Fishes Large and Small
The big Xiphactinus swam effortlessly through the clear, warm waters of the Western Interior Sea in a solitary, never-ending search for its next meal. Ten years old and nearly four meters long, it had not yet reached full size, but it was already larger than any of the other species of fish in this ocean except the giant ginsu sharks. Although still wary of the occasional prowling shark, the X-fish's only other major competitor for the larger fish that it preyed upon were the large marine lizards that shared the same waters. The X-fish had lived long enough and grown too large for any of the big marine lizards to be interested in it as a meal. Capable of taking very large prey as they were, the lizards were unlikely to attack anything larger than they could swallow, and the X-fish's massive body was well beyond what they could eat. The X-fish had on occasion taken a newly birthed mosasaur, but it generally avoided their family groups. Its favorite prey was other fish, especially some of the larger varieties that were closely related to his own kind. A large meal satisfied the X-fish's hunger and provided the energy necessary to support its constant swimming.
The X-fish's senses alerted it to a group of Gillicus feeding nearby and close to the surface. As it swam closer, it could see that the two-meter-long predatory fish had surrounded a school of much smaller fish, compressing them into a shimmering globe of millions of tiny forms. The Gillicus swam in circles around the trapped school, keeping it from spreading out or escaping. Occasionally some of the Gillicus would dart through the mass of little fish with their jaws wide open, gorging themselves on the sudden abundance of prey. Their attention diverted, they did not notice the X-fish's approach from the dark waters beneath them. Gauging their movements, the X-fish selected its victim and accelerated swiftly upward, meeting the other fish almost head-on. Opening its large mouth at the last moment, it seized the smaller fish's head from below. As its massive lower jaw closed, the large, conical teeth at the front punctured the thin bones covering the head of the Gilli-cus and kept it from getting away. The smaller fish struggled briefly, but the shock of impact had stunned it into submission. The X-fish held on to its prey for several moments, then quickly repositioned it so that it was pointed headfirst into the larger fish's mouth. Then it began to rapidly open and close its jaws, drawing the smaller fish deeper and deeper into its gullet. A shower of silvery scales, dislodged from the Gillicus by the teeth of the X-fish, glittered in the sunlight as they drifted away.
At first, swallowing the big Gillicus was relatively easy, but then the X-fish began to have problems. At nearly two meters in length, the Gillicus weighed more than a hundred pounds. It was probably the largest prey the X-fish had ever eaten. Once the head and pectoral fins of the Gillicus had passed into the larger fish's throat, however, there was no turning back. The bony pectoral fin rays of the Gillicus were folded back tightly against its body. Any attempt to reverse direction would cause them to unfold and catch in the muscular esophagus of the larger fish. The relatively large body of the Gillicus now filled the mouth and throat of the X-fish and made it difficult for water to reach its gills. The last two feet of the Gillicus, including its bony tail, still protruded beyond the jaws of the X-fish. Barely alive but tightly confined within the bony skull and forebody of the larger fish, the Gillicus thrashed about occasionally as it was swallowed. A final spasm caused one of its sharp fin rays to pierce the esophagus of the X-fish.
Twice as long as the Gillicus and much more massive, the X-fish struggled to swallow its prey. Unable to adequately replenish the oxygen in its body with the smaller fish lodged in its throat, however, it was quickly weakening. Slowly the smaller fish was moved deeper and deeper into the gullet of the larger one. Finally the bony tail moved entirely into the mouth, and the gills of the X-fish began to function normally again. Now the prey began to move more easily into the stomach of the X-fish, but something was wrong. The large fish swam in ever-slower circles as it finished swallowing the Gillicus. During its futile struggle to break free, a fin spine of the prey had punctured something vital in the X-fish. Within a few minutes, the X-fish stopped swimming, rolled over on its back as it died, and then sank headfirst toward the muddy bottom below.
