Haplocanthosaurus sacral 4 remix

August 22, 2012

No time for anything new, so here’s a post built from parts of other, older posts.

The fourth sacral centrum of Haplocanthosaurus CM 879, in left and right lateral view. This is part of the original color version of Wedel (2009: figure 8), from this page. (Yes, I know I need to get around to posting the full-color versions of those figures. It’s on my To Do list.)

Note the big invasive fossa on the right side of the centrum. The left side is waisted (narrower at the middle than the ends) like most vertebrae of most animals, but has no distinct fossa on lateral face of the centrum. What’s up with that? Here’s an explanation from an old post (about another sauropod) that still fits:

Now, this asymmetry is also weird, but it’s expected weirdness. Pneumaticity seems to just be inherently variable, whether we’re talking about human sinuses or the facial air sacs of whales or the vertebrae of chickens. It appears that the form of pneumatic features is entirely determined by local tissue interactions, with little or no genetic control of the specific form. Think of it this way: genes prescribe certain developmental events, and those events bring tissues into contact–such as pneumatic epithelium and bone. The morphology of the bone arises out of that interaction, and each interaction of bone and pneumatic epithelium has the potential to produce something new. In this case, the diverticula on the left side of the vertebral column come from the lungs or air sacs on the left, and those on the right side come from the lungs or airs sacs on the right, so it’s really two sets of diverticula contacting the bone independently. The wonder, then, is not that pneumatic bones are so variable, but that we see any regularities at all.

5 Responses to “Haplocanthosaurus sacral 4 remix”

  1. Meaning, “shape” of the fossa or foramen, or even extent of invasion, is not a useful phylogenetic feature.

    unless it is.

  2. Mike Taylor Says:

    Matt’s written pretty extensively about what character and are not reliably diagnostic of pneumaticity. I’ll leave it to him to summarise. (In fact, Matt, it would make a pretty good tutorial: How To Recognise A Pneumatic Foramen.)

  3. I think I can see his reasoning, and have read the literature on this. Sepcifically, the clear laminae surrounding the right fossa. Problematically, both fossae appear equally deep, and were probably equally invaginated by a diverticula. It has been my understanding, from both the developmental arguments controlling how we think about these things, that it is how further the diverticulae invades the bone structure that produces laminae, not that it does so.

    The fossa itself most likely is caused by a diverticula, regardless of the lack of strong laminar margins, sharp edges, etc., just as unequal left-right size and position of diverticulae produce different laminae, as discussed in Wilson’s recent “laminar capture” paper. When, instead of a sharp lamina, we find a round margin or feature — especially between two apparently “interrupted” laminae — we can presume that a diverticulum leaves more than just teeth features in the bone, as indeed it must to produce the fossae in the first place. Perhaps I am preaching to the choir, and this isn’t being debated, that it is not actually the argument that only ONE of these fossae is pneumatic in nature, and the other ISN’T? Perhaps instead the argument is that the bone of the right side hasn’t grown around the diverticula, and thus doesn’t produce the lamina, but does on the right side; but this being the definition for “diverticulum”.

    Mind you, I am going by sources that Matt should agree with, given Wedel et al., 2012:

    By working frommodern bird dissections Britt (1993) proposed a list of five osteological correlates of postcranial skeletal pneumaticity (PSP) in modern birds. These were: large foramina, fossae with crenulate texture, thin outer walls, broad tracks with smooth or crenulated texture, internal chambers communicating with the exterior via foramina. However, most of these features are not absolutely diagnostic. Thus diagnosis of PSP using these criteria is an exercise in probability: the greater the number of correlates present, the more likely the presence of PSP. Only foramina that open into internal chambers have been accepted as incontrovertible evidence for the presence of PSP (O’Connor 2006). However it is not necessary for PSP to take this form. Some pneumatic fossae of modern birds are simple shallow fossae that lack sharp rims (e.g., Wedel 2007: fig. 9). This fossa is taken to be pneumatic because: (i) it occupies a known site of pneumatic invasion of a diverticulum from the cervical air sac (see O’Connor and Claessens 2005: fig. 1); (ii) its counterpart on the right side of the same bone is a fully−diagnostic pneumatic foramen; (iii) no other soft tissue structure is known to excavate the dorsal surface of avian cervical vertebrae.
    (emphasis added)

  4. Excuse me, make that Yates et al., for Yates, Wedel and Bonnan, in Acta Pal Pol 57(1):85-100.

  5. […] pneumatic features, like the asymmetric fossae in this sacral of Haplocanthosaurus (featured here), in the tails of Giraffatitan and Apatosaurus (from Wedel and Taylor 2013b), and in the […]

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: