The musculoskeletal system of the caudal fin in basal Actinopterygii: heterocercy,diphycercy, homocercy

The caudal fin represents the posteriormost region of the vertebrate axis and is one location where forces are exerted to the surrounding medium. The evolutionary changes of its skeleton have been well analyzed in gnathostomes and revealed transitions from heterocercal to diphycercal and homocercal tails. In contrast, we only know little about the evolutionary transformations of the muscular system of the caudalis and about possible ways of force transmission from anterior myomeres to the caudal fin. The goals of this study are to gain insight into evolutionary transformations of the musculoskeletal system in the four basal actinopterygian groups (Cladistia, Chondrostei, Ginglymodi, and Halecomorphi) and to identify likely pathways of force transmission to the tail. In this context, the connective tissue of the myosepta is considered to be an essential part of the musculoskeletal system. For the first time, this system is analyzed for the whole postanal region. The use of microdissection techniques and polarized light microscopy revealed the collagen fiber architecture and the insertions of all postanal myosepta from cleared and stained specimens. The collagen fiber architecture is similar in all investigated specimens and thus represents the primary actinopterygian condition. All parts of postanal myosepta are dominated by longitudinally arranged myoseptal tendons (lateral and myorhabdoid tendons) that span several vertebral segments. This architecture supports the view that posterior myosepta are well designed to transfer muscular forces that are generated in anterior myomeres. In contrast to the uniform myoseptal architecture, the musculoskeletal system differs between the four basal actinopterygian groups. Among them, chondrosteans have retained the plesiomorphic condition of actinopterygian tails. For the remaining taxa several evolutionary novelties in the musculoskeletal system of the tail are revealed. Most of these have evolved independently in the cladistian and neopterygian stem lineage. In these groups extensions of all epaxial and hypaxial parts of myosepta are present that insert on caudal fin rays. This remarkable contribution of epaxial muscle masses to the caudal fin organization is in contrast to the skeletal organization, that largely derives from hypaxial material only. In contrast to former studies the hypochordal longitudinalis muscle is shown to be a synapomorphy of Halecostomi (Halecomorphi + Teleostei). The morphological framework presented here allows to generate new hypotheses on the function of caudal fins that can be tested experimentally.