Octopus origins

Some of the earliest relatives of today’s octopuses, squid and cuttlefish had only two tentacles and measured no longer than a child’s little finger, a new study suggests.

Previously, scientists didn’t know where to put Nectocaris pteryx on life’s family tree because the species was known only from one fragmentary, poorly preserved fossil unearthed from the 505 million-year-old Burgess Shale of British Columbia, says Jean-Bernard Caron, a paleontologist at the Royal Ontario Museum in Toronto. “That specimen had some characteristics of arthropods and some of chordates,” the group that includes vertebrates, he notes. “No one knew how to classify it.”

Now, after scrutinizing more than 90 additional fossils of the creature, including many that are more complete and better preserved, Caron and museum colleague Martin Smith suggest in the May 27 Nature that Nectocaris belongs near the base of the cephalopod family tree. That group includes today’s octopi and squids, as well as long-extinct creatures such as ammonites and belemnites.

The newly described specimens were unearthed from the Burgess Shale formation since 1984 at a site near where the first Nectocaris fossil was discovered early last century.

The new fossils show that the two-tentacled Nectocaris measured, on average, less than 4 centimeters long from tip to tip, or about two-thirds the length of an adult’s little finger. The flexible tentacles, which had a textured surface that presumably enhanced its ability to grasp and manipulate prey, accounted for slightly more than half of the creature’s length, says Caron.

Nectocaris had flexible fins that ran the length of its body, muscle-driven fringes of tissue similar to those sported by today’s cuttlefish. “Its shape was very much like a squid,” says Caron.

Unlike today’s cephalopods, Nectocaris apparently didn’t have a tough beak. What’s more, none of the fossils include remains of a radula, a rasping tongue covered with small toothlike structures, but a few species of modern cephalopods don’t have radulae either. In Nectocaris, the radula may have been so small that it wasn’t readily preserved, Caron notes.

“The seeming lack of a radula in Nectocaris is therefore a difficulty for the cephalopod interpretation” of this species, Stefan Bengtson, a paleontologist at the Swedish Museum of Natural History in Stockholm, comments in Nature. “Further studies … should try to sort out whether a radula is truly lacking or just inconspicuous,” he adds.

On the plus side, though, Nectocaris does sport one feature found only in cephalopods: a siphonlike tube of tissue that could have been used to direct water flow — the animal kingdom’s version of jet propulsion.

When the first specimen of Nectocaris was described in 1976, that siphon was thought to be a protective shield over part of the body — one reason that the species was originally considered to be related to arthropods. But the new fossil specimens show that the siphon on the bottom of the creature is flexible and can be pointed in nearly any direction, says Caron.

“The siphon shows that this creature lived in the water column, not on the bottom,” says Amélie Scheltema, a marine biologist at Woods Hole Oceanographic Institution in Massachusetts who specializes in invertebrates.

Despite the presence of a siphon, Scheltema is cautious about saying that Nectocaris is an ancient relative of today’s cephalopods. “We really need more info before we know exactly where to place them,” she notes.