Figure 1.

Hypothesized evolutionary transformations of the morphology and function of the axial system in craniates. Data were compiled from various sources (see text) and mapped onto a simplified phylogenetic hypothesis based on [71]. Character states plesiomorphic for craniates are indicated by arrows. -- Axial skeleton (rectangles): Notochordates (i.e., Cephalochordata + Craniata) ancestrally possess a notochord, eponymous for the group. In early vertebrates, cranio-caudally uniform vertebral elements evolved (VE). In gnathostomes, the axial skeleton is regionalized. A trunk (= dorsal, D) and tail region (caudal, CD) are distinguished in gnathostome fishes, while a cervical (C), truncal, sacral (S), and caudal region are present in early tetrapods. In mammals, the truncal region is further subdivided into a thoracic (T) and a lumbar (L) region. -- Axial musculature (circles): Gross anatomy and fiber orientation: Transformations in the arrangement of the perivertebral musculature are illustrated by schematic cross-sections showing the gross-anatomical changes (left) and cartoons of a few body segments in lateral perspective illustrating the changes in muscle and/or fiber arrangement (right). Dorsal and ventral parts of the myomeres are innervated by separate rami of the ventral root in agnathan fishes (light and dark brown). In each segment, muscle fibers span longitudinally between adjacent myosepta. In gnathostomes, the dorsal and ventral myomere parts are morphologically separated by the horizontal septum (pink) resulting in epaxial (ep) and hypaxial (hy) muscles. Likely associated with the evolutionarily new requirements to stabilize the body against long-axis torsion, deeper muscle fibers are obliquely oriented. In non-amniote tetrapods, the epaxial musculature retained its segmental organization in contrast to the hypaxial musculature, which comprises the polysegmental subvertebral (sv) and the abdominal wall muscles (the latter are not shown here). The majority of the epaxial fibers connects adjacent myosepta longitudinally, while deeper fibers run at different angles. In amniotes, the epaxial musculature is reorganized into three longitudinal and polysegmental muscle tracts (tr: transversospinal, lo: longissimus, ilc: iliocostalis). In mammals, the transversospinal muscle is subdivided into several entities forming the transversospinal system (trs). The mammalian ventrovertebral musculature is strengthened by the psoas major (ps). -- Axial muscle function (diamonds): The plesiomorphic function of the axial musculature is to mobilize the body in the horizontal plane. The horizontal and torsional moments that result from the evolution of fins and a heterocercal tail, which tend to laterally bend the trunk and cause long-axis torsion, respectively, have to be counteracted by the axial muscles in gnathostome fishes. In tetrapods, as a consequence of the evolution of supporting limbs and transition to land, the axial muscles additionally function to globally stabilize the trunk against inertial and extrinsic limb muscle forces as well as against gravitational forces. Note that the evolution of limbs preceded the transition to land. In tetrapods with a sprawled limb posture, extrinsic limb muscle forces in the horizontal plane are relatively large. The greater agility and maneuverability as well as an increased importance of limb action for body propulsion, likely requires the axial muscles to dynamically stabilize the trunk to a greater extent in amniotes than in non-amniote tetrapods. Associated with the evolution of sagittal mobility and a parasagittal limb posture in mammals, the axial muscles additionally function to globally stabilize the trunk against sagittal components of extrinsic limb muscle action as well as against inertia. Furthermore, the axial musculature mobilizes the trunk in the sagittal plane during asymmetrical gaits.