An Ultimate Guide to Xiphactinus: The Bulldog Tarpon

Leave a comment / / Updated on: 24th September 2023

Name MeaningSword RayHeightN/A
PronunciationZai-fak-ti-nusLength4.6 to 6 meters (15 to 20 feet)
EraMesozoicLate CretaceousWeight900 to 1,000 kilograms (2,000 to 2,200 pounds)
ClassificationActinopterygii, Ichthyodectiformes, IchthyodectidaeLocationNorth America and South America 

Xiphactinus Pictures

Xiphancitnus | estt via iStock

The Xiphactinus

Gage Beasley Prehistoric's Xiphactinus Concept
Gage Beasley Prehistoric’s Xiphactinus Concept

Sharks are the undisputed top predator in today’s marine ecosystem.

But back in the Cretaceous Period, large bony fish were among the most dominant animal species in the world’s oceans.

Xiphactinus is one of such remarkable bony fish genera.

This massive fish grew to an average length of about five to six meters (16–20 feet), making it one of the largest bony fish of the Cretaceous Period. 

Scientists also consider the Xiphactinus to be one of the most ferocious sea creatures to have ever existed.

Gage Beasley's Prehistoric Shirt Collection
Gage Beasley’s Prehistoric Shirt Collection
Gage Beasley's Prehistoric Plush Collection
Gage Beasley’s Prehistoric Plush Collection

It had powerful bulldog-like jaws with large fangs at the front that was useful for impaling prey during an attack.

Xiphactinus ruled in the Western Interior Seaway, the prehistoric sea that covered much of North America during the Cretaceous Period. 

This article provides an overview of one of the fiercest bony fish in the paleontological record. 

Physical Characteristics

Gage Beasley Prehistoric's Xiphactinus Size Comparison Chart
Gage Beasley Prehistoric’s Xiphactinus Size Comparison Chart

Xiphactinus was a much bigger and toothier version of present-day tarpons. 

Interestingly, both fish species are not closely related, so their similarities are only superficial.  

It had an elongated and streamlined body which is expected of a fast-swimming predatory fish.

The Xiphactinus’ streamlined body is similar to that of modern-day barracudas. 

This impressively large fish reached lengths of about 15 to 20 feet (4.5 to 6 meters) and weighed around 1,500 to 2,500 pounds (680 to 1,130 kilograms) on average. 

Restoration of X. audax
Restoration of X. audax | Fishboy86164577 via Wikipedia CC BY-SA 4.0

One of the most distinctive features of this monster fish is its massive strongly-upturned jaws, which were armed with numerous sharp and pointed teeth. 

These teeth were perfect for catching and holding onto their prey. 

Xiphactinus had wing-like pectoral fins as well as a distinct Y-shaped anal fin. 

It also had pelvic fins similar to modern-day bony fish. 

The Xiphactinus’ tail was heterocercal, meaning the upper lobe of the tail was larger and more prominent than the lower lobe. 

This tail shape would have helped with stability and improved maneuverability during swimming.

Habitat and Distribution

Xiphactinus was one of the dominant fish genera that lived in North America’s Western Interior Seaway. 

This large body of water covered most of central North America and Canada, dividing the entire continent into two distinct eastern and western land masses.

The prehistoric sea stretched from the Arctic Ocean to the Gulf of Mexico. 

The Xiphactinus range would have covered parts of present-day North Dakota, Kansas, South Dakota, Nebraska, and Texas.

The Western Interior Seaway was relatively shallow compared to modern oceans and would have provided diverse habitats for various marine organisms, including the Xiphactinus.

This massive fish may have also lived in the Patagonian seas of the Southern Hemisphere, with fossils discovered in Venezuela and Argentina. 

Xiphactinus was a pelagic predator. 

Xiphactinus swimming in the pelagic sea
Xiphactinus swimming in the pelagic sea | ДиБгд via Wikimedia Commons

This means it lived in the open waters far from the shores of the Western Interior Seaway. 

The streamlined body and powerful tail made it a fast and agile swimmer, a major advantage in the open ocean environment of the Late Cretaceous. 

The prehistoric seas where the Xiphactinus lived were temperate compared to today’s seas. 

The global climate was generally warmer during the Late Cretaceous effect. 

This has been attributed to greenhouse effects due to high carbon dioxide content. 

The Western Interior Seaway was a shallow and enclosed body of water. 

