'Kraken' octopus that lived at the time of the dinosaurs was a 62-foot-long apex predator of the ocean

Scientists have identified enormous finned "kraken" octopuses that may have reached up to 62 feet (19 meters) long. The behemoths prowled the oceans during the Cretaceous and could be the largest invertebrates ever discovered.

Fossil jaws revealed distinctive markings that suggest these kraken-like octopuses used their powerful jaws to crush hard-shelled prey. That, combined with their gigantic size and evidence of intelligence, put them top of the marine food chain, according to a study published Thursday (April 23) in the journal Science.

This finding suggests scientists need to rethink the oceanic pecking order during the Cretaceous period (145 million years to 66 million years ago).

"These findings revise the view of the Cretaceous ocean as a world dominated only by large vertebrate predators," study co-author Yasuhiro Iba, a paleontologist at Hokkaido University in Japan, told Live Science in an email. "They show that giant invertebrates — octopuses — also occupied the top of the food web."

Other experts say these size estimates are the upper end of a large possible range. Even so, the discovery raises questions about the oceanic landscape of the Cretaceous, such as how these species could grow so large, and whether even larger marine species existed after the Cretaceous period, they said.

Hunting down the apex predators

Species at the top of the food chain shape ecosystems, with their prey responding by evolving protection measures, such as hard shells. Understanding which species held the apex position is essential for understanding how Cretaceous marine ecosystems functioned, Iba said.

Until now, the top dogs were all assumed to be vertebrates, such as mosasaurs and plesiosaurs. However, the lack of preserved evidence from soft-bodied octopuses has made their position in the Cretaceous food chain a complete mystery, the authors wrote in the study.

"Octopuses are known today as highly intelligent animals, but they are extremely difficult to study in deep time because they lack hard external shells," Iba said. "A major motivation for this study was to reveal this almost invisible history of octopuses."

For the study, the researchers reassessed 15 fossilized octopus jaws previously unearthed in Japan and Vancouver Island. They also discovered 12 new Cretaceous fossil octopus jaws in Japan using state-of-the-art digital fossil-mining technology. Combined, these revealed two species of extinct finned octopuses: Nanaimoteuthis jeletzkyi and Nanaimoteuthis haggarti.

The N. jeletzkyi fossils were unearthed in rocks dating to between 100 million and 72 million years ago, pushing back the oldest known octopuses by around 5 million years, and finned octopuses by 15 million years, the authors wrote in the study.

The team then compared the size, shape and wear marks on all 27 jaws with modern-day octopuses to reconstruct their body size, feeding behavior and position in the food web.

The size of living octopuses' mantles — the bulging organ sac sitting above their eyes — is related to the length of their jaws. The total length of living long-bodied finned octopuses is around 4.2 times their mantle length.

Iba and his colleagues used this to estimate just how bulbous N. jeletzkyi and N. haggarti mantles were. From there, they could calculate their possible total length of the long-dead creatures.

Based on the largest jaw for each species, the team estimated the maximum length of N. jeletzkyi was around 10 feet to 26 feet (3 m to 8 m), while N. haggarti was approximately 23 feet to 62 feet (7 m to 19 m). This makes N. haggarti potentially the largest invertebrate discovered to date, and "among the largest body sizes of all organisms in the Cretaceous oceans," the authors wrote in the study. (Modern-day giant squid, Architeuthis dux, reach around 40 feet (12 m) long, and Cretaceous mosasaurs reached approximately 56 feet (17 m) long.)

The kraken jaws also showed signs of intensive wear, with patterns indicating that these animals were dismantling hard-shelled prey using their whole jaws. The front tips on both species' jaws were ground down on one side by as much as 10% of their total size, based on reconstructions. This lopsided loss suggests lateralized behavior, which is linked to being brainier, the authors said in the study.

"These were not just giant octopuses, but giant, intelligent, and highly formidable marine predators," Iba said.

However, while experts applauded the digital fossil-hunting techniques used in the study, they questioned the size estimates of each species.

The study researchers estimated the size of N. jeletzkyi and N. haggarti using "error prone" averages of jaw-to-mantle and mantle-to-total-body size relationships of living species, meaning their results produced a large possible size range for both species, René Hoffman, a paleontologist focusing on fossil cephalopods at the Ruhr University Bochum in Germany, told Live Science in an email.

Their enormous size also does not necessarily mean that these invertebrates were the top predator, Hoffman added.

Christian Klug, a professor of paleontology and expert in cephalopod evolution at the University of Zurich in Switzerland, agreed. While the estimates are within the range of what is possible, he said that some uncertainty is inevitable. "There is no doubt that Nanaimoteuthis was a huge and efficient predator," he told Live Science in an email, but only focusing on the maximum total size "lets one forget that it is conceivable that they may have not reached ten meters."

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