The snakebird lacks external nares, breathes through its mouth

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).

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

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.

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