22 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



protraction in relatively firm earth. Galliform and anseriform birds 

 display a long retroarticular process of the mandible but neither 

 group has an obvious need for powerful depression of the lower jaw. 

 Rather, the process may be related to kinetic action through the 

 depressor mandibulae muscles. 



The examples just given represent birds in which there is a need 

 for strong, extensive, or repeated protraction and in which a combina- 

 tion of features associated with the depressor mandibulae is adapted 

 to accomplish or to aid protraction. Other examples can be found 

 among the passerine birds (see illustrations in Beecher, 1953, pp. 302, 

 318), but the elucidation of the adaptive radiation within these groups 

 remains a challenge for the anatomist. 



Evolution of the interlocking jaw articulation.— Bock (1964, 

 p. 37) postulated an obligatory coupling of the jaws through the 

 postorbital ligament and thus found no explanation for the presence 

 of two strict coupling mechanisms (postorbital ligament and jaw 

 articulation) in the same species or for the evolution of jaw-articula- 

 tion coupling. Explanations for these phenomena are possible, how- 

 ever, with the knowledge that coupling may be bypassed and that the 

 postorbital ligament serves functions other thaja coupling. The ful- 

 crum provided by the postorbital ligament causes the posterior portion 

 of the depressed mandible to be rotated upward and forward. One 

 result is that the mandible pushes the quadrates forward at least 

 during the initial phases of depression, but another consequence is 

 that a firm contact of the jaw and quadrate surfaces is assured through- 

 out motions of the lower jaw. Without the postorbital ligament (or 

 some functional equivalent) the anterior surfaces of the jaw articula- 

 tion would tend to separate during depression of the mandible. As 

 the condyles of the jaw articulation may serve to coordinate jaw 

 motions, the ligament would enhance coordination by keeping the 

 articulation surfaces in close apposition. The postorbital ligament 

 could thereby play an important part in the evolution of the inter- 

 locking grooves and ridges of the jaw articulation. Support for the 

 lower jaw through interlocking in a species such as Balaeniceps is lost 

 quickly if the rami are spread slightly or the quadrates displaced 

 anteromedially at any given position of the lower jaw, and it cannot 

 be assumed that ligamentary supports are unnecessary in specialized 

 "articulation-coupled" forms. The postorbital ligament is a structure 

 that probably developed early in the evolution of modern birds and 

 has played a role in the evolution of increased articulation-coupling 

 in various groups while being reduced or lost in others. Its loss may 

 be associated with the development of maximum independence of 

 jaw motion. 



