Xiphactinus: The Beautiful Bull of the Sea

File:XiphactinusDB cropped.png - Wikimedia Commons
An artists impression of Xiphactinus, showcasing its characteristic face, jaws and Tarpon-like body.
Image Credit: Dmitry Bogdanov, https://commons.wikimedia.org/wiki/File:XiphactinusDB_cropped.png

The large (but masked and socially distanced) crowd bubbled with excitement as the artist they had come to see readied himself for the presentation of his masterpiece. Behind him is a large rectangular box covered by a brown sheet. His nervous hands sending flutters through the sheet he is holding, he readies himself for the biggest moment of his life.

“This is my latest and greatest work. An unknown fish pulled from the deep ocean, presented and preserved in exquisite detail in the box behind me using a formalin solution!”.

“I give you. Beauty!”

The artist pulls off the sheet, revealing a 5 metre long fish with upturned, bulldog like jaws filled to the brim with razor sharp teeth! The crowd are in complete shock. Beauty is not a word that comes into their mind. After the surprise wears off the crowd grumble their disappointment.

“Is this another example of unfathomable modern art? One naysayer says.

“Didn’t Damien Hirst once do something like this?” another asks

“I don’t get it? It’s just a weird looking fish?” yet another comments

After a while the crowd, once expectant and now disappointed, move on. The sounds of their footsteps carrying them away from the scene are accompanied by the cries of despair of the disappointed artist. Eventually only one person remains. A shaggy haired man who looked like he hadn’t had a haircut for months stares intently at the fish. He approaches the artist, who is wiping away his tears.

“Firstly, that is indeed beautiful!”

“Oh really?! That means so much! Thank you!” the artist replies with excitement.

“Secondly” the shaggy haired man continues. “Where did you get this fish?! It’s supposed to have been extinct for 66 million years?!!”

First discovered in Kansas, USA, in 1870 and named by Professor Joseph Leidy of the University of Pennsylvania; Xiphactinus (Latin for “Sword-Ray”) was a huge fish that had the size and power to compete with the large Sharks and even the medium sized Mosasaurs that it shared the oceans with. Two species of Xiphactinus are currently known to science. The first is Xiphactinus audax. This was the first species discovered, and is the larger of the two. X.audax had a wide geographical range, with fossils being discovered across North America from Saskatchewan in Canada, to Texas, New Jersey, Mississippi, Georgia and Delaware in the United States. The second species is Xiphactinus vetus. This species was discovered much more recently in 1997 and is known from the Eastern United States. This large range is just the North American distribution however! Fragments of an upper jaw bone and vertebrae from Xiphactinus audax were discovered in Patagonia in Argentina, South America, and described in 2020. These finds have expanded its range much further south west than was previously thought. Furthermore Xiphactinus fossils have also been unearthed in Western Europe and even as far as Australia. This almost worldwide distribution indicates that you would’ve had a good chance of spotting a Xiphactinus no matter where you ventured in the Late Cretaceous seas and that this multi-fanged fish was an incredibly successful animal for its time.

The most striking feature of Xiphactinus was undoubtedly its short, bulldog like face complete with a protruding and upturned lower jaw. This face was attached to a sleek, streamlined body complete with a “wing-like” pair of pectoral fins, a backward pointing dorsal fin, downward facing pelvic and anal fin, a broad tail and smooth scales. In essence Xiphactinus would’ve looked like a modern day Tarpon but larger and with a blunter, more fanged-teeth filled face. Just like the tarpon Xiphactinus was built for speed. Powerful strokes from its tail accelerated it through the water, and combined with jaws filled with large and sharp teeth would’ve made Xiphactinus a formidable hunter. This appearance is unique and begs the question; who was Xiphactinus related to? Well, Xiphactinus was a member of the Teleosts, a large group of bony fish which are also known as the “Ray-Finned Fish”. Teleosts are a massively successful group, so much so that they make up nearly 96% of all modern fish species, and nearly half of all modern vertebrate species. Yes, this includes ALL mammals, birds, reptiles, amphibians and other fish alive today! Within this huge Teleost group Xiphactinus belonged to a family known as the Icthyodectidae; a family of fish that became totally extinct at the end of the Cretaceous period 66 million years ago, leaving no living descendants.

