant snakebird, which swims underwater and stabs its fish prey with its sharp beak, striking like a snake. It breathes only through the mouth as the bones and other hard tissues around the nostrils are overgrown. The feathers do not shed water, so some time is spent drying the feathers prior to flying. Snakebirds are related to grebes (genus: Aechmophorus) and loons (genus: Gavia, Fig. 2).

Figure 2. Skull of the common loon (Gavia stellata) showing the primitive state, with large external nares and antorbital fenestra.

Figure 2. Skull of the common loon (Gavia stellata) showing the primitive state, with large external nares and antorbital fenestra.

The large number and length of cervical vertebrae
in snakebirds (Fig. 3) is more or less matched only by flamingoes (genus: Phoenicopterus) by convergence.

Figure 3. Anhinga anhinga skeleton. Note the large number of cervical vertebrae. These enable the snake-like darting of the sharp skull while attacking prey underwater.

Figure 3. Anhinga anhinga skeleton. Note the large number of cervical vertebrae. These enable the snake-like darting of the sharp skull while attacking prey underwater.

Hackett et al. 2008 nested loons with penguins.
While close, the large reptile tree (LRT, 1562 taxa) nests loons + grebes derived from terns (genus: Thalasseus) and sisters to kingfishers (genus: Megaceryle) + jabirus (genus: Jabiru) and murres (genus: Uria) + penguins (genus: Aptenodytes). Among these taxa, only Jabiru experiences a reversal in having such long, stork-like legs, a primitive trait for extant birds.

Figure 1. The rostrum of Spinosaurus. Note the maxilla rising to close off the elongate naris into a reduced anterior and posterior opening.

Figure 4. The rostrum of Spinosaurus. Note the maxilla rising to close off the elongate naris into a reduced anterior and posterior opening.

Footnote:
Another aquatic dinosaur taxon that expanded its maxilla to shut off its nostrils was Spinosaurus (Fig. 4) as we learned earlier here.


References
Hackett S et al. 2008. A phylogenetic study of birds reveals their evolutionary history. Science 320:1763–1768.
Kennedy M et al. 2019. Sorting out the Snakebirds: The species status, phylogeny, and biogeography of the Darters (Aves: Anhingidae). Journal of Zoological Systematics and Evolutionary Research (advance online publication)
doi: https://doi.org/10.1111/jzs.12299 https://onlinelibrary.wiley.com/doi/10.1111/jzs.12299

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