Enlarge / Reconstruction of the orthocone ammonite Baculites compressus. A new study reveals that the creatures likely lived a vertically oriented lifestyle.

David Peterman

The fossil record is chock-full of the fossilized remains of spiral-shelled ammonoids, whose shapes are reminiscent of rams’ horns. But there was another type of ammonoid with long, straight, uncoiled shells, known as orthocones, that particularly flourished during the early Paleozoic. Prior reconstructions have depicted these creatures as being horizontal swimmers, similar to today’s squid.

But a new investigation that involved dropping 3D-printed models into water tanks reveals that most species of orthocones would not have been able to swim well horizontally. Instead, the creatures likely led a vertically oriented lifestyle, moving leisurely up and down through the water column to hunt and sometimes executing rapid upward dodges as needed to avoid predators, according to a recent paper published in the journal PeerJ.

Co-authors David Peterman and Kathleen Ritterbush are paleontologists at the University of Utah. They previously developed digital models of ammonoids with coiled shells to investigate the evolution and lifestyle of these creatures. This time, they’ve turned their attention to a species of orthocone cephalopods (Baculites compressus) that lived during the Cretaceous Period. The authors hypothesized that there must be some adaptive benefit to having a straight shell, since the spiral shell of the orthoconic ammonoids has evolved several times in different lineages found in the fossil record.

There are hundreds of genera of orthocones. Little is known about their soft-body characteristics, but prior studies have concluded that their mass would have been distributed toward the front of the body chamber. It’s also known that early cephalopods expelled jets of water from their mantle cavity to move around in the water. They had mineral deposits that may have served as counterweights, thereby influencing the hydrostatics of the creatures in some way. “They were major components of marine ecosystems, yet we know very little about their swimming capabilities,” said Peterman.

Peterman and Ritterbush posited that it would have been difficult for such creatures to swim horizontally, limiting them to living vertically oriented lives. To test that hypothesis, the paleontologists built four 3D-printed hydrostatic models of prehistoric orthocones, relying on 3D scans of fossils to inform their design. The 2-foot-long models all had the same centers of buoyancy, since the external volumes were the same, but their centers of mass and bismuth counterweights were different in order to explore the balances of soft tissue and air-filled voids that the orthocone would likely have maintained in life.

Four experimental runs were performed in the deepest end of the so-called “crimson lagoon,” the University of Utah’s 50-meter lap pool. The team held the models with extendable tongs and then released them into the water. The researchers set up an underwater camera rig to record the movements of the models as they moved in the water. Once released, the models primarily moved in vertical directions, apart from some slight skewing in the horizontal direction due to the small currents created when the researchers removed the release mechanism.

Peterman and Ritterbush were both surprised at how stable all the models proved to be when it came to maintaining a vertical orientation. “Any amount of rotation away from their vertical orientation is met with a strong restoring moment, so many species of living orthocones were likely unable to modify their own orientations,” said Peterman. “Furthermore, the source of jet thrust is situated so low that, during lateral movement, much energy would be lost due to rocking.”

The authors found that the cephalopods could sink slowly down the water column. “This condition would have allowed low-energy movement and feeding for vertical migrants, while also providing suitable speeds to pounce on [slower] benthic from above,” they wrote. Furthermore, the orthocones may have been able to thrust themselves upward quite rapidly at times, peaking at 1.2 m/s, or 2.1 body lengths per second.

The authors think the occasional high-speed upward thrust might have helped these otherwise low-energy animals evade predators, so they compared their experimental results with the time they thought would be needed to escape modern predators similar to the now-extinct varieties that likely fed on orthocones. To successfully evade most predators (akin to whales or crocodiles, for instance), an orthocone would have to wait until the last possible moment to execute an upward thrust. Otherwise, the attacking predator could easily adjust its trajectory in time to capture the orthocone. (The researchers’ analysis did not consider repeated attacks by a predator.)

For predators with both speed and quick maneuverability, akin to today’s dolphins and some sharks, even the upward dodge would likely be insufficient. In such cases, “perhaps it was more favorable for an orthoconic cephalopod to hide in its shell rather than attempting to vertically escape,” they wrote. “Therefore, vertically escaping from larger predators that mark certain death is likely a last resort for orthoconic cephalopods.”

The Utah team also conducted similar experiments in water tanks with 3D-printed models of smaller cephalopods known as torticones, which have long shells shaped like a corkscrew. These creatures probably also led vertically oriented lives, although the experiments revealed that those corkscrew shells allowed them to be “masters of rotation,” per Peterman. That idea contradicts a prior assumption that torticones crawled along the ocean floor, much like modern-day mollusks.

According to Peterman, even the act of breathing (aka “gill ventilation”) would have been sufficient to start a gentle spin in a torticone. The torticone models would rotate one way while ascending and the other while descending. The authors suggest that a gently rotating descent would have helped the animals feed on plankton and similar organisms.

“These experiments transform our understanding of ancient ecosystems,” said Peterman. “Rather than crawling along the seafloor like snails, or swiftly swimming like modern squid, these animals were assuming rather unique lifestyles. These experiments refine our understanding of these animals by painting a picture of ancient seascapes dotted with pirouetting helical cephalopods and vertically oriented orthocones.”

DOI: PeerJ, 2021. 10.7717/peerj.11797  (About DOIs).

https://arstechnica.com/?p=1786458