Cuticle formation of the adductor muscle attachment in a podocopid ostracod, as an example of a calcified arthropod

The ostracod muscle scars have been used as important characters to define the evolutionary lineages of ostracods from the Palaeozoic to the Recent, though the formation and calcification of their muscle attachment have never been investigated. The present paper describes, for the first time, the cuticle formation or muscle attachment in a podocopid ostracod, as an example of a calcified arthropod. The formation of the muscle attachment structure in a podocopid ostracod progresses through the nearly same pathway as in other arthropods. The muscle connects to the cuticle by means of a specialised epidermal cell, the tendinal cell. Strong desmosomes adhere the muscle cell to the tendinal cell, while the tendinal cell is connected to the cuticle by hemidesmosomes with intracuticular fibres (tonofibrillae). The deposition of the new pre-exuvial cuticle takes place around the extended intracuticular fibres and these fibres maintain the connection between the tendinal cell and the old cuticle. At posteedysis the cuticle of the carapace begins its calcification, and immediately and rapidly increases its thickness, but the calcification of the muscle scars evidently progresses later than other parts of the procuticle, due to the fact that part of the tendinal cell is fully occupied with force-transmitting structures, like microtubules. Only the parts not engaged in force-transmitting action have enough space in the cell to store the granules to start the calcification. Furthermore, the less organic matrix in the procuticle of the muscle attachment area also contributes to the delay of calcification. The results of this paper provide information on cuticle formation in the calcified arthropods, which can be applied to fossil taxa, and show the relationships between the calcified and organic matrix during calcification. In addition, this study suggests that arthropod muscle scars, including in the fossil state, are useful characters when discussing the construction of muscular systems at the cellular level, and available as stable criteria through ontogeny for the comparative morphology of the exoskeleton.