On measuring head length in fishes, amphibians and reptiles and on modified calliper rules



The definition of head length in ichthyology and herpetology varies along four dimensions. Three may be taxon-dependent: (1) the point selected as ‘the anterior end of the head’ (2) the location of ‘the posterior end of the head’ and (3) the degree of inclusion of soft structures at the posterior end of the head. But (4) is technical and has caused much confusion: whether to measure parallel to the long axis, or ‘point-to-point’, at an angle to the axis. In order to facilitate measurements which are standardized in this dimension, modified calliper rules are introduced.