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The upper jaw, or maxilla [12] [13] is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible (lower jaw), which is also a fusion of two halves at the mandibular symphysis. In bony fish, the maxilla is called the "upper maxilla," with the mandible being the "lower maxilla".
In all jawed fish (gnathostomes), the first arch pair (mandibular arches) develops into the jaw, the second gill arches (the hyoid arches) develop into the hyomandibular complex (which supports the back of the jaw and the front of the gill series), and the remaining posterior arches (simply called branchial arches) support the gills.
Jawless fish and sharks only possess a cartilaginous endocranium, with the upper and lower jaws of cartilaginous fish being separate elements not attached to the skull. Bony fishes have additional dermal bone, forming a more or less coherent skull roof in lungfish and holost fish. The lower jaw defines a chin.
This arch divides into a maxillary process and a mandibular process, giving rise to structures including the bones of the lower two-thirds of the face and the jaw. The maxillary process becomes the maxilla (or upper jaw, although there are large differences among animals [11]), and palate while the mandibular process becomes the mandible or lower jaw.
Guiyu oneiros, the earliest known bony fish, lived during the Late Silurian, 425 million years ago. [1] It has a combination of both ray-finned and lobe-finned features. Bony fish are characterized by a relatively stable pattern of cranial bones, rooted, medial insertion of mandibular muscle in the lower jaw.
The four classical muscles of mastication elevate the mandible (closing the jaw) and move it forward/backward and laterally, facilitating biting and chewing. Other muscles are responsible for opening the jaw, namely the geniohyoid , mylohyoid , and digastric muscles (the lateral pterygoid may play a role).
Pandas show elaboration of the crushing features of the dentition. The molars are broad, flat, multi cuspid teeth and are the main grinding surface. Jaw action is not a simple crushing one but rather a definite sideways grinding. Panda jaws have a large zygomatico-mandibularis muscle, which is responsible for the sideways movement of the jaw.
Tuna and other fast-swimming ocean-going fish maintain their muscles at higher temperatures than their environment for efficient locomotion. [60] Tuna achieve muscle temperatures 11 °C (19 °F) or even higher above the surroundings by having a counterflow system in which the metabolic heat produced by the muscles and present in the venous ...