The Lacrimal-Ectethmoid Bone Complex in Birds: A Single Character Analysis

A single character analysis was undertaken of the lacrimal-ectethmoid bone complex in birds. In order to investigate the structure, variation, function, and possible evolution of the complex, more than 2700 specimens representing all but a few avian families were examined. The lacrimal and ectethmoid bones exhibit much variation within the class. The great structural diversity appears to be the result of design and accidental (chance-based) evolutionary factors. Among the former is selection for the formfunction complex associated with several important biological roles: (1) a brace for the palatines to prevent breakage during feeding or other activities using the palate, (2) a brace, for the jugal bars to prevent breakage and to contribute to stabilization during kinesis, (3) a retractor-stop mechanism to prevent abnormal retraction, and (4) protection for the eye and for attachment of periorbital connective tissue. The lacrimal-ectethmoid complex serves as an example of several phylogenetic principles. A jugal brace can be brought about independently by the lacrimal or by the ectethmoid, thus illustrating the notion of multiple evolutionary pathways. Also, the evolution of a retractorstop notch in quite unrelated families is a remarkable case of convergence. Examples of parallelism, independent origin, and reversal are also discussed. The complex apparently has greater taxonomic value at the family and subfamily levels than at the ordinal or generic levels. However, no taxonomic decisions should be based only on single characters.