Using Shark Teeth to Compare Past and Present Shark Populations Along the Southern Coast of Brazil

Quaternary fossil shark (Neoselachii: Galeomorphii and Squalomorphii) diversity from southern Brazil

Sheron Medeiros, Maria Cristina Oddone, Heitor Francischini, Débora Diniz, Paula Dentzien-Dias

Summarized by Max Raynor, a 4th year undergraduate student pursuing a bachelor’s degree in geology from the University of South Florida. Max currently works for a surveying company in Tampa, where he focuses on making digital maps of the Earth’s surface and ocean floor.  When he isn’t studying geology or working, he enjoys fishing, collecting and curating vintage clothing, and playing tennis.

What was the hypothesis being tested? Scientists used fossil shark teeth to quantify differences between shark populations throughout the Quaternary Period (the past 2.58 million years of Earth’s history). The shark teeth used for this study were collected along the beaches of the Rio Grande do Sul Coastal Plain (RSCP), which extends along Brazil’s southernmost shorelines. Scientists compared and contrasted structural differences between shark teeth to test hypotheses on changing climate conditions throughout the Quaternary and how changes over time affected shark populations along the Rio Grande do Sul. 

What data were used? The data collected in this study included 3,611 shark teeth that had been found on the beaches of the RSCP since 1996. Using the simple technique of manually collecting shark teeth from the beach, researchers were able to find a variety of species to use for this study.

Methods: Participating in a well-documented data collection process known as “beachcombing”, researchers scanned the exposed beach area and picked out shark teeth by hand. The gravelly nature of beach grains, as well as the less-than-perfect condition of many of the teeth found, made it increasingly difficult to find desirable samples over time. The collected teeth were subsequently sent to a laboratory where they were sorted and classified by species. A classified tooth would be analyzed from two different views: where a tooth was adjacent to the tongue (lingual), and where a tooth was adjacent to the inside of the mouth (labial). The characteristics of a tooth from these angles provide the information necessary to correctly identify the corresponding shark species.

Results: By observing the characteristics of the teeth sampled, scientists identified 3,611 teeth belonging to13 different species of shark in the dataset (Fig. 1). While about ¾ of the data were able to be identified to the species, some were only able to be identified to the genus, and some teeth were rendered unidentifiable due to physical alterations and erosion over time. The order Lamniformes represented just 3 of the 13 taxa identified, but was responsible for 2,390 teeth sampled, or 66.18% of the dataset. Carcharius taurus, commonly known as the sand tiger shark and belonging to Lamniformes, was the most abundant species overall with 2,027 identified teeth. With respect to species diversity, the majority of the diversity belonged to the shark order Carcharhiniformes, which represented 8 of the 13 species identified. Carcharhinus leucas, also known as the bull shark, was the most abundant species of the Carcharhiniformes with 191 teeth sampled. 11of the 13 species identified are still found in the region, indicating that the shark community and climate conditions of the RSCP throughout the Quaternary have been fairly similar over the past 2.58 million years.

Figure A: Pie chart of shark orders sampled, from highest to lowest percentage of teeth found per order: Lamniformes (2,390 teeth), Carcharhiniformes (821 teeth), Hexanchiformes (10 teeth), Squatiniformes (2 teeth), and 388 teeth that were unable to be identified to the species.Figure B: Pie chart of the number of teeth sampled from each species, from most teeth found per species to least: Carcharius taurus (2,027 teeth), Carcharadon carcharias (283 teeth), Carcharhinus leucas (193 teeth), Carcharhinus brachyurus (90 teeth), Isurus oxyrinchus (80 teeth), Sphyma (51 teeth), Carcharhinus longimanus (21 teeth), Galeocerdo cuvier (18 teeth), Notorynchus cepedianus (10 teeth), Galeorhinus galeus (3 teeth), Squatina (2 teeth), and Rhizoprionodon (1 tooth). There were 444 Carcharhinus teeth that could not be identified to a species, as well as 388 unidentified teeth.
Pie charts depicting (A) The orders of shark represented by teeth collected in this study and the number of samples belonging to each (B) The species of shark represented by the amount of teeth identified per species in this study.

Why is this study important: The diversity of shark species along the RSCP is important to note because it supports hypotheses posed in other studies that climate conditions have changed little throughout the Quaternary in this region. The two species found in the study that are not current residents of the RSCP, Carcharodon carcharias (Great White Shark) and Carcharhinus longimanus (Oceanic Whitetip Shark) are noteworthy, because they live in open oceanic environments today and are rarely found in the coastal RSCP, The presence of oceanic sharks such as these indicate higher sea levels along the RSCP at times throughout the Quaternary Period compared to present day. Periods of cooler and warmer weather were drivers of changes in sea level and climate changes throughout the Quaternary, resulting in periodic occurrences of shark species that migrated from both warmer and colder waters.

Broader Implications Findings from this study will allow paleontologists and biologists alike to assess how coastal and oceanic shark populations respond to a changing climate and marine ecosystem. Further research on this using different methods than beachcombing could potentially identify different results, as beachcombing can sometimes favor the collection of larger teeth, as it ismore obvious it is to the eye of the collector. Bulk collecting, collecting sediment and sorting it in the lab, may capture different results, but this will require future research. 

Citation: Medeiros, S., Oddone, M. C., Francischini, H., Diniz, D., & Dentzien-Dias, P. (2023). Quaternary fossil shark (Neoselachii: Galeomorphii and Squalomorphii) diversity from Southern Brazil. Journal of South American Earth Sciences, 122, 104176.