The genesis of external and internal nostrils

The origin of choanae
(internal nostrils) from the primitive dual external naris of basal vertebrates was ‘settled’ over a decade ago with the observation that Kennichthys (Fig. 1; Zhu and Ahlberg 2004; Janvier 2004) had a naris/choana at the rim of its jaws. At the time, Kennichthys was thought to be an osteolepiform and basal to tetrapods. In other words, that was the phylogenetic context at the time.

Figure 1. From Janvier 2015, evolution of the internal naris (choana) from primitive dual external nares.

Figure 1. From Janvier 2004, evolution of the internal naris (choana) from primitive dual external nares. No bony ray fin fish, sharks and placoderms are shown here, distinct from the LRT in figure 2.

However, a novel phylogenetic context
arises by greater taxon inclusion in the large reptile tree (LRT, 1548 taxa; Fig. 2).

Figure 2. Subset of the LRT focusing on basal vertebrates. Colors indicate various naris configurations. Some palates are not known.

Figure 2. Subset of the LRT focusing on basal vertebrates. Colors indicate various naris configurations. Some palates are not known.

The LRT indicates
that Kennichthys was a terminal taxon, not leading to higher taxa. Polypterus and Powichthys have/had a single external and internal naris. This pattern leads by homology to tetrapods and osteolepiforms. A reversal took place with a traditional osteolpiform, Onychodus (Fig. 3), which has dual external nares, like ray fin fish, AND an internal naris. Thereafter the internal naris disappears and dual external nares are retained.

FIgure 1. Several views of the Onychodus skull.

FIgure3. Several views of the Onychodus skull. Note the twin external nares AND the internal naris. 

Lungfish have/had two internal nares.
Some higher, predatory placoderms have a single external naris (not sure about the palate). A clade including living lizardfish (Trachinocephalus), Cheirolepis, and spiny sharks by convergence seem to have one external naris (not sure about the palate).


References
Chang M and Zhu M 1993. A new Middle Devonian osteolepidid from Qujing, Yunnan. Mem. Assoc. Australas. Palaeontol. 15 183-198.
Janvier P 2004. Wandering nostrils. Nature 432:23–24.
Zhu M and Ahlberg P 2004. The origin of the internal nostril of tetrapods. Nature 432:94-97.

wki/Stensioella
wiki/Kenichthys

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