Long before the first dinosaurs roamed the earth, the oceans were full of creatures known as ammonites. Scientists have now created a number of robotic ammonites, to see how the different shell shapes they evolved affected their movement through the water.
Ammonites belonged to the cephalopod group of marine invertebrates, current members of which include octopi, squid and cuttlefish. Unlike those examples, however, ammonites had protective outer shells – and those shells didn't maintain one consistent shape throughout the fossil record.
Led by postdoctoral fellow David Peterman and Asst. Prof. Kathleen Ritterbush, a team at the University of Utah recently set out to determine how the different shell shapes affected the animals' locomotion. In order to do so, the scientists created free-swimming robotic ammonites.
Each one consisted of a 3D-printed polymer shell with a watertight inner chamber, inside of which were electronics including batteries, a microcontroller, a motor, and an impeller-driven water pump. There were also air-filled voids and counterweights, in order to replicate the weight distribution of the existing nautilus – it's the only present-day cephalopod with a shell.
What's more, the robots were neutrally buoyant. This means that when placed in the water, they neither sank to the bottom nor floated to the surface.
Their shell shapes included a serpenticone, which combined tight whorls with a narrow shell; a sphaerocone, which featured a few thick whorls and a wider, almost spherical shell; and a somewhere-in-between oxycone, which combined thick whorls with a narrow, streamlined shell.
Each model was initially placed in an underwater clamp in a pool, then released so it could jet its way through the water. As it did so, its movements and position in three-dimensional space were recorded by an underwater video camera. Each model made about a dozen individual runs.
When the footage was analyzed, it was found that each shape had its own strengths and weaknesses. The narrower shells, for example, produced less drag and were more stable when moving straight through the water. The wider shells, while making for slower, less energy-efficient travel, could change direction more easily – a characteristic that may have helped the ammonites catch prey or escape predators.
"These results reiterate that there is no single optimum shell shape," said Peterman. "Natural selection is a dynamic process, changing through time and involving numerous functional tradeoffs and other constraints. Externally-shelled cephalopods are perfect targets to study these complex dynamics because of their enormous temporal range, ecological significance, abundance, and high evolutionary rates."
A paper on the research was recently published in the journal Scientific Reports.
Source: University of Utah
