PRESERVATION OF MARINE VERTEBRATES IN THE LATE CRETACEOUS (CENOMANIAN-TURONIAN) TROPIC SHALE OF SOUTHERN UTAH

The Tropic Shale of southern Utah is a late Cretaceous formation that represents the westernmost extent of the Cretaceous Western Interior Seaway. The Tropic is well-known for the presence of invertebrates, particularly ammonites and oysters. A remarkably diverse vertebrate fauna of turtles, sharks, fish, and plesiosaurs has been recently revealed through work by personnel from the Museum of Northern Arizona. The vast majority of the specimens have been discovered as float and are therefore typically heavily weathered (corroded, abraded, and discolored) from surface exposure. The most common specimens discovered in float are isolated shark teeth. Teeth of one particular genus, Ptychodus, are far more common than the others, likely due to their robust nature (Ptychodus had teeth designed for crushing shells). Shark bones have not been discovered, which is predictable given the low preservation potential of cartilage. Other vertebrate remains found in float include isolated turtle, fish, and plesiosaur bones, which are also heavily weathered.

While fragmented remains in float are vastly more common, several skeletons have also been discovered, including one fully articulated fish skeleton and several plesiosaur skeletons in various states of articulation. Nearly all of the vertebrate specimens discovered are heavily fractured and compressed from swelling and shrinking of the clays and pressure of the overlying rock during burial. Many of the major skeletal elements show signs of scavenging, although it is possible that these markings were made before death. No evidence of bioturbation was found near any of the skeletons.

Overall, the concentration of vertebrate fossils in the Tropic Shale consist mainly of fairly robust skeletal elements, suggesting that taphonomic durability played a large role in preservation. Scavenging marks on bones from each major skeleton signify that most carcasses were disturbed by predators as they fell towards the ocean floor, which almost certainly decreased the overall number of skeletons preserved. In addition, in situ skeletal material was isolated from biological activity and found in fine-grained black shale, indicating that those carcasses largely undisturbed by scavenging came to rest on an anoxic ocean floor, an ideal setting for preservation.