My T. rex skeleton model and general dinosaur anatomy

I was just looking through a couple of photos of my miniature T. rex skeleton model I got for Christmas present last year, when I felt a weird urge to dismantle it to rebuild it – this time without the manual. At the same time, I thought it to be a good opportunity to make a blog post related to vertebrates for once. So, prepare for a quick guide through some key features of the anatomy of dinosaurs!

First, I disassembled the model and mixed the pieces into a mess.

Second, I realised the easiest way to make sense of this jumble was to organise the different parts into categories.

The next step was to complete the trunk, or, more specifically, the axial skeleton – i.e. the entire skeleton excluding skull, limbs and limb girdles. I can imagine that this must be the most tricky part of a real skeleton,
but this model has a metal rod with the sacral (hip) vertebrae at the centre. The sacrals are naturally fused together, forming a stiff chunk of bone at the hip centre, giving good support for heavy bodies. The other (loose) vertebrae are conveniently numbered, so that the order in which to put them on the central rod is clear. However, I tried to make it some challenge by first guessing the order, based on size (generally, the vertebrae decrease in size from the centre toward the snout and tail tip – or, in formal words, they decrease in size anterioposteriorly, or rostrocaudally, if you want to be even more fancy), but that did not go very well…

There are four main types of vertebrae in the dinosaur body: the fused sacrals (hip), the fairly stiff caudals (tail), the sturdier dorsals (back), and the more flexible cervicals (neck). They have pretty distinct shapes, specialised for different functions, and so forth. I have never looked much into vertebrae, so I don’t have much to say here, but, looking at them separately, one can see some rather obvious similarities and differences.

A caudal vertebra.

A dorsal vertebra.

A cervical vertebra.

All three types have three processes on their uppermost (or dorsal, meaning toward or beyond the back – for most animals, this means up, since their trunks are held vertically, unlike us humans… and kangaroos) side, one facing straight up and the other two tilting outward. In formal terms, I would say that the central protuberance points dorsally and the other two dorsolaterally. (Lateral means away from the plane that runs through the middle of the body, dividing it in two similar halves; lateral is contrasted to medial, which instead means toward this central plane. Simplified, lateral means outward and medial means inward.)

However, they differ noticeably on their lower (or ventral, meaning toward or beyond the belly) portion. Here (bare with me), the caudals have a single ventral process, the cervicals two ventrolateral projections, and the dorsals lack any – instead, they have two ventrally curving ribs protruding from their lateral sides.

Generally, the vertebrae are uniform shape-wise within their respective types, but the two first cervicals are noticeably different from the rest, and are therefore given special names: the first one – closest to the head – is the atlas, and the successive one is called the axis.

The three first cervical vertebrae. From the left, we have the atlas, the axis and the third cervical.

I do not really know their special functions, I am merely aware of the difference. The feature visible on this model is that the central dorsal process has expanded laterally, even fusing with the lateral processes in the atlas. I have a feeling that they are made to lock tightly to the first few vertebrae, providing stability near the point where the head join is.

Once assembled, you can see how the vertebrae interlock tightly to its neighbours. Real vertebrae have loads of pits, sockets and projections that attach to adjacent vertebrae in manners I don’t understand very well yet, so I will leave it at that.

Assembled cervicals.

Assembled dorsals (left) and cervicals (right).

Assembled caudals.

The model came with a solid skull to be mounted at the end of the neck, keeping the vertebreae behind in place. This was quite unfortunate, because I could use some more practice on the skull bones: I am only properly acquainted with the bones around the mouth.

 

Dinosaur jaws are truly fascinating, especially those of the herbivores (!), which evolved various strategies to collect and process food. There is so much worth mentioning on this topic that it requires a post on its own. Thus, I will leave the skull anatomy for another time.

Instead, we move on to the appendicular skeleton, which is the limbs and its girdles (the connection between limbs and the body). Naturally, there are paired hind- and forelimbs, and one girdle for each pair.

The pelvic (hip) girdle attaches the hindlimbs to the body between the gut and the tail. It is a three-front articulation between three separate bones, with a hole (called the acetabulum) in the centre, where the thigh bone attaches.

