Xiphactinus

Xiphactinus is an ichthyodectid ichthyodectiform ray-finned bony fish from the Early-Late Cretaceous of the entire world. It was officially named in 1870 by Joseph Leidy. It was one of the largest predatory fish during the Cretaceous, as well as one of the most widespread and successful.

## Physiology

Xiphactinus resembled a larger, bulkier wolf herring. It was a fish with a long, laterally-flattened body, triangular pectoral (arm), pelvic ("leg"), dorsal (back), and anal (rear) fins, a forked, crescent-shaped caudal (tail) fin, and a midsized head with large, upturned, toothy jaws. Its body would have been covered in scaly skin.

## Diet

Xiphactinus was a predator, preying on smaller fish, mollusks, diving birds, and small mosasaurs. Its teeth were small yet sharp, and were used to get a hold of struggling prey as the ray-finned fish swallowed them whole.

## Ecology

Xiphactinus would have filled an ecospace similar to that of a marlin, being a marine ambush predator that was capable of putting on extreme bursts of speed and preying on a wide variety of animals within its size range as well as migrating long distances in order to keep up with prey; both fish are similar in body shape, with a sturdy, elongated, streamlined body, triangular pectoral fins that could fold back against the body, and a crescent-shaped caudal fin attached to a narrow peduncle (stalk). Similarly to marlin, Xiphactinus would have mainly eaten fish and mollusks small enough to be swallowed whole; any prey items unable to be ingested in one go would have been spiked by its teeth, allowing it to slowly slide its prey inside of its mouth, one bite at a time. While it mainly would have preyed on smaller animals, the discovery of stomach contents in one specimen showed that it ate an ichthyodectid roughly 1/3 its body length, which confirms that it would have tackled larger prey from time to time; interestingly enough, the smaller fish was not digested, which possibly indicates that the larger fish died of internal injury due to the struggles of its prey rupturing an organ. Although it was a formidable predator, it was not at the top of the food chain, as larger carnivores such as mosasaurs and sharks often preyed on it, with a specimen with noticeable bite damage showing that it fell prey to the large mackerel shark Cretoxyrhina; in fact, the discovery of two more specimens involving one preserved inside one another indicates that it was also cannibalistic, meaning that smaller individuals would have had to worry about being eaten by larger ones.

## In popular culture

Due to documentaries and books focusing on prehistoric animals, Xiphactinus has become notably popular as an extinct predatory fish; while it is overshadowed by Dunkleosteus and *Otodus megalodon*, it appears in many documentaries, such as *Sea Monsters: A Prehistoric Adventure* and *River Monsters*. So far, its biggest public appearance was in the 3rd episode of the 2003 documentary *Chased by Sea Monsters*, where it preys on a flock of Hesperornis alongside a Squalicorax and an Eonatator; as well as this, it leaps out of the water in an attempt to prey on a Pteranodon flying overhead, and chases Nigel Marven (the explorer of the prehistoric seas) as he rides on an Archelon before being scared off by an approaching swarm of Tylosaurus.