The diet of Xiphactinus included Fish, Small Marine Reptiles, Ancient Seabirds (e.g. Hesperornis, a flightless human sized seabird from the USA) and even Pterosaurs. These potential prey items would have not been easy to catch. But Xiphactinus had a secret weapon. It is theorized that it was endothermic, meaning that it could generate and maintain a higher body temperature than the surrounding environment (in a similar way to mammals and birds). This is actually not unheard of for a fish, who are usually thought to be exothermic, meaning their body heat is determined largely by their surroundings. Bluefin Tuna, Swordfish and Great White Sharks are also able to maintain a higher body temperature, independent of their environment. This strategy gives them the potential to produce the heat (and therefore energy) required to be fast active predators who can swim at high speeds. With this in mind maybe Xiphactinus could’ve leapt out of the water to grab flying animals or while hunting water bound animals in a manner akin to a Great White Shark! Obviously this is speculative behaviour, but what a sight that would have been if it did pull off such manoeuvres! Some remarkable fossils of Xiphactinus have allowed palaeontologists to gain further insight into its hunting behaviour. One fossil, discovered in 1952 at Smokey Hill in Kansas, USA, and stored in the Sternberg Museum (also in Kansas), preserves a complete 4 metre long Xiphactinus skeleton in the process of swallowing a 2 metre long fish named Gillicus. That’s right this Gillicus was half the size of Xiphactinus! It seems that this Xiphactinus perished due to a combination of choking and its internal organs being punctured by the struggling Gillicus. Such a hunting strategy would also helped explain the large fang like teeth and upturned jaw. The teeth would’ve pierced and held the animal in place while the up and down movement of its lower jaw would’ve helped Xiphactinus gulp down its prey. With this beautiful fossil in mind, it’s almost a good thing that Xiphactinus isn’t swimming around in today’s oceans. Being swallowed alive by one would not have been a fun way to go!

File:Xiphactinus audax Sternberg Museum.jpg
The “Fish within a fish” fossil of a Xiphactinus and a Gillicus on display at the Sternberg Museum in Kansas, USA.
Image Credit: Spacini, https://commons.wikimedia.org/wiki/File:Xiphactinus_audax_Sternberg_Museum.jpg

Despite its size and fearsome appearance Xiphactinus was NOT the top predator in its seas. A Xiphactinus audax individual, estimated to have been “only” 3 metres long, discovered in Kansas, and described in 2004, was found to have a shark tooth embedded in its third vertebrae. This tooth belonged to an estimated 3.1 metre long specimen of a Late Cretaceous shark called Cretoxyrinha. What seems to have happened is that the shark inflicted a powerful bite into the back of the Xiphactinus, breaking off and embedding one of its teeth in its vertebrae in the process. While it is not clear whether the shark was actively hunting Xiphactinus, or if it was just scavenging its remains, it is clear is that the two species not only co-existed in the same place and at the same time but also actively interacted with each other. As well as Cretoxyrinha, Xiphactinus would’ve had to look out for other large oceanic predators. One such group were the Mosasaurs; Marine Reptiles that were closely related to lizards and snakes. These Mosasaurs included the 13 metre long Tylosaurus and the 15 metre long Mosasaurus (see my article on Mosasaurus for more about these fascinating sea faring reptiles!), both of whom were powerful predators with strong bites. All of these animals lived together in a large sea known as the “Western Interior Seaway”. This was an ancient sea that covered the middle of North America, and was so big that it split the continent into two large islands; Laramidia to the west (which is where the famous dinosaurs Tyrannosaurus and Triceratops lived) and Appalachia to the east. With Xiphactinus, Cretoxyrinha and the Giant Mosasaurs lurking in the water it’s no wonder that Nigel Marven in the BBC documentary “Sea Monsters” called this Late Cretaceous Sea “Hells Aquarium”! Despite Tyrannosaurus rex stalking Laramidia at the time, you arguably would have been better off sticking to the land!

