Whassup with your segmented lamina, Uberabatitan ribeiroi?
July 30, 2008
One of the newest sauropods (if not the newest, in terms of publication date) is the Brazilian titanosaur Uberabatitan ribeiroi Salgado & Carvalho, 2008 from the Maastrichtian Marília Formation. Uberabatitan scores reasonably high in the ‘quality and quantity of vertebral information preserved’ stakes, with a few nicely preserved cervicals known, some complete and partial dorsals, and an assortment of caudals (Salgado & Carvalho 2008). As is the case with virtually all titanosaurs that include good vertebrae, several characters of the vertebrae are diagnostic. And one of these characters interests me in particular: the podls (postzygodiapophyseal laminae) in the cervicals are described as being ‘segmented’, consisting of separate diapophyseal and zygapophyseal segments. The zygapophyseal segment extends anterodorsally over the diapophyseal segment, and there appears to be a space of flattened bone in between the two segments (you should be able to see this in A in the figure here; the zygapophyseal segment is labelled podl (z) and the diapophyseal segment is labelled podl (d). This is Text-fig. 5 from Salgado & Carvalho (2008)). This is all a bit weird and hasn’t been reported in a sauropod before, and what’s really weird is that the presence of an ‘incomplete’ lamina would seem to fundamentally contradict the reason for a lamina to exist in the first place.
Two primary hypotheses have been entertained as explanations for the presence of laminae. One is that laminae serve a mechanical role, and somehow help to support the weight of the rest of the vertebra (and its associated soft tissues) by being aligned along the primary axes of stress. At the same time, their ‘supporting’ function may have allowed bone in other areas of the vertebra to become reduced, thereby lightening the whole structure. As Wilson (1999) stated, this hypothesis has never really been tested and the fact that laminae are present in saurischians with small, lightweight vertebrae (like birds and other coelurosaurs) counts against a strictly mechnical role for the laminae. However, the fact that laminae increase in complexity and number as sauropods become longer-necked might suggest at least some supportive role for them.
However, laminae also seem to play a crucial role in pneumaticity, in that they partition the different air sacs. Actually, as Witmer (1997) noted, it may be that, as air sacs develop during growth, they act as opportunistic pneumatising machines, resorbing as much bone as is possible and hence leaving the laminae behind as key supportive elements. I don’t really fancy getting involved in an intricate discussion of evo-devo (one of my least favourite subjects, sorry), so will stop there on that line of inquiry but – whatever the developmental process – the end result is that laminae bound air sacs. Here’s where we come back to Uberabatitan. If the laminae form the bony ‘boundaries’ that ‘contain’ the air sacs, what the hell is going on when a lamina is split into two and has a big gap in the middle? Maybe the air sac here (it would have been bound ventrally by the posterior centrodiapophyseal lamina) was reduced, and only tucked away under the anterior, diapophyseal part of the podl. Maybe the air sac was absent: to confirm this, you’d need detailed information on the medial wall of that part of the vertebra bound ventrally by the pcdl and dorsally by the podl. Whatever, we seem to have something else new, and weird going on here. Your thoughts would be appreciated.
References
- Salgado, L. & Carvalho, I. S. 2008. Uberabatitan riberoi, a new titanosaur from the Marília Formation (Bauru Group, Upper Cretaceous), Minas Gerais, Brazil. Palaeontology 51, 881-901.
- Wilson, J. A. 1999. A nomenclature for vertebral laminae in sauropods and other saurischian dinosaurs. Journal of Vertebrate Paleontology 19, 639-653.
- Witmer, L. M. 1997. The evolution of the antorbital cavity of archosaurs: a study in soft-tissue reconstruction in the fossil record with an analysis of the function of pneumaticity. Journal of Vertebrate Paleontology 17 (Supplement to No.1), pp. 73.
