Another mystery: embossed laminae and “unfossae”
December 7, 2009
Broadly speaking, pneumatic sauropod vertebrae come in two flavors. In more primitive, camerate vertebrae, modeled here by Haplocanthosaurus, the centrum is a round-ended I-beam and the neural arch is composed of intersecting flat plates of bone called laminae (lam above; fos = fossa, nc = neural canal, ncs = neurocentral suture; Ye Olde Tyme vert pic from Hatcher 1903).
In more derived, camellate vertebrae, the centrum and neural arch are both honeycombed with many small air spaces. This inflated-looking morphology is very similar to that seen in birds, like the turkey we recently discussed. The fossae and foramina on the outside tend to be smaller and more numerous than in camerate vertebrae, as shown here in a titanosauriform axis from India (Figure 3 from Wilson and Mohabey 2006). The green arrows show that the fossae visible on the external surface are excavations or depressions into the honeycombed internal structure of the bone.
External fossae on bones can house many different soft tissues, including muscles, pads of fat or cartilage, and pneumatic diverticula (O’Connor 2006). Pneumatic fossae are often strongly lipped and internally subdivided and may contain pneumatic foramina, which makes them easier to diagnose (but they may also be simple, smooth, and “blind”, which makes them harder to diagnose as pneumatic). But in all of these cases we are usually talking about the same thing: a visible excavation into a corpus of bony tissue, which may have marrow spaces inside if it is apneumatic, or air spaces inside if it is pneumatic (the corpus of bone, not the dent). That’s probably how most of us think about fossae, and it would hardly need to be explained…except that sometimes, something much weirder happens.
Consider this cervical of Brachiosaurus (this is BYU 12866, from Dry Mesa, Colorado). Brachiosaurus and Giraffatitan have an in-between form of vertebral architecture that my colleagues and I have called semicamellate (Wedel et al. 2000); the centrum does have large simple chambers (camerae), but smaller, thin-walled camellae are also variably present, especially along the midline of the vertebra and in the ends of the centrum. As in Haplocanthosaurus, the neural arch is composed of intersecting plates of bone; unlike Haplocanthosaurus, these laminae are not flat or smooth but are instead highly sculpted with lots of small fossae. Janensch (1950) called these “Aussenkaverne”, or accessory outside cavities, because and they are smaller and more variable than the large fossae and foramina that invade the centrum.
And that’s the weird thing. As the red arrows in the above image show, the “Aussenkaverne” are not excavations or depressions into anything, except the space on the other side of the lamina (which in life would have been occupied by another diverticulum). The neural arches of Brachiosaurus and Giraffatitan are not excavated by fossae, they’re embossed, like corporate business cards and fancy napkins.
What’s up with that!? We tend to think of pneumaticity as reducing the mass of the affected elements, but the shortest distance between two vertebral landmarks is a smooth lamina. These embossed laminae actually require slightly more bony material than smooth ones would.
As you can see above, the outer edges of the laminae are thick but the bone everywhere else is very thin. Maybe, like the median septa in pneumatic sauropod vertebrae, the thin bone everywhere except the edges of the laminae was just not loaded very much or very often, and was therefore free to get pushed around by the diverticula on either side, in the sense of being continually and quasi-randomly remodeled into shapes that don’t strike us as being very mechanically efficient. But also like the median septa, the thin parts of the laminae are only rarely perforated (but it does happen), for possible (read: arm-wavy) reasons discussed in the recent FEA post. And maybe the amount of extra bone involved in making embossed laminae versus smooth ones was negligible even by the very light standards of sauropod vertebrae.
Another question: since these thin sheets of bone were sandwiched in between two sets of diverticula, why are the “unfossae” always embossed into them, in the medial or inferior direction? Why don’t any of them pop out laterally or dorsally, looking like domes or bubbles instead of holes, like Mount Fist-of-God from Larry Niven’s Ringworld? Did the developmental program get accustomed to making fossae that went down and into a corpus of bone, and just kept on with business as usual even when there was no corpus of bone to excavate into? I’m only half joking.
I don’t have good answers for any of these questions. I scanned this vert a decade ago and I only noticed how weird the “unfossae” were a few months ago. I’m putting all this here because “Hey, look at this weird thing that I can only wave my arms about” is not a great basis for a peer-reviewed paper, and because I’d like your thoughts on what might be going on.
In Other News
The Discovery Channel’s Clash of the Dinosaurs premiered last night. I would have given you a heads up, except that I didn’t get one myself. I only discovered it was on because of a Facebook posting (thanks, folks!).
COTD is intended to be the replacement, a decade on, for Walking With Dinosaurs. I’m happy to report that one of the featured critters is Sauroposeidon. I grabbed a couple of frames from the clips posted here.
I haven’t seen the series yet, because I don’t have cable. But I’m hoping to catch it at a friend’s place next Sunday night, Dec. 13, when the entire series will be shown again. With any luck, I’ll have more news next week.
Finally, I got to do an interview at Paw-Talk, a forum for all things animal. I’m very happy with how it turned out, so thanks to Ava for making it happen. While you’re over there, have a look around, there’s plenty of good stuff. Brian Switek, whom you hopefully know from this and this, is a contributor; check out his latest here.
- Hatcher, J.B. 1903. Osteology of Haplocanthosaurus, with a description of a new species, and remarks on the probable habits of the Sauropoda, and the age and origin of Atlantosaurus beds. Memoirs of the Carnegie Museum 2:1–72.
- Janensch, W. 1950. Die Wirbelsaule von Brachiosaurus brancai. Palaeontographica (Suppl. 7) 3:27-93.
- O’Connor, P.M. 2006. Postcranial pneumaticity: an evaluation of soft-tissue influences on the postcranial skeleton and the reconstruction of pulmonary anatomy in archosaurs. Journal of Morphology 267:1199-1226.
- Wedel, Mathew J., Richard L. Cifelli and R. Kent Sanders. 2000. Osteology, paleobiology, and relationships of the sauropod dinosaur Sauroposeidon. Acta Palaeontologica Polonica 45(4): 343-388.
- Wilson, J. A. and Mohabey, D. M. 2006. A titanosauriform axis from the Lameta Formation (Upper Cretaceous: Maastrichtian) of central India. Journal of Vertebrate Paleontology 26:471–479.