Postcranial Skeleton

Although dental and cranial anatomy have generally received more attention in mammalian paleontology than has the postcranial skeleton, the skeleton is a critical source of information on body size, locomotion, habitat preference, and many other aspects of paleobiology. Postcranial characters are also playing an increasingly significant role in phy-logenetic analyses, as it becomes more accepted that these features are no more subject to homoplasy than are dental or cranial features (Sanchez-Villagra and Williams, 1998). The skeleton (Fig. 2.4) can be divided into axial and appendicular parts. The axial skeleton comprises the skull and trunk, including the vertebral column, sternum, and ribs. The ap-pendicular skeleton encompasses the limbs and limb girdles.

The segmented vertebral column provides support and flexibility and protects the spinal cord. It is also closely associated with locomotion. In mammals it is differentiated into five regions, each with its own distinctions: cervical, thoracic, lumbar, sacral, and caudal, abbreviated as C, T, L, S, and Ca, respectively (Fig. 2.5). Nearly all mammals have seven cervical vertebrae, a remarkable conservatism probably resulting from developmental constraints (Galis, 1999); the only exceptions are found among sloths (six to nine) and manatees (six). Other regions are much more variable.

Among extant mammals, thoracic vertebrae may number 9-25 (usually 12-15), lumbars 2-21 (usually 4-7), sacrals 3-13 (usually 3-5), and caudals 3-50 (Flower, 1885; Lesser-tisseur and Saban, 1967a; Wake, 1979). As a rule, individual vertebrae consist of a body (centrum) and a vertebral (neural) arch bearing a median dorsal spinous process and two pairs of articular processes (zygapophyses). The anterior or prezygapophyses face more or less dorsally or medially, whereas the postzygapophyses face ventrally or laterally From the side of the arches or centra extend the transverse processes. Both neural arch and transverse processes tend to be much reduced in most of the tail. In the thoracic and lumbar regions, an additional process, the metapophysis (mamillary process), may project from the prezygapophysis, and an anapophysis (accessory process) may extend caudally below the postzygapophysis.

Cervical vertebrae are distinguished by having a very large vertebral foramen (for passage of the spinal cord) and foramina in the transverse processes (except C7) through which the vertebral arteries pass en route to the cranial cavity. In most mammals the cervical centra tend to be short, but in some mammals, such as the giraffe, they are very long. The first two cervicals, called the atlas (C1) and axis (C2), are diagnostic of mammals. The ringlike atlas, which lacks a centrum, articulates with the occipital condyles and allows flexion and extension of the head. The axis has an anterior projection, the dens (odontoid process), which is a neo-morphic addition to the atlas centrum rather than its homologue (Jenkins, 1969a). The dens is held by ligaments against the ventral arch of the atlas, serving as a pivot for rotation of the head-atlas complex. The neural spine of the axis tends to be very prominent. Cervical ribs are present in monotremes and some primitive fossil mammals.

Thoracic vertebrae are readily distinguished because they articulate with ribs. The head (capitulum) of each rib artic

Monotreme Skeleton
Fig. 2.4. Skeleton of a generalized mammal, Eocene Phenacodus. (Modified from Osborn, 1898a.)
Caudal Skeleton

LUMBAR SACRUM CAUDALS

Fig. 2.5 Mammalian vertebrae. Key: art., articular; Ca, caudal; for., foramen; pr. or proc., process; tr., transverse. (Modified from Jayne, 1898.)

LUMBAR SACRUM CAUDALS

Fig. 2.5 Mammalian vertebrae. Key: art., articular; Ca, caudal; for., foramen; pr. or proc., process; tr., transverse. (Modified from Jayne, 1898.)

ulates at the junction of two centra, and the tubercle (tu-berculum) of the rib articulates with the transverse process. Thoracic vertebrae have progressively larger centra and smaller vertebral foramina, moving caudally in the series. The spinous processes of the anterior thoracics tend to be high and posteriorly inclined. At the caudal end of the series, the orientation changes, becoming somewhat anteriorly directed. Near the end of the series is a transitional vertebra with a vertical spine, called the anticlinal vertebra.

The lumbar vertebrae typically have the largest and longest centra. The transverse and spinous processes are well developed and anteriorly directed. Metapophyses and anapophyses tend to be most prominent in this region.

