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

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The skeleton of the jaws and neurocranium of sturgeons (Acipenseridae) are connected only through the hyoid arch. This arrangement allows considerable protrusion and retraction of the jaws and is highly specialized among ray-finned fishes (Actinopterygii). To better understand the unique morphology and the evolution of the jaw apparatus in Acipenseridae, we investigated the development of the muscles of the mandibular and hyoid arches of the Siberian sturgeon, Acipenser baerii. We used a combination of antibody staining and formalin-induced fluorescence of tissues imaged with confocal microscopy and subsequent three-dimensional reconstruction. These data were analyzed to address the identity of previously controversial and newly discovered muscle portions. Our results indicate that the anlagen of the muscles in A. baerii develop similarly to those of other actinopterygians, although they differ by not differentiating into distinct muscles. This is exemplified by the subpartitioning of the m. adductor mandibulae as well as the massive m. protractor hyomandibulae, for which we found a previously undescribed portion in each. The importance of paedomorphosis for the evolution of Acipenseriformes has been discussed before and our results indicate that the muscles of the mandibular and the hyoid may be another example for heterochronic evolution.

Section: Discussionsupporting

confidence: 93%

Development of the muscles associated with the mandibular and hyoid arches in the Siberian sturgeon, Acipenser baerii (Acipenseriformes: Acipenseridae)

Warth

Hilton

Naumann

et al. 2017

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“…These muscles include the m. adductor hyomandibulae (originating from the lateral wall of the otic capsule and inserting onto the medial surface of the hyosymplectic) and the m. adductor operculi (originating from the connective fascia just lateral to the otic capsule and inserting onto the medial surface of the opercle). Unlike Lepisosteus (Konstantinidis et al, ; Lauder, ), no distinct m. adductor arcus palatini muscle could be identified in A. spatula —that is, the m. adductor hyomandibulae does not have any fibers that insert onto the palatoquadrate (Figure ). Like Lepisosteus (Konstantinidis et al, ; Lauder, ), the adductor operculi shows no sign of subdivision (Figures and Figure S11), and no specialized jaw opening muscle corresponding to the m. levator operculi in Amia could be identified.…”

Section: Resultsmentioning

confidence: 99%

“…Unlike Lepisosteus (Konstantinidis et al, ; Lauder, ), no distinct m. adductor arcus palatini muscle could be identified in A. spatula —that is, the m. adductor hyomandibulae does not have any fibers that insert onto the palatoquadrate (Figure ). Like Lepisosteus (Konstantinidis et al, ; Lauder, ), the adductor operculi shows no sign of subdivision (Figures and Figure S11), and no specialized jaw opening muscle corresponding to the m. levator operculi in Amia could be identified. Instead, the m. adductor operculi intermingles fibers with the opercular membrane, which also subsequently fuses with the m. hyohyoideus superior to form a complete opercular valve around the margins of the opercular flap (Figure ).…”

Section: Resultsmentioning

confidence: 99%

“…Within this scarf joint, a pair of cartilaginous processes extends dorsolaterally from the ethmoid cartilage to support lateral flanges of the overlying frontal bones (Figures c and S4). These processes correspond to a thickened, anteriorly tapering portion of the palatoquadrate cartilage lying in a dorsal groove of the ectopterygoid, the pars autopalatina (sensu Konstantinidis et al, ; Figure S4). In kinematic simulations, this unusual ethmoid‐autopalatine articulation allows mediolateral sliding, while restricting dorsolateral rotation of the palate (Figure c).…”

Section: Resultsmentioning

confidence: 99%

“…However, LAP also originates partially on the suborbital bones of the cheek, although to a lesser degree than the condition reported for Lepisosteus (Lauder, ). LAP also has small subdivisions extending posteriorly, the m. protractor hyomandibulae (sensu Konstantinidis et al, ), and a portion extending directly between the metapterygoid and the hyosymplectic (Figure S12), which in Lepisosteus is reported to be entirely ligamentous (Arratia & Schultze, ). With these muscular connections between the palatoquadrate and hyosymplectic‐cheek, it is possible that LAP serves to conjoin movements of the palatoquadrate and hyosymplectic, which are particularly flexible relative to each other (see above).…”

