Comparative embryological study of pectoral girdle and thorax in amniotes

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Dorsal and ventral dermis of carapacial ridge (CR) of turtle embryo were innervated by dorsal and lateral cutaneous nerve respectively, showing that there is less strict constraint between innervation pattern and cell lineage, because boundary between somite derived dermis and lateral plate derived dermis corresponds to ventral edge of the CR. In chicken, mouse and turtle, scapula anlage had rostral and caudal processes. The caudal process made a scapula blade in these animals. The rostral process gave rise to acrominal process in chick and mouse and rostro-ventral process in turtle, suggesting that the process of turtle pectoral girdle is homologous to the acromion. This result was supported by expression pattern of transcriptional factor, three dimensional topological relationships with nerve and muscle anlage and morphology of the recently discovered oldest turtle. The scapula anlage of mouse had extra process in rostro-dorsal part from which supraspinous fossa developed, showing that the fossa is novel structure among amniotes.