Or, how a single lateral fossa becomes two foramina: through a finely graded series of intermediate forms. Darwin would approve. The ‘oblique lamina’ that separates the paired lateral foramina in C6 starts is absent in C2, but C3 through C5 show how it grows outward from the median septum. How do I know it grows outward, instead of being left behind during the pneumatization of the more posterior cervicals? Because with very few exceptions, all neosauropod cervicals start out with a single lateral fossa on each side, as illustrated in this post. But many of them end up with two or more foramina. Diplodocus is a nice example of this (from Hatcher 1901: plate 3):

I should clarify that the vertebrae above show that character transformation in this individual, at this point in its ontogeny. The vertebrae of CM 555 are about two-thirds the size of those of CM 3018, the holotype of A. louisae. In CM 3018, even C4 and C5 have completely divided lateral fossae, corresponding to the condition in C6 of CM 555.

As Mike and I discussed in our 2013 neural spine bifurcation paper, isolated sauropod cervicals require cautious interpretation because the morphology of the vertebrae changes so much along the series. The simple morphology of anterior cervicals reflects both earlier ontogenetic stages and more primitive character states. As Mike says, in sauropod necks, serial position recapitulates both ontogeny and phylogeny. So if you have a complete series, you can do something pretty cool: see the intermediate stages by which simple structures become complex.

If you’re thinking this might have something to do with my impending SVPCA poster, you’re right. Here’s the abstract.

For more on serially increasing complexity in sauropodomorph cervicals, see this post.

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Just got the APP new issue alert and there are three papers that I think readers of this blog will find particularly interesting:

That’s all for now, just popping in to let people know about these things.

Anterior view. Dorsal is to the upper right. The neural spine and left transverse process are missing.

Here’s a closeup of the condyle. The outer layer of cortical bone is gone, allowing a glimpse of the pneumatic chambers inside the vert. The erosion of the condyle was probably inflicted post-excavation by relatively unskilled WPA workers, whose prep tools were limited to chisels, penknives, and sandpaper. Because the bones from the Kenton localities are roughly the same color as the matrix, the preparators sometimes did not realize that they were sanding into the bones until the internal structure was revealed. Bad for the completeness of this specimen, but good for pneumaticity junkies like me, because this baby is too big to be scanned by any but the largest industrial CT machines.

For other posts on the giant Oklahoma apatosaur, see:

Here’s my face.

I went to the dentists’ office recently for a regular checkup and cleaning, and when my dentist learned that I taught human anatomy, he volunteered to send me a high-res copy of my panoramic x-ray. I couldn’t think of any plausible scenario wherein someone could use it for evil, and it has lots of cool stuff in it besides teeth, so decided to post it so I could yakk about it.

First things first: my teeth are in pretty good shape. I had to have my wisdom teeth (3rd molars) pulled back in 2009, and my upper 1st molar on the left has a root canal and a porcelain crown, which stands out bright white on the radiograph. Everyone else is present and looking good. If it’s been a while since you’ve covered this, the full human dentition consists of 2 incisors, 1 canine, 2 premolars, and 3 molars on each side, top and bottom, for a total of 32 teeth. Because I’ve had all four 3rd molars removed, I’m down to 28.

I could go on and on about the cool stuff in this image. Here are 12 things that stand out:

