Cold water giants: Megalodon may have adapted its size to water temperature

Millions of years ago, the megalodon shark was the ocean’s top predator. Before going extinct just 3.6 million years ago, Otodus megalodon could reach lengths of up to 18 meters. Its modern kin, the 4 meter-long great white shark, almost seems puny in comparison. 

However, there is reason to contend that not all megalodon were enormous. In some areas in Spain, the United States, and Panama, scientists discovered large deposits of relatively small megalodon teeth. Researchers have interpreted this as evidence of nurseries, postulating that the small teeth belonged to juvenile megalodon.

Now, a new study published in the journal Historical Biology shows that these smaller teeth might not come from baby megalodon after all. They might be the remains of smaller adults who lived in warmer waters. Led by Dr. Kenshu Shimada from the University of Chicago, the authors illustrate a striking pattern: The cooler the water, the larger the sharks. The results challenge how scientists think about the evolutionary drivers for body size in ancient sharks – and their modern counterparts.

Relating megalodon teeth length to body size

Like most marine fish, megalodon have skeletons made mostly of cartilage that decomposes quickly after death. Tooth enamel, however, stays preserved. It also provides insight into feeding patterns, as sharks constantly shed and regrow teeth based on their hunting behavior.

In the early 2000s, scientists used modern great white sharks as a proxy to create handy equations that relate tooth length to body size. For the equations to be accurate, researchers must correctly identify a fossil’s tooth positions in a megalodon jaw. Since most megalodon teeth are found individually– with an entire jaw being a coveted, rare discovery – this determination can be tricky. 

Revising equations

When researchers in the mid-2010s extrapolated body size estimates from tooth fossils, they used a linear regression equation that related the tooth crown’s height to the shark’s total length. They also measured lateral teeth whose position on the jaw could be hard to determine, leading to less reliable estimates of body size.

Dr. Shimada and his co-authors decided to re-examine a wide range of megalodon data using only the anterior teeth, which are easier to identify and can provide more reliable estimates of body size. Their goal was to recalculate body sizes and compare them to the original estimates to see if the type of tooth measured changed the body size estimate.

The researchers examined data from 80 teeth spanning a wide geographic range, including fossils from Southern California, Maryland, Northeastern Spain, Peru, Panama, Chile, North Carolina, and Florida. The specimens diverged in size, and teeth from the purported megalodon nurseries sat in the mix.

The geographic breadth of the specimens allowed the researchers to classify the assemblages into three climatic time bins: a “hot” period in the mid-Miocene, a “warm” period in the late Miocene, and a “cold” period in the early Pliocene. This climate range allowed the researchers to ask another question: Do body size trends in megalodon vary based on ocean basins, latitudes, and time? They also wanted to know whether water temperature correlates with body size. To study this, the scientists used an accepted model to convert latitudinal data into sea-surface temperature while accounting for differences in each period’s climatic variables.  

Bergmann’s rule 

Remarkably, the body length estimates were almost identical to the original measurements, no matter which equation the researchers used.

However, when the authors considered sea-surface temperature for each assemblage, they noticed an interesting pattern. In each of the three time periods studied, mean length of the tooth was inversely related to sea-surface temperature. In other words, the warmer the water, the smaller the tooth — and therefore, the smaller the shark. Though the smallest sharks were still big by today’s standards (between 4 and 10 meters long), they were significantly smaller than some of their counterparts, who grew as long as 15 meters.

In fact, almost all of the potential megalodon nurseries identified before lived in warmer regions or during warmer times, meaning that smaller adult megalodon might have been mischaracterized as juveniles. Plus, the largest estimated megalodon individuals came from relatively high latitudes (North Carolina, South Carolina, and Chile) that had cooler estimated temperatures.

This pattern is consistent with the ecological concept called Bergmann’s rule – a  generalization that explains the trend that larger animals are found in cooler climates. The rationale is that larger animals have a smaller surface-to-volume ratio and therefore can retain heat more easily than smaller animals, a distinct advantage in cooler climates. Although Bergmann’s rule has been demonstrated for terrestrial and marine animals, this study would be the first instance of the rule applying to sharks.

The implications of this pattern go on to inform many other aspects of megalodon biology. Living with a large body changes how a megalodon swims, which prey it hunts, and the type of metabolic demands it must meet. If Bergmann’s rule does apply to gigantism in megalodon, it means that cool temperatures constitute an important ecological and evolutionary driver for the ancient sharks. 

Modern lessons from the megalodon

Megalodon’s story has important implications for the fate of modern marine creatures. Although the megalodon study used a small sample size concentrated almost entirely in the western hemisphere, the results suggest that water temperature affects sharks’ body size — and every other related biological characteristic. 

General ecological and biological drivers do not wane over time. The same rules that applied to megalodon will influence the behavior of today’s biggest fishes. Therefore, as temperatures warm because of climate change, apex predators like sharks will probably shift their habitats to areas where their body size will serve them better – polar latitudes with cooler waters. Armed with a newfound understanding of their biology, we can better create plans to conserve and manage these iconic species and the marine habitats they will soon call home.