However despite being incredibly successful and widespread, Xiphactinus would end up being lost to extinction. 66 million years ago a large asteroid 10km wide smashed into the Yucatan Peninsula in Mexico. This resulted in an extinction event known as the “K/T” Extinction Event, which was so devastating that an estimated 70% of all living species at the time went extinct. While it is best known for wiping out all of the Non-Avian (or “non-bird”) Dinosaurs it also had a massive effect on marine life. When the meteorite smashed into the earth it led to the release of massive amounts of sulfur from impacted rocks into the atmosphere, causing a worldwide “rain out” of sulfuric acid. This resulted in a big drop in the pH of the oceans, making them more acidic. This ocean acidification in turn prevented calcifying foraminifera and other tiny invertebrates from making their shells (as the low pH would dissolve the shells before they formed). Furthermore a number of plankton and algae species sensitive to pH changes were badly affected, leading to a mass die off of these species. These tiny organisms may not seem like much but they are the foundations for the survival of all marine life further up the food chain. Once these small species disappeared, there was a massive ecological collapse. This was because the fish that ate the plankton died off from starvation, and then in turn fish that ate those fish died off. It was this horrible domino effect that ultimately ended up causing the extinction of Xiphactinus as eventually there was not enough food to support them. In fact marine life was so badly affected by the K/T extinction event that it would take roughly 3 million years for marine ecosystems to fully recover.

In conclusion, Xiphactinus may not have been the most attractive of prehistoric animals, but it was unique, innovative and successful. It deserves to be regarded as an iconic prehistoric animal, and perhaps the most successful large predator of the Late Cretaceous seas!

File:Styxosaurus and Xiphactinus.jpg - Wikimedia Commons
Xiphactinus would’ve co-existed with many strange creatures in the Western Interior Seaway, including Styxosaurus; a Marine Reptile that was a member of the Plesiosaur group.
Image Credit: ABelov2014, https://www.deviantart.com/abelov2014/art/Styxosaurus-Xiphactinus-audax-var-1-658020267

References/Further Reading

Ferrón 2019: a paper that built upon previous work and provided further evidence for endothermy in Xiphactinus

Humberto G. Ferrón (2019) Evidence of endothermy in the extinct macropredatory osteichthyan Xiphactinus audax (Teleostei, Ichthyodectiformes), Journal of Vertebrate Paleontology, 39:6, DOI: 10.1080/02724634.2019.1724123

Shimada & Everhart 2004: a paper reporting on a Xiphactinus fossil that possesses bite marks made by a large shark

Shimada, K., & Everhart, M. J. (2004). Shark-bitten Xiphactinus audax (Teleostei: Ichthyodectiformes) from the Niobrara Chalk (Upper Cretaceous) of Kansas. The Mosasaur, 7, 35-39.

Schwimmer & Stewart 1997 paper describing the second species of Xiphactinus; Xiphactinus vetus.

Schwimmer, D., et al. (1997). “Xiphactinus vetus and the distribution of Xiphactinus species in the eastern United States.” Journal of Vertebrate Paleontology – J VERTEBRATE PALEONTOL 17: 610-615.

Everhart, Hageman & Hoffman 2010 journal article talking about another “fish within a fish” fossil discovery similar to the Xiphactinus/Gillicus specimen.

Everhart, Michael J., et al. “Another Sternberg ‘Fish-within-a-Fish’ Discovery: First Report of Ichthyodectes Ctenodon (Teleostei; Ichthyodectiformes) with Stomach Contents.” Transactions of the Kansas Academy of Science (1903-), vol. 113, no. 3/4, 2010, pp. 197–205. JSTOR, www.jstor.org/stable/41309609. Accessed 3 Jan. 2021.

Henehan et. al. 2019 paper on the ocean acidification that occurred in the worlds oceans during the K/T extinction event 66 million years ago.

Henehan, M. J., et al. (2019). “Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact.” Proceedings of the National Academy of Sciences 116(45): 22500-22504.

An online copy of a chapter from Richard Cowans 1999 book titled “History of Life” which details the effects of the K/T extinction

Cowan, Richard, “The K/T Extinction”, History of Life, 1999, www.ucmp.berkeley.edu, https://ucmp.berkeley.edu/education/events/cowen1b.html

The Prehistoric Wildlife website factfile on Xiphactinus

Prehistoric Wildlife, “Xiphactinus”, www.prehistoric-wildlife.com, http://www.prehistoric-wildlife.com/species/x/xiphactinus.html

A short National Geographic profile on Xiphactinus

National Geographic “Xiphactinus audax”, Animals Photo Ark, nationalgeographic.com, https://www.nationalgeographic.com/animals/prehistoric/xiphactinus/

A University of Pennsylvania archives fact file on Professor Joseph Leidy, who first described and named Xiphactinus in 1870

University of Pennsylvania, “Joseph Mellick Leidy”, www.archives.upenn.edu.com, https://archives.upenn.edu/exhibits/penn-people/biography/joseph-mellick-leidy