The sacrum is the only part of the vertebral column in which the individual elements are typically fused. The number of fused vertebrae varies considerably among taxa and sometimes involves "sacralization" of adjacent caudal or lumbar vertebrae. The sacrum articulates with the ilia at a tight-fitting synovial joint mainly involving the first sacral.

The tail is a particularly variable part of the vertebral column, which can differ dramatically in both vertebral number and size. The caudal centra tend to be shorter and more robust proximally, and cylindrical and elongate distally. Proximal caudals usually have neural arches, transverse processes, and zygapophyses, which are greatly reduced or lost distally (see, e.g., Youlatos, 2003). Haemal arches, or chevron bones, project ventrally from between the centra in some mammals.

The ribs of extant mammals consist of a bony portion, which articulates with the vertebral column, and a costal cartilage (sternal rib), between the ventral end of the rib and the sternum. The sternal ribs are normally ossified in some primitive mammals, such as monotremes and xenarthrans. As already mentioned, most ribs have two articular surfaces for the vertebrae, the capitulum (which meets the demi-facets on adjacent vertebral centra) and the tuberculum (which articulates with the transverse process). The tubercles decrease in size caudally so that only a capitulum remains on some posterior ribs. Posterior ribs may join preceding ribs rather than having a separate sternal attachment, or may be "floating," with no attachment to the sternum.

The sternum is a segmented, midline bony structure, which articulates with the shoulder girdle at its cranial end and with the ribs between successive sternebrae. The first sternebra, or manubrium, is commonly enlarged; the last is the xiphisternum.

The limbs of mammals have diversified for a wide variety of locomotor and other functions, an appreciation of which requires an understanding of comparative anatomy

Mammal Scapula
Fig. 2.6. Scapulae of three mammals. Dagger (f) indicates extinct taxon. Scale bars = 1 cm. (Modified from Rose and Emry, 1993.)

An excellent comparative account of the limb skeleton of diverse mammals is presented by Lessertisseur and Saban (1967b). Each limb consists of the limb girdle and proximal, intermediate, and distal segments. The shoulder (pectoral) girdle is simplified in most mammals compared with other tetrapods, consisting of only the scapula (Fig. 2.6) and clavicle; the (posterior) coracoid, formerly a separate element, is incorporated as a process of the scapula. The clavicle forms a strut between the sternum and the scapula. The scapulae, however, have no direct bony connection to the trunk, but are suspended by muscles on the sides of the anterior thoracic region. In living mammals, except mono-tremes, the scapular spine divides the outer surface into supraspinous and infraspinous fossae, and an acromion process projects from the distal end of the spine. Monotremes are primitive, however, in retaining separate anterior and posterior coracoids and an interclavicle, as in some therap-sids. Moreover, they have no scapular spine and no distinct supraspinous fossa; the anterior margin of the scapula is homologous with the spine of other mammals. In all mammals the scapula articulates with the head of the humerus at the glenoid fossa.

Distal to the shoulder girdle the forelimb skeleton consists of the humerus, the radius and ulna, and the manus (Figs. 2.7-2.10). Many surface features of the humerus are related to muscle attachments (e.g., greater and lesser tuberosities [tubercles]; deltoid and pectoral crests or a combined deltopectoral crest; teres tubercle; medial and lateral epicondyles; supinator crest, also called the lateral supra-condylar ridge or brachialis flange). So, too, are the ulnar

Divergent Talar Ridge
Fig. 2.7. Left humeri of extant mammals (not to scale). Ratufa and Arctictis are arboreal, Ursus is generalized, Sus and Canis are cursorial, and Taxidea and Dasypus are fossorial.
Ursus Humerus Tuberculum
Fig. 2.8. (A) Radioulnar joint of extant mammals; (B) ulnae of extant mammals. Taxa same as in Fig. 2.7 except Tapirus (cursorial). (From O'Leary and Rose, 1995.)

olecranon process and the bicipital tuberosity and certain crests of the radius. The elbow is a complex joint involving three articulations: between the humeral trochlea and the ulna (a hinge), the humeral capitulum and the radial head (often a pivot), and the proximal radius and ulna (a potential gliding joint; Fig. 2.8). In higher primates, as well as some other arboreal mammals, the radius has substantial freedom to rotate on its long axis, allowing pronation (in which the distal radius crosses over the ulna so the palm faces downward or backward) and supination (in which the radius and ulna are parallel and the palm faces upward or forward). In most mammals the forearm and manus are normally held in the pronated position, and in some the elbow joint is modified to restrict or prevent supination.