As a result, it experienced significant fluctuations throughout the Cretaceous Period and was constantly changing. 

Behavior and Diet

Xiphactinus long slender body
Xiphactinus long slender body | Julian Johnson via Fossil Era CC License

Xiphactinus had a long and slender body similar in shape to a torpedo. 

This suggests that it was an agile swimmer. 

It likely used its tail and large pectoral fins to propel itself through the water, chasing down prey with impressive speed and agility. 

Experts have estimated the likely top speed of this massive fish to be about 60 kilometers per hour (37 miles per hour). 

In addition to being a fast swimmer, Xiphactinus was probably capable of leaping out of the water, similar to modern dolphins. 

Xiphactinus is considered a voracious marine predator. 

Given its size, it was probably capable of feeding on any of the smaller animals present in its ecosystems. 

This would have included small bony fish, ancient sharks, turtles, and even marine reptiles like the mosasaurs that were alive during the Cretaceous Period. 

Xiphactinus hunting in the pelagic zone
Xiphactinus hunting in the pelagic zone | Cougarcat via RiverMonsterFandom

In addition to hunting prey actively, Xiphactinus probably scavenged on the remains of other animals as well. 

Possible prey of the Xiphactinus are well known. 

That’s because fossils of this fish have been found with partially digested prey in their stomachs. 

One of the most famous instances of Xiphactinus fossils with its gut content preserved is that of a 13-foot Xiphactinus individual preserved with a 6-foot Gillicus arctuatis in its gut. 

Experts believe the Xiphactinus probably died shortly after eating this fish due to injuries sustained while feeding on its oversized prey. 

Xiphactinus was most likely a solitary predator. 

Unlike some modern predatory fish that tend to exhibit schooling behavior to increase hunting efficiency, Xiphactinus was large enough to take down prey on its own. 

Life Cycle

Xiphactinus in its mating state
Xiphactinus in its mating state | Julian Johnson-Mortimer via ICR

Xiphactinus reproduced sexually, exhibiting an oviparous mode of reproduction. 

This means they laid eggs that hatched outside the mother’s body, just like most fish species do today. 

Not a lot is known about the reproduction and mating behavior of the Xiphactinus

But it most likely involved the female releasing eggs into the water, which the male then fertilized. 

They exhibited no parental care, and the fertilized eggs hatched into juvenile fish on their own. 

Juvenile Xiphactinus were considerably smaller and more vulnerable compared to their full adult size. 

They probably lived in nurseries in shallow waters close to the shoreline. 

This would have provided some level of protection against large predators. 

Depiction of a Xiphactinus swallowing a Gillicus
Depiction of a Xiphactinus swallowing a Gillicus | Apokryltaros via Wikipedia CC BY 2.5

Juvenile fishes likely preyed on small fishes and marine invertebrates. 

As they grew bigger, they would venture into deeper waters and develop an active predatory habit. 

Teeth fossils of juvenile Xiphactinus individuals have been found and studied by scientists. 

Experts noticed no notable changes in their dentition as they grew older. 

This implies that the juveniles were piscivores like their parents, with the only difference being that they ate smaller prey present in their shallow nursery. 

Evolution and History

Xiphactinus eating a prehistoric eel | Hammerbro123 via DinopediaFandom

Xiphactinus belongs to the order Ichthyodectiformes.

This is a group of prehistoric ray-finned fish that were quite abundant during the Late Cretaceous Period.  

The evolutionary history of this order of fish can be traced back to the Early Jurassic Period, around 200 million years ago. 

However, the genus Xiphactinus itself only emerged during the last few decades of the Cretaceous Period, approximately 100 million years ago, and was alive till the end of the period about 65 million years ago.

Xiphactinus evolved to become a much larger fish compared to its primitive ancestors within the Ichthyodectiformes order, such as the Thrissops and Allothrissops

These early members of the teleost group were relatively small. 

Most of them were between one and five meters (3–15 feet). 

Xiphactinus compared to other ichthyodectids
Xiphactinus compared to other ichthyodectids | Dmitry Bogdanov via Wikipedia CC BY 3.0

Xiphactinus exceeded this size range, becoming the apex predator in its ecosystem. 

In keeping with its role as an apex predator, Xiphactinus developed a formidable dentition characterized by specialized teeth for hunting and capturing prey. 

Interactions With Other Species

The marine environment of the Western Interior Seaway, where the Xiphaticus lived, was a very rich and diverse one. 

