Last time, we looked at the bones of the sauropod skeleton, and I mentioned that “thanks to the wonder of homology, it doubles as a primer for dinosaur skeletons in general”.  To prove it, here everyone’s favourite vulgar, overstudied theropod Tyrannosaurus rex, in L. M. Sterling’s reconstruction from Osborn 1906:plate XXIV, published just one year after the big guy’s initial description.  (The pose is somewhat outdated, but it’s a classic):

Click through for the full-sized version (2897 by 1755 pixels), which — like yesterday’s Camarasaurus — you are welcome to print out and hang on your wall as a handy reference.  (Sterling’s original is out of copyright; I hereby make my modified version available under the CC-BY-NC-SA licence.)

The thing to notice is that the Camarasaurus and Tyrannosaurus have exactly the same bones, excepting only that theropods had gastralia (belly ribs) and sauropods probably did not.  If you doubt it, here are the two animals composited together.  Print it out!  Print lots of copies!  Hand them out to your friends!

References

Osborn, Henry Fairfield.  1906.  Tyrannosaurus, Upper Cretaceous carnivorous dinosaur (second communication).  Bulletin of the American Museum of Natural History XXII:281-296 and plate XXXIX.

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We should have done this long ago.  Back in the early tutorials, we covered skeletal details such as regions of the vertebral column, basic vertebral anatomy, pneumaticity and laminae, but we never started out with an overview of the sauropod skeleton.

Time to fix that.  This is numbered as Tutorial 15 but you can think of it as Tutorial Zero if you prefer.  Thanks to the wonder of homology, it doubles as a primer for dinosaur skeletons in general.

Overview

Here is a complete, labelled sauropod skeleton, modified from Erwin S. Christman’s reconstruction of Camarasurus supremus in Ostrom and Mook 1921:plate LXXXIV:

Click through for the full-sized version (2897 by 1280 pixels), which you are welcome to print out and hang on your wall as a handy reference.  (Christman’s original is out of copyright; I hereby make my modified version available under the CC-BY-NC-SA licence.)

Since that’s a lot to take in all at once, we’ll walk through the regions of the skeleton: the head and neck, the rest of the vertebral column, the forelimb and girdle, and the hindlimb and girdle.  But first, a little bit of …

Skeletal nomenclature

Skeletons consist of bones.  The study of skeletons and of bones is called osteology.  There are several ways of dividing up the skeleton into manageable chunks.  One is to consider cranial vs. postcranial bones.  In this division, cranium just means skull (though see below) and postcranium means “everything except the skull”.  Here at SV-POW!, of course, we consider skulls beneath our notice, so this division seems silly to us.  We have been known to refer to the skull as the prepostcranium on occasion.

A more useful division of the skeleton is into axial and appendicular.  The axial skeleton includes the skull, hyoid apparatus (little bones in the neck that anchor tongue and throat muscles), vertebrae, ribs and chevrons (i.e. everything on the midline), and the appendicular bones are those of the limbs and their girdles, i.e. shoulders and hips.  (I learned only very recently that, although they seem to be part of the forelimb girdle, the sternal plates are actually part of the axial skeleton, being related to the ribs rather than the shoulders.)

Head and neck

Let’s start with the head.  Although “cranium” is sometimes used to mean the whole head, as noted above, it more strictly refers to the rigid upper portion of the skull which attaches to the neck and includes the upper jaw.  The lower jar, which moves independently, is called the mandible. Both of these units are made up of many smaller bones.  There is of course much, much more to say about skull anatomy, but that is another tutorial for another day.  For now, we will just pretend that the skull is made of two lumps of bone and move swiftly onwards.

The back of the skull articulates with the neck, which is part of the spine, or vertebral column.  All vertebrates have a spine; and in all tetrapods it’s divided into neck, trunk (or torso), sacrum and tail.  The spine is composed of vertebrae: those in the neck are called cervical vertebrae, or cervicals for short; those in the trunk are called dorsal vertebrae (in crocs and mammals these are further broken down into the thoracic vertebrae, which bear mobile ribs, and the lumbar vertebrae which do not); those in the sacrum are called sacral vertebrae and those in the tail are called caudal vertebrae.  But you already know that if you read Tutorial 1.

