Origin of Tetrapod Limbs

Abstract

The skeletal plan ox the crossop-terygian paired fins and of the primitive tetrapod limb is interpreted as an axial series of bones (mesomeres), each with one or more obliquely placed pre-axials (radials). No deletions appear to be necessary in accounting for the derivation of the tetrapod limb, nor is it thought necessary to add any postaxial elements. The transformation of pectoral and pelvic fins to limbs is essentially alike, except for the different angles of knee and elbow; little torsion and no inversion occurred in either. Indeed it is suggested that the primary mechanical features of amphibian locomotion were already present in the completely aquatic fish. Limb muscles are restored in the pectoral fin of Eusthenopteron; its postaxial flanges are considered as areas of insertion of muscles arising from myomeres of the body.