It included all types of living organisms, from plankton to apex predators. 

There were marine invertebrates, including ammonites and belemnites, as well as different fish species. 

Ammonite at sea
Ammonite at sea | estt via iStock

These were the most likely prey for the Xiphactinus

Although it was predominantly piscivore, this giant fish probably fed on turtles and marine reptiles as well. 

Prehistoric birds and flying reptiles (pterosaurs) were also present in the Late Cretaceous environments. 

They hunted for fish from the surface of the Western Interior Seaway. 

Since the Xiphactinus was capable of leaping out of the water to catch prey, it may have preyed on these birds as well. 

Bony fish like the Xiphactinus were among the dominant predator species in the Cretaceous waters; they were not at the very top of the food chain. 

Mosasaurus | MR1805 via iStock

They shared the marine ecosystem with giant reptiles such as the Mosasaurus, Elasmosaurus, and ichthyosaurs. 

These reptiles were at the top of the aquatic food chain of the Cretaceous Period and probably preyed on Xiphactinus individuals. 

Sharks such as the Squalicorax and Cretoxyrhina were present as well. 

The largest of them were around the same size as an average Xiphactinus, meaning they would have competed for the same food and other resources. 

The sharks probably hunted young Xiphactinus individuals and scavenged on the remains of dead adults. 

On at least one occasion, scientists have found Xiphactinus remains in the belly of a large Cretoxyrhina specimen. 

Cultural Significance

Fossil of a Xiphactinus audax at the Tellus Science Museum near Cartersville, Georgia
Fossil of a Xiphactinus audax at the Tellus Science Museum near Cartersville, Georgia | JJonahJackalope via Wikipedia CC BY-SA 4.0

The first fossil of the Xiphactinus was discovered in 1870 by Professor Joseph Leidy.

Since then, several more specimens have turned up, especially in the Niobrara Chalk Beds of Kansas. 

Fossils of this giant fish have been severally studied to understand the evolution of the bony fishes as well as the nature of the Late Cretaceous Marine ecosystem. 

The ecological relationship between the Xiphactinus and other fish species that were alive during the Late Cretaceous is among the most fascinating things about this marine predator. 

Xiphactinus is often described as a voracious (possibly gluttonous) predator. 

This is due to the discovery of fossils of this fish that died while trying to swallow prey much bigger than its skull. 

One of the most renowned examples of this is the famous fish-within-a-fish fossil which shows a Xiphactinus that died while swallowing a 6-foot-long Gillicus. 

Fish within a fish fossil at Denver Museum of Nature and Science
Fish within a fish fossil at Denver Museum of Nature and Science | David Wipf via Wikipedia CC BY 2.0

The fish-within-a-fish fossil is still on display at the Sternberg Museum of Natural History.

There are several other remarkable museum exhibits showcasing this giant fish. 

This fish has also appeared in some documentaries, such as the BBC’s Sea Monsters and season 2 of Apple TV+’s Prehistoric Planet, released very recently.

Xiphactinus was nominated as the State fossil of Kansas in 2010. 

However, the Tylosaurus was picked instead.

Despite not being so popular to the general public, Xiphactinus is one of the most ferocious bony fish genera ever discovered.


Xiphactinus was a large predatory fish that lived during the Late Cretaceous Period. 

It was superficially similar to present-day tarpons but was significantly bigger. 

The main habitat of this fish was the Western Interior Seaway which covered most of North America during the Late Cretaceous Period. 

Xiphactinus is one of the largest bony fish to have ever lived. 

It was also one of the fiercest predators of the Late Cretaceous marine ecosystem. 

Xiphactinus preyed mainly on fish, but its diet probably included invertebrates, turtles, and marine reptiles as well. 

It has a reputation as a massive predator whose voracious appetite occasionally got it into trouble. 

The discovery and subsequent study of this massive fish provide a glimpse of the intricate fish-eat-fish marine ecosystem of the Cretaceous Period. 


Did Xiphactinus become extinct with the dinosaurs?

Yes, Xiphactinus, alongside many other prehistoric creatures (including the dinosaurs), became extinct at the end of the Cretaceous Period during the end-cretaceous extinction event. 

What does the name Xiphactinus mean?

The name “Xiphactinus” means “sword ray.” 

It is derived from the Greek words “Xiphos,” which means “sword,” and “Actinos,” which means means “ray” or “spine.” 

It most likely refers to the long sword-like body of this fish.


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