Polyisomerism, Anisomerism, and the Evolution and Development of the Vertebrate Head Muscles

Abstract

Early vertebrates like cyclostomes (lampreys and hagfish) have many muscles that appear as linear series of similar and equivalent structures, also called polyisomeres. In the first half of the 20th century William K. Gregory postulated that there is an evolutionary trend to reduce the numbers of repeating structures and/or to modify them into more specialized structures, which he designated as anisomerism. Similarly, “Williston's Law” is a trend towards a reduction in number and/or specialization in function of the bones of the skull. Interestingly, most of the discussion about anisomerism, polyisomerism, and Williston's Law focus on hard tissues. Here we provide an encompassing overview of the cephalic muscle evolution in vertebrates and show that there is (1) a trend to have less polyisomeric (serially similar) and more specialized muscles (anisomerism), and (2) a parallel between ontogeny and phylogeny, i.e. the same trends observed in evolution are observed in development. That is, it is important to stress that the decrease in polyisomeric muscles and increase in specialized muscles does not reflect the total number of cephalic muscles in the adult specimen. For example, during the cephalic muscle evolution from the last common ancestor of gnathostomes to humans 27 polyisomeric muscles were lost, but a single muscle (interhyoideus) got specialized into numerous muscles that are present in the adult human (ca. 22 muscles of facial expression).