Section: Resultsmentioning

confidence: 99%

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Feeding kinematics and morphology of the alligator gar (Atractosteus spatula, Lacépède, 1803)

Lemberg

Shubin

Westneat

2019

Living gars are a small clade of seven species that occupy an important position on the actinopterygian phylogenetic tree as members of Holostei, sister‐group to teleosts, and exhibit many plesiomorphic traits used to interpret and reconstruct early osteichthyan feeding mechanisms. Previous studies of gar feeding kinematics have focused on the ram‐based, lateral‐snapping mode of prey capture found in the narrow‐snouted Lepisosteus genus, whereas this study focuses on a member of the broad‐snouted Atractosteus sister‐genus, the alligator gar (Atractosteus spatula, Lacépède, 1803). High‐speed videography reveals that the feeding system of alligator gars is capable of rapid expansion from anterior to posterior, timed in a way to generate suction, counteract the effects of a bow‐wave during ram‐feeding, and direct a unidirectional flow of water through the feeding system. Reconstructed contrast‐enhanced μCT‐based cranial anatomy and three‐dimensional modeling of linkage mechanics show that a lateral‐sliding palatoquadrate, flexible intrasuspensorial joint, pivoting interhyal, and retractable pectoral girdle increase the range of motion and expansive capabilities of the alligator gar feeding mechanism. Reconstructions of muscular anatomy, inferences from in vivo kinematics, and in situ manipulations show that input from the hyoid constrictors and hypaxials play an important role in decoupling and modulating the dual roles of the sternohyoideus during feeding: hyoid retraction (jaw opening) and hyoid rotation (pharyngeal expansion). The alligator gar possesses an intricate feeding mechanism, capable of precise control with plesiomorphic muscles that represent one of the many ways the ancestral osteichthyan feeding mechanism has been modified for prey capture.

Development of the skull and pectoral girdle in Siberian sturgeon,Acipenser baerii, and Russian sturgeon,Acipenser gueldenstaedtii(Acipenseriformes: Acipenseridae)

Warth

Hilton

Naumann

et al. 2017

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The head is considered the major novelty of the vertebrates and directly linked to their evolutionary success. Its form and development as well as its function, for example in feeding, is of major interest for evolutionary biologists. In this study, we describe the skeletal development of the cranium and pectoral girdle in Siberian (Acipenser baerii) and Russian sturgeon (A. gueldenstaedtii), two species that are commonly farmed in aquaculture and increasingly important in developmental studies. This study comprises the development of the neuro-, viscero- and dermatocranium and the dermal and chondral components of the pectoral girdle, from first condensation of chondrocytes in prehatchlings to the early juvenile stage and reveals a clear pattern in formation. The otic capsules, the parachordal cartilages, and the trabeculae cranii are the first centers of chondrification, at 8.4mm TL. These are followed by the mandibular, then the hyoid, and later the branchial arches. Teeth form early on the dentary, dermopalatine, and palatopterygoid, and then appear later in the buccal cavity as dorsal and ventral toothplates. With ongoing chondrification in the neurocranium a capsule around the brain and a strong rostrum are formed. Dermal ossifications start to form before closure of the dorsal neurocranial fenestrae. Perichondral ossification of cartilage bones occurs much later in ontogeny. Our results contribute data bearing on the homology of elements such as the lateral rostral canal bone that we regard homologous to the antorbital of other actinopterygians based on its sequence of formation, position and form. We further raise doubts on the homology of the posterior ceratobranchial among Actinopteri based on the formation of the hyoid arch elements. We also investigate the basibranchials and the closely associated unidentified gill-arch elements and show that they are not homologous. J. Morphol. 278:418-442, 2017. © 2017 Wiley Periodicals, Inc.