Sauropod Vertebra Picture of the Week 
July 31, 2008 at 1:32 am
If, in fact, the air sacs expanded into space occupied by less-stressed bone, then the laminae would have been the minimal remaining bone necessary to provide structural support. In other words, there is no need for laminae to keep air sacs separate; rather, the presence of a lamina indicates a manifest structural need to preserve some actual bone at that spot. A gap in a lamina, then, would just mean that the stresses acted only on/through the remaining bits, leaving the bone that had been at the gap available to be scavenged. In the gap the sacs would abut comfortably against one another.
This developmental model makes me feel much better about the complexity of the vertebrae. It worried me to imagine all that structure encoded genetically. Here we see instead the complexity arising as a growth process responding to each vertebra’s immediately environmental conditions. Genes, rather than coding every detail directly, code the initial, undifferentiated shape, and the aggressiveness of the air sac expansion.
However, this also seems to call into question the diagnostisticity of vertebral features. Some structural details might depend more on an individual animal’s feeding strategies, and thus its vegetative environment, than its lineage.
July 31, 2008 at 5:00 am
It worried me to imagine all that structure encoded genetically. Here we see instead the complexity arising as a growth process responding to each vertebra’s immediately environmental conditions. Genes, rather than coding every detail directly, code the initial, undifferentiated shape, and the aggressiveness of the air sac expansion.
From everything we know about the development and morphology of pneumatic bones–all of them, in everything–this seems to be the case.
However, this also seems to call into question the diagnosticity of vertebral features. Some structural details might depend more on an individual animal’s feeding strategies, and thus its vegetative environment, than its lineage.
Yes, absolutely. It means that characters relating to pneumaticity are probably a lot more plastic and less reliable than other characters. It also means that if you see some weird fossa or lamina on a vertebra, it’s hard to know how important it is unless it’s present on more than one vert. Or even on the other side of the same vert, as Xenoposeidon shows.
That’s not a dig at any of the current generation of cladistic analyses. There is a whole load of pneumaticity-related variation that hasn’t been coded. That used to bug me. Now I think it’s probably pretty smart.
Compounding the problem is that we don’t have a great handle on how variable pneumatic characters are in extant vertebrates. The consensus is basically “pretty darn variable”, but except for human sinuses, postcranial pneumaticity in chickens, and skull-roof pneumatization in some birds (which is used to assess ontogenetic age), I don’t know of any published studies that focus on intraspecific variation in pneumatization. So when we find variability in the fossil record, we have no baseline against which to weigh it.
Which is why I keep telling people that if they’re interested in this stuff, there are mountains and mountains of things to be done. All of my work is like an underpowered laser slowly melting a tiny tunnel through an iceberg of ignorance. There may be immense truths lurking just a little way off my path, so I fan the beam around as much as I can and hope something lights up. But there will always be a huge element of crapshoot. Same as the rest of science, I suppose.
Stay tuned!
July 31, 2008 at 7:01 am
You’re like a boy playing in a road cut, diverting yourself in now and then finding a smoother condyle or a prettier postzygopophysis than ordinary, whilst the great quarry of truth lies all undiscovered before you.
August 1, 2008 at 5:50 pm
Dumb noob question: If air sacs are “opportunistic pneumatising machines, resorbing as much bone as is possible,” how do they “know” when they’ve resorbed as much bone as possible, without going overboard? How did sauropods avoid being pneumatized to death by the aggressive expansion of their own air sacs?
Or maybe they eventually would die of overpneumatization if they lived long enough, like an elephant’s fate is sealed by their dentition. Big old dinosaurs fading away into air, the “ghosts” of the natural world… :o
August 1, 2008 at 6:54 pm
Hi Brad,
That’s not a dumb question at all! In fact, it’s crucial.
Pneumatization seems to be held in check by the growth of new bone, in response to biomechanical stress. Here’s an example: when some elderly people lose their teeth and go on a soft or liquid diet, the maxillae are no longer exposed to the stress of chewing, and the maxillary sinuses expand to the point where the walls of the maxillae are extremely thin. Witmer (1997) suggested that the form of a pneumatic bone can be viewed as the outcome of a competition between bone growing in response to biomechanical stress, and pneumatic diverticula opportunistically proliferating.
