David Peterman, Kathleen Ritterbush and their colleagues at University of Utah took 3D printed reconstructions of fossil cephalopods to actual water tanks (including a University of Utah swimming pool) to see how their shell structure may have been tied to their movement and lifestyle.
“Thanks to these novel techniques,” says Peterman, a postdoctoral scholar in the Department of Geology and Geophysics, “we can trudge into a largely unexplored frontier in paleobiology. Through detailed modeling, these techniques help paint a clearer picture of the capabilities of these ecologically significant animals while they were alive.”
They found that cephalopods with straight shells called orthocones likely lived a vertical life, jetting up and down to catch food and evade predators. Others with spiral shells, called torticones, added a gentle spin to their vertical motions.
The researchers are veterans of this style of “virtual paleontology,” having worked with digital ammonoid models and 3D printed versions to test hypotheses about their evolution and lifestyles. Most ammonoids have coiled shells, like today’s chambered nautilus, and darted around the ocean in all directions.
But in their most recent study published in the journal PeerJ the researchers explored a different shell shape—the straight-shelled orthocone. Straight shells evolved several times in different lineages throughout the fossil record, suggesting they had some adaptive value.
“This is important because orthocones span a huge chunk of time and are represented by hundreds of genera,” Peterman says, and many reconstructions and dioramas show orthocones as horizontal swimmers like squid. “They were major components of marine ecosystems, yet we know very little about their swimming capabilities.”
The team took 3D scans of fossils of Baculites compressus, an orthocone species that lived during the Cretaceous, and designed four different digital models, each with different physical properties.
They adjusted the centers of mass within the models, representing the balances of soft tissue and air-filled voids that the orthocone would likely have maintained in its life.
The resultant 3D printed models were nearly 60 centimeters (2 feet) long. The researchers released the models underwater and filmed their movements.
The results showed clearly that the most efficient method of movement was vertical, since moving side to side created a lot of drag. “I was surprised by how stable they are,” Peterman says. “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.”
The results also showed that orthocones may have been capable of high velocities among shelled cephalopods. Similar to modern squids or the modern Nautilus, early cephalopods moved around thanks to a jet of water being expelled from the mantle cavity via a flexible funnel. That could have come in handy in evading predators. Looking at the results of the pool experiments and calculating the time needed to escape modern predators (as stand-ins for the orthocones’ long-extinct predators), they found that orthocones may have been able to jet upward fast enough to evade animals similar to crocodiles or whales. They may not have been as lucky against fast swimmers like sharks, however.
So most species of orthocones couldn’t have lived a horizonal-swimming lifestyle. “Instead,” Peterman says, “species without counterweights in their shells assumed a vertical life habit, either feeding near the seafloor or vertically migrating in the water column. While orthocones were not as athletic or active as modern squid, they could have maintained the ability to thwart predators with upward dodges.”