Summary of Trophic interactions of sharks and crocodylians with a sea cow (Sirenia) from the Miocene of Venezuela
Written by: Aldo Benites-Palomino, Gabriel Aguirre-Fernández, Jorge Velez-Juarbe, Jorge D. Carrillo-Briceño, Rodolfo Sánchez, and Marcelo R. Sánchez-Villagra.
Summarized by: Dimitar Peev. Dimi lives in Scarsdale, New York and is a student at Binghamton University. He is currently seeking a bachelor’s degree in biology. He is also in the education minor and plans to be a teacher. Dimi likes to run and is a big fan of the Arctic Monkeys.
What data were used? A group of three scientists found the remains of a dugongine sirenian, or a sea cow. This creature is very closely related to the manatee, which more of us are familiar with. Parts of the skull, spine, and rib cage were found and preserved well, due to a low energy environment that the sea cow was buried in. Multiple bite marks were found on this fossil, and it was collected in the Agua Clara Formation near Coro in Venezuela. It is thought to be from the Miocene, between 5 million and 23 million years ago. A shark tooth of the species Galeocerdo aduncus was found with this specimen as well.
What was the hypothesis being tested? Scientists wanted to identify the specific marks on the sea cow fossil and understand what event or animal could have caused the marks. While it is impossible to say what definitely caused the marks on this sirenian, the scientists formulated hypotheses that used evidence from other fossils and prior knowledge of animals that lived in the Miocene to paint a picture of what could have happened.
Methods: Scientists measured the depth, length, and orientation of the bite marks. There were three types of bite marks identified. The first bite mark is a shallow, circular depression that may feature some minor to moderate breaks in the surrounding bone. The second is a broad, curved cut that begins with a deeper, rounded pit. The third is an elongated, narrow opening with a triangular shape in the cross section. Based on the fossils found, the researchers focused on the larger bite marks on the specimen.
Results: The first bite marks seen on the dugongine sirenian are three punctures near the upper body on the fossil. They are around eight mm wide and are thought to be produced from a single biting event. Because the marks have a semicircular shape to them and they are very symmetrical, they are thought to be crocodylian marks. This is because the sharp, carnivorous teeth could make this bisected shape on the sea cow. The second marks are also thought to be crocodylian, identified by two large incisions that could be made from the tearing or rolling of prey by crocodiles (See figure 1). The third and final category of bite marks are ones that have a V-shape and are skinny and extended, hinting at teeth dragging across bone and leaving a very specific mark. This type of bite is known to be made by sharks, specifically tiger sharks, because of their narrow teeth. Because of a tooth found near the fossil, the shark is likely to be a Galeocerdo aduncus, which lived during the Miocene to Pliocene epochs around 10–3 million years ago. Even though it is extinct today, it is known to be an ancient relative of the modern tiger shark Galeocerdo cuvier.
Why is this study important? This study is important because it shows us the trophic levels involved with sirenians and their predators. Trophic levels are the organization of animals and plants in the food chain and how energy flows through an ecosystem. Because of this study, we can learn more about the interactions between sharks, sea cows, and crocodylians, and it can lead to an understanding on how these creatures lived millions of years ago. The interactions between predator and prey can tell us more about how each of these animals evolved in the past. This is an important find for the fossil record because there are very few specimens that show multiple sources of predation on one creature.
Broader Implications beyond this study: In the entire fossil record that we have access to today, there are few samples of creatures that are prey to multiple predators. This specific sirenian proves that it was prey to sharks and crocodiles, which is such an important connection to make when studying trophic levels of the Miocene. Today, the manatee is the closest living relative to the sirenian, and it has no natural predators in the modern world. Looking back into the fossil record can tell us why and when the sirenians stopped being hunted. The more predators and prey we can overlap and connect in the past, the better understanding we have about how these animals lived back then and today.
Citation: Benites-Palomino, A., Aguirre-Fernández, G., Velez-Juarbe, J., Carrillo-Briceño, J. D., Sánchez, R., & Sánchez-Villagra, M. R. (2024). Trophic interactions of sharks and crocodylians with a sea cow (Sirenia) from the Miocene of Venezuela. Journal of Vertebrate Paleontology. https://doi.org/10.1080/02724634.2024.2381505