Only therian mammals wear a nose fully separate from the mouth
A nose is both air conditioner and smell detector: external nostrils feed a nasal cavity that filters, warms, and samples the world before lungs see it. Humans sit it mid-face; infants especially lean on it while nursing. Fully separate protruding noses are a therian-mammal trademark.
A vertebrate nose has two parts: the external nose on the face and the nasal cavity inside the head. Air enters through paired nostrils, or nares, which widen inside into nasal fossae lined with turbinates and smell-sensing mucosa. The cavity connects to the paranasal sinuses, blind air pockets that buffer pressure and add moisture, and continues back into the pharynx, the junction where breathing and swallowing routes meet.
As the first stop before the lungs, the nose conditions air. Nostril hairs catch dust, smoke and airborne germs; mucus adds moisture and helps hold temperature; capillaries warm and humidify each breath so the alveoli can swap oxygen for carbon dioxide. On the way out, the same capillaries reclaim some of the moisture. In humans the nose sits in the middle of the face and gives infants a second airway while they suckle. Dogs exploit their wet noses as compasses: cold receptors sense which side is cooled most, pointing to the source of a smell.
Other vertebrates build the organ differently. In amphibians and lungfish the nostrils lead to small sacs opening into the roof of the mouth, though lungfish breathe through their mouths and do not use them for air. Crocodilians have an unusually long nasal chamber that lets them breathe while partly submerged. Birds, generally poor smellers, have small olfactory chambers, and doves and fowl often shield their nostrils with a horny cover.
Mammals fused the two nasal cavities into one that in many species fills up to half the skull's length. A palate sealed off the old upper mouth and handed that space to the nose, which now holds coiled, scroll-like turbinates. Primates, bats and cetaceans reduced their noses again and smell comparatively poorly. A protruding nose fully separate from the mouth is found only in therian mammals, and may have evolved from the front of the upper jaw of synapsid ancestors.
Source: Nose
