What do sperm whales have to do with Late Miocene sharks? A lot, it turns out.
It’s a showdown as old as time: the great behemouth of a sea mammal versus the ancient monster of a shark. These shark-cetacean trophic interactions span eons, preserved as bite marks in our fossil record to be discovered by researchers around the world. Most of these fossils are isolated or fragmentary finds that contain limited information about major trophic patterns or roles in the past. In a New articlescientists provide evidence of shark-focused foraging in the form of tooth bite marks on physteroid fossil bones from the late Miocene of Peru (about 23 million to 2.5 million years ago ).
Today, Peru is the third largest country in South America, after Brazil and Argentina. As thousands of visitors flock here to take a look at the Inca citadel of Machu Picchu, this site is just a flash in the history of this country – or its surroundings. The oceans then, as now, were teeming with life. From the smallest organisms to the largest predators, physteroid toothed whales were one hell of a foe to contend with. Still, something definitely did, as their scattered remains were discovered in the Pisco Formation. A geological formation located in the Ica Desert on the southern coast of Peru, it is a globally significant fossil deposit known for its remarkable Cenozoic marine Fossil-Lagerstätten worldwide.
It was no surprise to see sperm whales (Physeteroidea) here. Physteroids are among the oldest lineages of crown cetaceans, and while living sperm whales are represented by only three species (Physeter macrocephalus, Breviceps of Kogia and Kogia sima), their fossil record provides evidence for an ecologically diverse array of different forms. But what was surprising were the marks on the bones.
This remnant of ancient life helps researchers today by providing direct evidence of trophic interactions and competition in the fossil record. These recently published the results indicate that the sharks targeted the foreheads of these whales to actively feed on their lipid-rich nasal complexes. Sperm whales are well known for their noses which make up about a third of the animal’s total length and weight. You would think that would give you an incredible sense of smell, but not all toothed whales have an olfactory bulb. Instead, this head is filled with spermaceti, which have incredible resonance abilities that allow them to find prey by echolocation (a biological sonar used by several animals besides sperm whales). These predators are famous for diving into the depths to hunt for a meal by emitting high-frequency clicks that will then bounce off an object, returning an echo that provides a location of that object (aka the prey). It is essential for sperm whales in catching prey; and causes them to attack themselves. The sperm whale’s enlarged fatty frontal organs are a perfect target, as lipids are the main sources of metabolic energy in sharks.
“The main organs of this complex are the spermaceti and the melon, structures rich in fats and oils, but also strongly regulated by facial muscles,” explained the study’s lead author, Aldo Benites-Palomino, a paleontologist. at the University of Zurich. “Most bite marks were found on bones that would be adjacent to these soft tissue structures, such as the jaws, or around the eye, indicating that the sharks were actively targeting this region.”
The bite marks varied, indicating that several species of sharks enjoyed this feast. Some probably belonged to the fearsome Megalodon, while others seem to coincide more with modern sharks. But why did they go after sperm whales when baleen whales were also present in the Miocene? “During the Miocene, baleen whales were small, but sperm whales would have been a perfect fat depot due to their highly enlarged, lipid-rich nasal organs,” Benites-Palomino said. “Similar bite patterns can be recognized on other Miocene physteroid fossils around the world, suggesting that sharks actively exploited physteroid carcasses as sources of fat.”