![Vomer and choane](https://upload.wikimedia.org/wikipedia/commons/3/3e/Vomer_and_choane.jpg)
The choanae (singular choana), posterior nasal apertures or internal nostrils are two openings found at the back of the nasal passage between the
nasal cavity
The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal c ...
and the
throat
In vertebrate anatomy, the throat is the front part of the neck, internally positioned in front of the vertebrae. It contains the pharynx and larynx. An important section of it is the epiglottis, separating the esophagus from the trachea (windpipe ...
in
tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct theraps ...
s, including humans and other mammals (as well as
crocodilians
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period (Cenomanian stage) and are the closest living ...
and most
skinks
Skinks are lizards belonging to the family Scincidae, a family in the infraorder Scincomorpha. With more than 1,500 described species across 100 different taxonomic genera, the family Scincidae is one of the most diverse families of lizards. Sk ...
). They are considered one of the most important
synapomorphies
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
of
tetrapodomorphs
The Tetrapodomorpha (also known as Choanata) are a clade of vertebrates consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish. Advance ...
, that allowed the passage from water to land.
In animals with secondary palates, they allow breathing when the mouth is closed.
[ Janvier, Philippe (2004) "Wandering nostrils". ''Nature'', 432 (7013): 23–24. ] In tetrapods without secondary palates their function relates primarily to olfaction (sense of smell).
The choanae are separated in two by the
vomer
The vomer (; lat, vomer, lit=ploughshare) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right max ...
.
Boundaries
A choana is the opening between the
nasal cavity
The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal c ...
and the
nasopharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struct ...
.
It is therefore not a structure but a space bounded as follows:
*
anteriorly and
inferiorly
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...
by the
horizontal plate
The horizontal plate of palatine bone is a quadrilateral part of the palatine bone, and has two surfaces and four borders.
Surfaces
The superior surface, concave from side to side, forms the back part of the floor of the nasal cavity.
The inf ...
of
palatine bone
In anatomy, the palatine bones () are two irregular bones of the facial skeleton in many animal species, located above the uvula in the throat. Together with the maxillae, they comprise the hard palate. (''Palate'' is derived from the Latin ''pa ...
,
* superiorly and posteriorly by the
sphenoid bone
* laterally by the
medial pterygoid plate
The pterygoid processes of the sphenoid (from Greek ''pteryx'', ''pterygos'', "wing"), one on either side, descend perpendicularly from the regions where the body and the greater wings of the sphenoid bone unite.
Each process consists of a me ...
s.
* medially by the Vomer
Etymology
The term is a latinization from the Greek χοάνη, "choanē" meaning
funnel
A funnel is a tube or pipe that is wide at the top and narrow at the bottom, used for guiding liquid or powder into a small opening.
Funnels are usually made of stainless steel, aluminium, glass, or plastic. The material used in its construct ...
.
Choanae in different animals
Early bony fishes (~420 mya) had two pair of nostrils, one pair for incoming water (known as the anterior or incurrent nostrils), and a second pair for outgoing water (the posterior or excurrent nostrils), with the olfactory apparatus (for sense of smell) in between. In the first
tetrapodomorphs
The Tetrapodomorpha (also known as Choanata) are a clade of vertebrates consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish. Advance ...
(~415 mya) the excurrent nostrils migrated to the edge of the mouth, occupying a position between the maxillary and premaxillary bones, directly below the lateral rostral (a bone that vanished in early tetrapods).
In all but the most basal (primitive) tetrapodomorphs (or "choanates"), the excurrent nostrils have migrated from the edge of the mouth to the interior of the mouth. In tetrapods that lack a secondary palate (basal tetrapods and amphibians), the choanae are located forward in the roof of the mouth, just inside the upper jaw. These internal nasal passages evolved while the vertebrates still lived in water.
In animals with complete secondary palates (mammals, crocodilians, most skinks) the space between the primary and secondary palates contain the nasal passages, with the choanae located above the posterior end of the secondary palate.
In animals with partial secondary palates (most birds and reptiles), the median ''choanal slit'' separates the two halves of the posterior half of the palate, connecting the nasal cavity with the buccal cavity (mouth) and the pharynx (throat).
Fish
Most
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...
do not have choanae, instead they have two pairs of external nostrils: each with two tubes whose frontal openings lie close to the upper jaw, and the posterior openings further behind near the eyes. A 395-million-year-old fossil lobe-finned fish called ''
Kenichthys
''Kenichthys'' is a genus of sarcopterygian fish from the Devonian period, and a member of the clade tetrapodomorpha. The only known species of the genus is ''Kenichthys campbelli'' (named for the Australian palaeontologist Ken Campbell), the fi ...
'' ''campbelli'' has something between a choana and the external nostrils seen on other fish. The posterior opening of the external nostrils has migrated into the mouth.
In lungfish, the inner nostrils are regarded as an example of
parallel evolution
Parallel evolution is the similar development of a trait in distinct species that are not closely related, but share a similar original trait in response to similar evolutionary pressure.Zhang, J. and Kumar, S. 1997Detection of convergent and paral ...
. The fossil lungfish ''
Diabolepis
''Diabolepis'' (or ''Diabolichthys'') is an extinct genus of very primitive lungfish which lived about 400 million years ago, in the Early Devonian period of South China. ''Diabolepis'' is the most basal known dipnoan.
A rather small fish, the ...
'' shows an intermediate stage between posterior and interior nostril and supports the independent origin of internal nostrils in the lungfish.
Hagfish
Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...
es have a single internal nostril that opens inside the mouth cavity, while
chimaeras have open canals that leads water from their external nostrils into their mouths and through their gills.
References
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Skull