Complex shell patterns helped octopus ancestors fine-tune their buoyancy

April 13 (UPI) -- Ammonoids, an ancient group of mollusks, used the wavy lines found inside their shells to fine-tune their buoyancy, according to a new study.

Prior to their disappearance at the end of the Cretaceous Period, ammonoids, ancestors of today's octopus, squid and cuttlefish, were a constant presence in Earth's oceans for more than 340 million years.

Over the course of their time on Earth, the mollusks evolved increasingly complex patterns on the insides of their shells. By the Age of the Dinosaurs, the lines on ammonoid shells -- called sutures -- looked like the frilled edges of kale leaves.

Previously, scientists estimated suture complexity aided the structural integrity of ammonoid shells. But new research, published Tuesday in the journal Scientific Reports, suggests suture complexity allowed ammonoids to more precisely control their buoyancy.

Researchers tested their hypothesis with 3D-printed models of different ammonoid shells.

"These hypotheses couldn't be tested without being able to create incredibly accurate models of these intricate features," lead study author David Peterman said in a news release.

"The 3D printed models allow the fabrication of incredibly intricate chamber walls that have details comparable to the living animals," said Peterman, a postdoctoral scholar in the department of geology and geophysics at the University of Utah.

To design the models, researchers relied on ancient fossil imprints, as well as close examination of the shells of nautiloids, distant relatives of ammonoids.

When ammonoids first appear in the fossil record, during the Devonian Period, their sutures appear straight. Over time, they appear wavier and wavier.

Previously, scientists surmised this growing complexity helped buttress the shell's layers, boosting strength and structural integrity.

But the authors of the latest study suggest the wavier lines effect on surface tension -- and in turn, buoyancy -- was more important than any structural benefits.

In the lab, scientists weighed their 3D ammonoid models before dunking them in water, draining them and then weighing them again. The process helped scientists estimate the amount of water helped by surface tension.

The tests showed ammonoids with complex sutures held more water via surface tension, which scientists estimate would have allowed the mollusks to more precisely control their buoyancy.

Ammonoids reached their complexity peak just prior to their extinction at the end of the Cretaceous Period. Only the simply sutured relatives of the modern nautilus survived, though scientists suspect other factors were key to its survival.

Moving forward, researchers said they hope to build increasingly functional ammonoid models in order to better understand how the ancient mollusks adapted to different ecosystems, and why the sea creatures ultimately disappeared.

"While we won't be able to revive these animals like the dinosaurs in Jurassic Park, computer simulations and experiments such as these are the closest we will get to bringing these ecologically significant cephalopods back to life," said Peterman.