The Diplodocus Discussion
The sauropods in the Diplodocus group have a generalized body anatomy (with their long horizontal necks) unlike any living species I know of,  so they have conclusively demonstrated they have the capacity to evolve an anatomical feature which is uniquely their own (the very long horizontal neck) and different from any existent species. And I am not aware of any convincing argument for why they have the long horizontal neck, because it poses some intriguing biomechanical challenges and no apparent advantage has been described to justify those challenges. It can't even see it's own feet as it walks, so it likely must memorize the ground as it moves forward and then calculate that a rock or hole it sees will need to be avoided about 6 to 8 steps later by the feet so far behind the head. I still marvel at how it knows what it's stepping on.
Holding anything out extended far away from the body support (the legs) in a horizontal direction is tremendously tiring, because of the leverage and gravity. The way it holds the extended neck and head outward makes me tired just looking at it, so I wonder constantly what evolutionary design circumstance caused it's evolution. It's a weird animal, pure and simple, and a biomechanical engineering miracle that still defies any complete analysis of why it came to evolve.
The paper by Lawrence M. Witmer, titled "Nostril Position in Dinosaurs and other Vertebrates and its significance for Nasal Function" does not specifically discuss the diplodocus-type dinosaurs, but the paper by Knoll, Galton, and Lopez-Antonanzas references another paper by Witmer discussing the proposed nasal anatomy of Diplodocus allowing the nasal openings to emerge on the front of the face and low close to the mouth (but not extended beyond the face into a trunk).
The second paper, "Paleoneurological evidence against a proboscis in the sauropod dinosaur Diplodocus" by Fabien Knoll, Peter M Galton, and Raquel Lopez-Antonanzas, does focus almost entirely on Diplodocus, so it will be referenced more thoroughly here. It is a fascinating paper, and certainly deserves to be read and appreciated. Both papers are described in detail, bottom of THIS PAGE.
Starting Assumptions
It does, however, seem to take certain assumptions as the basis of it's arguments. Within the framework of these assumptions, the paper seems to be correct, and so I have no disagreement with the authors in that regard. But the discussion should also put into perspective the alternatives to those basic assumptions.
One assumption is a  trunk on a Dilpodocus, if existing, would have the anatomical complexity of the elephant trunk, and thus need a greatly enlarged facial nerve to feed it, and the study refutes that assumption with its detailed description of the brain and nerve structure of the Diplodocus. But what if the Diplodocus had a prehensile appendage (a trunk) of far less biomechanical complexity than the elephant's trunk, and had an anatomical design of it's own unique configuration, non-mammalian? Prehensile hydrostatic organs are not exclusive to mammals, and convergent evolution has resulted in prehensile hydrostatic organs in many invertebrates. Indeed, the tongue itself, in any animal form, could be considered a simplified hydrostatic organ, often performing food gathering functions.
The fact that the Diplodocus body design is unique for a terrestrial animal, and has no existent comparative example, establishes the president that this animal evolves structures unique to it's needs, dissimilar from any existing mammal. So a second uniquely evolved feature of anatomy, especially one attached to and thus functionally relevant to the known unique anatomical feature (the long horizontal neck), does not seem to me an unreasonable assumption to hypothesize.
So I do not find it unreasonable to speculate that they may also have evolved a trunk-like appendage in their own unique anatomical version, perhaps mechanically simpler than an elephant's trunk (and thus not needing as large a facial nerve) but still capable of assisting the animal in food gathering.
When I think things through, I like to outline (and sometimes illustrate) the options I think are under consideration.
On Diplodocus, the options I see are:
1. The nostril openings of the skin are right above the nostril openings of the skull, the old fashioned design illustrated in most dinosaur books.
2. The nostril openings are as Witmer described, forward and low close to the mouth, while the nostril openings in the skull are far higher and further back up on the skull top. This novel concept requires a breathing passage of flesh and tissue outside the skull, which would tend to be a vulnerable place when most animals have the nasal passage inside the skull. So I would ask, why is it this way, if merely to get the nostril openings of the skin in the right place, implying the nostril openings in the skull are in the "wrong place"?
3. The nostril openings are on the tip of some modest prehensile appendage and the nostril openings of the skull are higher and back to support muscle attachments to curl the appendage into the mouth. Everything's in the "right place" but studies of Diplodocus skulls seem to give no indication of muscle attachment for such proposed structures, an obvious weakness to this argument.
4. The nostril openings are on a prehensile proboscis as muscular and complex in sensory tactile perceptions as an elephant's trunk. The absence of a large facial nerve seems fairly conclusive that this option can be discounted.
The Chart below illustrates the four described options.
Are the Nostril Openings in the Skull in "The Wrong Place"?
I keep returning to the most fundamental issue, that if there's no trunk or prehensile appendage, why are the nostril openings of the skull in "the wrong place" They should be in "the right place", supporting Witmer's analysis of forward and lower close to the mouth as the preferred or expected position. And I feel the long horizontal neck design, so biomechanically curious, is a second and related issue which is unresolved, and which has direct relevance to the simplified trunk hypothesis, because it supports a potential for this animal to evolve unique configurations of anatomy unlike any existing species.
So, personally, I remain of the opinion this issue is not settled or conclusively proven, and most options (especially 2 and 3, above) are on the table, while the option of a proboscis of muscular and sensory complexity equaling an elephant is reasonably discounted. The first option above I think is refuted by the simple biomechanical study of what noses do, and how they should be structured to accomplish those purposes. This analysis of what good noses do (in my original notes on this site), and compared to the old concept of Diplodocus with nostrils atop the head, has not been refuted by either of these papers, and Witmer;s paper tends to concur with my notes, in terms of the superiority of nostril openings being low toward the mouth and as far forward as possible on the face.
It might also be noted that in the conclusion of the paper by Fabien Knoll, Peter M Galton, and Raquel Lopez-Antonanzas, the authors stated that "We conclude that the anatomy of the head and the endocranial cavity unambiguously indicates that a proboscis (at least a large muscular one) was not present in Dilpodocus. This also holds true for Camarasaurus, which had necessarily a small facial nerve, but not for Brachiosaurus (although the presence of a proboscis in the latter genus has not been convincingly argued, this hypothesis remains)"
So they conclude a "large, muscular proboscis" (my option #4 above)was not on Diplodocus, but they did not discount a smaller and simplified variation of a trunk-like proboscis (my option #3, above), if it contained far fewer muscles and less tactile sensation nerves than an elephant's trunk, and thus needing a much smaller facial nerve (presumably).
And they do not discount at all the prospect that Brachiosaurus had one, as I illustrated and described in my other notes and image gallery. And this leads me to the next commentary, on the Brachiosaurus.

Sauropod's Trunk First Notes      Updated Notes      Brachiosaurus Discussion     Models Illustrated