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A new study in the 3 August issue of the journal Science suggests that the external fleshy nostril of dinosaur noses may have been perched far forward, in a "rostral" position, rather than towards the back of their bony nose opening.
In fact, the rostral nostril appears to be the norm for reptiles, birds, and mammals, making it one of the few rules of anatomical construction among animals, says study author Lawrence M. Witmer of Ohio University.
The finding could have implications for how dinosaurs breathed, smelled, and regulated their body temperature and water loss, perhaps providing clues to how they adapted to different environments.
As a human, the rostral nostril might not seem like a big deal: our own bony nostril is relatively small, leaving little doubt about where the fleshy nostril could be placed. Among dinosaurs such as the long-necked sauropods, duck-billed dinosaurs, and horn-crowned dinosaurs such as Triceratops , however, the bony nostril opening can be more than two feet long, stretching half the length of the skull. So where within such a huge bony nostril was the dinosaur's smaller fleshy nostril located?
Traditionally, the fleshy nostril has been placed at the back of the bony nostril opening in dinosaurs, but this positioning may have more to do with history than biology, according to Witmer.
Early paleontologists thought that the big sauropods were amphibious, and may have used an upward-facing nostril placed dorsally, or at the back of the bony opening, as a snorkel as they submerged themselves in deep water. Although more recent interpretations suggest these dinosaurs were landlubbers, the idea of a dorsal nostril has remained untouched.
"It's an idea that no one has tested before, although the nose in general has been the focus of a lot of attention in vertebrates," says Witmer.
With no living dinosaurs to study directly, Witmer looked at the relationship of the fleshy and bony nostril in dinosaurs' closest living relatives--45 species of birds, crocodilians, and lizards. To simultaneously visualize and compare flesh and bone in individuals of these species, Witmer painted their fleshy nostrils with latex and sprinkled them with barium sulfate so that they could be seen on x-ray film alongside their bony counterparts.
In virtually all cases, the fleshy nostril perched out in front, close to the upper margin of the mouth. When Witmer looked at turtles and mammals, he found that the rostral nostril was the rule for those groups as well.
"It quickly became clear that the findings were going to apply much more generally than to just dinosaurs, which was sort of unexpected. Evolution is such a quirky thing, and it's quite rare to find a broad rule of anatomical construction like that," says Witmer
Witmer found independent confirmation of the dinosaur rostral nostril in the pattern of grooves and pits within the bony nostril, signatures of an intricate network of blood vessels in the nasal region. Crocodiles, lizards, turtles, and mammals all have extensive blood flow to the nose, particularly to a region of erectile tissue that is associated with the fleshy nostril. In dinosaurs, these traces of blood flow to the erectile tissue are all located forward or forward and downward within the bony nostril.
Why would the rostral nostril be the rule across such diverse groups of animals? Witmer suggests that the rostral nostril is perfectly positioned to take on a number of biological functions.
The mucous membranes, blood vessels, and bony architecture of the nose form a complicated apparatus that warms and humidifies air on its way to the lungs, cools blood that passes through on its way to the brain, conserves water, and filters out foreign particles, among other tasks.
A rostral nostril can convey airflow across the entire length of the nose, taking full advantage of all these features. If the fleshy nostril were located towards the back of the bony nostril, the airstream would bypass the apparatus, making for a poorly designed nose, says Witmer.
The fleshy nostril also plays a key role in the important sense of smell. A forward-facing fleshy nostril can capture information right in front of an animal as it greets its environment. And with the nostril near the mouth, the sense of smell can combine forces with the highly sensitive sense of touch along the mouth's margin.
One of the very few exceptions to the rostral nostril rule in Witmer's study, the monitor lizards, may support the importance of nostril position for smell. The monitor lizard fleshy nostril is placed towards the middle or back of its bony nostril, diverted by an enlarged chemosensory organ. While this placement could hinder the collection of smells from the environment, sensory data collected by the chemosensory organ may compensate for this diminished sense.
This research was supported in part by NSF and the Ohio University College of Osteopathic Medicine.