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Key Concepts: Osteology II (The Hindlimb)

September 17, 2009 3 comments

In a recent post, we gave an introduction to the osteology of the forelimb. Now, we’ll round out that series with a consideration of the hind limb. Fortunately, many of the concepts are the same, so we’ll be able to move more quickly.

As you may recall, the forelimb was divided into a pectoral girdle, a big proximal bone (the humerus), two more distal long bones (radius and ulna), and a hand (manus) consisting of some carpals, metacarpals, and phalanges (with some modified into unguals). The same pattern follows for the hind limb, with a pelvic girdle, a big proximal bone (the femur), two more distal long bones (tibia and fibula), and a foot (pes) consisting of some tarsals, metatarsals, and phalanges (again, with some modified into unguals). Easy, isn’t it?

Centrosaurus hind limb (after Brown 1917)

Centrosaurus hind limb (after Brown 1917)

First, let’s take a look at the pelvic girdle. In dinosaurs, as in humans and pretty much every other limbed vertebrate, the pelvis includes three elements on each side: ilium, pubis, and ischium. Looking at the whole structure in side view, the ilium is the top bone, and the latter two are on the bottom. Where the three bones meet, their surfaces form the limb socket, which is called the acetabulum. The ilium is a pretty big, usually flat structure, that anchors the pelvis (and thus the limb) to the vertebral column. Lots of thigh and butt muscles also attach to it. Of the two bottom bones, the pubis is the front (anterior, sometimes called cranial) one. A big deal has been made of its difference in its orientation between ornithischian and saurischian dinosaurs – in ornithischians, most of the bone is directed backwards, and in most saurischians (with the exception of birds and their close allies) the bone is directed forwards. Finally, there is the ischium (which a classically-grounded anatomy professor of mine liked to note is correctly pronounced with a hard “k” sound, rather than the “ish-ee-um” that most folks use). For various reasons (namely, all of the processes and extra bumps render accurate comparison of measurements difficult), we won’t be doing much with the pelvis in the present study. So, let’s move on to the femur.

The femur, just like the humerus, is a single robust bone that articulates with the limb girdle, its head fitting into the acetabulum proximally, and with two other elements distally. Sometimes, there is a little backwards-directed hangy process from the middle of the shaft, called the fourth trochanter.

The tibia and fibula are the developmental homologues of the forelimb’s radius and ulna. Unlike mammals, dinosaurs lack a kneecap (patella) floating over the proximal end of the tibia and distal end of the femur.

Centrosaurus pes (after Brown 1917)

Centrosaurus pes (after Brown 1917)

The “foot” is called the pes (Latin for “foot”), and is very slightly differently configured than for the manus. The hind limb’s equivalent of carpals are called tarsals – and unlike the condition up front, the tarsals are actually rather important and frequently ossified elements. In fact, they are so ossified that they usually fuse right on to the tibia and / or fibula. The two major tarsals in ornithischians are the astragalus (capping the tibia) and the calcaneum (capping the fibula, or at least floating in its general vicinity). Because the astragalus is so often fused to the tibia, many authors measure tibia length with the astragalus included.

Instead of metacarpals, we now have metatarsals. The numbering system is the same as for the manus, except they’re abbreviated as “MT.” So, the first (innermost) metatarsal, equivalent to the one associated with our big toe, is MT-I. Phalanges are handled quite similarly, with IV-2 being the second most proximal phalanx on the fourth digit. And once again, the final phalanges are often modified into unguals.

And that’s all there is to know about ornithischian dinosaur limb osteology!

Centrosaurus pes (after Brown 1917)

Centrosaurus pes (after Brown 1917)

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Key Concepts: Osteology I (The Forelimb)

September 15, 2009 16 comments
Forelimb of the horned dinosaur Centrosaurus apertus

Forelimb of the horned dinosaur Centrosaurus apertus

Osteology is the study of bones. Recognizing that not everyone here is completely familiar with all of the relevant names and features, this post will cover a brief tutorial of limb osteology and terminology in dinosaurs.

Broadly speaking, anatomists usually divide the skeleton into three sections: cranial (the head); axial (the vertebral column and ribs, although embryological and evolutionary histories mean that parts of the skull are sometimes lumped in here); and appendicular (the limbs). Presently, we’re only interested in the latter.