  1. The mandibular condyle, which is the articular end of the mandible that fits into the mandibular fossa, a shallow socket on the inferior surface of the temporal bone, to form the temporomandibular joint (TMJ). There’s an articular disk made of fibrocartilage inside the joint, which separates it into two fluid-filled spaces, one against the condyle and one against the fossa. This allows us to do all kinds of wacky stuff with our lower jaws besides simply opening and closing them, such as slide the jaw fore and aft or side to side. This is a strong contrast to most carnivores, which bite down hard and therefore need a jaw joint that works as a pure hinge. See this post for pictures and discussion of the jaw joint in a bear skull.
  2. The coronoid process of the mandible, which is a muscle attachment site. A few fibers of the masseter and buccinator muscles can encroach onto the coronoid process, but mostly it is buried in the temporalis, one of the primary jaw-closing muscles. Put your fingers on the side of your head a little above and in front of your ear and bite down. That muscle you feel bulging outward is the temporalis. Back in the 1960s, Melvin Moss (1968) discovered that if he removed the temporalis muscles from newborn rats, the coronoid processes would fail to develop. Moss’s ambition was to discover the quanta of anatomy, which in his view were “functional matrices” – finite sets of soft tissues related by development and function, which might contain “skeletal units” that grew because of the morphogenetic demands of the functional matrices. His tagline was, “Functional matrices evolve, skeletal units respond”. Not all of Moss’s ideas have aged well in light of what we now know about the genetic underpinnings of skeletal development, but he wasn’t completely wrong, either, and functional matrix theory is still an interesting and frequently productive way to think about the interrelationships of bones and soft tissues. For more horrifying/enlightening Moss experiments on baby rats, see this post.
  3. The mandibular angle, which is another muscle attachment. The medial pterygoid muscle attaches to the medial surface, and the masseter attaches laterally. You can feel this, too, by putting your fingers over your mandibular angle and biting down – that’s the masseter you feel bulging outward. Note that the angle flares downward and outward on either side of my jaw. This flaring of the angle tends to be more pronounced in males than in females, and it is one of many features that forensic anthropologists (like the one I belong to) take into account when attempting to determine biological sex from human skeletal remains. Like most sexually dimorphic features of the skeleton, this is a tendency along a spectrum of variation rather than a binary yes/no thing. There are women with flared jaw angles (Courtney Thorne-Smith, probably) and men with slender mandibles, so you wouldn’t want to sex a skeleton by that feature alone.
  4. The mandibular canal, a tubular channel through the mandible that houses the inferior alveolar artery, vein, and nerve. This neurovascular bundle provides innervation and blood supply to the tooth-bearing part of the mandible and to the teeth themselves, and emerges through the mental foramen to provide sensory innervation and blood supply to the chin.
  5. The upper surface of the hard palate, formed by the palatine process of the maxilla anteriorly and by the palatine bones posteriorly. The palate is the roof of the mouth and the floor of the nasal airways.
  6. The median septum of the nasal cavity, formed by cartilage anteriorly, the perpendicular plate of the ethmoid bone superiorly, and the vomer posteriorly and inferiorly.
  7. The blue lines are the inferior margins of my maxillary sinuses – air-filled spaces created when pneumatic diverticula of the nasal cavity hollow out the maxillae. You have these, too, as well as air spaces in your frontal, ethmoid, sphenoid, and temporal bones. It looks like many of the roots of my upper molars stick up into my maxillary sinuses. This is not an illusion, as shown below.
  8. When I had the root canal on my left upper 2nd molar, the endodontist filled the pulp cavities of the tooth roots with gutta-percha, a rigid natural latex made from the sap of the tree Palaquium gutta. Gutta-percha is bioinert, so it makes a good filling material (it was also used to insulate transoceanic telegraph cables), and it’s radiopaque, which allows endodontists to confirm that the cavities have been filled completely. The other teeth show the typical structure of a dense enamel crown, less dense dentine forming the bulk of the tooth, and radiolucent pulp cavities containing blood vessels and nerves.
  9. This is the rubber bit I gripped with my incisors to keep my teeth apart and my head motionless while the CT machine rotated around me to make the scan. Not that cool in a science sense, but I figured it deserved a label.
  10. Note that the roots of the canines go farther into the jaws than those of the other teeth. This is true for all four canines, it’s just easiest to see with this one. This is a pretty standard mammalian thing, for taxa that still have canines – they tend to be big and mechanically important, so they have deep roots. Even though our canines are absolutely and proportionally much smaller than those in the other great apes, we can still see traces of their earlier importance, like these deep roots.
  11. In places you can see the trabecular internal structure of my mandible clearly. As someone who geeks out pretty much anytime I get a look inside a bone, this tickled me.
  12. The remains of an alveolus or tooth socket. I had my 3rd molars out almost a decade ago, and by now the sockets will have mostly filled in with new trabecular bone. But you can still see the ghostly outline of at least this one – a sort of morphogenetic trace fossil buried inside my mandible. I assume that in another decade or two this will have disappeared through regular bone remodeling.

Here’s a closeup of my left upper 2nd premolar and first two (and only remaining) molars. The gutta-percha filling the pulp cavities of the three roots of the 1st molar is obvious. The disparity in root length is mostly illusory – this was an oblique shot and the two ‘short’ roots are foreshortened.

Here’s the same image with the roots of the 2nd molar traced in pink, and the inferior margin of the maxillary sinus traced in blue. It’s not that uncommon for upper molar roots to stick up into the maxillary sinuses. That was true of my 3rd molars as well, and when I had them taken out, the endodontist had to put stitches into my gums to close the holes. Otherwise I would have had open connections between my oral cavity and maxillary sinuses, which would have sucked and been dangerous. Nasal mucus in the maxillary sinuses could have drained into my mouth, and food I was chewing could have been forced up into the sinuses, where it would have decomposed and caused a truly vile sinus infection.

