Schematic representation of the vertebral column of penguins representing the general vertebral formula and its numerical variations. A General vertebral formula of Pygoscelis; B Numerical variation with extra vertebra in the caudal region, e.g. Pygoscelis papua; C Numerical variations with a missing vertebra in the thoraric region, e.g. Pygoscelis papua; D Numerical variation with a missing vertebra in the thoracolumbar segment of the synsacrum, e.g. Pygoscelis papua; E Numerical changes in the thoracic region and the lumbar segment of the synsacrum occur due to homeotic transformations. There is a variation in the number of the vertebrae within regions (an extra thoracic vertebra and a missing vertebra in the lumbar segment of the synsacrum), but not provokes a change in the total number of vertebrae, e.g. Pygoscelis papua; F General vertebral formula of Spheniscus; G Numerical variation with an extra vertebra in the caudal segment of the synsacrum , e.g. Spheniscus magellanicus; H Numerical variation with two extra vertebrae in the free caudal region, e.g. Spheniscus magellanicus; I Numerical variation with a missing vertebra in the thoracic region, e.g. Spheniscus magellanicus; J Numerical changes in the thoracic and free caudal region occur due to homeotic transformations. There is a variation in the number of the vertebrae within regions (an extra thoracic vertebra and a missing free caudal vertebra) but not provokes a change in the total number of vertebrae, e.g. Spheniscus magellanicus; K General vertebral formula of Aptenodytes/Eudyptes. Abbreviations: C – cervical vertebrae; CT – cervicothoracic vertebrae; T – thoracic vertebrae; S – synsacrum; Ca – free caudal vertebrae; P – pygostyle.