The Making of the Mammalian Nose

As far as anatomical ventilation systems go, our noses are pretty impressive. Scrolled sheets of bone called turbinates can take much of the credit for that. Covered in soft tissue called conchae, these delicate structures not only warm and humidify incoming air, but, deep in the nasal cavity, house specialized tissue that aids our sense of smell. And thanks to the exquisite preservation of a 227 million year old protomammal, paleontologists are starting to understand when and how these marvelous little bones evolved.

The skull, found in the Triassic rock of Brazil, belonged to a small, weaselish animal named Brasilitherium. This was not a mammal. Not quite. Brasilitherium was a cynodont – part of the larger, varied group of vertebrates called synapsids to which we also belong. It was close to the predecessors of the first mammals – called mammaliaforms – but didn’t fall inside that group. And that’s what makes its nose so interesting.

Using micro-CT scanning technology, Senckenberg Forschungsinstitut und Naturmuseum Frankfurt paleontologist Irina Ruf and colleagues were able to look inside the skull of Brasilitherium to see if the protomammal’s nose contained any trace of turbinates. Paleontologists expected the structures to be there on the basis of specialized ridges inside other protomammal skulls, but no one had ever found direct evidence of the turbinates those ridges were supposed to support. Part of that is because the turbinates were probably made of cartilage early in their evolution, and therefore more likely to decay away, not to mention the rarity of relatively intact skulls to study.

The skull of Brasilitherium was a better candidate to investigate the puzzle, especially since the protomammal had a secondary palate that separated the nose from the mouth and therefore had an encapsulated the space where the turbinates would have been. And the researchers were in luck. Ruf and colleagues were able to identify shards of bone inside the protomammal’s large nasal cavity that were likely from two different parts of the turbinates. This is the first time anyone has found direct evidence of turbinates in a protomammal.

CT scans showing the inside and outside of the Brasilitherium skull. From Ruf et al., 2014.
CT scans showing the inside (side view) and outside (bottom view) of the Brasilitherium skull. From Ruf et al., 2014.T

The turbiantes probably performed the same functions in Brasilitherium as in modern mammals. The turbinates and overlying soft tissue at the front warmed and moistened incoming air, while the structures towards the rear of the protomammal’s capacious nasal cavity picked up scents. This latter portion was probably especially important to Brasilitherium, as the protomammal’s brain anatomy suggests that it had a well-developed sense of smell.

But that’s not all. Ruf and colleagues also found peculiar hollows above the protomammal’s secondary palate. These, the scientists suggest, would have been the bony boundaries of the Jacobson’s organ – a sensory structure specially attuned to picking up pheromones. Ever see your housecat sniff something and grimace? They’re using their Jacobson’s organ to get a better sense of that object, or who marked it. Perhaps Brasilitherium did the same all the way back in the Triassic.

A virtual model of the inside of a Brasilitherium skull, with the ossified turbinate fragments in blue and red. From Ruf et al., 2014.
A virtual model of the inside of a Brasilitherium skull, with the ossified turbinate fragments in blue and red. From Ruf et al., 2014.

So Brasilitherium had turbinates and a Jacobson’s organ, meaning that these curious bits of anatomy evolved sometime before 227 million years ago and were later inherited by mammals. But Brasilitherium also had some relatively archaic traits. The protomammal’s external nostrils were separated by a barrier of bone – a condition later lost in mammals – and Brasilitherium lacked a bony separation between the nasal cavity and the brain called the cribiform plate.

This puts Brasilitherium in a very interesting position. The protomammal had a mosaic of features that connect it to both its ancestors and later cousins. In other words, the protomammal had a nose full of transitional features that help outline how our own nostrils came to be as they are. The evolution of jaw bones to the delicate anatomy of our inner ear is already a classic story of synapsid evolution. Perhaps, with the help of Brasilitherium and kin, the mammalian nose will become the next example of evolution’s great transformations.

[For more turbinate research, check out this post on the presence of these structures in dinosaurs.]

Reference:

Ruf, I., Maier, W., Rodrigues, P., Schultz, C. 2014. Nasal anatomy of the non-mammaliform cynodont Brasilitherium riograndensis (Eucynodontia, Therapsida) reveals new insight into mammalian evolution. The Anatomical Record. 297: 2018-2030.

2 thoughts on “The Making of the Mammalian Nose

  1. Hi, great post, very illuminating!
    I got a question: As far as I know turbinates are interpreted as a sign for endothermy. Does this mean, that Brasilitherium was warm-blooded? And furthermore, does the presence of turbinates in an ornithischian shed some new light on the endothermy of dinosaurs?

  2. Very, very interesting: the age of Brasilithrium, with its probable vomeronasal organ, is remarkably close to when vomeronasal sensory neurons began to diversify in what were to become mammals – see Grus WE, Shi P, Zhang Y-P, and Zhang J. 2005 Dramatic variation of the vomeronasal pheromone receptor gene repertoire among five orders of placental and marsupial mammals Proc. Nat. Acad. Sci. 2005 102: 5767–5772

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