| Abstract: | Finches have two major types of seed husking, each with two phases: crushing and seed husk disposal. Recent observations show that both Spice Finches and Greenfinches husk seeds by a crushing mechanism, but the Greenfinch, Carduelis chloris, applies mediolateral movements of the lower mandible, especially during seek husk disposal, while such movements are not-found in the Spice Finch, Lonchura punctulata. A detailed comparison of the anatomies of their jaw apparatuses elucidates that numerous discriminating characters form an integrated complex that parallels the difference in seed husking. These focus on 1) a more flattened shape of the articular facets in the quadratomandibular articulation and a lack of the postorbital ligament, resulting in decoupling of the quadratomandibular joint facets, and release of the pseudotemporal muscle from jaw elevation only and 2) adductor mandibulae and pterygoid muscles with an increased medial component in the Greenfinch. Muscles built heavier in the Greenfinch than the Spice Finch, especially mediolaterally oriented muscles, are in concurrence with muscle asymmetries found in crossbills (Loxia sp.), a finch-species employing unilateral mandibular rotation. These two groups of differences are conditions to transform a dorsoventrally oriented into an improved, mediolaterally oriented husking design. These features, though in concert with many others, are proposed as they key innovations that released the increase of crushing forces by extending forces from jaw muscles and improvement of the disposal of the crushed shell by left-right lower mandible manipulation. Such an improved husking capacity may have opened the vast food-array of hard-shelled, dicotyledon seeds, allowing extensive trophic radiation. J. Morphol 232:1–33, 1997. © 1997 Wiley-Liss, Inc. |
