The Developmental Pattern of the Musculature Associated with the Mandibular and Hyoid Arches in the Longnose Gar, Lepisosteus osseus (Actinopterygii, Ginglymodi, Lepisosteiformes)

This is the first in a planned series of studies in which we examine the cranial muscle ontogeny of exemplar taxa of actinopterygian clades to obtain a better understanding of the evolution of the cranial musculoskeletal system within the Actinopterygii. The Longnose Gar, Lepisosteus osseus, is a member of the basal actinopterygian family Lepisosteidae. Juvenile and adult gars are highly derived and anatomical characters can easily be misinterpreted, which makes a comparison with other taxa difficult. Highly complex adult structures such as the cranial skeleton and musculature are organized more simply early in development, making comparisons and homology assessments easier. Established methods such as clearing and double staining are widely used to study skeletal structures. However, methods to analyze the ontogeny of soft tissues are scarce. To study the development of the cranial musculature of L. osseus, we used a combination of 3D-reconstruction of soft tissue mCT scans and whole-mount antibody staining. The elongation of the palatoquadrate and the dentary that form the long snout of gars begins late in ontogeny. However, the adductor mandibulae complex, which separates from the mandibular muscle primordium initially as a single portion, does not follow the extension of the palatoquadrate. We also show that the ceratomandibular ligament that attaches the ceratohyal with the retroarticular process of the lower jaw is homologous with the mandibulohyoid ligament.