Sauropod nostrils: Where were they?

Short answer:
For whatever reason, derived sauropods shifted the external naris away from the mouth. It would appear illogical to extend soft nostrils back close to the mouth, as Witmer 2001 proposes, over the exterior of the maxillary basin (Fig. 1), which varies greatly (Fig. 2).

Figure 1. From Witmer 2001 showing brachiosaur sauropod skull, colors added. Witmer suggests the nostril might have been located at point 'B' in the maxillary basin (blue) rather than in the external naris (red).

Figure 1. From Witmer 2001 showing brachiosaur sauropod skull, colors added. Witmer suggests the nostril might have been located at point ‘A’ of ‘B’ in the maxillary basin (blue) rather than in the external naris (red).

Witmer 2001 proposed an anterior nostril position
within the nasal basin anterior to the bony external naris in sauropods (positions A and B in Fig. 1, green dot in Fig. 2) and a similar anterior position in other dinosaurs based on an anterior position in most lepidosaurs, crocs and birds. In every photo example presented by Witmer the nostril forms only a small opening relative to the bony external naris.

Witmer 2001 also provided several exceptions to that pattern:

  1. “Cormorant (Phalacrocorax) simply lacked a ßeshy nostril altogether (a diving adaptation)
  2. The bony nostril of geckos is so small that the fleshy nostril occupied almost its entire extent.
  3. The most significant exception was among monitor lizards (Varanus). Some species (e.g., V. griseus, V. dumerili, V. exanthematicus) have a fleshy nostril located in the middle to caudal half of the much enlarged bony nostril.”
  4. Witmer concludes: “Given the diversity of amniotes, one would expect to find additional exceptions.”

As everyone knows,
all tetrapods are capable of inhaling and exhaling through the mouth, which becomes important in panting for internal cooling and when exercise requires more oxygen. The external naris is principally for olfaction and the anterior position of the nostril within the naris maximizes the amount of soft tissue that can be exposed to incoming odors and pheromones.

Figure 1. Four sauropods with external nares identified in pink, internal nares in blue.

Figure 2. Four sauropods with external nares identified in pink, internal nares in blue, Witmer’s proposed nostril in green. Note the external naris already forms a restriction to the airway. For whatever reasons, more derived sauropods phylogenetically shift the nares away from the mouth. Thus there seems to be little reason to imagine the nostrils maintaining an anterior position, nor any reason to further restrict the dimensions of the nostril. When dipping the head down to drink, the internal naris were able to fill with water that drained into the throat whenever the skull was elevated.

A tracing of the external and internal nares in sauropods
(Fig. 2) and a simplified guess connecting the two in lateral view, shows

  1. the elevation of the external naris (pink) relative to the internal naris (blue)
  2. the spacious airway (blue) in sauropod skulls.
  3. the reduced airway proposed by Witmer (green) if skin extended the external naris to the anterior nasal basin
  4. the easy drainage of rainwater if allowed to directly enter the nostrils (pink) in sauropods (probably unimportant, but thought I’d mention it since most nostrils/nares, except whales and crocs, are anterior to lateral, not dorsal)
  5. When dipping the head down to drink, the internal naris were able to fill with water that drained into the throat whenever the lips were sealed and the skull was elevated. That is marginally different from the ostrich drinking behavior (below).
  6. Based on the ostrich example, the sauropod nostril may have extended from 1/3 to 2/3 the area of the external naris in brachiosaurs, to the entire naris in the relatively small external naris of Diplodocus (Fig. 2).

Witmer 2012 (YouTube video below)
provided an ostrich skull in which tissue labeled ‘airway’ completely filled the external naris.

Unfortunately,
the Witmer video does not show the nostril seen in an ostrich photo (Fig. 3). Confusing. That should have been somehow clarified, because the nostril is present in vivo, not in the µCT scan. Added January 22, 2019: The external naris above is the yellow patch at the far anterior tip of the naris. Thank you JB.

Figure 3. Ostrich skull compared to ostrich head with nostril appearing within the external naris.

Figure 3. Ostrich skull compared to ostrich head with nostril appearing within the external naris. The skull may belong to a younger ostrich with a higher cranium than the adult shown here. Note the nostril is about 1/3 the size of the external naris. This may be instructive considering the small head on the end of a long neck on this ostrich, comparable to the small head and long neck in sauropods.

Added January 22, 2019: The following image of a young ostrich
still does not fit the Witmer 2001 ostrich skull. Even when distorted to fit the skull (Fig. 4) the naris does not match the red patch provided for clarification. Something is wrong here. Who can help?

Figure 4. Baby ostrich naris still does not match patch from Witmer 2012 video.

