by Catalina Pimiento, Doctoral Student, Florida Museum of Natural History, University of Florida
My most recent study on Megalodon, the largest shark that ever lived, was published in the open access journal PLoS ONE last month.This work was the result of a collaborative project with Chris Clements, an expert in mathematical methods that calculate times of extinction, and is part of a larger project where I intend to reconstruct the extinction of this giant shark.
Why do I study the extinction of Megalodon?
Megalodon is a very important species because it was an apex predator. Apex predators are animals that are at the top of the food chain, and have no threats from other predators. These animals maintain ecosystem stability as they control the populations of their prey. Therefore, their removal produces cascading effects (affecting all trophic levels) with catastrophic effects.
Given its importance, the extinction of apex predators has been extensively studied by modern ecology. However, the temporal and spatial scales of these studies are usually limited. Consequently, what we know about the extinction of apex predators is based on population declines or local extirpations, as opposed to actual extinctions. Hence, the study of the extinction of the Megalodon has the potential to offer a broader perspective, not just because it is a giant and cosmopolitan species, but because it has an extensive fossil record spanning for millions of years.
That is why I have been studying the extinction of Megalodon for the last several years. But in order to better understand the extinction of this animal, the first step is to know when it happened. The extinction of species is something we cannot observe directly. Although some scientists use the date of the most recent fossil as a proxy for their extinction time, the fact is that species can survive long after the last time they were recorded. To calculate the most likely extinction date, several methods based on their last records (or sightings) have been developed.
Last year, I had the privilege to attend an international ecology conference (INTECOL) in London. There, I attended a talk on an experimental study that tested the effectiveness of the methods that have been proposed to calculate dates of extinction. Given that I work with the largest shark that ever lived, it was a bit ironic that I found a talk about protists so interesting. In any case, I decided to invite the speaker (Chris Clements) to the talk I was giving the next day. In my talk, I mentioned my studies on the evolution of body size of Megalodon, and my intentions to do a meta-analysis of the fossil record to reconstruct its extinction. Chris and I met later that day and decided to study the extinction time of Megalodon combining the methods he works with, and my ideas and results of the meta-analysis.
We started by collecting the most recent records of the species. For that, we used the Paleobiology Database (PaleoBioDB) as a platform. As this database was incomplete (for Megalodon), we collected all articles reporting the occurrence of species, and added them to the PaleoBioDB. All these data are available to the public.
Once we constructed the dataset (# 20 in the PaleoBioDB), we evaluated each record to make sure to include in the analysis only those with sufficient evidence on the age of the fossils. With this subset of data, we used the Optimal Linear Estimation model (OLE), which has been used before to calculate the extinction time of the Dodo. This method calculates the time of extinction based on the distribution of the most recent records. Given that in our case, the records do not have an absolute date, but instead, they span in a range of time, we re-sampled the age of each record 10,000 times from the highest to the lowest age.
The results suggest that the Megalodon became extinct around 2.6 million years ago. Since it has been suggested this species interacted with marine mammals, we proceeded to compare the results with the known evolution and diversification patterns of cetaceans.
What a surprise!
We noticed that our calculated extinction time for Megalodon coincides with the Pliocene and Pleistocene boundary. During the Pleistocene, modern baleen whales reached giant sizes. Therefore, in our study we suggest that the size, and therefore the ecological role of modern whales, was established once the world’s largest shark, Megalodon, became extinct.
For now, our study only provides the time of the extinction and recognizes the coincidence with the evolution of gigantism in modern whales. Knowing if an event caused another is our next step.
The new paper on Megalodon extinction is available here.
Read Catalina’s paper on ancient Megalodon nurseries here.