In some kinds of tetrapods, including all dinosaurs, the cervical vertebrae have backward-pointing ribs; these are called the cervical ribs.  Birds have these (in reduced form) and so do crocs and mammals, but they are absent in at least some lizards and turtles. Contrary to popular belief, mammals do have bicipital (two-headed) cervical ribs, they are just very short and fused to the vertebrae. Even most human osteology textbooks refer to them as transverse process. But developmentally and functionally they are ribs; they bound the transverse foramina through which the vertebral arteries pass, and they anchor deep neck muscles. The “cervical ribs” that occasionally crop up as a pathology in humans are large, mobile, thoracic-style ribs, and represent segmentation anomalies during early development.

The cervical vertebrae are numbered backwards from the head. Each cervical can be identified by number, so that the tenth is called “cervical 10”, or C10 for short.  Sauropods have between eleven and nineteen cervicals — a lot more than the feeble seven that nearly all mammals have, but well short of the seventy or so that Elasmosaurus could boast.

In most tetrapods, the cervicals from C3 backwards are similar in shape, although they tend to get bigger as they approach the torso; but the first two are distinctive, so they have special names.  C1 is called the atlas — easy to remember as it holds up the head, just as the titan Atlas held up the sky (not the Earth as often thought).  It doesn’t really look like a vertebra at all, being ring-shaped and (in sauropods) tiny.  C2 is called the axis.  It looks much more like a normal vertebra, but has an odd articulation at the front, a distinctive blunt spike that the atlas sits on (it also has small prezygapophyses for the neural arch elements of the atlas–these little bits of bone are often lost in fossil skeletons).  It’s smaller than the succeeding vertebrae — unlike the situation in mammals, in which the axis is ususally the largest cervical — and has a big, blade-like neural spine.

Torso and tail

The vertebral column continues back from the base of the neck, as the torso, which consists of dorsal vertebrae.

In the region of the hips, several vertebrae fuse together: this is true to some extent in most or all tetrapods, but in many groups it’s only two or three vertebrae that fuse, whereas in sauropods (and most dinosaurs) it’s four or more.  This set of fused vertebrae is the sacrum, and the vertebrae that make it up are the sacral vertebrae.

Behind the sacrum is the tail, which is composed of caudal vertebrae.  Hanging beneath these — or, specifically, between the intervertebral joints — are transversely flattened bones called chevrons or haemopophyses.  These exist in most reptiles, but have been lost in most mammals. (They do exist in wallabies, but they are a very different shape.) Developmentally the chevrons mirror the neural arch, and form a canal for the caudal aorta in the same way that the neural arch forms a canal for the spinal cord.

Just as the cervical vertebrae have cervical ribs, so the dorsal vertebrae have dorsal ribs.  These are longer and more vertically oriented than the cervical ribs.  The sacral vertebrae, too, have sacral ribs, but you rarely see them because in lateral view they are obscured by the ilium — as is the case here.  You might, then, wonder whether the caudal vertebrae have caudal ribs, but the answer is not clear.  The first few caudals, at least, do have lateral processes, but surprisingly there is no consensus about what they actually are: ribs that are fused to the vertebrae, or paraphophyses/diapophyses that are fused together.  See the overview in Wilson (1999:642).

How can you tell where the neck ends are the torso begins?  The traditional answer is that the first dorsal vertebrae is the first one with a “free” (i.e. unfused) rib, but it’s not always that clear.  Although cervical ribs generally fuse to their vertebrae and dorsal ribs rarely or never do, there are plenty of exceptions — for example, the last few cervical ribs of the Mamenchisaurus hochuanensis holotype appear unfused.  Also, in specimens where the cervicodorsal transition is well preserved, it’s apparent that the switch from short backward-directed cervical ribs to long downward-directed dorsal ribs may be abrupt, between adjacent vertebrae, or a gradual transition spread out over several vertebrae. Since the shoulder girdle bones don’t articulate with the torso, that clue’s also unavailable, so all in all it can be hard to nail down where the transition was.  You just sort of know it when you see it.

The final axial bones are the sternal plates, which belong somewhere in the breast area.  The exact placement and orientation of these bones is not agreed, and they are rarely if ever preserved in place.

