Xiphactinus is one of the great also-rans of pop paleontology. An enormous fish that swam the long-vanished sea which once blanketed the middle of North America during the Late Cretaceous, it isn’t exactly a household name but it is still cool enough to be regularly included in museum displays, books, and documentaries. The reason why is attributable to a beautiful specimen discovered half a century ago.
Paleontologists have known about Xiphactinus for quite some time. The Philadelphia-based polymath Joseph Leidy named it in 1870, but the discovery that gave the fish its claim to fame occurred in 1952. Back then a field crew from the American Museum of Natural History was tagging along with the skilled fossil hunter George Sternberg in search of creatures that once swam the Cretaceous seas over what is now Gove County, Kansas. One of the AMNH crew members, Walter Sorenson, found the tail of one Xiphactinus peeking out from the chalky strata, but since his team had no time to dig the whole thing out the task of uncovering the rest of it went to Sternberg.
There was much more to the specimen Sorenson found than the tail. The whole fish was present, from snout to caudal fin, but it was another fish that made this Xiphactinus so remarkable. Imprisoned behind the bony bars of the bigger fish’s ribcage was a complete specimen of Gillicus arcuatus – the last meal of the bigger, bulldog-faced predator. As summarized by Michael Everhart in Oceans of Kansas, “The smaller, two-meter-long fish had been swallowed headfirst and rested entirely within the ribs of the four-meter Xiphactinus”, and the fact that the smaller fish had not been digested indicates that the predator died shortly after engulfing the prey. In fact, for those of you who have been reminded of “There was an old lady who swallowed a fly”, it’s worth noting that one hypothesis for why the Xiphactinus died so soon is that a spine of the Gillicus may have pierced a vital organ in the larger fish on the way down and killed its assassin. “I don’t know why she swallowed that Gillicus… perhaps she’ll die.”
Preserved gut contents like the famous “fish-within-a-fish” – on display at the Sternberg Museum of Natural History, though other such specimens have been found – are rare snapshots of prehistoric life. We can draw connections between prehistoric creatures and the food they ate based upon the shape of their teeth, the anatomy of their jaws, toothmarks on skeletons damaged by carnivores, chemical signatures within their bones, and other methods, but fossilized gut contents allow fuzzy glimpses into what individual animals were eating shortly before death. (In more recent animals, like woolly mammoths found frozen in Siberia, preserved gut contents can even be used to get some idea of what season the animal died in.) Of these examples, one of my favorites involves a three-part chain of carnivory from tens of millions of years before the time of Xiphactinus.
Around 299 million years ago, the area that is now southwestern Germany was dotted with rivers and lakes that were intermittently connected to the sea and to each other. These shifting pockets of aquatic connections played host to a mixture of freshwater animals and marine lineages which became adapted to freshwater environments thanks to frequent isolation from the ocean, including creatures paleontologists know as xenacanth sharks. These aquatic predators were not quite like sharks as we know them today. Xenacanth sharks had long, eel-like bodies fringed with long fins and decorated with long spines sticking out of their heads, and at least one died with quite a substantial meal in its stomach.
Described by paleontologist Jürgen Kriwet and co-authors in 2008, a specimen of the prehistoric shark Triodus sessilis had died with a stomach full of other bones. The fossil – preserved belly-up on a piece of iron-rich siderite – contained the partial skeletons of two young temnospondyl amphibians belonging to the genera Archegosaurus and Cheliderpeton. Vaguely salamander-like in appearance, these amphibians were common parts of freshwater ecosystems at the time, and, as luck would have it, one of the two had recently fed before it died. Whereas the shark had feasted on temnospondyls, one of the amphibians had just eaten a spiny variety of fish called Acanthodes.
As interpreted by Kriwet and colleagues, it appears that the amphibian ate the small fish a significant amount of time before it was itself consumed by the shark. The incomplete nature of the Acanthodes specimen indicated that it had already been digested considerably, but the better preservation of both amphibians hinted that the shark ate them shortly before it died. Despite what cable documentaries have led us to believe about the predatory behavior of extinct animals, this accumulation was not the result of a 1-2-3 stepwise process that was over in seconds – the different degrees of preservation and completeness indicate that there was some time lag between when the different organisms ingested their final meals.
So what does this tell us about who was on the menu in Permian lakes? That can be difficult to say, especially since the stomach contents represent the near-death diet of an individual animal. Are the gut contents representative of the animal’s regular diet, or was this accumulation unusual? Additional specimens are needed to be sure, but, assuming that this unique fossil represents regular interactions, it would seem that small fish were regular prey for small temnospondyl amphibians whereas larger sharks – and big, adult amphibians – were probably apex predators. As simplified and cliched as it is, I can’t help but think of Tennyson’s worry about “Nature, red and tooth and claw” when looking at a specimen like this. For denizens of Permian lakes, even predators were in danger of being eaten.
Top Image: A reconstruction of Xiphactinus at the Royal Ontario Museum in Canada. Photo by Flickr user Martin Cathrae.
Everhart, M. 2005. Oceans of Kansas. Bloomington: Indiana University Press. p. 72
Kriwet, J., Witzmann, F., Klug, S., & Heidtke, U. (2008). First direct evidence of a vertebrate three-level trophic chain in the fossil record Proceedings of the Royal Society B: Biological Sciences, 275 (1631), 181-186 DOI: 10.1098/rspb.2007.1170