Paleontologists have found a missing link in the evolution of air-breathing nostrils. A 395-million-year-old fossil fish demonstrates how the position of nostrils on the face moved to connect the nasal passages with the mouth--a transition that could have improved the fish's ability to smell and paved the way for breathing on dry land.
Nostrils are one way to tell fish from their relatives the tetrapods, which include all land vertebrates. Tetrapods have two external nostrils. Air passes through them into a common nasal cavity and passes into the mouth through an internal opening called the choana. Most fish have four nostrils--a pair on either side of the face. On each side, water enters through the front nostril, then passes through a nasal sac where scents are detected, before exiting through the rear nostril. For decades researchers have thought that tetrapods evolved a pair of entirely new openings in their palate that connected the inside of the mouth with the outside world through the front nostrils. Meanwhile, the rear nostrils disappeared or became the tear ducts.
A fossil fish from China, called Kenichthys, tells a different story. After piecing together a fairly complete specimen of the fish's skull, the researchers--Per Ahlberg, a paleontologist at Uppsala University in Sweden, and Min Zhu of the Institute for Vertebrate Paleontology and Paleoanthropology in Beijing, China--discovered that the rear nostrils of Kenichthys are located on the upper lip. That suggests that these nostrils gradually slid down the face and crossed the lip before ending up in the palate, the researchers report in the 4 November issue of Nature. "Kenichthys is a perfect intermediate stage" in the evolution of internal nostrils, Ahlberg says.
Kenichthys is a close relative of tetrapods, but it clearly wasn't breathing air. So why evolve internal nostrils? Ahlberg believes that internal nostrils would allow a fish to pull water through its nose by expanding its mouth, and that would let it smell even if it wasn't swimming. Even though Kenichthys had its nostrils on its lip, it could still have sucked enough water through to have an advantage in smelling, Ahlberg says.
The new material is a "remarkable discovery" that "clearly provides the first factual basis" that the internal nostrils of tetrapods are derived from the rear nostrils of fish, writes Philippe Janvier of the National Museum of Natural History in Paris, in a commentary in the same issue of Nature.