Shoulder and forelimb

The bones of the shoulder are the elongate scapula, or shoulder-blade, on the side of the torso; and the coracoid, lower down wrapping round to the front.  Together, these bones make up the shoulder girdle.  Unlike the pelvis, the shoulder is not fused to the bones of the torso, but would have been bound to it by ligament and muscle.  Because of this, the exact position of the scapula and coracoid are not known, and remain the subject of controversy.  The reconstruction above shows a fairly vertical scapula; some others make it more nearly horizontal.

Where the scapula and coracoid meet, they form a hollow on the underside, called the glenoid.  The head of the humerus fits in here; two parallel bones form the lower limb segment: the ulna and radius.  In sauropods, the ulna is a rounded triangle in cross-section, with a hollow on the front face of the triangle which the radius fits into.

At the bottom of the lower limb segment are the carpals, or wrist bones; then the manus, or hand.  The upper bones of the manus are the metacarpals, which in sauropods are held near-vertical in a semi-circular arcade with the hollow directed backwards and slightly inwards.  Below the metacarpals are the phalanges (singular phalanx); each finger may have multiple phalanges, but sauropods tend to have very few.  When the last phalax of a digit is claw-shaped, it’s called an ungual.

Because both forefeet and hindfeet have phalanges and unguals, we distinguish by saying manual phalanges and manual unguals for the bones of the forelimb, and pedal phalanges and pedal unguals for those of the hindlimb.

Hip and hindlimb

The pelvis, or hip girdle, is made up of three bones on each side: the ilium, on top, is roughly semi-circular; the pubis, at the front, and the ischium, at the back, are more elongate.  Where these three bones meet, they form a circular hole called the acetabulum, or hip socket.  Unlike the shoulder girdle, the pelvis is fused to the torso: specifically, the ilium is fused to the sacrum via the transverse processes of the sacral vertebrae and their sacral ribs.  The pubes and ischia do not fuse.

The femur, or thigh bone, has a head that projects into the acetabulum.  At the knee, it meets two parallel lower-limb bones, the tibia and fibula.  The former is the main weight-bearing bone and is nearest the midline.  The fibula sits to the side of it.  Unlike mammals, most reptiles including non-avian dinosaurs have no kneecap, or patella; but birds do. Sesamoids or “floating” bones like the patella seem to be evolved and lost more readily than the normally-connected bones of the skeleton.

Below these two bones are the tarsals, or ankle bones.  In sauropods there are one or two of these: a large, disc-shaped astragalus beneath the tibia, and sometimes a smaller globular calcaneum below the fibula.  (For some reason, the carpals don’t seem to have names.)  Beneath these is the pes, or hindfoot.  The upper bones of the pes are the elongate metatarsals.  Beyond these are the short pedal phalanges and unguals.

What did we miss?

The bones listed account for nearly all the skeleton.  There are, however, a few extra bones that are rarely recovered or not always present.  Clavicles, or collar bones, have been reported in the limb girdles of some sauropods.  Gastralia, or belly ribs, were probably present in all sauropods, but are fragile and very rarely preserved.  Finally, some sauropods had osteoderms — small, isolated bones embedded in the skin and serving as armour.  None of these are illustrated in Christman’s Camarasaurus.

Comparative osteology

Because the basic tetrapod body-plan is so conservative — many bones change size and shape, but it’s comparatively rare for bones to evolve away or for new ones to evolve — you can look at skeletons of all sorts of animals in a museum and recognise nearly all the bones I’ve listed here.  Birds, the closest living relatives of sauropods, have everything I’ve listed here, though their sternal plates have merged into a single big sternum and their forelimbs are obviously highly modified.  Crocs have everything.  Lizards have everything except cervical ribs.  Even mammals are surprisingly similar, though all the pelvis bones fuse together and the coracoid is lost (the coracoid process of the scapula in humans and other mammals is a different, non-homologous bit of bone).

In particular, you have nearly all the bones in a sauropod skeleton, though of course many of the bones are very different in shape, or fused together, and your tail is contemptible.  You might like to try re-reading this tutorial, finding all the relevant bones in your own body.  You have a few extras as well: most obviously, your kneecaps, but also extra bones in the wrist and ankle.

SEE ALSO: the same thing done for Tyrannosaurus.

References

Osborn, Henry Fairfield, and Charles C. Mook.  1921.  Camarasaurus, Amphicoelias and other sauropods of Cope.  Memoirs of the American Museum of Natural History, n.s. 3:247-387, and plates LX-LXXXV.

