PDF (Lo-Res) - Smithsonian Institution Libraries

Implications of the Cranial Morphology of Paleognaths
for Avian Evolution
ABSTRACT
In the early evolution of birds, bill formation produced a problem
for muscular control of the thin, elongated upper jaw. In particular,
it required a relatively high retracting force. Three sources
of this force evolved. (1) A powerful M. retractor palatini (especially
in Tinamiformes and Apteryx), originating primarily on the
vomer and pterygoid, developed to provide direct muscular connection
between the dermal palate and the cranial base. It apparently
evolved due to a joining of the medial portions of the
pterygoid and mandibular depressor muscles, which were aligned
by development of the proc. mandibulae medialis (a character
unique to birds). (2) The ancestral pseudotemporalis muscle developed
into two portions, a large postorbital portion and an almost
horizontally oriented intramandibular portion. Each portion seves
to increase the retraction ability of the muscle as a whole. (3) The
external mandibular adductor muscle developed, which, in neognaths,
is larger than either muscle previously mentioned. Its evolutionary
development was temporarily retarded by reduction of
one of its places of origin—the upper temporal arch.
Introduction
For more than a century, paleognaths have been subjected to
morphological studies in order to ascertain their apparently
primitive nature and to discover their position in avian phylogeny
(W.K. Parker, 1866; T.J. Parker, 1891; Pycraft, 1900; Mc­
Dowell, 1948; Hofer, 1945, 1950, 1955; de Beer, 1956; Webb,
1957; Muller, 1963; Bock, 1963; Cracraft, 1974; Yudin, 1970,
1978). I shall try to extract information on avian ancestry from
the comparative and functional morphology of the feeding apparatus
in paleognaths.
Nomenclature for species' binomials and English names of
modem birds follows Sibley and Monroe (1990).
ACKNOWLEDGMENTS.—In the process of this work I received
valuable assistance from the following individuals and
Felix Y. Dzerzhinsky, Faculty of Biology, Moscow State University,
Moscow 119899, Russia.
Felix Y. Dzerzhinsky
267
institutions. F. Vuilleumier and A.V. Andors (American Museum
of Natural History, New York, New York) and E.G. Kordicova
(Institute of Zoology, Kazakh Academy of Sciences, Almaty,
Kazakhstan) granted me access to the alcoholic specimen
of Apteryx sp.; K.A. Yudin and V.M. Loskot (Zoological Institute,
Russian Academy of Sciences, St. Petersburg, Russia)
provided me with specimens of Eudromia and Casuarius and
with several skulls of paleognaths, respectively. M.V. Bevolskaya
(Institute of Cattle-Breeding Askania-Nova, Ukraine)
permitted me to dissect the alcoholic heads of Dromaius novaehollandiae
(Latham), Rhea americana (Linnaeus), and
Struthio camelus (Linnaeus). A. Elzanowski (National Museum
of Natural History, Smithsonian Institution, Washington,
D.C.) and E.N. Kurochkin kindly assisted me in obtaining literature.
A.N. Kuznetsov helped me in editing the manuscript and
in translating it into English. S.C. Bennett, L.D. Martin, and
S.L. Olson read the English version with fruitful criticism and
helped me in editing it. I am sincerely grateful to all these persons.
This work was supported by The Cultural Initiative Foundation,
Moscow, and by The Russian Foundation of Basic Researches
(RFBR, grant N 96-04-50822).
Skeleto-muscular Consequences of Bill Formation
The adductory force of the mandible is transferred to the upper
jaw through a food object. Resistance of the upper jaw to
this force is produced (Figure 1) by a combination of tension
on the ventral stalk (premaxillary and maxillary bones with
palate caudally) and longitudinal compression of the dorsal
stalk (frontal projection of premaxillary and premaxillary processes
of the nasal bones). The longer the jaw grew, the greater
the forces became, and, due to jaw lightening, the stresses became
ever greater.
The active forces necessary for normal grasping of food
items must be supplied by muscles. The muscular force that
creates tension in the palate and upper-bill floor also can accomplish
ventral movement of the upper jaw by means of re-