In a developmental sense, it’s not that the roots of the teeth grow upward into the sinuses, it’s that the sinuses grow downward, eroding the bone around the roots of the teeth. This happens well after the teeth are done forming – the sinuses continue to expand as long as the skull is growing, and they retain the potential to remodel the surrounding bone for as long as we live. Even in cases like mine where the roots of the molars stick up into the sinuses, the tooth roots are still covered by soft tissue, including branches of the superior alveolar artery, vein, and nerve that enter the pulp cavities of the tooth roots through foramina at their tips.

If you ask your dentist for copies of your own dental x-rays, you’ll probably get them. If you do, have fun exploring the weird territory inside your head.

Reference

  • Moss, M. L. (1968). A theoretical analysis of the functional matrix. Acta Biotheoretica, 18(1), 195-202.
jvp-fig-12

Fig. 14. Vertebrae of Pleurocoelus and other juvenile sauropods. in right lateral view. A-C. Cervical vertebrae. A. Pleurocoelus nanus (USNM 5678, redrawn fromLull1911b: pl. 15). B. Apatosaurus sp. (OMNH 1251, redrawn from Carpenter &McIntosh 1994: fig. 17.1). C. Camarasaurus sp. (CM 578, redrawn from Carpenter & McIntosh 1994: fig. 17.1). D-G. Dorsal vertebrae. D. Pleurocoelus nanus (USNM 4968, re- drawn from Lull 1911b: pl. 15). E. Eucamerotus foxi (BMNH R2524, redrawn from Blows 1995: fig. 2). F. Dorsal vertebra referred to Pleurocoelus sp. (UMNH VP900, redrawn from DeCourten 1991: fig. 6). G. Apatosaurus sp. (OMNH 1217, redrawn from Carpenter & McIntosh 1994: fig. 17.2). H-I. Sacral vertebrae. H. Pleurocoelus nanus (USNM 4946, redrawn from Lull 1911b: pl. 15). I. Camarasaurus sp. (CM 578, redrawn from Carpenter & McIntosh 1994: fig. 17.2). In general, vertebrae of juvenile sauropods are characterized by large pneumatic fossae, so this feature is not autapomorphic for Pleurocoelus and is not diagnostic at the genus, or even family, level. Scale bars are 10 cm. (Wedel et al. 2000b: fig. 14)

The question of whether sauropod cervicals got longer through ontogeny came up in the comment thread on Mike’s “How horrifying was the neck of Barosaurus?” post, and rather than bury this as a comment, I’m promoting it to a post of its own.

The short answer is, yeah, in most sauropods, and maybe all, the cervical vertebrae did lengthen over ontogeny. This is obvious from looking at the vertebrae of very young (dog-sized) sauropods and comparing them to those of adults. If you want it quantified for two well-known taxa, fortunately that work was published 16 years ago – I ran the numbers for Apatosaurus and Camarasaurus to see if it was plausible for Sauroposeidon to be synonymous with Pleurocoelus, which was a real concern back in the late ’90s (the answer is a resounding ‘no’). From Wedel et al. (2000b: pp. 368-369):

Despite the inadequacies of the type material of Pleurocoelus, and the uncertainties involved with referred material, the genus can be distinguished from Brachiosaurus and Sauroposeidon, even considering ontogenetic variation. The cervical vertebrae of Pleurocoelus are uniformly short, with a maximum EI of only 2.4 in all of the Arundel material (Table 4). For a juvenile cervical of these proportions to develop into an elongate cervical comparable to those of Sauroposeidon, the length of the centrum would have to increase by more than 100% relative to its diameter. Comparisons to taxa whose ontogenetic development can be estimated suggest much more modest increases in length.

Carpenter & McIntosh (1994) described cervical vertebrae from juvenile individuals of Apatosaurus and Camarasaurus. Measurements and proportions of cervical vertebrae from adults and juveniles of each genus are given in Table 4. The vertebrae from juvenile specimens of Apatosaurus have an average EI 2.0. Vertebrae from adult specimens of Apatosaurus excelsus and A. louisae show an average EI of 2.7, with an upper limit of 3.3. If the juvenile vertebrae are typical for Apatosaurus, they suggest that Apatosaurus vertebrae lengthened by 35 to 65% relative to centrum diameter in the course of development.

The vertebrae from juvenile specimens of Camarasaurus have an average EI of 1.8 and a maximum of 2.3. The relatively long-necked Camarasaurus lewisi is represented by a single skeleton, whereas the shorter-necked C. grandis, C. lentus, and C. supremus are each represented by several specimens (McIntosh, Miller, et al. 1996), and it is likely that the juvenile individuals of Camarasaurus belong to one of the latter species. In AMNH 5761, referred to C. supremus, the average EI of the cervical vertebrae is 2.4, with a maximum of 3.5. These ratios represent an increase in length relative to diameter of 30 to 50% over the juvenile Camarasaurus.

