Musculoskeletal study of cebocephalic and cyclopic lamb heads illuminates links between normal and abnormal development, evolution and human pathologies

Abstract

This paper is part of the emerging field of Evolutionary Developmental Pathology, dedicated to study the links between normal and abnormal development, evolution and human pathologies. We analyzed the head musculoskeletal system of several ‘natural mutant’ newborn lambs displaying various degrees of abnormality, from mild defects to cebocephaly and to cyclopia, and compared them with humans. Interestingly, muscle defects are less marked than osteological ones, and contrarily to the latter they tend to display left-right assymetries. In individuals with cebocephalic and even cyclopic skulls almost all head muscles are normal. The very few exceptions are some extraocular muscles and facial muscles that normally attach to osteological structures that are missing in the abnormal heads: such muscles are instead attached to the ‘nearest topological neighbor’ of the missing osteological structure, a pattern also found in cyclopic humans. These observations support Alberch’s ill-named “logic of monsters” - as a byproduct of strong developmental/topological constraints anatomical patterns tend to repeat themselves, even severe malformations displayed by distantly related taxa. They also support the idea that mammalian facial muscles reverted to an ancestral ‘nearest-neighbor’ muscle-bone type of attachment seen in non-vertebrate animals and in vertebrate limbs, but not in other vertebrate head muscles.