By Victor Perez, Florida Museum of Natural History
At the early age of 6, I visited the Calvert Marine Museum in Solomons, Maryland and received my first shark tooth from their “Discovery Room” exhibit. The staff at the exhibit told me I could find my own shark teeth along the beaches in the nearby Calvert Cliffs and I quickly started to accumulate my own personal collection. Fifteen years later, I got the opportunity to work as a vertebrate paleontology intern at the Calvert Marine Museum, during which I began a research project on the evolution of Carcharocles megalodon and the loss of lateral cusplets.
The Calvert Cliffs extend for 25 miles along the Chesapeake Bay and are comprised of nearly continuous stratigraphic layers ranging from roughly 21 to 7.5 million years old (Ma). Experienced collectors had anecdotally reported that Carcharocles teeth with lateral cusplets become less common as one ascends in time from the oldest sediments at the north end of the cliffs to the younger sediments at the south end. We decided to test this observation by analyzing all Carcharocles teeth from the Chesapeake Bay region in the collections of the Calvert Marine Museum and the National Museum of Natural History of the Smithsonian Institution in Washington D.C.
In the oldest stratigraphic layers (~20.2 to 17.6 Ma) 87% of the Carcharocles teeth had lateral cusplets, whereas Carcharocles teeth in the youngest layers (~8.5 to 7.6 Ma) lacked lateral cusplets entirely (Figure 1). Our study found that over this roughly 12.5-million-year period lateral cusplets were gradually lost, rather than an abrupt transition from teeth with lateral cusplets to teeth without lateral cusplets. So the question then becomes “why were lateral cusplets lost?”.
If we look back to one of megalodon’s early ancestors, Otodus obliquus, roughly 60 million years ago, we can see a very different tooth morphology: lacking serrations, large triangular lateral cusplets, and a robust root. This tooth morphology would have a tearing-grasping function that would be ideal for feeding on fast swimming prey. In stark contrast, teeth of C. megalodon are uniformly serrated, lack lateral cusplets, and have broadly flattened triangular shape. This tooth morphology would be ideal for cutting and feeding on larger bodied prey.
As these features are developed in the Carcharocles lineage, whales and dolphins first appear and increase in abundance and size. While the immediate explanation is that megalodon developed cutting teeth to feed on large marine mammals, the question remains as to why this transition took so long. If there was a strong selective pressure towards teeth without lateral cusplets, you might expect that this transition would happen much faster. Consequently, the actual reason for why lateral cusplets were lost remains speculative.
Among the thousands of Carcharocles teeth from the Chesapeake Bay region in these museum collections, over 90% were donated by amateur/avocational paleontologists. In short, this study would not be possible without the efforts of the public, and their willingness to share their discoveries. In fact, roughly two dozen teeth used in this study were posted to the myFOSSIL database that would have otherwise not been known. With that in mind, I encourage everyone to share their discoveries with the world to support new collaborations and future research endeavors!
To learn more:
See more images in the Florida Museum press release “How Megalodon’s Teeth Evolved Into the Ultimate Cutting Machine”