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THE SHOULDER

     A number of animals possess limbs: fish have fins, tetrapods have legs, arthropods have a series of legs and other appendages, etc.  It is probable that the Precambrian ancestor of coelomates had at least some limb-like appendages from the body wall given the common use of distal-less (Dll) homeodomain proteins in the formation of limbs in arthropods, fish fins, tetrapod limbs, annelid parapodia, echinoderm tube feet, the ampullae of tunicates, and the lobopodia of oncyphorans (Panganiban, 1997; Shubin, 1997).

     Placoderms were the first fish to evolve pectoral and pelvic girdles.  The pectoral girdle included a scapulocoracoid element (Kardong, 2002, p. 323).

shark pectoral girdle

shark pectoral girdle

shark pelvic girdle pelvic girdle
 
  Acanthodians also possessed this scapulocoracoid in their pectoral girdle (Carroll, p. 89).  Actinopterygians and sarcopterygians possessed a pectoral girdle composed of a cleithrum, scapulocoracoid, and clavicle, (Kardong, 2002, p. 325; Carroll, p. 145).   Thus, the shoulder included elements of both endochondral and dermal bone, as in tetrapods (Carroll, p. 145).  Rhipidistian fish were the first to develop an interclavicle bone in their shoulder (Kardong, 2002, p. 327).
fish pectoral girdle fish pectoral girdle
 
    In amphibians, the shoulder lost its connection to the skull as a number of ancestral skull bones were lost, such as the supracleithrum, anocleithrum, extrascpaular (medial and lateral), and most of the opercular series (Carroll, p. 160).  As a result, the arms could move without simultaneously moving the head which probably improved the function of the head’s sensory structures.   The glenoid and acetabulum faced laterally rather than posteriorly (Carroll, p. 162) and the scapulocoracoid was composed of two centers of ossification, as opposed to one element in fish (Kardong, 2002, p. 329).
fossil girdles

Note the relative positions of the pectoral girdle and the skull in the fish and frog skeletons.

fish pectoral girdle fish pectoral girdle

frog

frog

fossil fish

salamander

salamander scapula

 

frog

frog

      In amniotes, the size of the clavicle and cleithrum was decreased and the shoulder became more agile (Carroll, p. 196).  Turtles are unusual in that their limb girdles actually develop inside their rib cage (the shell).

turtle

turtle

turtle

alligator

alligator

alligator

alligator

emu

emu

Synapsid reptiles evolved a second coracoid bone (and it is this coracoid which is retained as coracoid process in mammals) (Kardong, 2002, p. 329).  The cleithrum was reduced to a very thin bone in ophiacodont pelycosaurs and had become vestigial by the sphenacodont pelycosaurs (Kemp, 1982, p. 53).  The clavicle expanded medially (Kemp, 1982, p. 33).

The clavicle is produced as a result of fusion of dermal and endochondral components and results both from neural crest and mesoderm cell differentiation. It may be that the posterior dermal clavicle is a remnant of ancestral structures which were once more extensive (Matsuoka, 2005).

The cleithrum was persent in ancestral tetrapods and amniotes and has been subsequently lost in at least four lineages (salamanders, turtles, diapsids, and mammals). The cells which give rise to the endochondral spine of the scapula may be derivatives of those that once produced the dermal cleithrum (Matsuoka, 2005).

     In therapsids, the glenoid cavity and acetabulum became more open ventrally (Carroll).  Therapsids evolved an acromion process (Romer, p. 205).   In advanced cynodonts (such as Masetognathus depicted in the adjacent drawing), the glenoid cavity became more open (Kemp, 1982, p. 246), the coracoid bones were reduced (Kemp, 1982, p. 204), the clavicle attached to the acromion (Kemp, 1982, p. 199), and more movement was permitted at the shoulder (Kemp, 1982, p. 115).

cynodont

      In early mammals, the procoracoid was excluded from the glenoid fossa (Carroll, p. 403).  Monotremes still retain both coracoid bones and the interclavicle, unlike therian mammals (Carroll, p. 421).   Therian mammals evolved a spine of the scapula which is actually homologous to anterior border of monotreme and cynodont scapula.  The supraspinous fossa is a new structure which exists only in therian mammals (Carroll, p. 409).  The glenoid cavity became more open, increasing importance of rotator cuff muscles for the stabilization of the shoulder (Carroll).  The interclavicle was lost (Carroll; Kardong, 2002, p. 330).

      Protoeutherians such as Zalambdalestes lacked separate coracoid bones in the shoulder (Carroll, p. 447).  In eutherians, the supraspinous fossa is enlarged and located more laterally (Kemp, 1982, p. 305; Carroll).  The shoulder of Proconsul was similar to that of higher apes (Carroll).

      In the Late Pleistocene, the human humerus robusticity decreased, reflecting a reduction in typical loads borne by the arm and shoulder.  There is no such reduction in the robusticity of the femur (Trinkaus, 1997).  While the H. sapiens found around 100,000 years ago in Israel had reduced strength of the upper arm, the hand characters, such as the bases of metacarpals, were more like those of modern humans (Niewoehner, 2001).  In neanderthals, the glenoid cavity was similar to that of Homo sapiens sapiens but it was not as wide (Churchill, 1990).

platypus
platypus
platypus

bat

bat

 

cat

cat

cat
cat

mink

mink

monkey

monkey

monkey

human clavicle

human clavicle

human clavicle

human scapula

human scapula

human scapula
human scapula human scapula