Primordial octopus was up in arms – 10 instead of eight

For the roughly 300 known octopus species dwelling in the world’s oceans, having eight arms is a defining characteristic. But that is not the way it started.

Scientists said on Tuesday a fossil unearthed in central Montana of a species named Syllipsimopodi bideni represents the oldest-known relative of today’s octopuses and boasts 10 arms, with two twice as long as the other eight. The fossil, so well preserved that it reveals two parallel rows of suckers up and down each arm, dates to about 328 million years ago.

Syllipsimopodi, about 4-3/4 inches (12 cm) long, had a torpedo-shaped body and squid-like appearance though it was not closely related to squids, which appeared much later. It also is the oldest-known creature with suckers, which enable the arms to better grasp prey and other objects.

“The fossil greatly changes our understanding of how octopuses evolved and indicates that the earliest members of the group superficially resembled living squids,” said paleontologist Christopher Whalen, a postdoctoral fellow at the American Museum of Natural History in New York and Yale University and lead author of the study published in the journal Nature Communications.

Soft octopus bodies ordinarily do not lend themselves to fossilization, complicating the study of octopus evolution.

Octopuses, ranging from the one-inch (2.5 cm) star-sucker pygmy octopus to the 30-foot (9-meter) giant Pacific octopus, are known for their otherworldly appearance, with bulbous heads, large eyes and beak-like jaws. They are adept at camouflage – changing colors and even textures to mimic their surroundings – and can maneuver their bodies into tiny cracks and crevices. They also are capable of tool use and problem-solving.

“Octopuses are the most intelligent invertebrates, and among the most intelligent animals overall. It is fascinating to see what these unique animals started from evolutionarily,” Whalen said.

Syllipsimopodi pushes back by 82 million years the origins of a group called vampyropods that includes today’s octopuses and the world’s lone species of vampire squid, a misnomer because it is not a squid but rather an octopus cousin.

The very word octopus means eight feet. Syllipsimopodi represents the only member of the octopus lineage with 10 arms, meaning two were lost in later evolution. There are numerous similar examples in the history of life on Earth – such as the reduction in the number of digits seen in meat-eating dinosaurs or horses.

Today’s vampire squids have eight arms and two thin filaments that scientists long have considered vestiges of former arms. Octopuses do not have these vestigial filaments.

“Syllipsimopodi is the first fossil to demonstrate that, yes, vampyropods did ancestrally possess 10 arms as had been predicted,” Whalen said.

Two of Syllipsimopodi’s arms were about 1-1/2 inches long and the other eight half that length, a squid-like configuration.

“Capture of prey is facilitated by the two longer tentacles with the eight shorter arms helping to manipulate the prey and transport it to the beak,” said study co-author Neil Landman, an American Museum of Natural History invertebrate paleontologist.

Syllipsimopodi prowled the warm waters of a tropical bay – Montana at the time was situated close to the equator. It may have been a mid-level predator, eating smaller invertebrates.

Octopuses are cephalopods, a marine invertebrate group dating back to roughly 530 million years ago and distinguished by having arms or tentacles. Cephalopods today also include squids, cuttlefishes and nautiluses.

Syllipsimopodi lived during the Carboniferous Period, a time of important evolutionary changes in other marine life that included the appearance of more modern-looking fishes.

“Syllipsimopodi” means “prehensile foot” – its arms are an evolutionary modification of the foot of mollusks – and “bideni” recognizes US President Joe Biden, who had just been inaugurated when the study was submitted for publication.