Newly discovered fossil frogs shed light on the Amazonian environment 10 million years ago

Fossil Frogs from The Upper Miocene of Southwestern Brazilian Amazonia (Solimões Formation, Acre Basin)

Fellipe P. Muniz, Marcos César Bissaro-Júnior, Edson Guilherme, Jonas P. De Souza-Filho, Francisco R. Negri, and Annie S. Hsiou

Summarized by Ryan Taylor. Ryan lives in Clearwater, Florida, and is a student at the University of South Florida (USF) who is currently seeking a B.Sc. in geology. He also works as an assistant engineer for a civil engineering company in Tampa, Florida. One of Ryan’s favorite activities is going garage sale hunting every Saturday morning.

What was the goal of the paper? This study used newly discovered frog fossils, from the genus Pipa and Rhinella, from the southern Amazon to gain a greater understanding of what the biodiversity and the environment of the southern Amazon was like in the Late Miocene (6 to 11 million years ago). Scientists accomplished this by comparing the differences in the structure of the newly discovered Pipa and Rhinella to modern-day frogs of the same genus (a taxonomic rank above species).

What data were used?  Scientists used the bones of the frogs that were found at the Talismã site in the southern Amazonas of Brazil. These bones included the frogs’ ilium and ischium (hip bones), humeri (front legs), and other parts of the frogs’ skeletal structure. This fossil data was compared with data collected from previous studies for small, fossilized animals found in the same region. The researchers also collected samples of the bones of frogs that live in that area today to compare the differences in the structure against the fossil frogs that were found.

Methods:  The researchers recovered multiple samples of frog bones that were in the southern Amazonas region of Brazil by examining an exposed section of sediment layers (Fig. 1). Next, they identified differences in the bone structures of the fossil frogs as compared with the same species of living frogs that are found in the same area today. Using this data of the differences in bone structures, researchers performed an evolutionary tree analysis of the frogs they found to how they were related to other frogs alive today. This tree allows the researchers to see how the frog diversity changed over time to what we have today. They compared the bone structures of the newly found fossils to frogs known to be living there in the late Miocene supporting that the new frog fossils used to live there at that time too.

This figure shows a map of Brazil and focuses on a part of Brazil in the southwestern Amazon with the location of the fossil excavation and some of the rivers that are in the area. There is also a representation of a 5.3-meter-tall cross-section that was in the ground. This cross-section showed the clay/mud layers and the different types of fossils found in them. Reptiles were found between 1.12 and 0.68 meters deep. Fish, anurans (frogs), mammals, reptiles, and crustaceans (crabs) were found between 2.15 and 2.34 meters deep. More crustaceans were found between 2.34 and 4.89 meters deep.
Figure 1 shows the area where the samples of the new frog species were found. The layer in which the bones were found is helpful in finding the approximate age of the frog bones because each layer down from the top represents an older time in geologic history. The anurans (frogs) were found between 2.15 and 2.34 meters deep. Other animals that were found include: crustaceans, fish, mammals, and reptiles which all lived in the same time period the frogs lived.

Results: The researchers found two frog taxa here belonging to the genera Pipa and Rhinella. The results of this study showed that a diversity of frogs in genus Pipa lived in the southern Amazonas region. The Pipa fossils that were found are some of the oldest for the genus which supports a previous study that was done in Venezuela, showing Pipa also lived there in the late Miocene. The discovery of Pipa in the southern Amazonas gives an idea as to what type of environment this region had. Pipa is only known to live in aquatic environments near stagnate waters like lakes and swamps, showing that this is likely the type of environment that existed here in the late Miocene. Frogs in the genus Rhinella are not very dependent on aquatic environments and can live in a broader variety of habitats, but their tadpoles are dependent on a nearby water body. This study also found a possible new species of frog belonging to the genus Rhinella that also lived in the area. There are differences in the pelvis of the fossil Rhinella compared to today’s frogs, indicating that they are different species.

Why is this study important? This study is important because it showed what type of environment the southern Amazonas had in the late Miocene. They were able to see that the Amazonia used to have more lakes and was possibly less tropical, as compared to its modern-day rainforest environment. This study also added clarity to the evolutionary history of when these types of frogs may have evolved.

Broader Implications beyond this study: Any land species, like frogs, are not commonly preserved in the fossil record.  When these rarer fossils are found, they offer massive contributions to the scientific community.  

Citation: Muniz, F. P., Bissaro-Júnior, M. C., Guilherme, E., Souza-Filho, J. P., Negri, F. R., & Hsiou, A. S. (2022). Fossil Frogs from the Upper Miocene of southwestern Brazilian Amazonia (Solimões Formation, acre basin). Journal of Vertebrate Paleontology, 41(6).