Tutorial 15: the bones of the sauropod skeleton
September 7, 2011
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.
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 …
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.
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.
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.]