Also, I don’t know of any examples of things being pneumatized to death, but it is not impossible. The mastoid sinuses behind your ears are especially opportunistic, and there are cases of “pneumatocoels” where the pneumatic diverticula basically burrow out of the bone and create big air sacs in the surrounding tissue. If these are between the skin and the skull, the result is a big air-filled bulge, which is unsightly but not that dangerous. Intracranial pneumatocoels are worse; these happen with the diverticula invade the braincase and create an air-filled bubble between the brain and the skull. These can put pressure on the brain and could conceivably kill a person if left untreated.
I think that such developmental accidents must have been important in the evolution of pneumaticity. The inflatable air sac in the neck of the emu (pictures here) almost certainly started as a subcutaneous laryngeal pneumatocoel in a single individual, which was passed on to its descendants. The line between “developmental accident” and “evolutionary novelty” is probably defined only by whether the accident is harmful on one hand, or neutral or even beneficial on the other.
August 1, 2008 at 7:22 pm
Thanks, that does make a lot more sense when you add in the consideration of bone growing in response to stress!
August 2, 2008 at 1:04 pm
Those are weird things to know, about invasive/expansive sinuses and pneumatocoels in humans.
Thanks for widening my horizons (and eyes) thereby!
I guess parents could use that as another incentive to motivate children: “Chew your food thoroughly, don’t gulp it, or you know what? Your maxilli will grow thin and your sinuses will take over your head.”
Or maybe better not.
Are there any exercises we can do, to strengthen the relevant bones to combat that mastoid-sinus lust for lebensraum? Ear-waggling, head-swaying, etc?
Something to look out for, in future workout videos:
“And now, How to keep those invasive pneumatic cavities in check”.
August 6, 2008 at 4:58 pm
A rather boring suggestion that really needs to be made. Could it be that the so-called zygopophyseal part of the PODL is in fact the entire PODL, and the so-called diapophyseal part is a non-homologous accessory post-diapophyseal lamina? Or indeed a good, old-fashioned PCDL, like mother used to make? The figure is not really good enough to tell us one way or the other.
August 6, 2008 at 8:16 pm
A counter-suggestion that I think is at least as likely: what we’ve been calling the PODL is in fact two developmentally separate things, a zygopophyseal part and diapophyseal part, and we’ve never recognized it before because they (almost) always fuse into a single PODL.
If you look at basal archosaurs, crocs, prosauropods, and so on, you see a whole bunch of not-quite laminae that start out well enough and then just disappear before they ever make it to another apophysis. There could be a more-than-additive effect here, where many possible laminae could develop but the ones that do become the primary stress pipelines so they get strengthened while the other proto-laminae (which might not even rise to the level of observability on the outside of the bone) are eaten away by pneumatic diverticula because they’re not bearing the stress required to trigger the bone growth that would maintain them. And in this survival-of-the-most-stressed, it just happens that 99% of the time the PO part of the PODL meets up with the D part, and now they are bound together into a larger structure that further concentrates the stress and further canalizes the morphology.
The reason that the laminae start to drop in out in titanosaurs is that in addition to being devoured from without by pneumatic diverticula, the vertebrae are inflated from within. I don’t think the laminae are really going away, I think they’re becoming so fat and so well blended with the corpus of the vertebra that we don’t recognize them as laminae anymore. In the case documented in this post, I’ll bet that the internal septa between the camellae are bearing enough of the stress that the external lamina-looking part can get partly eaten away by the external diverticula, to the point that the PODL is divided back into its separate constituents.
How much is a hypothesis worth, anyway? Is it still just the $0.02? I don’t mean to puff myself up, but I think that one is worth at least $0.04.
Further discussion is not just welcome, it’s badly needed!
August 8, 2008 at 11:22 pm
To help understand this, we need a computer model of bone growth and air sac proliferation, with all of the genetic, developmental and realistic in vivo stresses factored in (also: growth curves, seasonal nutrient availability, and sometime calcium depletion for egg-formation).
Once someone has programmed that model, we could run it tweaking the variables and see what the laminae come out like.
A nice weekend project for someone…
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