Postcranial

The postcranial skeleton also contains a variety of modifications that are valuable in family, class, genus, and species identification. Form follows function, and this is especially true regarding locomotion; therefore, the lower portion of the limbs tends to be more modified in some species than in others, including the reduction in numbers of carpals and metacarpals, or the conversion of phalanges into hooves or claws.

Forelimb

Scapula

The scapula is typically located on the dorsolateral surface of most mammals. It is elongated to allow for muscle attachments. It is more elongated in some species, while more D-shaped or triangular in others.

Humerus

The humerus is the upper bone of the forelimb. The humeral head articulates with the scapula. It is a rounded structure that allows for rotation at the shoulder joint. The head tends to be more ovoid in species that require less rotation and more rounded in species that use their forelimbs for activities other than locomotion, such as digging, climbing, or swimming. The head tends to be oriented more posteriorly in quadrupeds for articulation with the downward-facing scapula, e.g., dog, while the head is more lateral in more upright animals, e.g., chimpanzee (Figure 11.19).

Radius and Ulna

The radius and ulna are the lower bones of the forelimb, and they tend to be of similar size. The radius is the more anterolateral bone, while the ulna is the more posteromedial bone. In a number of species, the ulna will fuse to the radius, becoming one bone, while in other species the ulna is vestigial and barely developed.

Figure 11.19 Chimpanzee and dog humeri.

Torso/Pelvis

Clavicle

This element is present in some mammals but is often vestigial or absent, so it is of limited use in species identification. Bony fish have clavicles; turtles incorporate them into the plastron, and they are found in birds in the form of the "wishbone.”

Vertebrae

The vertebrae tend to be of similar length from neck to pelvis and are often cylindrical in overall shape. All vertebrae have similar structures with some modification to those structures that is indicative of each segment of the vertebral column.

Cervical

Typically, there are seven cervical vertebrae.

• 1. C-l is called the atlas, and it articulates with the cranium.
• 2. C-2 is called the axis, and it allows the pivoting of atlas and rotation of the head. The overall shape of the dens, or the bony projection that the C-l pivots around, can suggest the degree of rotation allowed.
• 3. Thoracic: There are usually between 11 and 16 thoracic vertebrae. The ribs articulate with these vertebrae on the body.
• 4. Lumbar: There are typically between four and seven lumbar vertebrae. These vertebrae tend to have longer bodies than the thoracic vertebra.

• 5. Sacral: The sacrum is typically composed of three to six fused vertebrae, but it can be as few as one. The first sacral element articulates with the most caudal lumbar vertebra. The wings of the sacrum are called alae and they articulate with the os coxa at the auricular surface. However, the sacrum often fuses to the os coxa forming one solid structure.
• 6. Coccygeal: The number of caudal or coccygeal vertebrae varies depending on the length of the tail.

Sternum

The sternum, when present, articulates with the clavicle, if present, and the ribs. It directly articulates with the more cranial ribs, often called the true ribs, via individual cartilaginous attachments, and indirectly, via common cartilage, with the lower ribs, or false ribs. The body of the sternum can present as several bodies, or sternabrae, joined by cartilage, or these can fuse into one element.

Ribs

Typically, there are twice as many ribs as thoracic vertebra, so if there are 12 thoracic vertebrae there should be 24 ribs; two ribs, one left and one right, articulate with each thoracic vertebra. Often the first 7 ribs articulate directly with the sternum (true ribs), ribs 8, 9, and 10 articulate via common cartilage (false ribs), and ribs 11 and 12+ do not articulate with the sternum (floating ribs). Some species may have cervical or lumbar rib, but these tend to be much smaller than the other ribs and typically do not develop all of the anatomical features of the thoracic ribs. Various publications provide lists of expected numbers of ribs per species. It should be noted that domesticated cattle and hogs tend to have variable numbers of ribs, based on breeding for meat or not.

Ribs tend to come in two forms: either very broad and flat, like those found in cattle, or very round, like those found in canines.

Os coxae

The pelvis in quadrupeds tends to be long and narrow, which allows for muscle attachment and maximizes muscle function. The pelvis is further modified in bipeds and in swimming mammals, becoming almost nonexistent in animals like manatees and dolphins.

The os coxa is actually made up of three bones: the ilium, the wing or blade-like portion; ischium, the more bulbous arched portion; and pubis, the thinner arched portion. These three structures fuse to form one structure called the os coxa or innominate; the acetabulum or acetabular fossa is where the three meet—this is the hip socket. The iliac crest is the protruding "hip’’ of an animal.

Hindlimb

Femur

The femur is the upper portion of the hindlimb. The femoral head, which is oriented medially, articulates with the acetabulum of the os coxa; the femoral head tends to be more ball-shaped than the humeral head, which allows more rotation at this joint. In the middle of the femoral head is the fovea capitis, which is a small depression for the insertion of the ligamentum teres from the acetabular notch of the os coxae. The humeral head does not have a depression, which allows for easy distinction between the two in fragmentary elements.

Patella

The patella, or the kneecap, is a large sesamoid bone and functions to increase the level arm of the quadriceps muscle in the leg.

Tibia and Fibula

The tibia is the larger of the two lower leg elements and is the more anteromedial bone, while the smaller fibula is the more posterolateral bone. In a number of species, the fibula will fuse to the tibia, becoming one bone, while in other species the fibula is vestigial and is barely developed or is absent.

Fore and Hind Paws

Carpals and Tarsals

There are up to eight carpals and seven tarsals.

Metacarpals and Metatarsals

In the ancestral mammalian form, there are five metacarpals (MCs) or metatarsals (MTs); however, this number is reduced in various species, and in some there is only a single long metacarpal or metatarsal. In dogs that have had the hind dew claw removed, the 1st metatarsal and the associated phalanges are absent. In dogs, and many other species, when this 1st metatarsal is present it may appear as a fully formed digit similar to that seen in the forepaw, or it may be vestigial in appearance, sometimes only having the distal phalanx.

The 1st MC or MT tends to be the shortest, and the 3rd and 4th tend to be the longest. The shaft of the metacarpals tends to be flatter or more D-shaped in cross-section, and they are shorter and thicker overall, while the metatarsals tend to be more ovoid or rounded in cross-section and longer and thinner in appearance.

Phalanges

There are typically 14 phalanges; the first digit (thumb) has two, a proximal and distal phalanx, while the other four digits contain a proximal, intermediate, and distal phalanx. The number of phalanges is reduced as the number of fingers/toes is reduced, so horses only have three phalanges per foot, one proximal, one intermediate, and one distal. The phalanges get smaller as you progress from proximal to distal. The forepaw phalanx tends to be larger than the comparable hindpaw phalanx.

The number and length of these elements and the presence of claws or hooves is useful in identification. For example, plantigrade (habitually walk with sole on the ground) animals have the full complement of toes, and the metacarpals and metatarsals tend to be shorter in relation to radius/ulna or tibia/fibula; digitigrade (toe-walker) animals have the full complement of toes or a slight reduction; however, the length of metacarpals and metatarsals has increased; unguligrade (hoof-walkers; essentially walking on the nails of the toes) have reduced numbers of toes, and the metacarpals and metatarsals have reached their peak length (Figure 11.20).

Figure 11.20 Posterior limbs of a raccoon, cat, and horse demonstrating the relationship of tarsals, metatarsals, and phalanges.