If the ontogenetic changes in EI observed in Apatosaurus and Camarasaurus are typical for sauropods, then it is very unlikely that Pleurocoelus could have achieved the distinctive vertebral proportions of either Brachiosaurus or Sauroposeidon.

apatosaurus-cm-555-c6-centrum-and-arch-united

C6 of Apatosaurus CM 555 – despite having an unfused neural arch and cervical ribs, the centrum proportions are about the same as in an adult.

A few things about this:

  1. From what I’ve seen, the elongation of the individual vertebrae over ontogeny seems to be complete by the time sauropods are 1/2 to 2/3 of adult size. I get this from looking at mid-sized subadults like CM 555 and the hordes of similar individuals at BYU, the Museum of Western Colorado, and other places. So to get to the question posed in the comment thread on Mike’s giant Baro post – from what I’ve seen (anecdata), a giant, Supersaurus-class Barosaurus would not necessarily have a proportionally longer neck than AMNH 6341. It might have a proportionally longer neck, I just haven’t seen anything yet that strongly suggests that. More work needed.
  2. Juvenile sauropod cervicals are not only shorter than those of adults, they also have less complex pneumatic morphology. That was the point of the figure at the top of the post. But that very simple generalization is about all we know so far – this is an area that could use a LOT more work.
  3. I’ve complained before about papers mostly being remember for one thing, even if they say many things. This is the canonical example – no-one ever seems to remember the vertebrae-elongating-over-ontogeny stuff from Wedel et al. (2000b). Maybe that’s an argument for breaking up long, kitchen-sink papers into two or more separate publications?

Reference

Wedel, M.J., Cifelli, R.L., and Sanders, R.K. 2000b. Osteology, paleobiology, and relationships of the sauropod dinosaur Sauroposeidon. Acta Palaeontologica Polonica 45:343-388.

Wedel 2005 Morrison sauropod cervicals 1 - Diplodocus

When I was back in Oklahoma in March, I met with Anne Weil to see some of the new Apatosaurus material she’s getting out of her Homestead Quarry. It’s nice material, but that’s a post for another day. Anne said something that really resonated with me, which was, “I love it when you guys post about vertebral morphology, because it helps me learn this stuff.” Okay, Anne, message received. This will begin to make things right.

Wedel 2005 Morrison sauropod cervicals 2 - Barosaurus and centra shapes

I spent a week at BYU back in 2005, collecting data for my dissertation. One of the first things I had to do was teach myself how to identify the vertebrae of different sauropods, because BYU has just about all of the common Morrison taxa. These are the notes I made back then.

Wedel 2005 Morrison sauropod cervicals 3 - Brachiosaurus and Apatosaurus

I always planned to do something with them – clean them up, get them into a more usable form. There are a lot of scribbly asides that are probably hard for others to read, and it would be more useful if I put the easily confused taxa next to each other – Barosaurus next to Brachiosaurus, for example. And I didn’t go into serial changes at all.

Wedel 2005 Morrison sauropod cervicals 4 - Camarasaurus and Haplocanthosaurus

Still, hopefully someone will find these useful. If there are things I missed or got wrong, the comment thread is open. And if you want all four spreads in one convenient package, here’s a PDF: Wedel 2005 notes on Morrison sauropod cervicals

Mike and I leave for the Sauropocalypse tomorrow. I’m hoping to post at least a few pretty pictures from the road, as I did for the Mid-Mesozoic Field Conference two years ago. Stand by…

Back in 2013, when we were in the last stages of preparing our paper Caudal pneumaticity and pneumatic hiatuses in the sauropod dinosaurs Giraffatitan and Apatosaurus (Wedel and Taylor 2013b), I noticed that, purely by chance, all ten of the illustrations shared much the same limited colour palette: pale brows and blues (and of course black and white). I’ve always found this strangely appealing. Here’s a composite:

wedel-taylor-2013b-all-figures

I’m really happy with this coincidence. In fact I think I might get it printed up as a poster for my office.

(Thought: if I did, would anyone else be interested in buying it?)

Update (a couple of hours later)

At Matt’s suggestion, I switched the order of figures 7 and 8 (the last two on the third row) to get the following version of the image. It breaks the canonical order of the figures, but it’s visually more pleasing.

wedel-taylor-2013b-all-figures-v2

Now we should write an updated version of the paper that reverses the order in which we refer to figures 7 and 8 :-)

References

  • Wedel, Mathew J., and Michael P. Taylor. 2013. Caudal pneumaticity and pneumatic hiatuses in the sauropod dinosaurs Giraffatitan and Apatosaurus. PLOS ONE 8(10):e78213. 14 pages. doi:10.1371/journal.pone.0078213