The manus consists of the carpus, metacarpus, and phalanges (Fig. 2.10). Primitively the carpus comprises nine elements—arranged essentially in proximal and distal rows— some of which have been lost or fused in some mammals.

From medial to lateral (in the typically pronated manus of quadrupeds), the proximal row consists of scaphoid, lunate (lunar), cuneiform (triquetrum of human anatomy), and pisiform. Distally the radius articulates with the scaphoid and lunate, which are fused in some mammals, such as carnivorans, whereas the ulna usually articulates with the cuneiform and pisiform. Composing the distal carpal row (medial to lateral) are the trapezium, trapezoid, magnum (capitate in humans), and unciform (hamate of humans). In many primitive mammals a centrale is present as a separate element, usually between the scaphoid-lunate and trapezoid-magnum. Typically the trapezium articulates with meta-carpal I, the trapezoid with metacarpal II, the magnum with metacarpal III, and the unciform with metacarpals IV and V

Distal to the carpus are the digits, generally five in number, each of which has a metacarpal, and either two phalanges (in digit I, the pollex or thumb) or three (all others), resulting in a phalangeal formula of 2-3-3-3-3. The terminal or ungual phalanges are modified to bear claws, hoofs, or nails, and they vary considerably in form in relation to both phylogeny and function (Fig. 2.11; see also Fig. 2.17). The form and number of metacarpals and their phalanges also vary considerably among mammals. The forelimbs of many mammals have become modified in connection with other behaviors besides locomotion.

The pelvic girdle consists of the ilium, ischium, and pubis on each side, fused together to form a single innominate (hip) bone; the innominates articulate with the vertebral column at the sacroiliac joints, and with each other at the pubic symphysis (Fig. 2.12). All three pelvic elements meet and fuse within the acetabulum, which forms a socket for the femoral head. Primitive mammals, including extant monotremes and marsupials, also have epipubic ("marsupial") bones. Although epipubic bones have generally been assumed to be related to pouch support, a recent study indicates that they also (or alternatively) function as levers between abdominal muscles and the femur during locomotion (Reilly and White, 2003). Also associated with the pelvis is the baculum (os penis) found in many mammals, and used as a taxonomic character in rodents and carnivorans.

The femur, tibia and fibula, and pes (foot) make up the hind limb skeleton (Figs. 2.13, 2.14). The femur is generally the longest of the limb elements. It typically has three muscular processes on the proximal half, the greater, lesser, and third trochanters. Distally the femoral condyles articulate with the proximal tibia. The fibula may be strong and free from the tibia (joined at each end by synovial or fibrous joints), co-ossified at one or both ends, or reduced and virtually lost. There are seven tarsal bones: the astragalus (talus), calcaneus, navicular, cuboid, and three cuneiforms (Fig. 2.14). The astragalus (Fig. 2.15), which is supported by the calcaneus, articulates with the tibia proximally and the navicular distally. The navicular articulates with the three cuneiforms (ento-, meso-, and ectocuneiform), which in turn articulate with metatarsals I-III, respectively. The calcaneus articulates distally with the cuboid, which usually articulates

Primate Calcaneus

Fig. 2.9. Proximal and distal radius of Eocene mammals. First four columns are proximal radius in proximal and anterior views; last column is distal radius in distal view Differences in shape affect mobility of the radius and reflect locomotor diversity. (From Rose, 1990, and O'Leary and Rose, 1995.)

Fig. 2.9. Proximal and distal radius of Eocene mammals. First four columns are proximal radius in proximal and anterior views; last column is distal radius in distal view Differences in shape affect mobility of the radius and reflect locomotor diversity. (From Rose, 1990, and O'Leary and Rose, 1995.)

with metatarsals IV and V Perhaps more than any other part of the postcranial skeleton, the anatomy of the tarsals, particularly the astragalus and calcaneus, has played an important role in the determination of phylogenetic relationships of mammals (e.g., Matthew, 1937; Szalay, 1977, 1994). As in the manus, there are five metapodials (called metatarsals in the foot), and the same complement of phalanges as in the manus. The first pedal digit is the hallux, and it is often somewhat divergent from the other phalanges.

Several sesamoid bones are also associated with the limb skeleton. These are generally small, nodular elements encased within muscle tendons and located near joints. The best known is, of course, the patella (knee cap). Additional sesamoids are associated with various digital flexor tendons in both the manus and pes of many mammals. They usually serve to enhance leverage of the muscle in which they are contained.

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