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How did the jaw structure of palaeonisciforms affect the size of their mouth opening?
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chondrostean
Russian sturgeon (Acipenser gueldenstaedtii) swimming along the seafloor of the Black Sea.
In palaeonisciforms, the size of the mouth opening was directly influenced by the orientation of their jaw suspension, which was tied to the cheekbones covering the area between the eyes and the gill covers.1
The relationship between the jaw structure and the mouth gape was as follows:
* Oblique orientation: This configuration was associated with a wide mouth gape.1
* Nearly vertical orientation: This configuration was associated with a relatively smaller mouth gape.1
While these early chondrostean fishes possessed a consistent feeding mechanism design, the variation in their jaw suspension orientation allowed for differences in how they opened their mouths.1