Specialize or disappear: anisomerism, empirical evidence of the differentiation or loss of polyisomere structures during gnathostome evolution based on a comparative study of head and appendicular muscles (918.27)

Some authors have defended the notion of anisomerism, i.e. that there is an evolutionary tendency for the original polyisomeres of an organism (e.g., branchial arches, digits) to become differentiated so that phylogenetically derived organisms do not consist of a linear series of similar parts. This notion has been often related with the notion of functional uncouplings, i.e. that the differentiation of a polyisomere into an anatomically different structure allows this structure to not only have a new function but also to be liberated from its original functional complex and therefore to become functionally associated with other structures of the body. We provide here the first detailed empirical analysis of this issue based on data obtained from a comparison of the head and appendicular muscles of all major groups of gnathostomes. Our results support the idea that there is effectively a trend for the muscles associated for instance with the branchial arches or with the digits to specialize or, instead, to become lost in evolution. One example concerns our own species, Homo sapiens , which almost completely lacks any of the original similar polyisomere head muscles found in taxa such as sharks (e.g., branchial adductors), and that lacks many of the original similar polyisomere autopodial (hand/foot) muscles found in tetrapods such as salamanders or lizards (e.g., the flexores breves superficiales). Grant Funding Source : Faculty Start‐Up Package, Howard Univ. Coll. Medicine