Figure 5.1. The famous "fish-in-a-fish " specimen of a large Xiphactinus audax (FHSM VP-333) and its last meal (Gillicus arcuatus; FHSM VP-334) in the Sternberg Museum of Natural History, Hays, Kansas. The remains were collected from the Smoky Hill Chalk of Gove County in 1952.
While this story is fiction, the famous "fish-in-a-fish" specimen recovered by George Sternberg is a fact (Rogers, 1991, p. 248; Liggett, 200 1, p. 64). While a group from the American Museum of Natural History was on a field trip with George Sternberg in the spring of 1 952, Walter Sorenson of the AMNH found the caudal fin of a large Xiphactinus audax (FHSM VP-3 33) eroding out from the chalk in Gove County. Recovering the fossil would have taken far more time than the visitors had, so they graciously gave the specimen to Sternberg. Hours of meticulous field work under a hot summer sun by Sternberg and others from the university uncovered what was certainly one of the most complete specimens of Xiphactinus audax Leidy 1870 found up to that point (Bardack, 1 965, p. 40). Sternberg's patient work also revealed the last meal of the larger fish, a well-preserved Gillicus arcuatus (FHSM VP-334). The smaller, two-meter-long fish had been swallowed headfirst and rested entirely within the ribs of the four-meter (13-ft.) Xiphactinus (Fig. 5.1). We know that the smaller fish had not been there long before the larger fish died because it had not yet been digested. What caused the death of this Xiphactinus, and a surprising number of other specimens of the species, cannot be determined from their fossils, but it most likely involved an injury that occurred when the prey was swallowed. In the case of the Sternberg specimen, a fin from the struggling Gillicus could have pierced the heart or a major blood vessel of the Xiphactinus and killed it quickly, or there may have been damage caused to the gills of the larger Xiphactinus that were rapidly fatal. In any case, the fossil remains have preserved a very interesting and puzzling moment in time.
While Xiphactinus remains as complete and well preserved as the fish-in-a-fish specimen are relatively rare, a number of Xiphac tinus fossils have been found with a Gillicus as the last meal. Bar-dack (1965) surveyed eighteen relatively complete Xiphactinus specimens in museum collections and indicated that at least three (including the Sternberg specimen) had Gillicus remains inside. A similar-sized specimen (DMNH-1 667) in the Denver Museum of
Nature and Science had lived long enough to partially digest its last meal, also a large Gillicus. In fact, seven of the eighteen specimens surveyed included fish as stomach contents. The giant (17 ft.) Xiphactinus I discovered in 1996 (NAMAL 2 0 0 0-0925-009) also included the partially digested remains of another Gillicus. Preservation of stomach contents is rare in the fossil record (Cicimurri and Everhart, 2 001) and the relatively frequent discovery of Xiphactinus specimens containing the remains of their last meals raises the question of why so many of them died so quickly after eating.
The remains of fish are the most common vertebrate fossils found in the Smoky Hill Chalk. Russell (1988) suggested that about 60 percent of the Late Cretaceous specimens in museum collections were fish. While this number probably includes some cartilaginous fish specimens (sharks and ptychodontids) in those collections, my experience is that the percentage of fish specimens found in the field probably approaches 80 or 90 percent of the total number. The difference can be explained by the fact that museum specimens are generally more complete, while in the field bits and pieces of fish are likely not to be collected, even though they may be identifiable to species. If your field time or storage space is limited, you tend to be a bit more choosy in what you collect.
After reviewing my field notes for the years 1988-1995 when my wife and I were vacuuming up ("hoovering") almost everything we found, I found that 78 percent of 43 0 vertebrate remains we collected were fish (not including isolated shark and fish teeth) and 16 percent were mosasaur. The other 6 percent were more or less equally divided between pteranodons (2 percent), turtles (2 percent) and plesiosaurs (1 percent), with a single bird bone making up a small fraction of a percent. I will admit that I picked up every mosasaur specimen that I saw, but not every fish. It is important to note, however, that these specimens ranged from complete fish to a few vertebrae or just an identifiable piece of a fin or jaw. In size, the fish specimens ranged from tiny 10-cm (4-in.) fish inside clam shells to giant Xiphactinus specimens that were more than 4 m (13 ft.) long. Far and away the most common remains we observed during that time, however, were fish tails. These generally consisted of a complete, mostly undamaged caudal fin with a few attached vertebrae. Mostly these were from medium-sized fish, with an occasional fairly large Ichthyodectes thrown in. Some fish-eating predator was apparently biting or breaking off the bony, relatively nutritionless tails while swallowing the rest of the fish. Carpenter (1996, p. 44) reported severed tails as evidence of predation on fishes in the Pierre Shale. Our unofficial count over the years has been that we find about ten tails for every one skull. Needless to say, we gave up collecting fish tails a long time ago.