Figure 4. Baby ostrich naris still does not match patch from Witmer 2012 video.

The small head on the end of a long neck
of an ostrich is analogous to the small head and long neck of sauropods when it comes to breathing and drinking. In the ostrich the nostril is one third the size of the naris and located within the naris, more or less anteriorly. Drinking would have been similarly done, with similar problems to get over, like transferring a throat-full or snout-full of water to the stomach by elevating the head and neck.

In a future post
we’ll look, from a scientist’s perspective, why scientists shy away from attempting to replicate discoveries. On the other hand, I revel in testing published hypotheses because so often they leave their work unfinished or misguided one way or another. All the loose ends need to be tidied up.

References
Witmer LM 2001. Nostril position in dinosaurs and other vertebrates and its significance for nasal function. Science 293, 850-853. PDF

4 thoughts on “Sauropod nostrils: Where were they?

  1. A couple of things.

    The maxillary basin that you are referring to is known as the subnarial fossa. It is common to most amniotes to some degree or another, and it is consistently associated with narial soft tissues.

    Sauropods did not move back their bony external naris (more accurately: bony nasal aperture [BNA], since naris literally means nostril). Instead, they expanded the size of the BNA, turning it into a giant opening on the skull. Various lizards today (e.g., varanids, iguanids) show similar expansion of the BNA.

    A primary function of the nose is in air conditioning. Olfaction is limited to a specialized section of the nose in all amniotes. The majority of the amniote nose seems to operate more for warming and humidifying air, rather than the sense of smell.

    Airways are never that spacious, and are typically very compressed throughout the majority of the nasal passage. This aids in that air conditioning effect, and will also promote faster airflow.
    Your internal naris (choana, or more accurately: fenestra exochoanalis) is too big in your sauropod reconstructions. In particular, the fenestra exochoanalis in Diplodocus would have ended at the vomer and would not have continued onto the palate as you are showing. Sauropod internal nasal passages make an S curve as they go from nostril to throat.

    The nasal drinking hypothesis is interesting, but I don’t see why it would be done. I can’t think of any nasal-drinking animals today (even elephants only siphon water into the trunk so they can shoot it into their mouths). Olfactory epithelium doesn’t usually mix well with external water (as anyone who has had pool water, or a drink get up in their nose can attest).

    Your interpretation of the airway in the WitmerLab video is a bit off. The airway does indeed fill the BNA, but that whole region is not open to the outside world. You are seeing a cast of the potential spaces that the air could go. We did include the nostril in our reconstruction. You can see it in the video, it just doesn’t pop out as well from the rest of the nasal passage. I’ve e-mailed you an image that highlights the location of the nostril in this ostrich. As you alluded to, this is indeed from a young ostrich (a 2-month-old chick), so the proportions are a little more “baby-like” than in the adult.

    As for the overall size of the nostril, that is a problem that I have looked at before, and while I can’t say for certain how much space the nostril would take up, it does seem to at least be bounded by the BNA itself. On superficial examination, it at least seems that as the BNA gets larger, the proportion taken up by the nostril becomes smaller. The real interesting question (and one that I am currently researching) is why increase the size of the BNA at all?

    • Jason, thank you and sorry for the delay. Sometimes it takes awhile for me to get to anything else if I’m studying something interesting.
      1.The sub-narial basin on the premaxilla makes sense in Camarasaurus, but goes to high extremes in Brachiosaurus and low extremes in Diplodocus. So questions arise.
      2. Drinking through the nose has always been off the table, as your examples indicate. Whenever the sauropod head is underwater, if vertical, I can only imagine internal flaps keeping the internal nares closed. Otherwise water goes in. Struthio circumvents this by drinking with the skull horizontally.
      3. Why increase the size of the BNA? I’ve wondered that myself. The converse, reducing the naris, as in diving birds, makes sense. Toucan and hornbill nares, as you know are close to the eyes, far from the beak tip. Exceptions worth considering. http://reptileevolution.com/pteroglossus.htm

  2. Weird, I’m not sure what happened to my original comment that I posted (it said it was in moderation).

    Regarding why your photo of a baby ostrich doesn’t fit our video, I strongly suspect it is the angle you are trying to contort the image into. I can assure you that it fit the μCT scans of the baby ostrich that we used. The airway segmentation was cut off around the margins of the nostril, which resulted in the shape seen in the video.

    Maybe try the adult Ostrich from Witmer and Ridgely 2008. https://people.ohio.edu/witmerl/Ostrich_3D-PDFs.htm

    Again, all of these are data based from scans of real animals.

    • No doubt. As you know, I always want to confirm any way I can. That hypothesis was well presented and documented. I still wonder about sauropods. Everything was pushed back for reasons we may never know.

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