The hip shape has been historically used to distinguish between the two main dinosaur groups: saurischia (lizard-hipped) and ornithischia (bird-hipped). T. rex has a typical saurischian hip, with the pubis and ilium widely apart. The ornithischians, by contrast, usually have the pubis pulled backward so that it lies in line with and touching the ilium. However, this feature is not exclusive nor entirely consistent to either group (the lizard-hipped condition is widespread among other reptiles, and birds have an ornithischian hip, even though they belong to the Saurischia – in spite of what the names suggest; yes, this can be rather confusing, but the names were coined long before it was certain that birds belonged to the dinosaurs, and the ornithishian hip did look like that of a bird, so the name was merely natural, given the knowledge they had at that time), and there are even variants of the ornithischian hip that superficially look more like a saurischian hip (e.g. in advanced ceratopsians), so this division is not used by modern paleontologists. There is actually a much easier way (if you have the skull, that is): all ornithischians have a unique extra jaw bone attached outermost to the lower jaw – the predentary.

The legs consist of three bones: the femur (thigh bone), and the tibia (shin bone) and fibula (calf bone) attached next to one another under the femur.

There is not much to note about these bones. They are pretty straight. There is only one thing I wish to point out. You might be aware that all dinosaurs had their limbs straight beneath their bodies, like mammals and birds, but unlike reptiles and amphibians, which have their limbs sprawling out to the sides. (Some bipeds, such as humans, may be exceptions to this trend, with regards to their arms.)

Reptiles (here a Komodo dragon) and amphibians have a sprawling limb arrangement, in contrast to dinosaurs, 

birds and mammals, which have their legs and arms placed underneath their bodies. Picture from

 http://wisata.kompasiana.com/jalan-jalan/2011/10/23/hanya-komodo-saja-yang-harus-dipromosikan-ntt-lebih-dari-itu/

Mammals achieved this feature by essentially tilting their hips outward and downward, but dinosaurs did it the other way: they tilted the head of the femur so that it lies at a right angle from the shaft.

One major effect of this (which could also be due to some other thing about the hips) is that the dinosaurs could only rotate their legs back and forth along the join, not roll it around in virtually any direction like we can. This, in combination with the knee and dinosaur ankle joins being hinge joints (only opens and closes in one direction), gave the dinosaurs very poor turning ability. Humans, conversely, have very flexible hip and ankle joints that can roll and rotate rather widely. Dinosaurs were more built for running straight forward, rather than agile fetching.

Another noteworthy thing about dinosaur locomotion is that they all walked on their toes – said to be digitigrade – unlike most other animals, which walk on the whole foot – platigrade. This is why I though dinosaurs (and birds too) had a backward-turned knee when I was a child – but that is not their knee, it’s their heel. 

The leg bones attach to the foot via the tarsals, or ankle bones – the calcaneum and the astragalus – which connect to the metatarsals, or foot bones. There is always one metatarsal per toe, but the number of toes varied between four and five among different dinosaur groups (and number of toes that actually bore weight could be even less). For each toe, there was at least one phalanx – toe bone – but usually three or four. Dinosaurs then walked on their phalanges, with their metatarsals held off the ground. The outermost phalanx – the ungual – bore a claw or hoof-like structure. 

The forelimbs of this model came in one piece together with the entire pectoral girdle, probably because of T. rex’s arms being so puny. The principle is very similar to that of the hindlimbs: two girdle bones (the scapula, or shoulder blade, and the coracoid); one upper arm bone, the humerus, two lower arm bones, the radius and ulna, several wrist bones, or carpals (numbers vary), one metacarpal – palm bone – per finger, and one or more phalanges – also the name for the finger bones – on each finger, the ungual clawed or hoofed.

Something I have always found funny, for some reason, is that the dinosaur pectoral girdle was attached right on their rib cage.

Soon enough, however, I realised that is actually the same for us too. I got confused since our scapula is actually on our back, but still attached to our ribs, not to the vertebral column. It is wonderful how much you can learn about yourself by studying others.

I suppose that would be enough for now. There is, of course, much much more about dinosaurs that is absolutely super-awesome, so much that no one can take it all in one go. See this as an introduction to other texts that will surely appear. In time…