The recently published 637-page tome, At the Top of the Grand Staircase: The Late Cretaceous of Southern Utah (Indiana University Press: edited by Alan Titus and Mark Loewen) is filled with papers summarizing the geological and paleontological research on these rocks since the 1996 establishment of Grand Staircase–Escalante National Monument (GSENM). While I am a coauthor on several papers in this volume relating to new dinosaur discoveries, I also took the opportunity to finally publish my research on Utah’s freshwater elasmobranches (sharks and rays) that I had begun as a graduate student 35 years ago. This Survey Notes article briefly summarizes the results of this research.
The first extensive research on the vertebrate fossils in southern Utah was not on the dinosaurs, but was in the quest for the tiny fossil mammals that had lived in the shadow of the dinosaurs. The process of recovering these fossils involves breaking down fossiliferous rock by gently washing the rock, preferably soft shales, mudstones, or poorly consolidated sandstones, through fine screens and picking out the small identifiable bones and teeth from the residues under a microscope. In this way, collections of the entire fauna can be made—not just the mammals, but the fish, salamanders, frogs, lizards, turtles, crocodiles, and even dinosaur teeth.
This research, begun during the early 1980s, revealed not only that southern Utah preserved the most complete record of Late Cretaceous life on land, but that throughout the Cretaceous, Utah’s rivers hosted a variety of freshwater sharks and rays. As an aspiring young paleontologist, I was assigned the job of researching these teeth, as the “more interesting” groups of animals were already spoken for by more established scientists. As with the Cretaceous mammals, isolated teeth from the sharks and rays are useful for identifying species, and I realized that these collections contained new species that documented the entire evolutionary history for several lineages of freshwater elasmobranches. Unfortunately, the taxonomy of these fish was a mess, and it took many years of research to get a handle on this group of slippery organisms.
Today, most people would not expect to catch a shark or ray while fishing in one of North America’s rivers. However, in the tropics and in the southern hemisphere, freshwater sharks and rays are characteristic of many large river systems. The Fitzroy River in eastern Australia, for example, includes a variety of sharks, sawfish, and stingrays and can be thought of as a modern analogue for the Late Cretaceous rivers of southern Utah. A comparable diversity of freshwater elasmobranches is found to have been living in Utah’s rivers throughout the last 30 million years of the Cretaceous.
Hybodonts are an extinct group of sharks that were the dominant sharks for much of the Mesozoic Era following the near-extinction of sharks at the end of the Paleozoic Era. They are characterized by dorsal fin spines and small spines on their heads. There are two lineages of hybodont sharks. Hybodus is a moderately large species with a maximum length of 3 meters (10 ft), with teeth well adapted to grabbing fish. This is the only spined shark found in these rocks that was just as much at home in the ocean as it was in Utah’s rivers. Lonchidion is a smaller shark, usually less than 1 meter (3 ft) long, with long, low blade-like teeth better adapted to cutting up soft-prey animals.
Among the more advanced sharks, or neoselachians, were a number of orectoloboid species that probably can be divided into two evolutionary lineages. Orectoloboid sharks are represented today by the nurse, carpet, and cat sharks. They seem to have entered into the Cretaceous river systems from an ancestry of shallow coastal sharks about 90 million years ago. They were small sharks less than 1 meter (2 ft) long with a mouth packed with tiny 1–2 millimeter (0.1 in) teeth useful for grasping and shredding small prey. One lineage, Cantioscyllium, was characterized by teeth whose primary cusp is ridged and there are lateral cusps. Through time, species of Cantioscyllium have progressively subdued the ridges on their teeth. The other lineage has been referred to the modern cat shark Chiloscyllium and is characterized by teeth lacking lateral cusps, and whose primary cusp may have shoulders that are incipiently serrated.
Because the taxonomy of the modern cat sharks is complicated, with few differences in the teeth among the modern species, the assignment of these Cretaceous species to Chiloscyllium is tentative and based primarily on the taxonomy of other authors. The Upper Cretaceous of western North America represents the only time and place that this group of sharks is documented to have populated freshwater environments.
The bottom-dwelling rays have repeatedly invaded freshwater habitats throughout geological history since their origins in the Jurassic. The genus Myledaphus (“mill stone of the earth”), first recognized in the 1870s, reached the relatively large size of around 3 meters (10 ft) long prior to their extinction at the end of the Cretaceous. Initially, the large grinding teeth of this ray were thought to be the teeth of a large stingray. However, the discovery of a Myledaphus mummy at Dinosaur Provincial Park in Alberta, reported in 2005, proved that these were not freshwater stingrays, but a specialized group of the more primitive guitarfish.
The abundance of fossil teeth from Utah document a lineage of primitive rays linked by having crushing teeth with a distinct midline crest. They originated more than 100 million years ago at the end of the Early Cretaceous with small species (Cristomylus) that got progressively larger and developed more efficiently packed teeth (Pseudomyledaphus) with more complex grinding surfaces (Myledaphus) through the Late Cretaceous.
The last group is represented by two lineages of extinct Scelerorhynchiform sawfish. While looking superficially like modern sawfish, differences in the skulls and pectoral fins indicate that this group originated from guitarfish in the Early Cretaceous. They reached a greater diversity than modern sawfish and included very large species, prior to their extinction at the end of the Cretaceous. Modern sawfish (pristids) do not appear until the Eocene, about 45 million years ago. The most common Utah species are found throughout the Upper Cretaceous and have small teeth on their saw (or rostrum) and ridged, diamond-shaped teeth in their mouths. We assigned these species to the ptychotygonid Texatrygon, which appears to be related to Lebanopristis and Micropristis, which are known from complete specimens in the Middle East. Another taxon, the scelerorhychid Columbusia, is only known from its oral teeth and may have had a toothless rostrum more like that of a swordfish. These fish used their rostrums to grub for invertebrates in the sediment and to slash at small fish in more open water.
After being so characteristic of Utah’s Cretaceous river systems, all of these lines of freshwater elasmobranches went extinct with the dinosaurs at the end of the Cretaceous. Perhaps the continued success of the bony fish was a result of their much higher reproductive rates.