Xiphactinus

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Xiphactinus sometimes called the X fish was a massive, slender bony fish that grew to between 16 to 20 feet long. Its most frequently found roaming the western interior seaway but has been found on several other continents, suggesting worldwide distribution. It had a double-hinged jaw with an extra joint on the upper jaw allowing its teeth to face towards it’s prey then shift back into position as it chewing. With both sets of teeth facing forward it would ambush its prey at presumed speeds of up to 37 mph then proceeded to swallow it whole. One such example is the famous fish in a fish fossil where a Xiphactius swallowed a six-foot Gillicus, which was a smaller relative of Xiphactinus whole, leading and the live fish squirming, rupturing it’s organs and the two quickly sinking to the bottom before being buried by sediments. Xiphactinus is hypothesised to be endothermic or warm-blooded while most fish are ectothermic. Needing to be warm-blooded to maintain is more active lifestyle and it’s body as it was already adapted for maintaining heat from gigantothermy. Xiphactinus’s closest living relative is the Tarpon however, it has nightmarish dentition that resembles a goliath tigerfish. Xiphactinus' diet would consist of anything that could fit in its mouth like baby mosasaurs and turtles, sharks, squid, other fish, seabirds, and pterosaurs, with the Tarpon being known for jumping out the water for prey. You would think Xipactinus would be the apex predator of the oceans but it would occasionally be prey to Sharks and Mosasaurs.

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