Xiphactinus audax (zai-fact-in-us aw-dax), formerly known as Portheus mollosus, was the largest bony fish that ever lived, sometimes reaching a length of eighteen feet. Although distantly related to modern tarpons, it became extinct at the end of the Cretaceous period.These huge fish, along with mosasaurs, were the most ferocious inhabitants of the Niobrara Sea. Xiphactinus means “sword-ray” and was named by Joseph Liedy from a piece of a front (pectoral) fin. It is equipped with fang teeth, some over two inches long, and had jaws like a huge piranha.
The large Xiphactinus on display at Keystone Gallery (shown at right) was collected using the plaster slab method. The fossil bones are cleaned in the field, a frame is constructed, and plaster is then poured directly onto the bone and surrounding matrix. After the plaster has dried and set, the slab is dug under and loosened, then carefully turned over. This exposes the underside of the fossil specimen, which is then painstakingly prepared in the laboratory for exhibition.Xiphactinus had a voracious appetite and often ate fish whole. Greed sometimes got the best of them and some died with a meal inside. The world famous “Fish within a Fish,” collected by George F. Sternberg in 1952 and on display at the Sternberg Museum of Natural History in Hays, Kansas is a classic example of this. The fourteen-foot Xiphactinus swallowed a six foot Gillicus fish and died as a result of its gluttony.
One of the most common fish found in the Niobrara chalk is Gillicus (gill-ee-cuss). Attaining a length of six feet, this tarpon-like fish preyed on smaller fish even though it only had small, fringe teeth. Gillicus had large scales that covered its entire body. Many Gillicus fish tails are found in the chalk, adding credence to the theory that Xiphactinus made it a staple of its diet.
One of the most unusual fish from the Niobrara Sea is Saurodon (saw-roe-don), a long, thin predator fish with a strange skull. The head was large in proportion to its body with an extended lower jawbone (pre-dentary).
Complete Saurodon fish are very rare; possibly the only whole specimen was found and collected by Marion Bonner and now resides in the Los Angeles County Museum of Natural History. It is possible that more skulls are found because they were more massive than the thin body and were easily separated when the fish died and broke apart in the ocean bed.
A very odd fish from the Niobrara, Pentanogmius (pen-ta-nog-mee-us) ate clams and other mollusks. It was equipped with multiple flat jaws and palatine plates excellent for crushing invertebrate shell life. Pentanogmius averaged between three to five feet long.
In 2007, Chuck Bonner found a Pentanogmius with an articulated dorsal fin. Barbara’s field work was very itchy as the overburden (dirt and chalk above the fossil) was full of tiny centipedes and other things. The specimen was 5 and one-half feet long. This wonderful specimen of a Pentanogmius fossil fish was delivered to the Denver Museum of Nature and Science in September of 2008.
Although this fossil isn’t new to science, the sail part of it has never been found articulated (preserved intact). So this specimen is “new” to science as it adds more information to the fossil record. Usually Pentanogmius fish are scattered out, but this one was mostly perfect except the tail and lower fin are reconstructed. More photos of this specimen can be found here.
Pachyrhizodus (pack-ee-rye-zo-dus) is represented in the Kansas chalk by two main species. Pachyrhizodus caninus is the largest variety reaching lengths up to eight feet. It had a robust skull with large, curved teeth. The jaws of this fish have been mistaken for small mosasaur jaws.
Pachyrhizodus minimus is the smaller species of the genus, seldom reaching over three feet. This fish is noted for dense, compact scales.
Cimolichtys (sim-o-lick-tees) is in a close race with Gillicus for the most common fish found in the Kansas chalk. It is represented by several species, each having multiple jaws containing numerous rows of various-sized sharp teeth. Cimolichtys resembles a modern barracuda and was a fierce predator of smaller fishes.
This fish is actually related to salmon and has distinctive vertebrae with an hourglass shape in the middle. Cimolichtys had large, diamond-shaped scales that probably ran down its back. Three to five feet was the average length of Cimolichtys.
Stratodus (strat-toe-dus) is a a small, thin-bodied fish closely related to Cimolichtys. It is not as large and does not have the large back scales. Stratodus also has a multi-toothed upper front jaw that is layered with small teeth.
Apsopelix (app-sop-o-licks) is one of the smallest fish found in the Smoky Hill Chalk. Most specimens average between eighteen and twenty inches long. Smaller species of fish are found, but they are usually encased in large Inoceramus clam shells.
Apsopelix was a favorite food of larger predators and probably swam in schools. This fish is usually preserved whole with dense scales.
This fish usually measures under five feet long and is notable for its strange dentition (the number, sort, and arrangement of teeth). Enchodus (enn-ko-dus) has huge, fang-like teeth at the front of both upper and lower jaws. It was undoubtedly a nasty predator fish that could wreck havoc on smaller prey. Complete specimens of Enchodus are fairly rare, but isolated jaws and teeth are common.
Ichthyodectes (ick-thee-o-deck-tees) is closely related to both Gillicus and Xiphactinus. While averaging around the same size as Gillicusup to seven feet longit has much larger teeth, similar to Xiphactinus.
Unlike Xiphactinus, it has round, sharp, uniform teeth rather than multi-sized large teeth like its larger relative. Ichthyodectes also has smaller scales than either Gillicus or Xiphactinus.
One of the strangest fish from the Kansas Cretaceous Sea was the swordfish-like Protosphyraena (pro-toes-fie-ray-nuh).
The skull of Protosphyraena had a short, pointed snout, with flat, blade-like teeth jutting forward on both upper and lower jaws. The back part of the jaws has variously sized-and-shaped teeth.
Protosphyraena is also noted for its long pectoral fins with a distinctive scalloped leading edge. Vertebrae of this fish did not fossilize and there have been very few tails found.
©2006 Keystone Gallery / Photos ©Barbara Shelton unless otherwise noted