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How confident are you in a lack of swim bladders?

Are you aware that many large fish use gliding as a form of energy conservation. This is facilitated by pectoral fins. They can still possess swim bladders. Within the tuna family there are species with and without swim bladders. Indeed the albacore tuna with exceptionally long pectorals has lost it and relies on dynamic lift. Others such as the bluefin and yellowfin use a mixture of dynamic lift and buoyancy adjustment from their swim bladder. These fish are incredibly active but still find benefit in not relying solely on dynamic lift. Also unless the lift force acts through the centre of mass there will be a resultant moment causing a rotation. In sharks this could be the reason for the larger upper lobe on the tail which these fish seem to lack? My suggestion is that the larger pectoral fins could provide a means to balance the changes in hydrodynamic lift resulting from the changes of mouth open and mouth closed conditions?

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There are two things to consider with your question. The first is that comparing active teleost predators like tuna to Protosphyraena is not an apples-to-apples comparison. Not only are the ossification patters (and therefore density/distribution throughout the body) different, these fish also lack the ability to retract the pectoral fins flush against their body. This seemingly minor difference might have larger implications as the pectoral fins in pachycormids are permanently in the slipstream and therefore always interacting with the environment, unlike tuna and actually more like sharks.

You make a very good point with the center of gravity postulation. I'd answer that in the pursuit predators at least, the lift consideration is mostly addressed with the exceptionally large lateral caudal flukes (almost giving the tail a cruciform layout) providing an aft source of lift to counteract the upwards body rotation that might be caused by a forward center of lift based on pectoral fins alone. The vast majority of bone on these fish is in the cranial and pectoral area as well, so the densest portion of the animal will also be at or forward of the pectoral fins.

It's an exciting time to see these fish finally being looked at as organisms interacting in their environment rather than mere fossil specimens. While over 100 were evaluated in the study, it became clear that many more specimens are needed to answer more questions.

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