Relatively few remains of fish (or anything else) are com plete when found in the Smoky Hill Chalk. The multitudes of smaller fish that must have been present were most likely to be consumed whole while the larger ones were sometimes torn apart by sharks or other predators. Sometimes the fish bones we find are were partially digested (see Chapter 4 on sharks) or were found inside co-prolites. Once the remains reached the bottom, they generally were not disturbed by large predators. As might be expected, in most cases, the bones of larger fish had a better chance of being preserved as fossils. The major exception to that rule occurred when schools of small fish were trapped inside giant inoceramids when they died and were preserved intact (Stewart, 1990b). In some cases, notably the "fish in a fish" Xiphactinus mentioned above, the remains of a last meal are identifiable inside the larger fish. Evidence of scavenging is preserved as bite marks, severed bone, and associated shed teeth (usually shark).
The ecosystem of the Late Cretaceous was probably not unlike that of the Gulf of Mexico or similar warm-water areas around the Earth today. Immense populations of "sardine"-sized fish were necessary to support larger fish and the predators that fed on them. A Late Cretaceous marine food web for the Western Interior Sea might go something like this:
1. Algae as primary producers converted sunlight, carbon dioxide, and nutrients into biological materials for growth and reproduction. The calcitic shells of the algae, called coccolithophores, and the disk-shaped, calcite coccoliths that surrounded them accumulated in vast numbers to form the limey mud on the sea floor that eventually became chalk (Hattin, 1982)
2. Microscopic single-cell or multi-cellular animals fed on the algae. Their fecal (waste) pellets containing the indigestible cocco-lithophores and coccoliths of their prey (ibid.) also add to the limey mud below.
3. Tiny fish, small crustaceans, and the larvae of other marine animals, including squid and ammonites, fed on the microscopic protozoa and other primary consumers. Apparently there were no large, filter-feeding vertebrates during the Late Cretaceous.
4. Small fish, young fish of many species, and invertebrates (ammonites and squid) fed on the tiny fish.
5. Medium-sized fish fed on the small fish. Pteranodons, birds, young marine reptiles, ammonites, and squid also fed at this level.
6. Large fish, sharks, plesiosaurs, and small mosasaurs fed on the medium-sized fish and on invertebrates such as ammonites and squid. Sharks, cephalopods, and possibly mosasaurs also served as scavengers to recycle biological materials from the remains of dead animals.
7. Large mosasaurs and sharks fed on everything as the top predators.
Because we find the remains of the top predators (mosasaurs) quite often, we are fairly certain that there must have been a productive ecosystem running at full tilt to support them. The coastlines, estuaries, and swamps that bordered both coasts of the Western Interior Sea may have served as semi-protected hatcheries for untold numbers of smaller fish and invertebrates that were not preserved in the fossil record but had to be there to support the base of the food web. The remains we find as fossils in the Smoky Hill Chalk are only a small fraction of the abundant life that must have existed in the Western Interior Sea during the Late Cretaceous.
The specimens that are preserved are important indicators of what was going on in the oceans of Kansas. Large predators such as mosasaurs and Xiphactinus are probably overrepresented as fossils because their more robust, heavier bones improved their chances for preservation. Because they are there, however, we can view them as indirect evidence of the vastly larger number of smaller fish and invertebrates that lived and died as prey and left only a scant record.
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