The appendicular skeleton includes forelimbs and hind limbs. Let’s start at the front in this post, and work back in a subsequent post. But before we start that, we need to introduce one more set of terms: proximal and distal (see image for their context within the forelimb). This just refers to the position along a structure relative to the main part of the body. Proximal is close to the body, and distal is away from it. Considering the humerus (upper arm bone), the elbow is at the distal end and the shoulder is at the proximal end. Within the entire leg, your toes are at the distal end and the thigh bone is at the proximal end.

The forelimb includes the pectoral girdle as well as the limb bones themselves. In dinosaurs, the pectoral girdle includes a scapula, a coracoid and a sternal plate on each side. Humans have scapulae too (most of us know them as “shoulder blades”), but our coracoids have shrunk down to little nubbins (the coracoid processes) that are fused onto the scapulae themselves. We also have clavicles (“collar bones”) as part of our pectoral girdle, but ornithischians lack this bone (although theropods preserve part of it in the furcula, or “wishbone”). In all adult ornithischians, the scapula and coracoid are fused together, and the area where they meet forms the glenoid, or shoulder socket. If the bones are fused, their total combination is then called a “scapulocoracoid.”

The humerus (or “upper arm bone”) fits into the glenoid. It’s a long bone, expanded at both ends for various muscle and bony attachments. Lots of muscles—including the famous deltoids, lats, biceps, triceps, and pectoral muscles—attach here. The “midshaft” of the humerus is exactly that – the point at the middle of bone.

A pair of bones – the ulna and radius – form the forearm. They articulate with the distal end of the humerus. They’re pretty simple, rod-like bones in most cases. The ulna usually has a process (i.e. a sticking-out bit), called the olecranon, at its proximal end for attachment of the triceps muscle.

Finally, we have the hand – more properly called the manus (Latin for “hand,” strangely enough). The manus has carpals (wrist bones), metacarpals (joining the wrist to the digits), and phalanges. Each digit (or finger) is numbered starting at the thumb. The thumb (innermost digit, for ornithischians) is I (note the Roman numeral), the index finger II, middle finger III, ring finger IV, and pinkie V.

The most proximal elements within the manus (just distal to the ulna and radius) are called the carpals. They’re often just cartilage, and even when ossified are rarely preserved (they tend to float away if the skeleton becomes disarticulated). At any rate, they’re usually non-descript little round elements in ornithischians, and we’ll pretend these bones don’t exist for the purposes of our study.

Manus of the horned dinosaur Centrosaurus (modified after Brown 1917)

Right manus of the horned dinosaur Centrosaurus (modified after Brown 1917)

Next, we have the metacarpals. If you squeeze the palm of your right hand between the thumb and index finger of your left, these are the bones you’re feeling. The number of metacarpals is variable in many dinosaurs. Humans, and most ornithischians, have five metacarpals (and hence, five fingers in most cases). Most theropods have fewer. “Metacarpal” is often abbreviated as MC. So, the first metacarpal would be MC-I, and so on.

Finally, we come to the phalanges. A single element is most properly called a phalanx (not a “phalange,” although this archaic spelling is not technically incorrect – many older publications use the terminology). The phalanges are numbered by digit (I-V) as well as their position relative to the metacarpals (given by an Arabic numeral). For instance, I-1 is the first phalanx on the first digit, and III-2 is the second phalanx on the third digit. The second-to-last phalanx is sometimes referred to as the “penultimate” phalanx.

The distal-most (terminal) phalanx is often modified into a hoof or claw. These specially modified phalanges are usually called unguals, but they are numbered just the same as regular phalanges. Even if the third and final phalanx on the third digit is a huge claw, it’s still called manual phalanx III-3.

Finally, we should mention the sternal plates. These odd bones (probably equivalent to the sternum, or breast bone, of mammals) are usually floating at the front of the chest wall. The sternals sometimes look like kidney beans (in ceratopsids) or hatchets (in other ornithischians).

It’s a blizzard of terms, but a little practice should help you become completely conversant with all of the parts of the forelimb. In an upcoming post, we’ll tackle the hindlimb. Don’t worry – many of the concepts are the same!

Categories: Key Concepts, Tutorials Tags:
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