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 μCT 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.
Ontogenetic series of Lepisosteus osseus. The specimens are arranged in a developmental order rather than according to their actual size. The diameter of the (A) egg is 4.5 mm and lengths of other specimens are as follows: (B) 2.6 mm NL; (C) 4.8 mm NL; (D) 5.6 mm NL; (E) 7.8 mm NL; (F) 9.0 mm NL; (G) 13.2 mm NL; (H) 16.9 mm NL; (I) 16.3 mm SL; (J) 16.7 mm SL; (K) 18.6 mm SL; (L) 23.0 mm SL; (M) 34.0 mm SL.
(A) Lateral view of a reconstruction of a μCT scan of a 6.2 mm NL Lepisosteus osseus. (B) Lateral view of a confocal image of a 7.2 mm NL Longnose Gar. Abbreviations: prhy, hyomandibularis primordium; prma, mandibularis primordium.
Reconstructions of a μCT scan of a 10.0 mm NL Lepisosteus osseus. (A) Lateral view of a reconstruction of the skeletal elements and (B) of the musculature of the mandibular and hyoid arch. (C) Ventral view of the right side of the ventral portions of the musculature. (D) Lateral and (E) ventral views of confocal images of an 8.3 mm NL and (F) lateral and (G) ventral view of confocal images of a 9.5 mm NL Lepisosteus osseus. Abbreviations: am, musculus adductor mandibulae; coh d, musculus constrictor hyoideus dorsalis; coh v, musculus constrictor hyoideus ventralis; chy, ceratohyal cartilage; com d, musculus constrictor dorsalis; hys, hyosymplectic cartilage; ihy, interhyal cartilage; im, musculus intermandibularis; mc, Meckel’s cartilage; pq, palatoquadrate.
Lateral and ventral views of reconstructions of a μCT scan of a NL 12.8 mm Lepisosteus osseus. (A) Lateral view of the skeletal elements and (B) lateral view of the musculature of the mandibular and hyoid arch. (C) Ventral view of the right side of the ventral portions of the musculature. (D) Lateral and (E) ventral views of confocal images of a 10.9 mm NL Lepisosteus osseus. Abbreviations: am, musculus adductor mandibulae; chy, ceratohyal cartilage; coh d, musculus constrictor hyoideus dorsalis; com d, musculus constrictor dorsalis; hh, musculus hyohyoideus; hhy, hypohyal cartilage; hys, hyosymplectic cartilage; ih, musculus interhyoideus; ihy, interhyal cartilage; im, musculus intermandibularis; mc, Meckel’s cartilage; pq, palatoquadrate.
Lateral and ventral views of reconstructions of a μCT scan of a 16.1 mm SL Lepisosteus osseus. (A) Lateral view of the skeletal elements and (B) lateral view of the musculature of the mandibular and hyoid arch. (C) Ventral view of the right side of the ventral portions of the musculature. (D) Lateral and (E) ventral views of confocal images of a 16.1 mm SL Lepisosteus osseus. Abbreviations: aap, musculus adductor arcus palatini; ahy, musculus adductor hyomandibulae; am, musculus adductor mandibulae; aop, musculus adductor operculi; chy, ceratohyal cartilage; cml, ceratomandibular ligament; dop, musculus dilatator operculi; hh, musculus hyohyoideus; hys, hyosymplectic cartilage; ih, musculus interhyoideus; ihy, interhyal cartilage; im, musculus intermandibularis; lap; musculus levator adductor palatini; mc, Meckel’s cartilage; phy, musculus protractor hyomandibulae; porb, musculus preorbitalis; pq, palatoquadrate.
Lateral and ventral views of reconstructions of a μCT scan of a 22.0 mm SL Lepisosteus osseus. (A) Lateral view of the skeletal elements with a close up of the cml and (B) lateral view of the musculature of the mandibular and hyoid arch. (C and D) Lateral views of the musculature (superficial muscles removed in C). (E) Ventral view of the ventral portions of the musculature. (F) Similar view as in D, but with the hyosymplectic cartilage removed. Abbreviations: aap, musculus arcus palatini; ahy, musculus adductor hyomandibulae; am, musculus adductor mandibulae; aop, musculus adductor operculi; chy, ceratohyal cartilage; cml, ceratomandibular ligament; dop, musculus dilator operculi; hh, musculus hyohyoideus; hhy, hypohyal cartilage; hys, hyosymplectic cartilage; ih, musculus interhyoideus; ihy, interhyal cartilage; im, musculus intermandibularis; lap, musculus levator adductor palatini; mc, Meckel’s cartilage; phy, musculus protractor hyomandibulae; pm, musculus palatomandibularis; porb p, musculus preorbitalis profundus; porb s, musculus preorbitalis superficialis; pq, palatoquadrate.
Histological sections of 21.5 mm SL Lepisosteus osseus. (A, B, C) Sections from a mid-region of the head. (D) Section from a more anterior region. Abbreviations: am, musculus adductor mandibulae; bbr, basibranchial cartilage; chy, ceratohyal cartilage; hbr, hypobranchial cartilage; hh, musculus hyohyoideus; hys, hyosymplectic cartilage; ih, musculus interhyoideus; mc, Meckel’s cartilage; pch, parachordal cartilage; pm, musculus palatomandibularis; porb p, musculus preorbitalis profundus; pq, palatoquadrate; sth, sternohyoideus.
Contributor Notes
Associate Editor: G. Arratia.