Wilson, Jeffrey A.  1999.  A nomenclature for vertebral laminae in sauropods and other saurischian dinosaurs.  Journal of Vertebrate Paleontology 19(4): 639-653.  [Wilson used to have a freely available PDF on his site, but he seems to have removed it, and substituted a link to a paywalled PDF.]

Necks lie, redux

September 1, 2011

In a recent post I showed photos of the trachea in a rhea, running not along the ventral surface of the neck but along the right side. I promised to show that this is not uncommon, that the trachea and esophagus of birds are usually free to slide around under the skin and are not constrained to like along the ventral midline of the neck, as they usually are in mammals. Here goes.

Here’s figure 5 from van der Leeuw et al. (2001): a lateral x-ray of a duck, reaching up just a bit with its head and neck, possibly to get a bite or just look around. Click through for the unlabeled version.

There’s a LOT of stuff going on in this image:

  • As promised, the trachea (blue lines) is taking a very different path to the head than the vertebrae and skeletal muscles.
  • As usual for tetrapods, the neck is extended at the base in the caudal half and flexed at the head in the cranial half.
  • The epaxial (dorsal) muscles at the base of the neck are not tied down to the vertebral column so they are free to bowstring across the U-bend at the base of the neck (black arrow)–this was the point of the figure in the original paper. Although the gross outline of the neck also deviates from the vertebral column on the ventral side near the head, this is caused by the trachea and gullet approaching the pharynx, not because the hypaxial muscles are bowstringed across the curve.
  • As the post title intimates, this neck lies: the cervical vertebrae are significantly more extended than one would expect based on the external appearance of the neck alone. The red line shows the angle of the most strongly retroverted vertebra, which I measure at 48.5 degrees from vertical (41.5 degrees above horizontal)–slightly closer to horizontal than to vertical! We have seen this before, in most mammals and in a couple of small birds (see this post); here we see it even in a reasonably large, long-necked bird.
  • Worse, the gross outline of the neck–what one can see from the outside–lines up with nothing on the inside: the trachea is less curved and the vertebral column is more curved.

Same points again, this time in a chicken in an alert posture (Vidal et al. 1986: fig. 7). Here the most strongly retroverted cervical is 36 degrees from vertical (54 degrees above horizontal).

What’s all this got to do with sauropods?

First, it shows that even in animals with long, slender necks, it’s not enough to show a photo or painting of an extant animal and make assertions about what the cervicals are doing (necks lie, again). It’s even less defensible to make the dual assertions that (a) the gross outline of the neck shows the path of the cervicals and (b) the cervicals are in ONP, all based on a photo or painting of a living animal. The first point can only be established by radiography, and the second by manipulation of the skeleton, either physically or digitally. It may seem like I’m tilting at windmills here, but we’ve seen these very assertions made in conference talks. As always, we’ll follow where the evidence leads, but not until we see some actual evidence.

Second,  I am increasingly haunted by the idea that we are all waaay too influenced, even (maybe especially) subconsciously, by big mammals when we think about sauropods and their necks. Big mammals–like, say, horses and giraffes–have:

  • only 7 cervical vertebrae;
  • lots of big muscles that attach to the thorax and the head and cross the cervical column without attaching to it much or at all;
  • presacral neural spines that max out, height-wise, over the shoulders, creating withers;
  • alert neck postures that are elevated (like all tetrapods) but often short of vertical, with the vertebrae often held more-or-less straight through the middle section of the neck (camels are an obvious exception here).

In contrast, birds have:

  • many cervical vertebrae, from a 12  or so up to 27 or 28;
  • almost no muscles that span from thorax to skull;
  • presacral neural spines that rise monotonically to the synsacrum (except–maybe–in Giraffatitan);
  • alert neck postures that are S-shaped, with the craniocervical joint over or just slightly in front of the cervicodorsal junction.

Which group sauropods had more in common with is left as an exercise for the reader.

References

  • van der Leeuw, A.H.J., Bout, R.G., and Zweers, G.A. 2001. Evolutionary morphology of the neck system in ratites, fowl, and waterfowl. Netherlands Journal of Zoology 51(2):243-262.
  • Vidal, P.P., Graf, W., and Berthoz, A. 1986. The orientation of the cervical vertebral column in unrestrained awake animals. Experimental Brain Research 61: 549­-559.