Sacrum and Pelvic Girdle Development in Lacertidae

Sacrum and pelvis structure and development in four species of Lacertidae currently assigned to different genera are studied: Lacerta agilis, L. vivipara, L. saxicola, Podarcis muralis. Size, shape, and position relationships between the cartilaginous elements as well as patterns and sequence of chondrification and ossification are given. Two sacral segments are characterized by a number of markers such as the branching of iliac arteries, development of the paired lymphatic hearts, and degeneration of sacral myomeres beginning from the dividing sacral myoseptum. Lizard species differ one from another by the number of presacral vertebrae and the extent of ossification. Degeneration of sacral myomeres and formation of the sacral gap in myomeres play a crucial role in development of sacral ribs and in the direction of their growth. Sacral vertebrae are defined as vertebrae, which ribs or transverse processes develop at the place of the sacral gap. There are ribs vs. diapophyses in Lacertidae and most probably in other lizards. Caudal ribs are not found, but prerequisites for their development take place, so they might be expected as a rare anomaly in Lacertidae. There is a strict order of maturation of skeletal elements: vertebral centra and neural arches, ilium, pubis, ischium and sacral ribs, which has its reflection in the adult phenotype. Additional elements of the pelvic girdle, namely, epipubis, epiischium, and hypoischium develop along the axial fibrous ligament and undergo chondrification and calcification separately. A hypothesis that the overall shape of the pelvic girdle is formed under the influence of the visceral cavity is partly supported. Cause-and-effect explanations of sacrum and pelvis morphogenesis are given. Certain anomalies of sacral structure are described and discussed in developmental terms.