Stratigraphic Paleobiology of an Evolutionary Radiation: Taphonomy and Facies Distribution of Cetaceans in the last 23 Million Years
Stefano Dominici, Simone Cau and Alessandro Freschi
Summarized by Laura Martins. Laura Martins is a senior geology student at The University of South Florida. She plans to attend a Master program in Geophysics in the spring semester of 2021 out west. She dreams to work in seismological networking. She mostly spends the free days with her son and husband in different adventures such as visiting national parks, springs, Disney, and road trips!
What data were used? The study surveyed over 255 published papers associated with Neogene (~23-2 million years ago) cetacean (whales, dolphins, porpoises) fossils within a global context (excluding ones found in Southern America, due to a lack of fossil evidence)
Methods: All individual specimens found in the survey were sorted and classified by facies (rock type that represents certain environments) and time intervals (Miocene to Pleistocene Epochs) of deposition. The research also included the number and the preservation quality of bones per skeleton of each example. Even though South American fossils were predicted to have high quality preservation setting due to its hypoxic/anoxic depositional environments, it was set apart because of its lack of even distribution during the Neogene. The study collected a total of 255 specimens with absolute age data and 117 specimens with sedimentary facies data.
Results: The collected data was plotted in two graphs; the first represents the distribution of fossil over time intervals and the second shows the relation between facies (environments) vs time. These illustrations indicate a slight increase of cetacean fossils during the Miocene, followed by a vast increase during the Pliocene. However, by the early Pleistocene, the number of fossils dropped significantly. Consequently, the study conveyed that the highest abundance of cetacean fossils were collected in offshore marine mudstones and sandstones facies, whereas the lowest amount was related to shoreface sandstone facies. It implies that very shallow and very deep waters are not the greatest environments for preservation of these fossils. The study found that offshore mudstone and delta sandstone facies have the highest amount of bones per skeleton, suggesting that these facies are good preservation sites for cetaceans, due to high rates of sedimentation (deltas) and low-pressure settings (offshore) that would minimize decay and scavenging of the organisms. Finally, the research suggests that the remarkable drop off of cetacean fossils in the early Pleistocene might be affected by taphonomy factors (meaning, taphonomy might be making the drop in diversity more severe than it actually was).
Why is this study important? Cetaceans are the largest living marine animals that have ever lived. Through their fossil record, we can understand how their modern and extinct diversity and be explained by variations in taphonomy, taxonomy, loss of habitat, environment, climate and even massive extinction events. The study of this variation on the fossil record allows for the analysis of decay, preservation and environment settings of these large mammals, as well as the relationship of cetaceans with ecosystem changes, enabling the construction of evolutionary pattern trends.
The big picture: The study suggests that the peaks with the highest amount of cetacean fossils during late Miocene and Pliocene are correlated with an optimum climate. The vast drop of fossil localities during the late Pliocene accords with an extinction age. However, it is necessary to highlight that all of the evidence might be affected by taphonomy factors, such as scavengers contributing to loss of tissue and disarticulation.
Citation: Dominici et al. (2018). Stratigraphic paleobiology of an evolutionary radiation: taphonomy and facies distribution of cetaceans in the last 23 million years. Fossilia, Volume 2018: 15-17. https://doi.org/10.32774/FosRep-Pal.20.1810.051517