Figuring Out When the octopus and squid lost their shells

A 166-million-year-old fossil of an extinct relative of the squid. Credit Jonathan Jackson and ZoË Hughes/National History Museum of London

Shaped like a torpedo and about as swift, squids are jet-propelled underwater predators. Together with their nimble brethren, the octopus and cuttlefish, they make for an agile invertebrate armada.

But that was not always the case. Hundreds of millions of years ago, the ancestors of the tentacled trio were slow, heavily armored creatures, like the coil-shelled ammonites and the cone-shelled belemnites.

Alastair Tanner, a doctoral student at University of Bristol in England, wanted to better understand why those cephalopods lost their shells. But though both ammonites and the belemnites have left behind rich fossil records, their shell-less descendants have not.

So Mr. Tanner conducted a genetic analysis of 26 present day cephalopods, including the vampire squid, the golden cuttlefish and the southern blue-ringed octopus.

With the molecular clock technique, which allowed him to use DNA to map out the evolutionary history of the cephalopods, he found that today’s cuttlefish, squids and octopuses began to appear 160 to 100 million years ago, during the so-called Mesozoic Marine Revolution.

During the revolution, underwater life underwent a rapid change, including a burst in fish diversity. Some predators became better suited for crushing shellfish, while some smaller fish became faster and more agile.

“There’s a continual arms race between the prey and the predators,” said Mr. Tanner. “The shells are getting smaller, and the squids are getting faster.”

The evolutionary pressures favored being nimble over being armored, and cephalopods started to lose their shells, according to Mr. Tanner. The adaptation allowed them to outcompete their shelled relatives for fast food, and they were able to better evade predators. They were also able to keep up with competitors seeking the same prey.

Today most cephalopods are squishy and shell-less. The biggest exception is the nautilus. But though there are more than 2,500 fossilized species of nautilus, today only a handful of species exist.

Squid and octopus species number around 300 each, and there are around 120 species of cuttlefish. The differences in number, compared with the nautilus, indicates the advantages that these cephalopods may have gained over their shelled relatives, according to Mr. Tanner.

“It became a much more successful strategy to be a really high metabolism, very rapid moving animal,” Mr. Tanner said, “and they evolved into these really quite amazing things we see today.”

The study was published in the journal Proceedings of the Royal Society B: Biological Sciences.